Multiwavelength optical networks: architectures, design, and control

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Multiwavelength Optical Networks, Second Edition Updated and expanded, this second edition of the acclaimed Multiwavelength Optical Networks provides a detailed description of the structure and operation of modern optical networks. It also sets out the analytical tools for network performance evaluation and optimization for current and next generation networks, as well as the latest advances in enabling technologies. Backbone optical networks are evolving to mesh topologies utilizing intelligent network elements; a new optical control plane is taking shape based on GMPLS; and significant advances have occurred in Fiber to the Home/Premises (the “last mile”), metropolitan area networks, protection and restoration, and IP over WDM. Each of these is treated in depth, together with new research on all-optical packet-switched networks, which combine the speed of optics with the versatility of packet switching. Also included are current trends and new applications on the commercial scene (wavelengths on demand, virtual private optical networks, and bandwidth trading). With its unique blend of coverage of modern enabling technologies, network architectures, and analytical tools, the book is an invaluable resource for graduate and senior undergraduate students in electrical engineering, computer science, and applied physics, and for practitioners and researchers in the telecommunications industry. Thomas E. Stern is Professor Emeritus of Electrical Engineering at Columbia University, New York, and has served as department chair and technical director of Columbia’s Center for Telecommunications Research. A Fellow of the IEEE, he holds several patents in networking. He has also been a consultant to a number of companies, including IBM, Lucent, and Telcordia Technologies. Georgios Ellinas is an Assistant Professor in the Department of Electrical and Computer Engineering at the University of Cyprus, Nicosia. He has held prior positions as an Associate Professor at City College of New York, as a Senior Network Architect at Tellium Inc., and as a Senior Research Scientist at Bell Communications Research. He has authored numerous papers and holds several patents in the field of optical networking. Krishna Bala is currently the CEO of Xtellus, a company that manufactures fiber optical switches. Krishna was the co-founder and CTO of Tellium (NASDAQ: TELM), a successful optical networking company. Prior to that he was a Senior Research Scientist at Bell Communications Research. He holds a Ph.D. in electrical engineering from Columbia University.

Multiwavelength Optical Networks, Second Edition Architectures, Design, and Control THOMAS E. STERN Columbia University

GEORGIOS ELLINAS University of Cyprus, Nicosia

KRISHNA BALA Xtellus

CAMBRIDGE UNIVERSITY PRESS

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521881395 © Cambridge University Press 2009 This publication is in copyright. Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published in print format 2008

ISBN-13

978-0-511-48054-6

eBook (NetLibrary)

ISBN-13

978-0-521-88139-5

hardback

Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate.

To Monique, who has always been there for me. To our children and our grandchildren (T.E.S.) To my loving mother, Mary, and sister, Dorita, and the memory of my beloved father, Nicos (G.E.) To my wife, Simrat, and our children, Tegh and Amrita (K.B.)

Contents

Figures Tables Preface to the Second Edition Acknowledgments 1

2

page xvii xxix xxxi xxxv

The Big Picture

1

1.1 Why Optical Networks? 1.2 Objectives of an Optical Network Architecture 1.3 Optics versus Electronics: The Case for Transparent Multiwavelength Networks 1.4 Optics and Electronics: The Case for Multilayered Networks 1.5 Network Hierarchies 1.6 A Little History 1.7 Overview and Road Map

1 4 9 12 16 18 22

The Layered Architecture and Its Resources

28

2.1 Layers and Sublayers 2.2 Network Links: Spectrum Partitioning 2.3 Optical Network Nodes: Routing, Switching, and Wavelength Conversion 2.3.1 Static Nodes 2.3.2 Dynamic Nodes 2.3.3 Wavelength Converters 2.4 Network Access Stations 2.4.1 Transmitting Side 2.4.2 Receiving Side 2.5 Overlay Processors 2.5.1 Regeneration 2.5.2 Wavelength Interchange 2.6 Logical Network Overlays 2.6.1 SONET Networks 2.6.2 ATM Networks

29 34 39 40 46 63 67 70 71 74 76 76 77 79 81

viii

3

4

Contents

2.6.3 IP Networks 2.6.4 MPLS and Its Extensions 2.7 Summary 2.8 Problems

83 84 85 87

Network Connections

91

3.1 Connection Management and Control 3.1.1 Optical Connections 3.1.2 Logical Connections 3.2 Static Networks 3.2.1 Point-to-Point and Multipoint Connections 3.2.2 Packet Switching in the Optical Layer: The MAC Sublayer 3.2.3 Additional Comments on Broadcast-and-Select 3.3 Wavelength-Routed Networks 3.3.1 Routing and Channel Assignment 3.3.2 Routing and Channel Assignment Examples 3.4 Linear Lightwave Networks: Waveband Routing 3.4.1 Routing and Channel Assignment 3.4.2 Multipoint Subnets in LLNs 3.4.3 A Seven-Station Example 3.5 Logically-Routed Networks 3.5.1 Point-to-Point Logical Topologies 3.5.2 Multipoint Logical Topologies: Hypernets 3.6 Summary 3.7 Problems

96 100 100 102 104 111 121 122 124 128 133 135 140 143 151 153 156 162 163

Enabling Technology

165

4.1 Evolution of Transmission and Switching Technology 4.2 Overview of the Optical Connection 4.3 Optical Fibers 4.3.1 Principles of Guided-Wave Propagation 4.3.2 Optical Fiber Technology: Transmission Impairments 4.3.3 Solitons 4.3.4 Photonic Crystal Fibers 4.4 Amplifiers 4.4.1 Erbium-Doped Fiber Amplifiers 4.4.2 Raman Amplifiers 4.4.3 Semiconductor Optical Amplifiers 4.4.4 Amplification Trends in Metro Optical Networks: Amplets 4.5 Optical Transmitters 4.5.1 Lasers 4.5.2 Vertical Cavity Surface Emitting Lasers 4.5.3 Modulation Technology

166 167 168 168 174 187 188 190 191 198 201 204 205 205 211 212

Contents

4.6 Optical Receivers in Intensity-Modulated Direct-Detection Systems 4.6.1 Photodetectors 4.6.2 Front-End Amplifier: Signal-to-Noise Ratio 4.6.3 Digital Signal Detection: Noise, Interference, and Bit Error Rate 4.6.4 Analog Systems: Carrier-to-Noise Ratio 4.7 The End-to-End Transmission Channel 4.7.1 Modulation Formats 4.7.2 Forward Error Correction 4.7.3 Equalization 4.8 Coherent Optical Systems 4.9 Performance Impairments in a Network Environment 4.9.1 Cross-Talk 4.9.2 Signal Power Divergence 4.9.3 Chirp-Induced Penalty 4.9.4 Optical Filter Concatenation: Distortion-Induced Penalty 4.9.5 Polarization Mode Dispersion Impact on System Performance 4.10 Optical and Photonic Device Technology 4.10.1 Couplers and Switches 4.10.2 Reciprocity 4.10.3 Nonreciprocal Devices 4.10.4 Optical Filtering Technology 4.10.5 Multiwavelength Switch Technology 4.11 Wavelength Conversion and Signal Regeneration 4.11.1 All-Optical Wavelength Conversion 4.11.2 Opaque Wavelength Conversion and Signal Regeneration 4.12 Optical Switch Architectures 4.12.1 Space Switches 4.12.2 Wavelength-Selective Switches 4.13 Performance Evaluation: Methodology and Case Studies 4.13.1 Physical-Layer Simulation: Three-Step Approach 4.13.2 WDM Network Simulation Case Studies 4.14 Problems 5

ix

217 217 219 221 227 228 229 231 233 234 235 235 239 240 240 241 241 242 255 257 257 266 274 275 278 281 281 288 297 298 301 311

Static Multipoint Networks

324

5.1 Shared Media: The Broadcast Star 5.2 Representative Multiplexing and Multiple-Access Schemes 5.2.1 Time-Wavelength-Division Multiplexing/Multiple Access 5.2.2 Subcarriers 5.2.3 Code Division Multiple Access

324 327 328 336 352

x

Contents

6

5.3 Traffic Constraints in Shared-Channel Networks 5.3.1 Balanced Traffic 5.3.2 Unbalanced Traffic 5.4 Capacity Allocation for Dedicated Connections 5.4.1 Fixed-Frame Scheduling for Stream Traffic 5.4.2 Fixed-Frame Scheduling for Packet Traffic 5.5 Demand-Assigned Connections 5.5.1 Blocking Calculations in WDMA Networks 5.5.2 Blocking in Combined Time-Wavelength-Division Networks 5.6 Packet Switching in the Optical Layer 5.6.1 Uncontrolled Scheduling: Random Access 5.6.2 Scheduling with Loss 5.6.3 Lossless Scheduling: Reservations 5.6.4 Perfect Scheduling 5.6.5 Dynamic versus Fixed Capacity Allocation 5.7 The Passive Optical Network 5.7.1 ATM and Fixed-Frame PONs 5.7.2 Ethernet-Based PONs 5.7.3 WDM PONs 5.7.4 Optical-Wireless Access 5.7.5 Recent Trends 5.8 Summary 5.9 Problems

367 370 370 371 371 383 389 390 395 399 401 403 405 407 408 409 412 414 416 420 422 424 425

Wavelength/Waveband-Routed Networks

432

6.1 Introduction 6.2 Physical Topologies 6.3 Wavelength-Routed Networks: Static Routing and Channel Assignment 6.3.1 Flow Bounds: Matching the Physical and Logical Topologies 6.3.2 Nonblocking Stations 6.3.3 RCA as a Graph Coloring Problem 6.3.4 Rings 6.3.5 Ring Decomposition of General Mesh Networks 6.3.6 Multistar Wavelength-Routed Networks 6.3.7 RCA as an Optimization Problem 6.3.8 Heuristics for Static RCA 6.4 Wavelength-Routed Networks: Dynamic Routing and Channel Assignment 6.4.1 Some Basic Routing and Channel Assignment Algorithms

432 434 442 444 448 449 452 458 462 464 474 484 484

Contents

7

xi

6.4.2 Case Study: Bidirectional Rings 6.4.3 Performance of Dynamic Routing Rules on Meshes 6.4.4 Case Study: An Interconnected Ring 6.4.5 Routing Multicast Connections in WRNs 6.5 Linear Lightwave Networks: Static Routing Rules 6.5.1 Routing of Optical Paths 6.5.2 Optical Connections: λ-Channel Assignment 6.5.3 Significance of Nonblocking Access Stations in LLNs 6.5.4 Local Access to LLNs 6.5.5 Routing Waveband and Channel Assignment on the Petersen Network 6.5.6 Channel Assignment 6.5.7 Multistar Linear Lightwave Networks 6.6 Linear Lightwave Networks: Dynamic Routing Rules 6.6.1 Point-to-Point Connections 6.6.2 Routing Multicast Connections in LLNs 6.7 Problems

491 494 495 497 507 509 516 518 519

Logically-Routed Networks

576

7.1 Introduction: Why Logically-Routed Networks? 7.1.1 Multitier Networks: Grooming 7.2 Point-to-Point Logical Topologies: Multihop Networks 7.2.1 ShuffleNets 7.2.2 Families of Dense Logical Topologies 7.3 Multihop Network Design 7.3.1 Logical-Layer Design 7.3.2 Physical-Layer Design 7.3.3 Traffic Grooming in Point-to-Point Logical Topologies 7.4 Multipoint Logical Topologies: Hypernets 7.4.1 Capacity of a Multipoint Subnet 7.4.2 Families of Dense Hypernets 7.4.3 Kautz Hypernets 7.4.4 Hypernet versus Multihop 7.4.5 Multicast Virtual Connections 7.5 Hypernet Design 7.5.1 Logical-Layer Design 7.5.2 Physical-Layer Design 7.5.3 Traffic Grooming in Multipoint Logical Topologies 7.5.4 Multistar Realizations 7.6 Summary 7.7 Problems

576 581 585 587 589 591 591 594

521 528 540 544 544 558 568

597 607 611 613 615 628 631 632 632 634 637 639 641 642

xii

8

9

10

Contents

Survivability: Protection and Restoration

647

8.1 Objectives of Protection and Restoration 8.2 Current Fault Protection and Restoration Techniques in the Logical Layer 8.2.1 Point-to-Point Systems 8.2.2 SONET Self-Healing Rings 8.2.3 SONET Self-Healing Ring Interconnection Techniques 8.2.4 Architectures with Arbitrary Mesh Topologies 8.3 Optical-Layer Protection: Point-to-Point and Ring Architectures 8.3.1 Point-to-Point Systems 8.3.2 Self-Healing Optical Rings 8.4 Optical-Layer Protection: Mesh Architectures 8.4.1 Shared Optical Layer Line-Based Protection 8.4.2 Optical Path-Based Protection 8.4.3 Segment Protection 8.4.4 Survivability Techniques for Multicast Connections 8.5 Summary 8.6 Problems

648 650 650 654 657 663 669 669 672 677 679 692 700 702 703 706

Optical Control Plane

714

9.1 Introduction to the Optical Control Plane 9.1.1 Control-Plane Architecture 9.1.2 Control-Plane Interfaces 9.1.3 Control-Plane Functions 9.2 Overview of Multiprotocol Label Switching 9.2.1 Packet Transport through an MPLS Network 9.2.2 MPLS Protocol Stack 9.2.3 MPLS Applications 9.3 Overview of Generalized Multiprotocol Label Switching 9.3.1 Link Management in GMPLS 9.3.2 Routing in GMPLS 9.3.3 Signaling in GMPLS 9.4 Conclusions

716 719 719 721 722 722 727 728 729 731 734 742 751

Optical Packet-Switched Networks

756

10.1 Optical Packet-Switched Network Architectures 10.1.1 Unbuffered Networks 10.1.2 Deflection Routing 10.1.3 Performance Analysis of Deflection Routing 10.1.4 Buffering: Time Domain Contention Resolution 10.1.5 Buffering and Wavelength Conversion: Time/Wavelength Domain Contention Resolution

758 759 764 766 770 778

Contents

10.2

10.3

10.4

10.5

10.6 10.7 11

10.1.6 Comparison of Contention Resolution Techniques for Asynchronous OPS Networks 10.1.7 Hybrid Electronic and Optical Buffering OPS Enabling Technologies 10.2.1 Packet Synchronization 10.2.2 All-Optical 2R or 3R Regeneration 10.2.3 Optical Switching 10.2.4 Wavelength Conversion 10.2.5 Optical Header Processing 10.2.6 Optical Buffering OPS Network Testbed Implementations 10.3.1 CORD Testbed 10.3.2 KEOPS Testbed 10.3.3 WASPNET Testbed Optical Burst Switching 10.4.1 Just Enough Time Protocol 10.4.2 Just In Time Protocol 10.4.3 Contention Resolution in OBS Networks Optical Label Switching 10.5.1 All-Optical Label Swapping 10.5.2 Contention Resolution Techniques 10.5.3 OLS Network Implementations Conclusions Problems

xiii

782 784 787 788 788 788 789 789 789 791 791 793 796 798 801 803 806 808 809 811 811 820 822

Current Trends in Multiwavelength Optical Networking

828

11.1 Business Drivers and Economics 11.1.1 Cost Issues for WDM Point-to-Point Systems 11.1.2 Cost Issues for WDM Rings 11.1.3 Cost Issues for WDM Cross-Connect Networks 11.1.4 Open versus Closed WDM Installations 11.2 Multiwavelength Optical Network Testbeds 11.2.1 Optical Networks Technology Consortium 11.2.2 All-Optical Network Consortium 11.2.3 European Multiwavelength Optical Network Trials 11.2.4 Multiwavelength Optical Network 11.2.5 National Transparent Optical Networks Consortium 11.2.6 The Importance of the Testbeds in Driving the Telecommunications Infrastructure 11.3 Metropolitan Area Networks 11.3.1 Metro Network Unique Characteristics 11.3.2 Defining the Metropolitan Networking Domain

828 831 832 833 835 838 838 839 839 840 840 840 841 841 842

xiv

A

B

C

Contents

11.3.3 Metro Network Evolution 11.3.4 Metro Networking State of the Art 11.4 Long-Haul and Ultra Long-Haul Networks 11.4.1 Current Considerations in Wide Area Network Architectures 11.4.2 Some Recent Commercial Network Deployments 11.5 New Applications and Services 11.5.1 Wavelength-on-Demand 11.5.2 Virtual Private Optical Networks 11.5.3 Bandwidth Trading 11.6 Conclusions

844 847 854 854 856 858 858 858 859 861

Graph Theory

869

A.1 Graphs A.1.1 Cycle Double Covers A.1.2 Eulerian Graphs A.1.3 Planar Graphs A.1.4 Matchings in Graphs A.1.5 Graph Coloring A.1.6 Digraphs A.1.7 Moore Bounds A.1.8 Max Flow–Min Cut A.2 Hypergraphs A.2.1 Undirected Hypergraphs A.2.2 Directed Hypergraphs

869 872 872 873 873 874 875 875 876 877 877 878

Fixed Scheduling Algorithm

879

B.1 Column/Row–Expansion Algorithm B.2 Decomposition into Permutation Matrices B.3 Column/Row–Compression Algorithm

880 883 883

Markov Chains and Queues

884

C.1 Random Processes C.2 Markov Processes C.3 Queues C.3.1 The M |M |1 Queue C.3.2 The M |G |1 Queue C.3.3 Little’s Formula

884 885 887 888 888 889

Contents

D

xv

A Limiting-Cut Heuristic

890

D.1 The Multicommodity Flow Problem and Limiting Cuts D.2 A Heuristic D.2.1 Swap (X, Y ) D.2.2 Limcut

890 891 891 892

An Algorithm for Minimum-Interference Routing in Linear Lightwave Networks

893

E.1 The Image Network E.2 The Min-Int Algorithm E.3 Minimum Interference

893 894 895

F

Synopsis of the SONET Standard

896

G

A Looping Algorithm

900

Acronyms

903

Index

915

E

Figures

1.1 1.2 1.3 1.4 1.5 1.6 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 2.10 2.11 2.12 2.13 2.14 2.15 2.16 2.17 2.18 2.19 2.20 2.21 2.22 2.23 2.24 2.25 2.26 2.27 2.28

Multilayered network. Physical picture of the network. Layered view of an optical network. Alternative network approaches. Hierarchical network. Road map. Layered view of optical network connections. A typical connection. Client server associations in an optical transport network. Fiber resources. Wavelength and waveband partitioning of the optical spectrum. Network picture based on spectrum partitioning. Tree physical topologies. Directional coupler. A 16 × 16 star coupler. Static routing node. Directed star. Space switch connection matrices. Unidirectional ring. Crossbar switch. Clos switch. Recursion for Benes switch. An 8 × 8 Benes switch. Generalized optical switch. δ–σ linear divider-combiner. A node without loopback connections. Three-stage realization of a waveband-space switch. Multiwaveband directional coupler. WADM–NAS combination. Wavelength conversion as a linear operation. Wavelength interchanger. Wavelength-interchanging switch. WIXC implementation. Wavelength-routed network.

page 6 8 13 15 17 24 29 32 33 34 35 38 41 42 43 44 45 46 47 48 49 49 50 53 54 55 56 57 58 64 65 65 66 67

xviii

Figures

2.29 2.30 2.31 2.32 2.33 2.34 2.35 2.36 2.37 2.38 2.39 2.40 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 3.10 3.11 3.12 3.13 3.14 3.15 3.16 3.17 3.18 3.19 3.20 3.21 3.22 3.23 3.24 3.25 3.26 3.27 3.28 3.29 3.30 3.31 3.32

Network access station. Example of a logical connection between two NASs. Optical transmitter. Optical receivers. Heterodyne receiver and spectra. Overlay processor. Logically routed network. SONET DCS. ATM cell format. ATM switch connections. TCP/IP and OSI. Taxonomy of multiwavelength networks. End systems: full connectivity. Star physical topology. Bidirectional ring physical topology. The control plane in an optical network. Connection management system. Star coupler example. Time-shared medium. A TDM/TDMA schedule. TDM/T-WDMA. CSMA/CD. CSMA/CD collision. NAS equipped for packet switching. Packet switching in the optical layer. MAC protocol in the layered architecture. Wavelength-routed star network. Channel assignment example. Nonblocking access link. Bidirectional ring: single access fiber pair. Bidirectional ring: two access fiber pairs. A mesh network. Inseparability. Two violations of DSC. Inadvertent violation of DSC. Avoidance of DSC violations. Color clash. Seven stations on a mesh. Tree embedded in mesh. Multistar network. Embedded star on a bidirectional ring. Seven-node hypernet. Assumed channel spacings. A logical switching node in an optical network.

68 69 70 72 73 75 78 81 82 83 84 86 92 92 93 98 99 105 106 108 110 114 115 116 118 120 123 125 129 130 132 133 136 137 137 138 139 141 142 144 146 148 150 152

Figures

3.33 3.34 3.35 3.36 3.37 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 4.10 4.11 4.12 4.13 4.14 4.15 4.16 4.17 4.18 4.19 4.20 4.21 4.22 4.23 4.24 4.25 4.26 4.27 4.28 4.29 4.30 4.31 4.32 4.33 4.34 4.35 4.36 4.37 4.38

Eight-node ShuffleNet. ShuffleNet embedding. Details of ShuffleNet node. Twenty-two node hypernet. Hypernet embedding. A point-to-point optical connection. Refractive index profiles for fibers. Snell’s law. Ray propagation in a step-index fiber. Ray propagation in a graded-index fiber. Cylindrical coordinates. Commercial fiber cables. Attenuation as a function of wavelength. Broadening of pulses due to dispersion. Dispersion coefficients as a function of frequency. Limitations due to nonlinear effects in multiwavelength systems. Soliton. Three types of microstructured fibers. Basic erbium-doped fiber amplifier structures. Energy levels in EDFA. EDFA gain profile. Illustration of noise figure. Raman gain coefficient in bulk silica as a function of frequency shift. Hybrid distributed-discrete amplification. Signal and pump power in hybrid system. Fabry–Perot laser. Single-frequency lasers. Laser array. Typical VCSEL structure. Pulse and accompanying chirp. Mach–Zehnder interferometer. Typical structure of an EA-DFB transmitter. Absorption and chirp (linewidth enhancement factor) parameters versus reverse bias voltage for a typical EA-DFB transmitter. Photodiode. Transimpedance amplifier. Binary receiver. Typical waveforms in an IM/DD system. Eye diagram. Ideal detection. BER as a function of Q. Transmission channel processing operations. Modulation formats. FEC encoding/decoding functions.

xix

154 155 157 159 160 167 169 169 170 171 173 175 176 178 179 186 188 189 192 192 194 196 198 200 200 206 208 210 211 213 215 216 216 217 219 221 222 223 225 226 228 229 232

xx

Figures

4.39 4.40 4.41 4.42 4.43 4.44 4.45 4.46 4.47 4.48 4.49 4.50 4.51 4.52 4.53 4.54 4.55 4.56 4.57 4.58 4.59 4.60 4.61 4.62 4.63 4.64 4.65 4.66 4.67 4.68 4.69 4.70 4.71 4.72 4.73 4.74 4.75 4.76 4.77 4.78 4.79 4.80 4.81

Transversal decision-directed equalizer. Heterodyne receiver. Types of cross-talk. Power penalty with homodyne cross-talk. Controllable directional coupler. Mach–Zehnder switch. Two-stage Mach–Zehnder switch. Y-branch switch. Gate array switch. Laser-activated bubble switch element. 2D mechanical switch using micromachined mirrors. 3D MEMS switch. 3D gimbaled mirror. Liquid crystal holographic switch. Two hologram N × N liquid crystal holographic switch. Illustration of reciprocity. Optical isolator. Fabry–Perot filter and its spectral response. MI filter. MI filter array. FBG used as a drop filter. A Mach–Zehnder WADM. Arrayed waveguide grating. Acousto-optic tunable filter. Liquid crystal MWS. A MEMS-based WADM. An MI filter-based WADM. Wavelength-dilated switch. Optoelectronic wavelength converter. Performance of a difference frequency converter. Opaque conversion and regeneration. SA-based regenerator. Nonlinear Mach–Zehnder regenerator. Nonlinear optical loop mirror regenerator. Optical crossbar switch. Path-independent loss crossbar switch. Circuit layout for 8 × 8 optical crossbar switch. Router/selector. Benes switch. Orders of cross-talk. Enhanced performance switch. Space dilation. OADMs in a network.

233 234 237 238 245 246 247 248 249 250 251 252 253 254 255 256 258 260 262 263 264 264 266 267 269 270 272 273 275 277 278 279 279 280 282 282 284 285 285 286 287 288 289

Figures

Parallel and serial OADM architectures with capability for m wavelength add/drops. 4.83 Functional diagram of an OADM based on wavebands and wavelengths. 4.84 Typical B&S OADM architecture. Assumes (1+1) protection. 4.85 Typical 4 × 4 ROADM based on 4 × 1 wavelength selective switch and B&S-type of architecture. 4.86 Transparent OXC. 4.87 Opaque O-E-O OXC. 4.88 Opaque O-O-O OXC. 4.89 Hybrid waveband/wavelength switch. 4.90 Wavelength-domain simulation. 4.91 WADM chain. 4.92 WADM structure and simulation model. 4.93 Simulation results for the WADM chain. 4.94 Ring interconnect network architecture. Worst-case paths between A and B are indicated. 4.95 Histogram of all cross-talk terms accumulated at receiver B for the worst-case path of Figure 4.94. 4.96 Cross-talk-induced Q penalty in dB versus dominant cross-talk term power level. 4.97 Q-channel performance for the worst-case path of Figure 4.94 assuming OC-192 bit rate and EA-modulated transmitters. 4.98 A DWDM metro network deployment scenario. All rings represent typical SONET OC-12/48/192 designs. DWDM is deployed only between the superhub nodes (dark squares) in ring (solid) or possible mesh (dotted) configurations. 4.99 DWDM metro network case study based on the network deployment scenario presented in Figure 4.98. Nodes represent only superhub stations with typical distances (not shown to scale). 4.100 Simulation results for path A-F-D in Figure 4.99 comparing Q-channel performance with and without EDC. 5.1 Star networks. 5.2 A 3 × 3 example. 5.3 TDM/T-WDMA channel allocation schedules. 5.4 Illustrating channel reuse in an FT-TR system. 5.5 Illustrating optical spectral efficiency. 5.6 SCMA example. 5.7 Transmitting and receiving stations equipped for SCMA. 5.8 Subcarrier spectra. 5.9 Effect of OBI. 5.10 TDM/T-SCMA. 5.11 SCM/SCMA.

xxi

4.82

290 290 292 292 293 294 294 297 299 302 302 303 304 306 307 308

309

309 311 325 330 331 332 334 337 337 340 342 346 349

xxii

Figures

5.12 5.13 5.14 5.15 5.16 5.17 5.18 5.19 5.20 5.21 5.22 5.23 5.24 5.25 5.26 5.27 5.28 5.29 5.30 5.31 5.32 5.33 5.34 5.35 5.36 5.37 5.38 5.39 5.40 5.41 5.42 5.43 5.44 5.45 5.46 5.47 5.48 5.49 5.50 5.51 6.1 6.2 6.3 6.4

SCM/WDMA/SCMA. SCM/WDMA/SCMA example. Block diagram of a direct-detection CDMA system. Waveforms for a direct-detection CDMA system. Orthogonal optical codes. Parallel CDMA transceiver structure. CDMA with all-optical processing. Multidimensional codes. FBG encoder for FFH-CDMA. A 3D CDMA system. Realization of coherent optical CDMA. Shared-channel broadcast medium. Normalized traffic matrices. CASs for systems with a full complement of channels. CASs for Examples 4, 5, 6, and 7. Heterogeneous traffic scheduling. Logical multicast CAS. Single-server queue. Throughput versus traffic intensity. Markov chain model for demand-assigned traffic. Comparison of Engset and Erlang models. Normalized throughput versus traffic intensity. Normalized throughput versus traffic intensity. Matching time slots. Framed system blocking probabilities. Illustrating rearrangeability. Slotted ALOHA. Tell-and-go protocol. Lossless scheduling. Queues for perfect scheduling. Passive optical network. BPON frame. Transmission scenario in a BPON system. PON equipped for decentralized control. LARNet. RITE-Net. WDM PON. Integrated system for dual services. Dual services testbed. DWDM/TDM PON. Number of vertices in known maximal graphs. Thirty-eight-vertex graph. Tessellations of the plane. Undirected deBruijn and Kautz graphs.

350 351 354 356 357 359 359 361 361 363 364 367 370 376 378 379 382 385 387 391 393 394 395 396 397 398 403 404 406 407 410 412 413 416 417 418 419 421 421 423 435 435 436 437

Figures

6.5 6.6 6.7 6.8 6.9 6.10 6.11 6.12 6.13 6.14 6.15 6.16 6.17 6.18 6.19 6.20 6.21 6.22 6.23 6.24 6.25 6.26 6.27 6.28 6.29 6.30 6.31 6.32 6.33 6.34 6.35 6.36 6.37 6.38 6.39 6.40

d Construction for min . d as a function of N . Plot of min Internodal distances in random networks. Recursive grid. Hierarchical Petersen graph. Limiting cuts for four networks. Three-node network. Illustrating RCA in a wavelength-routed network. A four-fiber SPRING. A two-fiber SPRING. Bidirectional ring. Five-node WDM ring. Ring decomposition. Bridged ring overlay. A multistar network. Layered view of RCA. External traffic in flow conservation equations. Wavelength savings by increasing fibers. Mean values of N λ versus α. Minimum values of N λ versus α. Flow chart of the Monte Carlo algorithm. Time trace of Monte Carlo algorithm. An example of SPD routing. Blocking on an 11-node WDM ring. Gain in blocking; 11-node WDM ring, simulation. Fairness ratio; 11-node WDM ring, simulation. Fairness ratio improvement versus interchanger density; 11-node WDM ring with 32 wavelengths. Simulation and asymptotic analysis; 195-node interconnected WDM rings. Blocking improvement with wavelength interchange; 195-node interconnected WDM rings. Fairness ratio improvement with wavelength interchange; 195-node interconnected WDM rings. Fairness ratio improvement versus interchanger density; 195-node interconnected WDM ring, 32 wavelengths. Multicast connection in a transparent network. A P×P split-and-deliver switch. A P×P multicast-capable optical cross-connect based on a split-and-deliver switch. A P×P multicast-capable optical cross-connect based on splitter sharing. Multicasting in a network with sparse splitting capabilities.

xxiii

438 438 439 440 441 447 449 451 453 454 455 458 459 462 462 465 467 476 477 478 481 483 488 490 491 492 493 496 497 498 499 500 501 502 503 506

xxiv

Figures

6.41 6.42 6.43 6.44 6.45 6.46 6.47 6.48 6.49 6.50 6.51 6.52 6.53 6.54 6.55 6.56 6.57 6.58 6.59 6.60 6.61 6.62 6.63 6.64 6.65 6.66 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13 7.14 7.15 7.16 7.17 7.18

Petersen network. Structure of a nonblocking access station for an LLN. Optical paths. Optical connection hypergraph. Local access subnets on the Petersen network. Embedded star on tree T A . Waveband assignments: W = 5. Connection interference graph. Connection interference graph for Equation (6.60). Optical connection hypergraph. Fixed-frame scheduling for four LCs. Directed hypernet GKH (2, 8, 4, 4). Color clash. Illustrating inseparability. Illustrating Min-Int. Random network. Max Reuse versus Min Reuse channel allocation. k-SP routing. k-SP versus Min-Int routing. Blocking in networks with multifiber links. Blocking in networks with multiple wavebands. Example of a multicast connection. Example of a tree decomposition using MBFS-1. Example of a tree decomposition using MBFS-4. Illustrating routing on a tree. Blocking probability for multicast connections. Why logically-routed networks? A schematic of a point-to-point LRN. Two-tier architecture. The architecture of a grooming node with optical bypass. ShuffleNet: δ = 3, k = 2, N = 18. Maximum throughput per node for ShuffleNet. deBruijn and Kautz digraphs. A traffic matrix and matched LCG. ShuffleNet on Atlantis. Benefit of traffic grooming. A node in a SONET over WDM ring. Advantage of grooming static traffic in SONET over WDM rings. Construction of an auxiliary graph for grooming. Layered view of a hypernet. Hypernets. Illustrating fan-out in hypergraphs. Shuffle hypernet. Orders of K H (2, D, r ).

509 510 512 518 520 523 526 529 530 534 534 542 547 548 550 551 552 553 554 555 556 559 562 563 564 568 577 579 582 584 587 588 590 593 595 598 599 601 605 608 609 613 614 616

Figures

7.19 7.20 7.21 7.22 7.23 7.24 7.25 7.26 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8 8.9 8.10 8.11 8.12 8.13 8.14 8.15 8.16 8.17 8.18 8.19 8.20 8.21 8.22 8.23 8.24 8.25 8.26 8.27 8.28 8.29 8.30 8.31 8.32 8.33 8.34 8.35

Duality construction. Directed hypergraph construction via duality. Directed hypergraph construction via edge grouping. Tripartite representation of G K H (2, 42, 3, 28). Routing header. Comparison of hypernets and multihop networks. Multicast tree in D K H (1, 4, 2). Multicast-capable logical-grooming switch. Path versus line protection. (1 + 1) SONET protection. (1:1) SONET protection. (1:N) SONET protection. Single- and dual-access ring interconnection configurations. Two-fiber UPSR-to-UPSR ring interconnection. BLSR-to-BLSR ring interconnection. BLSR-to-UPSR ring interconnection. (1 + 1) Protection in the optical layer. (1:1) Protection in the optical layer. (1:N) Protection in the optical layer. (1 + 1) Optical protection and (1:N) electronic protection for a WDM system. Four-fiber WDM SPRING architecture. Four-fiber WDM SPRING surviving a link failure. Four-fiber WDM SPRING surviving a node failure. Two-fiber WDM SPRING architecture. A taxonomy of survivability schemes. Rerouting around a failed link. Directed cycles in a planar graph. Directed cycles in a nonplanar graph: K 5 . Face traversal for a planar national network. Orientable CDC of the ARPANet. Seven-node planar network with default protection switch settings. Seven-node planar network after a link failure. Failure recovery using the p-cycle approach. Generalized loopback example. (1+1) dedicated protection architecture. (1:3) shared protection in a mesh network. Spanning trees used in optical path protection. Shared risk groups. SRG classification. SLSP protection scheme. Examples of different types of islands centered on node 22. Examples of segment and path-pair protection of multicast sessions. Example illustrating the arc-disjoint and MC-CR algorithms.

xxv

618 619 620 622 624 628 631 638 651 652 653 654 658 660 661 662 670 671 671 672 673 674 675 676 677 682 683 683 684 686 687 687 688 691 692 693 697 698 699 701 702 703 704

xxvi

Figures

9.1 9.2 9.3 9.4 9.5 9.6 9.7 9.8 9.9 9.10 9.11 9.12 9.13 9.14 9.15 9.16 10.1 10.2 10.3 10.4 10.5 10.6 10.7 10.8 10.9 10.10 10.11 10.12 10.13 10.14 10.15 10.16 10.17 10.18 10.19 10.20 10.21

A mesh optical network. Example of an optical node architecture. Provisioning a connection between two routers through an optical network. Control plane architecture. Control plane interfaces. MPLS header format and MPLS packet format. Two LSPs in an MPLS packet-switched network. Label stacking. MPLS protocol stack. Link bundling illustration. LSP hierarchy in GMPLS. User-Network Interface. Provisioning in GMPLS. Path and Resv message flows in RSVP for resource reservation. RSVP message format. Protection signaling using GMPLS RSVP-TE. Optical packet-switching node. A generic OPS node architecture for an unslotted network. A generic OPS node architecture for a slotted network. An FDL-based synchronizer. A generic packet format for a slotted network. Optical packet contention. Petersen network graph. Paths from A to D. Input buffered optical packet switch with WSXC. Input buffered optical packet switch using multiple space switch planes. FDL input buffer. Example of head-of-the-line (HOL) blocking. Feed-forward delay line architecture. Feedback delay line architecture. Dump-and-insert buffer architecture. Typical packet sequence in DI buffers for a 4 × 4 optical switch. Generic node architecture with TOWCs at the input lines. Details of output buffers for TOWC switch. Packet-loss probability versus number of FDLs with and without wavelength conversion. Generic node architecture with TOWCs that are shared among input lines. Node architectures for different contention resolution schemes: single-wavelength delay line, multiwavelength delay line, wavelength

715 716 717 720 721 724 725 727 728 740 741 743 744 746 747 751 760 761 761 762 762 764 766 766 771 772 772 773 773 774 775 776 779 780 780 781

Figures

10.22 10.23 10.24 10.25 10.26 10.27 10.28 10.29 10.30 10.31 10.32 10.33 10.34 10.35 10.36 10.37 10.38 10.39 10.40 10.41 10.42 10.43 10.44 10.45 10.46 10.47 10.48 10.49 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9

conversion, and wavelength conversion with multiwavelength buffering. Switch architecture with electronic buffering and wavelength conversion. All-optical buffering and switching architecture. Physical implementation of the CRO device. CORD testbed. Packet format for the KEOPS project. Proposed unicast node architecture for the KEOPS project. Proposed multicast/broadcast node architecture for the KEOPS project. SLOB architecture. Each stage is a photonic switch element (PSE). WASPNET optical packet switch. Optical packet switching and Optical burst switching. OBS architecture concept. Just enough time (JET) protocol. Just in time (JIT) protocol. Segmentation of a burst. Packet-loss probability versus load for different contention resolution policies in OBS. OLS network. All-optical processor for OLS. OLS subcarrier transmission system. OLS network node. Network node architecture for an OLS testbed demonstration. FSK/IM orthogonal labeling scheme used in the STOLAS project. Optical router and optical label swapper used in the STOLAS project. Architecture of the edge-router in OPSnet. Architecture of the core-router in OPSnet. Core node configuration for label swapping and packet switching. OCSS technique. Experimental setup for multihop packet transmission and multirate payload. K 3,3 network. Six Central Offices, including two hubs, with capacity exhaust. Application of WDM point-to-point systems to alleviate capacity exhaust. Six Central Offices, including two hubs, with capacity exhaust. Economic case for WDM rings. Node in Central Office: Electronic cross-connect. Economic case for WDM optical cross-connect. Open WDM network architecture: Opaque network. Integrated closed WDM network architecture. Current legacy SONET/SDH design in U.S. metropolitan regions.

xxvii

783 784 790 792 792 794 794 795 796 797 799 800 802 804 807 808 809 810 812 813 814 814 815 816 817 819 819 820 822 831 832 833 834 835 836 837 837 842

xxviii

Figures

11.10 Metro WDM interconnected-ring simulation case study. 11.11 Typical ILEC metro network in the 2004 time frame. 11.12 Typical view of a superhub in the ILEC metro network of Figure 11.11 (2004 time frame). 11.13 Typical view of a current superhub in an ILEC metro network. 11.14 Typical deployment of 10 GbE technology in the metro environment. 11.15 Typical metro network ring architectures. 11.16 Typical vendor generic WADM node architecture for the metro network application space of Figure 11.15. 11.17 Metro network case study. 11.18 Wavelength-brokering operational model. 11.19 Infrastructure swapping. 11.20 Multicarrier recovery. A.1 A maximal independent set. A.2 The complete graph K 5 . A.3 The complete bipartite graph K 3,3 . A.4 Orientable cycle double cover for K 3,3 . A.5 Multigraphs: Non-Eulerian and Eulerian. A.6 Maximum matching of a bipartite multigraph. A.7 A diclique. A.8 A cut. B.1 Example of decomposition of an NQDS matrix. B.2 Example of fixed-frame scheduling. C.1 Two-state chain. C.2 Birth–death process. C.3 A queue. E.1 Image network. F.1 SONET STS-1 frame and overhead channels. F.2 Creating an OC-N signal. F.3 Structure of a concatenated SONET frame. G.1 Looping, first step. G.2 Final switch settings.

844 845 846 847 848 850 851 853 860 860 861 870 870 871 872 873 874 876 877 881 882 886 887 888 894 897 898 898 901 901

Tables

3.1 3.2 3.3 5.1 5.2 6.1 6.2 6.3 6.4 6.5 6.6 6.7 7.1 7.2 7.3 7.4 7.5 7.6

Tree routing on mesh Seven-station comparisons Wavelength assignments for ShuffleNet on a ring SCM/WDMA/SCMA example Multicast connections Orders of some graphs Comparative performance of three RCA heuristics Routing table Waveband routing in the Petersen graph: W = 3 Waveband routing in the Petersen graph: W = 5 Comparison of performance of various configurations of the Petersen network Sizes of trees generated with the MBFS-d algorithm ShuffleNet routing on Atlantis Relative costs for different ring networks Kautz hypergraphs Performance of K H (2, D ∗ , r ) Performance comparison of D K H (d, D, s) to multihop networks Hypernet trees on Atlantis

149 150 156 351 382 437 480 517 524 527 538 567 596 602 615 627 630 636

Preface to the Second Edition

The first edition of this book was published when optical networks were just emerging from the laboratory, mostly in the form of government-sponsored testbeds. Since then there have been fundamental changes in many aspects of optical networking, driven by the move from the laboratory to commercial deployment and by the twists and turns of the world economy. The investment climate in which optical networks have developed has had two major swings as of this writing. During the technology bubble that began at the end of the 20th century, investment in research, product development, and network deployment increased enormously. The activities during this time of euphoria produced advances in the technology base that would not have been possible without the extraordinary momentum of that period. At the same time, commercial network deployment provided a reality check. Some ideas that were pursued in the late 1990s dropped by the wayside because they did not meet the test of commercial viability, and new ones came along to take their place. When the bubble burst after less than a decade of “irrational exuberance,” the pendulum swung the other way. Investors and executives who a short time earlier thought the sky was the limit now wondered if demand would ever materialize for all of the fiber capacity in the ground. At this writing a more reasoned approach has taken hold; that seemingly elusive demand has materialized and, hopefully, a more rational and sustainable growth period will ensue. This is the context for the second edition. It is designed to build on the foundations laid out in the first edition while reflecting the new developments of the past 9 years: a maturing underlying technology, new tools for network control, and a recognition of the latest directions of optical network deployment and research. These new directions include cost-effective metropolitan area network architectures tailored to the strengths of current optical transmission and switching equipment, passive optical networks to bring high-speed access to the end user, hybrid optical/electronic architectures supporting the merging of multiwavelength and Internet technologies, and networks of the future based on all-optical packet switching. As in the first edition, the emphasis of this book is on concepts and methodologies that will stand the test of time. The first three chapters provide a qualitative foundation for what follows, presenting an overview of optical networking (Chapter 1), the multiwavelength network architecture and its supporting components (Chapter 2), and a high-level view of the different network structures considered throughout the book (Chapter 3). A more detailed picture is provided in the remaining chapters, with a survey of enabling technology (Chapter 4) and in-depth studies of the three basic network structures: static

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Preface to the Second Edition

multipoint networks (Chapter 5), wavelength/waveband routed networks (Chapter 6), and optical/electronic (logically routed) networks (Chapter 7). The remaining chapters complete the networking picture: survivability (Chapter 8), network control (Chapter 9), optical packet switching (Chapter 10), and current trends (Chapter 11). The first three chapters are suitable for the reader who wishes to gain an understanding of multiwavelength networks without delving deeply into the analytical tools for network design and the physical underpinnings of the optical technology. These beginning chapters, together with Chapter 11, would be suitable for a short undergraduate course for electrical engineers and computer science majors. The first seven chapters provide a largely generic framework for understanding network architectures, performance, and design in an abstract setting. An exception is Chapter 4, which surveys enabling technology from theory to practice, thereby providing the necessary background concerning the physical limitations and possibilities of the network technology. The material through Chapter 7, together with selected material from the remaining chapters (depending on the reader’s orientation), can form the basis of a comprehensive graduate course, introducing the student to the latest developments in the field and suggesting a host of different research directions. The networking developments since the publication of the first edition have served to reorient and expand our treatment in significant ways. r Recognizing the importance of current activity in the “last mile” (fiber to the home/premises), and in metropolitan area networks, we have added a new section on passive optical networks (PONs) in Chapter 5,1 and we have included new material in Chapters 4 and 11 to connect our generic networking approach to recent metro network developments. r Chapter 4 was substantially expanded and updated to provide a glimpse of the impressive new trends in photonic and electro-optic technology. Some of the new and/or expanded topics are photonic crystal fibers, Raman amplification, supercontinuum generation, amplification trends in metro networks, and forward error correction and equalization to improve transmission performance. There are also new and expanded sections on wavelength conversion and signal regeneration with emphasis on alloptical techniques, and a new section on microelectromechanical system (MEMS) devices. The treatment of optical switch architectures has been significantly enlarged with a focus on cost-effective architectures and opacity versus transparency. More emphasis is placed on the effects of signal impairments, including a new section on performance impairments in a network environment. Also, new case studies are included that illustrate methodologies for evaluating the performance of metropolitan area networks. r Chapter 8, on survivability, protection, and restoration, was extensively updated, consistent with the growing importance of optical layer fault management in current 1

It is interesting to note that the PON, epitomized by the broadcast star, was the first structure that demonstrated the possibilities of optical networking in the 1980s. However, it was largely ignored for large-scale network deployment until recently, when it has again come into its own as the vehicle of choice for extending optical networks to the end user.

Preface to the Second Edition

r

r

r

r

xxxiii

networks. It contains recent work on the subject, including shared line-based protection in mesh networks, path-based protection, ring-based protection, segment protection, the treatment of shared risk groups, and recovery of multicast connections. Chapter 9, describing the control plane, was added to present the latest developments in optical network control. It describes the control plane architecture as it has developed through the recent activities of several standards organizations. The chapter offers a detailed discussion of Multiprotocol Label-Switching (MPLS) and Generalized MPLS (GMPLS) as it applies to optical networks. Chapter 10, on optical packet-switched networks, was added to provide an introduction to this emergent field.2 It provides a window on a cutting-edge research area that has the potential to offer the next breakthroughs in optical networking. Chapter 11, on current networking trends (replacing the original Chapter 9), is a completely updated description of the current networking environment. This includes a historical perspective describing the pioneering network testbeds, business drivers, and current trends in metro, long-haul, and ultra long-haul netwoks. Included also are some new applications that have emerged on the commercial scene, such as wavelengths on demand, virtual private optical networks, and bandwidth trading. This edition places increased emphasis on the practical aspects of hybrid (i.e., electronic/optical and wavelength/waveband) architectures. This includes the importance of grooming, which is required to pack electronically multiplexed channels efficiently into an optical wavelength channel, and to pack wavelength channels into wavebands. Also, the existence of transparent (purely optical) and opaque (electronic/optical) alternatives to network design is stressed throughout. These practical aspects of networking have become important as optical networks have found their place in the real world.

Exercises are provided for most of the chapters, and many of them suggest avenues for future study. The book is meant to offer several different alternatives for study depending on the interest of the reader, be it understanding the current state of the field; acquiring the analytical tools for network performance evaluation, optimization, and design; or performing research on next-generation networks. 2

Although the idea of using packet switching in optical networks is not new, it has attracted renewed interest as the technology for purely optical packet processing has developed over the past few years, and the advantages of merging Internet and WDM technology have become apparent.

Acknowledgments

The first edition of this book had its origins in 1990, when we organized a small group within the Center for Telecommunications Research (CTR) at Columbia University, to investigate lightwave networks. Among the many colleagues, students, and friends who contributed in various ways to the first edition, there are several who have continued to interact with us in the preparation of the second edition. We specifically express our thanks to Eric Bouillet, Aklilu Hailemariam, Gang Liu, and G.-K. Chang. Special thanks go to Ioannis Roudas for useful discussions and comments. Mischa Schwartz, who was singled out as our guiding spirit in the first edition, is still an indefatigable contributor to communication networking and a continuing inspiration to us. We are especially indebted to Neophytos Antoniades for coauthoring Chapters 4 and 11 of the second edition. His understanding of the role of physical layer simulation and the evolution of optical networks in the metropolitan area domain provided invaluable additions to this edition. We also express our thanks to Phil Meyler at Cambridge University Press for his support and encouragement, and to Anna Littlewood at Cambridge and Barbara Walthall at Aptara for their help in putting everything together. Finally, Thomas Stern expresses his profound gratitude to his wife, Monique, for her everlasting support; Georgios Ellinas is deeply grateful to his mother, Mary, and sister, Dorita, for their unyielding support and understanding during this endeavor; and Krishna Bala is greatly indebted to his wife, Simrat, and children, Tegh and Amrita, for their patience and support.

Multiwavelength Optical Networks, Second Edition

1

The Big Picture

Since the beginning of the 21st century there has been a burgeoning demand for communications services. From the ubiquitous mobile phone, providing voice, images, messaging, and more, to the Internet and the World Wide Web, offering bandwidth-hungry applications such as interactive games, music, and video file sharing, the public’s appetite for information continues to grow at an ever-increasing pace. Underneath all of this, essentially unseen by the users, is the optical fiber-based global communications infrastructure – the foundation of the information superhighway. That infrastructure contains the multiwavelength optical networks that are the theme of this book. Our purpose is to present a general framework for understanding, analyzing, and designing these networks. It is applicable to current network architectures as they have evolved since the mid-1990s, but more importantly it is a planning and design tool for the future. Our approach is to use a generic methodology that will retain its relevance as networks, applications, and technology continue to evolve.

1.1

Why Optical Networks? Since the fabrication of the first low-loss optical fiber by Corning Glass in 1970, a vision of a ubiquitous and universal all-optical communication network has intrigued researchers, service providers, and the general public. Beginning in the last decades of the 20th century enormous quantities of optical fiber were deployed throughout the world. Initially, fiber was used in point-to-point transmission links as a direct substitute for copper, with the fibers terminating on electronic equipment. Glass fiber was and is the ideal medium because of its many superior properties: extraordinary bandwidth, low loss, low cost, light weight and compactness, strength and flexibility, immunity to noise and electromagnetic interference, security and privacy (it is difficult to tap them), and corrosion resistance. Although all of these qualities make the fiber a technological marvel, fibers do not become networks until they are interconnected in a properly structured architecture. For our purposes, an optical network is a telecommunications network with transmission links that are optical fibers, and with an architecture that is designed to exploit the unique features of fibers. (Most of the communication systems in use today, including many specialized networks such as cable TV and mobile phone systems, have optical fiber in them somewhere; however, this does not make them optical networks.) As we shall see, suitable architectures for high-performance lightwave

2

Multiwavelength Optical Networks

networks involve complex combinations of both optical and electronic devices. Thus, as used here, the term optical or lightwave network does not necessarily imply a purely optical network, but it does imply something more than a set of fibers interconnecting electronic switches. As optical and photonic technology has advanced, applications to point-to-point transmission have preceded advances in networking. For example, it was clear in the early years of optical fiber transmission that by introducing wavelength division multiplexing (WDM) on existing fibers the capacity of a fiber link could be increased manyfold at minimum cost. However, it was only since the early 2000s that the optical switching technology necessary to convert isolated fiber transmission links to optical networks matured sufficiently to permit the commercial deployment of these networks. In the mid-1990s, the optical network (as opposed to optical fiber transmission alone) was still a “blue sky” concept. New optical and photonic devices were being developed and incorporated into experimental networks. But full-fledged multiwavelength networks integrating optical transmission, switching, and user access were still in the research and development stage. At that time the technology push for networking was out in front, but demand for the seemingly unlimited capacity of these networks was essentially nonexistent. As this is being written, the promise of optical networking is finally being fulfilled. The demand pull for these networks has materialized. As low-cost broadband services are made available to the general public, demand for Internet-based applications continues to increase. Equipment manufacturers, communications carriers, and service providers have joined in moving optical networking from feasibility studies to commercial viability in both cost and performance. The focus in the networking community has now shifted to organization, control, manageability, survivability, standardization, and cost-effectiveness, a trend that reflects the maturing of the optical technology as well as the recognition that the optical network is the only way of supporting current and future demand. These networks have played a critical role in reducing communications costs, promoting competition among carriers and service providers, and thereby increasing the demand for new services. In addition to the technology push and demand pull, a number of other recent developments are contributing to the expansion and effectiveness of optical networks. One is the accelerating removal of the bottleneck in the “last mile” – the distribution network that is the bridge between the high-speed fiber core network and the end users. Until the last decade of the 20th century this distribution network – composed of twisted pairs of copper wires connecting each residential subscriber to the local telephone Central Office – was specifically engineered to a limited bandwidth of 3000 Hz. As a result the user bit rates were restricted to a tiny trickle. This low-speed access link separated the various high-speed communications and computing devices located on the premises of the end users (e.g., PCs, TV displays, and music/image/video storage equipment in the home) from the high-capacity network serving this equipment. Considering that the processors in today’s PCs operate at speeds six orders of magnitude faster than a low-speed access link, and the optical fibers in the network have bandwidths nine orders of magnitude wider than the bandwidth of the access link, it is obvious that access was – and is – a severe problem. As long as the last-mile bottleneck is present, the information

The Big Picture

3

superhighway is still a dirt road; more accurately, it is a set of isolated multilane highways with cow paths for entrance and exit ramps. The introduction of broadband access to residential customers by the telephone carriers and the cable operators is a step toward eliminating those cow paths. However, digital subscriber line (DSL) and cable modems are half-measures at best. Direct access to the fiber network by the end user [i.e., fiber to the home (FTTH) or business user] is the ultimate way of removing the bottleneck so that the network remains effective as demand for bandwidth grows. Although FTTH was deployed many years ago in a few demonstration projects, it did not take hold for several reasons, including cost and the absence of services of interest to the customers. Today that has changed because of the proliferation of broadband Internet services. Deployment of glass is now moving from the network core through fiber access networks to the end users. This will undoubtedly stimulate interest in new broadband services that take advantage of high-speed access and in turn produce demand for more bandwidth. At this writing, most of the world’s installed fiber capacity is underutilized – arguably due to the last-mile bottleneck. That should change rapidly as progress in the removal of the bottleneck results in a quantum jump in network traffic, making high-performance optical networks indispensable. Higher level issues such as deregulation, new ideas for improving the economics of networking, and standardization of control and management techniques in multivendor networks are also contributing to the growing effectiveness of optical networks. Deregulation, which began in the United States in 1984 with the dismantling of AT&T, has brought with it a new level of competition, with long-haul carriers, local carriers, Internet service providers (ISPs), and cable operators poaching on each other’s domains and using optical fiber capacity to do so. Bandwidth trading has been introduced as a way of improving the utilization of fibers and thereby optimizing profits. A carrier with idle capacity sells it to another carrier with excess demand. This type of exchange requires sophisticated control and management tools for network reconfiguration. More generally, any large network requires complex control and management systems and intelligent network elements for performance monitoring, network reconfiguration, and fault recovery. The systems, protocols, and equipment for performing these functions in traditional telephone and data networks were built over many years by the public carriers and equipment manufacturers. The new optical networks require similar tools, and this is especially important in multivendor environments. These are now making their appearance in the form of a proposed control plane for optical networks and protocols for systems management in these networks. As more sophisticated control and management functions are incorporated into optical networks the network operators are in a better position to offer high-quality service to their customers, improving the operator’s revenue stream and customer loyalty. Above all, the lessons of the past show us that tomorrow’s networks must be flexible and versatile enough to adapt to a continuing barrage of new and as-yet-unknown services. It is interesting to note that when optical networks were still in an embryonic form, the typical uses envisaged for them were high-tech applications such as high-resolution medical image archiving and remote supercomputer visualization – basically usages generated by a minuscule, elite segment of the population. Today these applications

4

Multiwavelength Optical Networks

represent but a tiny part of the global network traffic, submerged in a torrent generated by the common man, who has only recently gained access to the enormous opportunities our worldwide communication system has to offer. The networks we conceive today must be “futureproof ” so as to be ready for the next unforeseen developments.

1.2

Objectives of an Optical Network Architecture Today’s and tomorrow’s optical networks must provide the capacity, connectivity, and intelligence necessary to link together a global community of information providers and consumers. A well-designed network performs this function efficiently and reliably. To facilitate a systematic study of networks that achieve this goal, it is useful to formulate a generic model in the form of a multiwavelength network architecture (MWNA). As background for the MWNA we briefly review the current network structures and the services they support. Until the end of the second millennium, the world of networking consisted of two separate spheres: the traditional telephone networks mainly devoted to providing voice services (operated in a circuit-switched mode) and data networks (operated in a packet-switched mode) for communication between computers. Each type of network was specially engineered and optimized for its own type of service. Circuit switching was the preferred approach to voice transmission, because the voice signal was transmitted as a continuous stream, whereas packet switching was invented to carry data traffic because data signals were bursty in nature, making the circuit-switching approach very inefficient. Because the voice networks operated by the public carriers contained virtually all of the world’s installed communication capacity, the early data networks were constructed as overlays on these networks, running on lines leased from the public carriers – mainly AT&T in the United States and the government administrations in Europe. The traffic flow in the early data networks was minute compared to voice traffic – essentially confined to businesses, universities, and research laboratories. For this reason the main players in data networking were originally government, research, business, and educational organizations and data-processing equipment manufacturers. As optical fiber became the dominant transmission medium, various standards for exploitation of fiber were developed, including the synchronous optical network (SONET) standard in the United States and a similar synchronous digital hierarchy (SDH) standard in Europe. The SONET/SDH transmission, multiplexing and switching equipment, adapted primarily to circuit-switched applications, was soon augmented by asynchronous transfer mode (ATM) switches and Internet Protocol (IP) routers (cell-switched and packet-switched, respectively) to handle a wide variety of data and multimedia services. By the late 1990s the traditional separation of voice and data networks changed significantly. In a very short time we moved from a voice-centric world to a data-centric world, and, more importantly, the techniques of carrying data (packet switching) were extended to an infinite variety of services having no resemblance to those in the traditional computer world. Internet/Web services, running the gamut from interactive computer games through telemedicine to peer-to-peer file sharing, now use IP for transmitting anything

The Big Picture

5

from computer data to video (Internet Protocol TV; IPTV) to old-fashioned voice (Voice over IP; VoIP).1 This brings us to the characteristics and requirements of the services supported by the optical networks discussed here. These are extremely diverse in terms of connectivity, bandwidth, performance, survivability, cost, and a host of other features. Consider the common Internet services offered to the general public (e.g., e-mail and search engines). They serve a vast globally distributed user community. In terms of connectivity, these types of services push networking to its ultimate limits; any end user wants rapid connectivity to anyone or anything in the network. However, in terms of performance, they are undemanding – they can tolerate errors, delays, and occasional downtimes due to congestion, programming bugs, and equipment failure. Total costs may be high, but they are spread over an enormous user base resulting in a very low cost per user. In contrast, consider a different type of application, the virtual private network (VPN). This is a subnet carved out of a larger network by a telecommunications carrier and put at the disposal of a single enterprise, which typically controls and manages it. Consequently it has a much smaller user group with more intense utilization per user, far fewer active connections, and tighter control of network performance, including security and reliability. Customer costs per user will be higher, but this is offset by higher performance and more responsiveness to the needs of the customer. Another example is telemedicine, which requires high-quality communication (e.g., high fidelity medical image encoding and transmission, and rapid response) and where cost is secondary. Different requirements apply to public safety services (e.g., police, fire, and disaster relief), which depend on a high degree of survivability, fault recovery, and availability2 in the face of equipment and line failures, natural disasters, or malicious attacks. Transmission quality is secondary. Similar requirements hold to a lesser degree for financial services (e.g., banks and brokerage houses). In public safety and financial service applications, cost is not the primordial issue. To ensure satisfactory service, large users of network services (e.g., enterprises operating VPNs) enter into service-level agreements (SLAs) with the service providers. For example, the SLA might specify a level of availability, network delay, packet loss, and other features. These represent promises from the provider to the user, and as such they must be backed up by suitable controls within the underlying network to achieve the performance stated in the SLA. These controls are enforced within a large network by identifying differentiated services, that is, traffic flows that are singled out to be provided with a predictable quality of service (QoS) (e.g., limits on packet loss and delay). Traffic routed through a large network can be tagged to recognize its class of service (CoS), thereby facilitating the satisfaction of service requirements through mechanisms such as priority packet queueing, bandwidth allocation, and service recovery priority. 1 2

The increased interest in VoIP, because of its low cost and growing ubiquity is, to paraphrase Shakespeare, the most unkindest cut of all from the computer community to the traditional telephone carriers. Availability is the percentage of time that a network is operational. For example, “five 9s” (99.999%) availability, which is a goal for public carriers, implies 5.25 minutes of downtime per year.

6

Multiwavelength Optical Networks

Internet access

POTS

VPN

Music/video VoIP Telemedicine file sharing

Services Layer

Logical Layers

IP

IP

ATM

SONET Wavelength on demand

Physical Layer

Figure 1.1 Multilayered network.

The various functions executed by the network operator, such as load balancing and QoS-based traffic handling, are known as traffic engineering. Considering the wide diversity of these service requirements (and we have only mentioned a small sample), it is a challenge to support all of them on a single network. Yet this is not only possible, but it is generally the most efficient way of doing the job. As we will show, the building blocks are now available to assemble multiwavelength optical networks that can sustain large user populations with diverse service requirements of the type just described. This means that, ideally, the MWNA must be structured to offer a special set of features adapted to each service it supports. To see how this is achieved it is convenient to think of the network in terms of its constituent layers, with client–server relations between the neighboring layers. An illustration is the multilayered view shown in Figure 1.1. The architecture is composed of an underlying optical infrastructure – the physical layer – which provides basic communication services to a number of independent logical networks (LNs) residing in the logical layer. Each LN organizes the raw capacity offered by the physical layer, adapting it to the needs of the clients it serves, shown in the services layer of the figure. For example, the SONET network shown in Figure 1.1 uses optical wavelength channels provided by the physical layer, transmits optical signals on them, and carries multiplexed communication channels on those signals. The SONET channels can be tailored to support a wide variety of services; two services shown in the figure are plain old telephone service (POTS) and a VPN. In our example, the SONET layer also supports an ATM layer that in turn supports a client IP layer providing Internet access services to end users. Another independent IP network shown in Figure 1.1 is supported directly by the physical layer, providing a telemedicine service, VoIP, and a music/video file-sharing service. In addition, the

The Big Picture

7

physical layer provides purely optical connections directly to end users via demandassigned wavelengths (also known as clear channels), thereby bypassing the logical layer altogether. Thus, the logical layer shown in the figure contains several LNs; some are stacked in a client–server relationship, and others are independent of each other, offering specialized features to the service layer. Stacked logical layers; e.g., IP over ATM over SONET over WDM, have both advantages and disadvantages. For example, different services (e.g., POTS and VPNs) require channels running at different bit rates. The SONET layer supports these different speeds and in addition provides a grooming function, packing the diverse channels onto a common optical wavelength, using time division multiplexing. This “fills up” the wavelength channel for efficient utilization. However, stacked layers mean additional equipment, which is costly, introduces delays and potential points of failure, and is difficult to manage. Therefore, it is desirable to reduce superfluous layers wherever possible. For example, the IP equipment manufacturers propose to provide IP over WDM, short-circuiting the commonly used configurations involving stacked intermediate layers.3 Another view of an optical network is the physical picture of Figure 1.2, showing the network elements in the layers of Figure 1.1. Here the physical layer is portrayed for simplicity as a transparent purely optical core.4 The “glue” in the physical layer that holds the transparent optical network together fits roughly into two basic classes: the optical network nodes (ONNs), which connect the fibers within the network, and the network access stations (NASs), which interface end-user systems and other nonoptical equipment to the network. Shown as rectangles in Figure 1.2, the NASs (or stations for short) provide the terminating points (sources and destinations) for the optical signal paths within the physical layer. The communication paths continue in electrical form outside the purely optical part of the network, either terminating at end systems (for example, PCs, telephones, and servers) or traversing electronic multiplexing and switching equipment (e.g., ATM switches, IP routers, or SONET digital cross-connect systems [DCSs]), shown as hexagons in Figure 1.2. The ONNs (or nodes for short), shown as circles in Figure 1.2, provide the switching and routing functions that control the optical signal paths (also called lightpaths), configuring them to create desired source-destination connections. The stations and nodes contain the optoelectronic and photonic components of the network: lasers, detectors, couplers, filters, optical switches, amplifiers, and so on. These components work together with the fibers to produce the required optical signal connectivity. Although the underlying optoelectronic and photonic technologies have matured considerably since the mid1990s, they are not as well developed as their electronic counterparts. Thus, electronics (in the logical layer) is currently an equal partner with photonics (in the physical layer). 3 4

In many cases, superfluous LN stacking results from a reluctance of carriers to write off a large investment in legacy systems, which would require a complete revision of existing control and management structures. As will be seen, the physical layer in optical networks often includes electronic components in the form of signal regenerators or electronic switch fabrics, so it is not always purely optical nor is it completely transparent. We have more to say about purely optical signal paths and the meaning of transparency in Sections 1.3 and 1.4.

8

Multiwavelength Optical Networks

Internet access VoIP

Telemedicine

File sharing

IP POTS

VPN

A IP

S

Supercomputer

e o

NAS ONN IP

IP router

A

ATM switch

S

SONET DCS

Optical Ether

Figure 1.2 Physical picture of the network.

The line between the optical and electronic parts of the network has become fuzzy as technology has advanced, but at this point in our discussion we retain the simplified view that the physical layer is transparent and optical, whereas the logical layers are electronic and “opaque.” The electronic switching node plays the same role in the logical layer as ONNs play in the physical layer. Our generic term for an electronic switching node (each hexagon in Figure 1.2) is a logical switching node (LSN). The LSNs sort, multiplex, switch, and route signals in the various LNs. In this way they create virtual connections among the entities they serve. These entities may be service provider equipment or user equipment (end systems), as in the case where the LSNs are IP routers connecting ISP servers to customer PCs, or they may be higher layer switching nodes in a client network, as in the case of SONET DCSs serving ATM switches serving IP routers. Although the focus of this book is the physical layer and its optical components, the logical layers are an integral part of the overall network architectures we discuss here. Therefore, our MWNA includes the logical layers and their electronic components. An understanding of the design and operation of multiwavelength optical networks requires an awareness of the close coupling between the physical layer and the logical layers it serves. The networks we examine will generally be designed to serve large, heterogeneous, geographically dispersed user populations. Given this fact, and the various service requirements discussed, we can infer a list of general design and operating objectives:

The Big Picture

9

r Connectivity – Support a very large number of end systems – Support a very large number of concurrent connections, including multiple connections per station and per end system – Support multicast connections efficiently r Performance – High aggregate network throughput (hundreds of terabits per second) – High fiber transmission capacity (terabits per second) – High user bit rate (gigabits per second) – Small end-to-end delay – Low error rate (digital)/high signal-to-noise ratio (analog) – Low processing load in nodes and stations – Adaptability to changing and unbalanced loads r Technology: cost-effective design and utilization – Access stations: limited number of optical transceivers per station, limited complexity of optical transceivers, simple tuning techniques – Optical network nodes: high throughput, minimal signal impairment, and low complexity – Logical switching nodes: efficient channel grooming, simple packet-routing procedures, and controlled traffic load – Network: Limited number and length of cables and fibers, efficient use (and reuse) of the optical spectrum, controlled signal impairment in the physical layer, minimization of logical layer complexity. r Structural features – Scalability – Modularity – Survivability (fault tolerance) r Control and management – Efficient, rapid, automated connection provisioning, and reconfiguration – Built-in intelligence in the network elements for monitoring and control – Efficient and rapid automatic fault identification and recovery – An integrated network management system to monitor and coordinate all network layers As we look at existing and proposed network architectures, it is important to keep these goals in mind.

1.3

Optics versus Electronics: The Case for Transparent Multiwavelength Networks There are certain functions that come naturally to each technology. Referring to the somewhat idealized view of a network in Figure 1.2 based on the assumption of a purely optical physical layer, there is a clean separation between optical/photonic technology,

10

Multiwavelength Optical Networks

on the one hand, and electronic technology, on the other. The NASs represent the optoelectronic interface (denoted by the boundary labeled e/o) between the electronic domain (the equipment outside the purely optical portion of the network) and the optical domain, sometimes called the optical ether. This interface is the point of demarcation between the physical layer and the logical layers. In a typical purely optical physical layer the optical signal paths are as transparent as a piece of glass. The stations provide the basic functions of getting the light into the fibers (with lasers) and getting it out (with photodetectors). When the signals are in optical form, photonic technology is well suited to certain simple signal-routing and switching functions within the nodes. With static photonic devices, it is fairly easy to perform functions such as optical power combining, splitting, filtering, and wavelength multiplexing, demultiplexing, and routing. By adding suitable control, the static devices can be controlled dynamically (switched) at slow to fast speeds (milliseconds in the case of mechanical or thermal control and microseconds or nanoseconds in the case of electronic control). The enormous usable bandwidth of a single fiber (tens of terahertz) is at the same time a great asset and a great challenge. It is technologically impossible to exploit all of that bandwidth using a single high-capacity channel. Thus, to make efficient use of the fiber it is essential to channelize its bandwidth. This is most easily accomplished by superimposing many concurrent signals on a single fiber, each on a different wavelength; that is, by using WDM. Thus this book focuses on multiwavelength or WDM network architectures. The relative ease of signal manipulation in the wavelength (or optical frequency) domain, as opposed to the time domain, suggests that current optical technology is particularly suited to multiwavelength techniques. In WDM networks each optical transmitter (receiver) is tuned to transmit (receive) on a specific wavelength, and many signals operating on distinct wavelengths share each fiber – possibly more than 100 in dense WDM (DWDM) transmission systems. It should be observed that all photonic routing and switching functions within the optical domain in these networks are linear operations. Thus, at the optical level the network typically consists of only linear devices, either fixed or controllable. It is the property of linearity that makes multiwavelength networking simple and cost-effective. To distinguish these linear networks from other types of optical networks, we refer to them frequently as transparent optical networks. Typical nonlinear operations performed in networks include signal detection, regeneration, reading, and modifying the information in the signal, buffering, and logic functions (e.g., packet routing based on header information). Although many nonlinear functions can be performed in the optical domain with present-day technology the current state of the art for these nonlinear devices is not nearly as advanced as it is for linear components. For these reasons, we frequently use the terms transparent optical network and purely optical network interchangeably in this book.5 Nonlinearities make the signal path opaque rather than transparent. Some of the advantages of keeping nonlinear operations out of the signal path are (1) the 5

As the state of the art progresses photonic technology is becoming a viable alternative for many nonlinear signal processing operations, so that the linkage between “transparent” (i.e., linear) and “purely optical” is becoming tenuous.

The Big Picture

11

end-to-end optical path behaves as a literally transparent6 “clear channel” so that there is nothing in the signal path to limit the throughput of the fibers (a transparent channel behaves very much like an ideal communication channel with almost no noise and a very large bandwidth), (2) the architecture of the optical network nodes can be very simple because they have essentially no signal processing to do (optical node simplicity also means simplicity of network control and management), and (3) system upgrades involving changes in speed, format, and protocol are easily implemented. There are also downsides to transparency. First, problems caused by equipment failures tend to propagate throughout the network, making fault management a more complex issue than in nontransparent networks. Similarly, impairments such as switch cross-talk, noise, fiber dispersion, and nonlinear effects accumulate over long paths, limiting the geographic “reach” of an optical connection. Second, by definition, in-band information (e.g., control information carried in packet headers, such as source and destination address, sequence number, channel number, and parity check bits) cannot be used while the signal is in optical form. Because of this, a transparent physical layer cannot perform the various processing functions required in packet switching. It is important to note that the in-band control information carried with the data in IP (packet-based) or ATM (cell-based) networks is the key to achieving a high degree of virtual connectivity in these networks. Typically, many virtual connections are multiplexed on each network link and sorted (switched) on a packet/cell basis at each IP router or ATM switch using information contained in the packet or cell headers. Maintaining transparency in the physical layer eliminates the intelligence necessary to process this information and therefore tends to produce an optical “connectivity bottleneck” in transparent networks. For all of the aforementioned reasons, there is a case to be made for opaque optical networks [Bala+95]. The properties of electronics are complementary to those of optics. Electronic processing is ideal for complex nonlinear operations, but the limited speed of electronic and optoelectronic devices (e.g., electronic switches, memory devices, and processing units), and the high processing load imposed on electronics in broadband networks, causes a well-known “electronic bottleneck” in optical transmission systems. Putting an electronic termination on an optical fiber reduces the potential multiterabit-per-second throughput of the fiber to a multigigabit-per-second trickle: the maximum speed that can be expected of an electronic signal. This is the origin of the highway/cow path analogy we used in Section 1.1. More succinctly, optics is fast but dumb, whereas electronics is slow but smart. A final caveat: whenever we speak of enabling technology it must be understood that it is a fast-moving target. Thus, the state of the art is rapidly evolving in the direction of smarter optics and (somewhat) faster electronics. One consequence of smarter optics is that nonlinear operations can be introduced into a purely optical network using optical processing. For example, optical packet switching can be realized either through purely 6

Transparency implies that signals with any type of modulation schemes (analog or digital), any bit rate, any type of format, and using any kind of protocol can be superimposed and transmitted without interfering with one another and without their information being modified within the network. Opaque networks do not have these properties.

12

Multiwavelength Optical Networks

optical processing or a combination of electronics for header processing and optics for switching, resulting in an opaque optical network capable of very high speed packetswitched operation (see Chapter 10). Another example of smarter optics is the use of optical processing for signal regeneration and wavelength conversion within an otherwise linear signal path (see Chapter 4). Faster electronics is a more questionable issue, because as we push the electronic speed limits, costs rise rapidly.

1.4

Optics and Electronics: The Case for Multilayered Networks Because of the size and complexity of the networks we are considering, and because of the fact that lightwave technology alone cannot satisfy our networking objectives, we now return to the multilayered model of Figure 1.1 with a more detailed look at the logical layers. Elaborating on the discussion of Section 1.2, we continue with the assumption of a clear separation between optics (in the physical layer) and electronics (in the logical layers); that is, the network has as its physical foundation a multiwavelength purely optical network. Superimposed on the physical layer are one or more LNs, each of which is designed to serve some subset of user requirements and is implemented as an electronic overlay superimposed on the physical layer. Just as a transparent optical network has a physical topology composed of ONNs and fiber links, a logical network has a logical topology composed of LSNs and logical links. A logical link is an electronic transmission channel joining two LSNs. It is carried on an optical path provided by the underlying physical layer. Each LN organizes the connections it offers to its clients in a specific way, with its own layered architecture. Different LNs may be managed independently or in coordination with others. Typically, an LN will provide services to end systems in the form of virtual connections traversing paths in the logical topology. In MWNAs involving stacked LNs a logical layer acting as a server for a client logical layer (e.g., the ATM layer serving the IP layer in Figure 1.1) will offer its services to the client layer in the form of virtual connections, which can be used as logical links in the client layer. The physical layer makes a large pool of bandwidth available to the LNs in the form of transparent endto-end connections. These high-bandwidth optical channels may be used to provide a dedicated communications backbone for an LN as in IP over WDM, or the channels may be demand assigned for temporary activities such as response to changing traffic distributions or recovery of faults. In descending the layers in the optical network architecture, several connections in each client layer are typically multiplexed on a single connection in the corresponding service layer, resulting in fewer but “thicker” connections carried on the optical paths in the underlying physical layer. This results in a connection granularity ranging from fine to coarse as we move down the stack. In each LN the electronic switching equipment acts as a “middleman,” taking high-bandwidth channels offered to it by the layer below it and organizing them into lower bandwidth channels with a format acceptable and cost-effective for the end users and/or client layer it serves. For example, the layer in Figure 1.1 providing IP over WDM makes use of wavelength channels offered to it by

The Big Picture

Logical Layer Physical Layer

LL1

LL2

13

LL3

Optical Layer Fiber Layer

Figure 1.3 Layered view of an optical network.

the physical layer and “packages” the bandwidth so as to support the flow of IP packets among its end users. Sophisticated network users requiring high bandwidth and the flexibility of a clear channel can dispense with the services of an LN to obtain direct access to demand-assigned wavelength channels as shown in the figure, without the intervention of an electronic middleman. Returning to the physical picture, Figure 1.2 illustrates in more detail how the end users interface to the network through various layers of logical (electronic) switching equipment, and then through the NASs to the physical layer. For example, a user of Internet access services is shown accessing the network via an IP router on top of an ATM switch on top of a SONET DCS, which uses the services of the transparent physical layer. At the other extreme, a supercomputer is shown accessing the physical layer directly through an NAS. In Figure 1.3, the architecture of the overall network is shown in a more formal view, reduced to its constituent layers, each one providing support for the layer above it, and using the services of the layer below it – a natural extension of the idea of layered architectures to optical networks. The layering formalism is introduced here to partition a complex set of interactions among network components into a small number of more manageable pieces. The characteristics of the logical layers (LLs) depend on the architectures of the various LN overlays. The physical layer, representing a purely optical network, is now shown divided into an optical layer and a fiber layer. The former contains the optical connections supported by the fibers, and the latter embodies the layout of the physical infrastructure itself: the fibers, switches, and optical transceivers. In Chapter 2, the layered view of optical networks is expanded into a complete multiwavelength network architecture. Why is a hybrid approach required for optical networks? Although some early structures proposed for local area networks (LANs) and metropolitan area networks (MANs) were purely optical, the current state of the art suggests that neither optics nor electronics alone can provide all the desired features listed in Section 1.2. Using current technology, purely optical wavelength-selective switches are capable of interconnecting as many as 100 fibers operating in a DWDM mode, switching and routing each wavelength independently and yielding aggregate throughputs in the range of many terabits per second while supporting hundreds to thousands of optical connections running at speeds of the order of 10 Gbps each. On the other hand, electronic packet/cell switches are currently limited in throughput to approximately 100 Gbps.7 However, they can support a much larger number of relatively low-bit-rate virtual connections. 7

A similar, but not as severe, throughput limitation applies to “opaque” optical switches that are often used in the physical layer to replace purely optical ONNs. An opaque switch converts the optical signals to

14

Multiwavelength Optical Networks

The difference in the two approaches is the granularity we spoke of earlier. Optical switches are operated most easily in a circuit-switched mode, in which the informationbearing units (optical wavelength channels) being switched are few but large (in bandwidth) and the holding times for a given switch configuration are long (seconds or more). Circuit-switched operation of the optical nodes is perfectly suitable for the physical layer shown in Figures 1.1 and 1.3. Dedicated connections supporting the various LNs are normally held in static configurations for durations of hours, days, or more. Demand-assigned connections are held typically for minutes or hours. Thus, the number of circuits being set up and taken down per unit time is relatively small. This type of operation requires little processing and provides a high aggregate throughput. Conversely, electronics is employed in situations in which there are many information units (e.g., individual packets or cells) being switched per unit time. Because the units are typically small (in number of bits) and because each unit is processed individually, this leads to a heavy processing load, with a relatively low throughput limited by the processing power of the switch. Wide area networks (WANs) must handle both large and small information units: hence the need for marrying both electronic and optical switching technologies. The hybrid approach exploits the unique capabilities of each while circumventing their limitations. To illustrate the importance of combining optical and electronic technology let us explore the demands placed on large networks. Broadband WANs must be capable of supporting high connectivity, high throughput, and heterogeneous traffic mixes, and they must be flexible in meeting changing demands and unforeseen circumstances such as equipment and link failures. Consider a WAN serving as a backbone that interconnects a large number of users. To reduce access costs, the traffic from small users should be aggregated before it enters the WAN, through the intermediary of an access network – a high-speed LAN, a fiber access network, or an electronic switch (e.g., an IP router owned by an ISP). Each of these represents a gateway to the WAN, and there might be 10,000 of these in a network of global scale, each one serving hundreds of active users. The aggregate traffic in and out of each gateway might require connectivity to perhaps 1000 other gateways on the network at any one time, and the total traffic injected into the network might be of the order of tens of terabits per second. (This example corresponds in orders of magnitude to present-day numbers on the Internet.) Let us examine the optical, electronic, and hybrid alternatives for supporting these users. Consider first a network supporting gateway interconnection, using purely optical switching. It would resemble a modified version of Figure 1.2, with the access gateways attached directly to NASs without the intervening (electronic) LSNs, as shown in Figure 1.4(a). Costs dictate that there will be considerably fewer optical nodes than NASs (i.e., each node may serve many stations). In the purely optical case, each NAS interfaces one or more gateways to the backbone, so that the NAS must connect its gateways to the other gateways on the network through individual optical connections. This could require millions of relatively low-bit-rate connections (as many as 10,000,000 for electronic form for purposes of switching, and in the process it performs signal regeneration, wavelength conversion, and signal monitoring as well. Although this makes an opaque switch much more versatile than a transparent optical ONN, it also produces an electronic bottleneck within the node.

The Big Picture

Gateway

Gateway

Gateway

Gateway

Gateway

15

Gateway Gateway

Gateway

Gateway

Gateway

Gateway

Gateway

Gateway

Gateway e

Gateway

o

Gateway IP IP

IP

NAS

IP

ONN

IP IP

Gateway

Optical Ether

IP IP

IP router

IP

Gateway IP

IP Gateway Gateway

Gateway

Gateway

Gateway

Gateway

(a) Purely optical switching

(b) Electronic switching

Gateway

Gateway

Gateway

Gateway

Gateway Gateway

Gateway

Gateway

Gateway

IP

IP

e

IP

o

Gateway

NAS ONN IP

Optical Ether

IP router

IP IP

Gateway

Gateway

Gateway

Gateway

Gateway

(c) Hybrid network Figure 1.4 Alternative network approaches.

the connectivity postulated here). It is the high-connectivity requirement that makes a purely optical approach very difficult. To implement this connectivity, each station must maintain connections, to 1000 other NAS/gateways (assuming 1 gateway per NAS). This means either equipping each station with that many optical transceivers or providing extremely rapid optical connection switching. No matter how it is realized, this degree of connectivity is well beyond the reach of current optical technology. A purely electronic version of Figure 1.2 has its own problems. This would be implemented with electronic rather than optical switches at the network nodes, as shown in Figure 1.4(b), where the ONNs have been replaced by IP routers. This reduces the physical layer to a set of isolated point-to-point transmission links terminating on the electronic routers. Electronics can easily support the required connectivity via virtual connections. However, the electronic processing bottleneck at the switches makes it difficult and expensive to sustain the required multiterabit throughput on the backbone. Because optical switching is still in the early stages of penetrating large networks, current architectures are tilted more to the electronic side (with multiple stacked logical layers) than the optical side. However, pressures of increasing demand, performance, cost-effectiveness, and fault tolerance are moving networks toward hybrid configurations of the type shown in Figure 1.2.

16

Multiwavelength Optical Networks

In a hybrid architecture both high connectivity and high throughput are achieved easily and efficiently. For example, an LN composed of IP routers provides the necessary connectivity through sorting and routing packets at each LSN (see Figure 1.4[c]). The physical layer supports the required throughput over dedicated, high-bandwidth (optical) backbone connections carrying aggregated traffic on logical links between the IP router ports. This example illustrates the fact that the test of a good lightwave network is whether it can achieve both high throughput and high connectivity at a reasonable cost. Another advantage of the hybrid approach is the versatility achieved by combining optical and electronic technology. This is especially important from the point of view of reconfigurability. For example, frequent changes in fine granularity traffic routing due to changes in customer demand are most easily accommodated by switching in the logical layers; i.e., by using the intelligence built into the IP routers. However, rerouting of masses of coarse granularity traffic due to equipment or link faults is more quickly and efficiently handled in the physical layer by resetting the optical paths through the ONNs using automatic fault recovery mechanisms. Without a reconfigurable layer underneath them, the logical links between the IP routers are fixed. We have an IP network whose logical topology is written in stone.8 Conversely, without an intelligent logical layer above it, the physical layer cannot manipulate the fine-granularity traffic flows among end users. In other words, the logical layers need a reconfigurable physical layer just as the physical layer needs reconfigurable logical layers.

1.5

Network Hierarchies Just as there is a case to be made for a multilayered network architecture, there is also a rationale for a hierarchically structured network. Traditional carriers have all found hierarchies to be useful. In telephone networks, individual subscribers are connected to a nearby Central Office, and Central Offices in the same area are interconnected by a MAN, typically spanning a region of a few hundred kilometers at most. These are usually in the form of rings. Finally, the MANs are interconnected by long-haul networks with mesh topologies. These include ultra long-haul transmission links such as transoceanic cables. In going from local subscriber access through MANs to long-haul networks, efficiency and manageability demand that the granularity of the connections increases, just as it does in going down the layered architecture of Figure 1.1. The new optical network architectures have similar hierarchies as shown in the example of Figure 1.5. The mesh network in the center is the core long-haul network, which joins the MANs in the form of rings. End users connect to the MAN through access networks joined to the MAN at gateways indicated by the shaded circles in the figure. These access networks might be in optical form (e.g., p
assive optical networks [PONs]) or electrical form (e.g., traditional LANs or electronic switches). Their purpose is to aggregate traffic from individual users for more efficient and cost-effective transmission 8

One of the reasons for the use of stacked LNs such as ATM and SONET underneath an IP logical network is that the lower logical layers can be used for offering a reconfigurable logical topology to the IP network, partially replacing the functions of a reconfigurable physical layer.

The Big Picture

17

Access point ONN

Figure 1.5 Hierarchical network.

on the network. A characteristic of the hierarchical structure is that as one moves closer to the end user, the number of entities attached to the network grows exponentially. Referring to Figure 1.5, there are many MANs attached to the core, many access networks attached to each MAN, and many end users attached to each access network. Conversely, the amount of traffic originating from each entity declines exponentially as one moves closer to the end user. Because there are so many end users with modest individual traffic demands, the equipment located close to the end users in the access networks must be inexpensive and simple but does not need to be high capacity or high performance. Conversely, the long-haul network contains relatively few optical links and ONNs (the clear circles in Figure 1.5), but they must support high throughput with carefully controlled signal quality and high reliability. For example, transmission on long-haul links is cost-effective when a large amount of capacity is packed into each fiber. This means using large numbers of closely spaced wavelengths; i.e., DWDM with subnanometer wavelength separations and high bit rates (10 Gbps or higher) on each wavelength. Achieving these numbers over distances of thousands of kilometers is not a trivial feat and requires costly transmitting, amplifying, and receiving equipment. The MAN must meet criteria somewhere in between these two extremes. For example, a standard called coarse wavelength division multiplexing (CWDM) is adapted to MANs and uses spacings of the order of 20 nm. This wide spacing combined with fairly short transmission distances means that relatively inexpensive transmission equipment can be used in these applications. In addition to the cost–performance issues in the various levels of a hierarchy, there are important control and management issues. The complexity of control and management in a large network grows rapidly with the number of entities being managed. For example, in real-time decision making for fault recovery, speed is of the essence, but speed is compromised if the fault recovery system must deal “microscopically” with every

18

Multiwavelength Optical Networks

fine-grain active connection in the network. If all traffic among end users in a large network was transported and controlled in the form of low-speed individual connections (e.g., kilobits per second for a voice call) the number of connections to keep track of in the high-capacity long-haul core would be overwhelming, making the core network unmanageable. Instead, as we move from the end user, through the MANs and into the core, various levels of multiplexing are used, so that the fine-grain end-user connections are bundled into a much smaller set of coarse granularity connections on the longhaul links. Manageability is maintained because there is a small number of high-speed connections (and a small amount of hardware) to control rather than an enormous number of low-speed connections. On the other hand, control and management in a lower level of the hierarchy – a single MAN or access network – can be handled more or less autonomously, dealing only with the relatively small number of “local” entities without an overall view of the complete network. Another way of seeing this is in terms of the multilayered view in Figure 1.1. In a high-speed core of an optical network, control activity is largely confined to high-speed highly multiplexed connections in the physical layer. In moving out to the end users the control functions shift to the higher logical layers, but because the view is local, the total number of entities being controlled is still relatively small.

1.6

A Little History The idea of a high-speed optical transmission system (in free space) was considered as early as 1958, when the laser was conceived [Schawlow+58], and guided wave optical transmission was exhibited in the laboratory in the mid-1960s [Kao+66]. However, practical optical transmission systems did not become possible until the production of the first low-loss fibers and the invention of the semiconductor laser diode, both around 1970. By refining the optical transceivers and reducing fiber loss, the effectiveness of unamplified optical transmission systems (measured in bit-rate-distance product) grew roughly at an exponential rate from the early 1970s to the late 1980s, with bit rates as high as 8 Gbps over distances of 100 km achieved in the mid-1980s [Miller+88]. The first optical-fiber transatlantic cable (using electronic repeaters) was laid in 1988. The distance limitations due to fiber attenuation disappeared in the late 1980s, almost overnight, with the emergence of the erbium-doped fiber amplifier (EDFA) [Desurvire+87, Mears+86, Mears+87]. Over the ensuing years interest in long-distance optical transmission using EDFAs grew rapidly [Saito+90]. In laboratory experiments, in which long distances are simulated using closed loops with amplification, and in which fiber dispersion effects are eliminated using solitons, transmission distances have been extended essentially without limit. For example, [Nakazawa+93] reported a 10-Gbps soliton system operating over a total distance of 106 km. During the late 1970s to the middle 1990s, fiber transmission capacity on a single wavelength roughly doubled each year. Afterward, the focus was on multiwavelength transmission, resulting in a significant jump in aggregate transmission bit rates to the terabits-per-second range in the late 1990s using WDM. A recent example of

The Big Picture

19

high-capacity long-distance transmission is reported in [Charlet+04]. It involved a DWDM system using 149 channels at 50-GHz spacing running at 42.7 Gbps each, for a total capacity of 6 Tbps over 6120 km. While the transmission limitations in both capacity and distance were being surmounted, important developments were taking place at other levels as well. The SONET and SDH standards were developed in the late 1980s [Ballart+89, Boehm90]. Both of them pertain to optical transmission links carrying synchronous bit streams terminated by electronic switches. Soon after SONET and SDH came on the scene, the concept of a broadband integrated services digital network (B-ISDN) was introduced as a means of supporting all sorts of multimedia services on a common network [CCITT92]. In the 1990s, much activity was devoted to developing ATM as the preferred transport service for B-ISDN. The cell-based transport technique in ATM (essentially a fixed-length, fast packet-switching system) lent itself at the time to a wide variety of multimedia applications and at the same time was well adapted to high-speed switching techniques [de Prycker91]. All of the developments discussed so far were carried out within the traditional voice communications carrier community. However, the early data networks also influenced the structure of today’s optical networks. The first data networks were developed in the 1970s mainly for business users, utilizing packet switches designed by computer equipment manufacturers to work together in a closed network environment employing proprietary protocols. Examples were IBM’s SNA (Systems Network Architecture) and Digital Equipment’s DEC-NET. Typical applications were airline reservation systems and timeshared computing. Governmental organizations joined with ARPANet in the United States, Datapac in Canada, and Cyclades and Transpac in France. Although Datapac and Transpac were public data networks, ARPANet and Cyclades were experimental and contained the precursors of today’s dominant global packet-switched network – the Internet. Today’s Internet routing equipment and IP, the Internet Protocol, evolved from the hardware and software developed in the 1970s for ARPANet, which was a pioneering communication network sponsored by the Advanced Research Projects Agency (ARPA) of the U.S. government. ARPANet was developed as an experiment in computer resource sharing, and its driving force was the computer community. In a competing development, the International Standards Organization (ISO) Open Systems Interconnection (OSI) Reference Model was created in the 1980s as an attempt by international organizations to produce a common standard that would move data communications out of the proprietary networking world, making multivendor networks feasible. However, at the same time that the OSI model was being promoted worldwide by international standards bodies, the Internet was quietly and spontaneously spreading throughout the world. Proprietary networking protocols, which had previously become de facto standards were displaced by IP, the language of the Internet. ISPs became key players in the networking community, and the World Wide Web, invented in 1989, provided the impetus for a flood of new services requiring a high-capacity, high-connectivity infrastructure, and high-speed user access. The new multimedia applications necessitated broadband access, and, as mentioned in Section 1.1, the telephone and cable carriers responded, respectively, with DSL and cable access. Going beyond these copper-based

20

Multiwavelength Optical Networks

systems there has been considerable activity by the local exchange carriers (LECs) in fiber-based access: fiber to the curb or node (FTTC or FTTN), building (FTTB), cabinet (FTTCab), and home (FTTH).9 Although most of the developments discussed so far occurred in the logical layers of the network, they were and are continuing to produce challenges throughout the layers of the network architecture in terms of increased network loads and ever escalating performance requirements. It is ultimately the task of the underlying optical infrastructure to respond to these challenges. Spurred by the early developments in optical and photonic technology, interest in purely optical networks began in the mid-1980s [Henry89]. However, the technological barriers to the deployment of large-scale networks remained formidable until the advent of the fiber amplifier. Systems efforts during the pre-EDFA era were focused on a simple architecture appropriate for LANs or MANs: the broadcast-and-select network [Mukherjee92a]. In a typical network of this type, each access station is equipped with a single laser transmitter capable of generating light at a fixed wavelength and contains a single optical receiver capable of being tuned to the wavelength of any transmitter. Signals from all transmitting stations are combined in a centrally located optical star coupler, a passive device that broadcasts an attenuated version of the combined signals back to each receiver. By selecting the appropriate wavelength, each receiver can accept the signal injected by the corresponding transmitter, thereby creating a transparent connection from the transmitter to that receiver. Probably the earliest prototype of a broadcast-and-select network was LAMBDANET [Goodman+86, Goodman+87]. Broadcast-and-select networks do not scale well to large sizes primarily because they rely on rapid tuning of optical transceivers over a wide range of wavelengths, they waste optical power, and, most important, they make poor use of the optical spectrum. More general mesh topologies were soon proposed to eliminate the constraints of broadcast-and-select networks. In these networks, alternate paths together with wavelength routing produce possibilities for reuse of the optical spectrum [Hill88] as well as recovery from failures. At about the same time that wavelength routing was proposed, the multihop concept was suggested to obtain high connectivity without requiring expensive, tunable optical transceivers. Multihop networks were early examples of the hybrid approach described in Section 1.4 [Acampora87, Mukherjee92b], relieving the connectivity bottleneck at the optical level by adding packet or cell switches in an electronic LN overlay. From the late 1980s to the present, activity intensified in optoelectronic and photonic technology in the demonstration and deployment of new network architectures and, most recently, in optical network standardization, control, and management. Some of the important recent technological advances occurred in fiber technology and amplification, extending the usable optical fiber spectrum over a contiguous window from 1200 to 1600 nm. Other advances occurred in microelectromechanical systems (MEMS). These devices were the basis of new high port-count optical switches, tunable filters, and related devices. A wide variety of high-performance devices for WDM 9

Early demonstrations of fiber access date back to 1981 with FTTH by Northern Telecom and 1989 with passive optical access networks by British Telecom.

The Big Picture

21

based on guided-wave technology were also developed, including tunable lasers, filters, integrated switch fabrics, and optoelectronic subsystems such as integrated optical receivers. As the fundamental multiwavelength technology for transparent networks was maturing, experimental work was (and is) continuing in more speculative areas entailing nonlinear optical devices for opaque networks. Units such as all-optical switches, optical logic devices, and optical storage elements are of interest to move the nonlinear operations now executed in electronics down to the optical level. (Among the enablers of these devices are photonic crystal fibers [PCFs] whose structures can be tailored to produce a variety of highly nonlinear effects.) Applications are in optical packet switching and optical computing. In the 1990s, ambitious optical network testbeds were deployed in the United States, Europe, and Japan, involving the maturing multiwavelength technology as well as incorporating the management and control equipment necessary for making these networks operational and reliable [IEEE93, IEEE96, IEEE98]. These testbeds showed for the first time that optical technology could be taken out of the laboratory to produce cost-effective operational networks. Many of the technologies and concepts developed in the testbeds led to commercial products and network deployments that are basic components of our current network infrastructure. Undoubtedly the rapid advances in the enabling technology for optical networks, and its accelerated commercialization at the end of the 1990s, can be largely attributed to the massive infusion of capital to the various players during the “technology bubble” at that time. This produced a host of new start-ups and spin-offs as well as expansions of ongoing activities in the large equipment manufacturers. Although many of the companies that originated the new products have disappeared, the technological progress remains, and will serve as a foundation for the networks of the future. The period of economic consolidation following the bursting of the bubble brought with it a more down-to-earth view of networking, essentially focusing on the areas of prime concern to the network operator and the customer: cost-effectiveness, reliability, fault recovery, and manageability. Focus on costs has led to a trend toward eliminating superfluous electronic and electro-optic components in networks (e.g., the use of IP over WDM without intervening logical layers) and improving operating efficiencies through enhanced traffic grooming and fault recovery techniques. Focus on cost has also produced a concentration on bread-and-butter issues such as economic designs in MANs, in contrast to the “hero experiments” in the 1980s, where the objective was to break records for long-haul transmission – at any cost. Focus on manageability has resulted in solid advances in control and management, including taking control techniques designed for the logical layers of the network and adapting them to the physical layer. As an example, the control protocol known as Generalized Multiprotocol Label Switching (GMPLS) has been proposed for application to control functions in the physical layer. As its name implies, GMPLS is a generalization of Multiprotocol Label Switching (MPLS), which was designed as an improvement over the packet-forwarding techniques used in IP networks.

22

Multiwavelength Optical Networks

This recent activity provides the context for this book: an advancing and maturing technology base, a steady increase in demand for network capacity and performance, and an industrial base that has positioned itself to meet these demands.

1.7

Overview and Road Map Lightwave networks can be characterized broadly in terms of three basic features: (1) physical (fiber) topology, (2) functionality in the links, the network nodes, and the access stations, and (3) control algorithms for assigning, routing, and multiplexing connections. In keeping with the focus of this book on the physical layer, we elaborate on these issues here, emphasizing how they apply to the optical infrastructure. A rich physical topology provides many alternate paths among access stations, increasing the aggregate capacity of the network as well as its potential survivability and adaptability to changing load patterns. But the properties of the physical topology cannot be exploited without sufficient functionality in the links, the network nodes, and the access stations. Also, logical network overlays are generally required to adapt the bandwidth offered by the physical layer to the needs of the end users. By link functionality we mean good transmission properties (large bandwidthdistance product). Useful functional properties of nodes and stations include controllable switching and multiplexing features. Without controllability in the nodes and stations, optical channel assignments and signal paths must remain fixed at all times so that connections are frozen, and the network has no flexibility in responding to changing conditions. Conversely, a high degree of node and station controllability under the supervision of a network control and management system improves the efficiency of resource utilization, allows the network to maintain satisfactory performance in the face of fluctuating demand, and enables it to reconfigure itself in case of component failures. Of course, controllability implies the existence of suitable control algorithms to coordinate the functions of the various network entities. Three basic features – topology, functionality, and control – interact closely to influence overall network performance. As might be expected, there are many opportunities for cost–performance trade-offs. Thus, high functionality in the nodes and access stations improves performance, but this comes at a price. The same can be said of the richness of the physical topology. Also, to compensate for the limitations of the network resources, one can attempt to optimize performance through sophisticated control algorithms. However, optimality generally comes at the price of controller complexity. Thus, the cost–performance trade-offs involve complex interactions among all the basic features. In a field of engineering where the technology is mature (e.g., in digital electronics), systems can be understood, analyzed, and designed with only a limited understanding of the physical principles involved. (A designer of a personal computer is not particularly concerned with electromagnetic theory.) However, when the enabling technology is rapidly evolving, as in the case of optical networks, a more thorough understanding of technology and its relation to system performance is required. Thus, a good grasp of

The Big Picture

23

optical networks requires an understanding of the interrelations between two bodies of knowledge that traditionally have been treated separately: the physics of the underlying devices and the mathematical methodology required to analyze, design, and control systems incorporating these devices. The emphasis of this book is on methodology rather than devices. However, we weave the physical and mathematical sides of networking into an integrated whole by linking physical constraints with performance analysis and design concepts whenever possible. In addition, we attempt to integrate current practice, generic models, and futuristic concepts. By emphasizing linkages across traditional lines, our intention is to break down the compartmentalization that tends to hinder progress. We recognize that integration across a broad range of material presents a challenge to the reader. The interconnections make it difficult to isolate sets of topics matched to the background and interests of each reader, and some readers may feel caught in a tangled web. To help the disoriented traveler, we provide a simple road map in Figure 1.6 as a guide through the labyrinth. As shown in the figure, the first three chapters are required for an understanding of the rest of the subject matter. These are accessible to readers with only a limited background in networking and physical principles. After the broad view presented in this chapter, Chapter 2 introduces the multiwavelength network architecture, describing the layers of connectivity in a wavelength division multiplexed network. The chapter focuses on the functionality of optical network elements – links, nodes, and access stations – and their relation to network performance. Chapter 3 gives an overview of the various layers of network connections. Purely optical networks are discussed first, starting from the simplest (static) networks and then considering the two controllable classes: wavelength-routed networks and linear lightwave networks. The former class supports point-to-point connections, whereas the latter supports multipoint connections, representing a more general view of transparent optical networks and their functionality. Because the physical layer alone is generally not sufficient to serve the needs of network users, the chapter concludes with a discussion of logically routed networks – the multilayered networks of Figure 1.1, consisting of electronic overlays on an optical infrastructure. Chapters 2 and 3 are largely qualitative and serve as introductions and “pointers” to material explored quantitatively in later chapters. The rest of the book may be read more selectively. It is linked to the earlier material, as shown in Figure 1.6. For those with only a limited understanding of the physical side of networking, Chapter 4 provides a concise treatment of physical principles and device technology. It is not intended to be all-encompassing, because other works are solely devoted to these topics; for example, see references [Agrawal02, Saleh+07]. The material in the first part of the chapter (through Section 4.9) focuses mainly on fundamentals and generic concepts. The latter part of the chapter is devoted to current technology with a focus on physical and technological constraints that limit network performance. These include limits on WDM channel-packing density, optical receiver performance, geographic reach of optical connections, and technological limitations imposed on optical switches. The chapter concludes with methodologies for performance evaluation.

24

Multiwavelength Optical Networks

Chapter 1 The Big Picture

Chapter 2 The Layered Architecture and Its Components

Chapter 3 Network Connections

Chapter 4 Enabling Technology

Chapter 5 Static Multipoint Networks

Chapter 6 Wavelength/ Waveband Routed Networks

Chapter 7 Logically Routed Networks

Chapter 8 Survivability: Protection and Restoration

Chapter 9 Optical Control Plane

Chapter 10 Optical PacketSwitched Networks

Chapter 11 Current Trends in Multiwavelength Optical Networking

Figure 1.6 Road map.

The remaining chapters are more specialized. Chapters 5 through 7 present a thorough treatment of the generic multiwavelength optical network.10 Static networks based on shared optical media are covered in Chapter 5. The chapter discusses multiplexing and multiple-access techniques, traffic flow constraints, capacity assignment, packet switching in the optical layer, and access network applications of passive optical networks. Wavelength-routed and linear lightwave networks are examined in Chapter 6, with discussions of routing and channel assignment, as well as the relationship between optical switch functionality and network performance. Chapter 7 deals with logically routed networks. It is here that the overall multilayered network design problem appears for the first time. We present methodologies for designing a logically routed network to satisfy a prescribed traffic requirement while observing the constraints imposed by a given fiber topology, the limitations of the network components, and the limited capacity of the available optical spectrum. 10

The necessary background in graph theory, Markov chains, and queueing theory is included in Appendices A and C, and some algorithms for special aspects of network switching, provisioning, and control appear in Appendices B, D, E, and G.

The Big Picture

25

Chapter 8 considers the very important issue of optical network survivability and fault recovery, specifically ring and mesh topologies and line and path-based recovery techniques. Chapter 9 examines the issue of network control, focusing on the most recent proposals for the optical network control plane, based on GMPLS. Chapter 10 is the most forward-looking part of the book, dealing with recent progress in optical packet, burst, and label switching. We conclude in Chapter 11, tying the generic concepts of earlier chapters to recent trends in network deployment. An infinite variety of additional engineering issues arise when operating networks in the real world, and these are often missed in an abstract view of things. Parts of Chapters 4 and 7, the opening part of Chapter 8, and all of Chapters 9 and 11 provide examples of contemporary technology, network design, and network operation, as well as the trade-offs between optical and nonoptical networking solutions. These will be of particular interest to those involved in near-term network deployment. However, this is the most “perishable” material in the book. For example, Chapter 9 is important for an understanding of current standardization efforts in network control. However, these are continuing to evolve as this is being written, so that techniques of optical network control and management can be expected to change and progress in future network deployments. Some of the more advanced and speculative sections in the book may be skipped initially by readers learning about optical networks for the first time. In Chapter 5, Section 5.2.3, Code Division Multiple Access, and parts of Sections 5.5 and 5.6, which deal with demand-assigned connections and packet switching in the optical layer, can be bypassed by those new to the field. The same is true for parts of Chapter 6, including the material on ring decomposition in Section 6.3.5, optimization in Section 6.3.7, and some of the more specialized topics on LLNs in the latter part of the chapter. In Chapter 7, most of the material on point-to-point logically routed networks will be of interest to all readers, whereas Sections 7.4 and 7.5 on hypernets are more futuristic in nature than the rest of the chapter. The latter sections of Chapter 8, on optical layer protection in mesh topologies, are currently important open research areas, and the same can be said of all of Chapter 10. Readers involved in research on next-generation networks would normally focus on the complement of the subject matter of interest to the novice. Thus, for example, rings, which are “old hat” to the researcher, could be skimmed in favor of more advanced topics; for example, optimization and LLNs in Chapter 6. Researchers might also focus on hypernets in Chapter 7, general optical layer protection in Chapter 8, and the full range of optical packet-switching issues in Chapter 10. In summary, the reader is invited to customize an itinerary suited to his or her interests in exploring the fascinating world of optical networking. We hope you enjoy the journey.

Bibliography [Acampora87] A. S. Acampora. A multichannel multihop local lightwave network. In Proceedings of the IEEE Global Telecommunications Conference (Globecom), pp. 1459–1467, Tokyo, Japan, November 1987.

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[Agrawal02] G. P. Agrawal. Fiber-Optic Communication Systems, Third Edition (Wiley Series in Microwave and Optical Engineering). New York: Wiley 2002. [Bala+95] K. Bala, R. R. Cordell, and E. L. Goldstein. The case for opaque multiwavelength optical networks. In Proceedings of the IEEE/LEOS Summer Topical Meeting, Keystone, CO, August 1995. [Ballart+89] R. Ballart and Y.-C. Ching. SONET: Now it’s the standard optical network. IEEE Communications Mag., 27(3):8–15, 1989. [Boehm90] R. J. Boehm. Progress in standardization of SONET. IEEE Lightwave Communication Systems, 1(2):8–16, 1990. [CCITT92] CCITT. Recommendation i.121, Broadband Aspects of ISDN. Geneva: CCITT, 1992. [Charlet+04] G. Charlet, E. Corel, J. Lazaro, A. Klecamp, R. Dischler, P. Tran, W. Idler, H. Mardoyan, A. Konczykowska, F. Jorge, and S. Bige. WDM transmission at 6 Tbps capacity over transatlantic distance, using 42.7 Gbps Differential Phase-Shift Keying without pulse carver. In Proceedings of the IEEE/OSA Optical Fiber Commun. Conf., PDP 36, Los Angeles, CA, February 2004. [de Prycker91] M. de Prycker. Asynchronous Transfer Mode: Solution for Broadband ISDN. Chichester, UK: Ellis Horwood, 1991. [Desurvire+87] E. Desurvire, J. R. Simpson, and P. C. Becker. High-gain erbium-doped travelingwave fibre amplifier. Optics Lett., 12(11):888, 1987. [Goodman+86] M. S. Goodman, H. Kobrinski, and K. W. Lo. Application of wavelength division multiplexing to communication network architectures. In Proceedings of the IEEE Int’l Conf. Commun. (ICC), pp. 931–934, Toronto, Canada, 1986. [Goodman+87] M. S. Goodman, C. A. Brackett, R. M. Bulley, C. N. Lo, H. Kobrinski, and M. P. Vecchi. Design and demonstration of the LAMBDANET system: A multi-wavelength optical network. In Proceedings of the IEEE Global Telecommunications Conference (Globecom), pp. 1455–1458, Tokyo, Japan, November 1987. [Henry89] P. S. Henry. High-capacity lightwave local area networks. IEEE Communications Mag., 27(10):20–26, 1989. [Hill88] G. R. Hill. A wavelength routing approach to optical communications networks. In Proceedings of the IEEE Conference on Computer Communications (Infocom), pp. 354–362, New Orleans, LA, March 1988. [IEEE93] IEEE. Special issue on broad-band optical networks. IEEE/OSA J. Lightwave Technol., 11(5/6), 1993. [IEEE96] IEEE. Special issue on multiwavelength optical technology and networks. IEEE/OSA J. Lightwave Technol., 14(6), 1996. [IEEE98] IEEE. High-capacity optical transport networks. IEEE J. Select. Areas Commun., 16(7), 1998. [Kao+66] C. K. Kao and G. A. Hockham. Dielectric-fiber surface waveguides for optical frequencies. Proc. IEEE, 113(7):1151–1158, 1966. [Mears+86] R. J. Mears, L. Reekie, S. B. Poole, and D. N. Payne. Low-threshold, tunable CW and Q-switched fibre laser operating at 1.55 µm. Electron. Lett., 22(3):159, 1986. [Mears+87] R. J. Mears, L. Reekie, I. M. Jauncey, and D. N. Payne. Low-noise erbium-doped fibre amplifier operating at 1.54 µm. Electron. Lett., 23(19):1026, 1987. [Miller+88] S. E. Miller and I. P. Kaminow (Editors). Optical Fiber Telecommunications II. New York: Academic Press, 1988.

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[Mukherjee92a] B. Mukherjee. WDM-based local lightwave networks—Part I: Single-hop systems. IEEE Network, 6(3):12–27, 1992. [Mukherjee92b] B. Mukherjee. WDM-based local lightwave networks—Part II: Multihop systems. IEEE Network, 6(4):20–32, 1992. [Nakazawa+93] M. Nakazawa, K. Suzuki, E. Yamada, H. Kubota, Y. Kimura, and M. Takaya. Experimental demonstration of soliton data transmission over unlimited distances with soliton control in time and frequency domains. Electron. Lett., 29(9):729–730, 1993. [Saito+90] S. Saito, T. Imai, T. Sugie, N. Ohkawa, Y. Ichi-hashi, and T. Ito. An over 2,200 km coherent transmission experiment at 2.5 Gbps using erbium-doped fiber amplifiers. In Proceedings of the IEEE/OSA Optical Fiber Commun. Conf., Paper PD2, San Francisco, CA, January 1990. [Saleh+07] B. E. A. Saleh and M. C. Teich. Fundamentals of Photonics, Second Edition, (Wiley Series in Pure and Applied Optics). New York: Wiley, 2007. [Schawlow+58] A. L. Schawlow and C. H. Townes. Infrared and optical lasers. Phys. Rev., 112(6):1940, 1958.

2

The Layered Architecture and Its Resources

Ultimately, the performance of a network is limited by the quantity and functionality of its physical resources. In this chapter we examine the various functions performed in a multiwavelength network, emphasizing the role of the optical resources (located in the physical layer of Figure 1.3) in providing connectivity and throughput. For the most part we use the terms transparent optical, purely optical, and just optical interchangeably to refer to entities in the physical layer. The implication is that there is a clean break between the underlying technology and functionality in the physical layer and that in the logical layer. The physical layer contains optical components executing linear (transparent) operations on optical signals, whereas the logical layers contain electronic components executing nonlinear operations on electrical signals. In reality, as mentioned in Chapter 1, the picture in real networks is more nuanced. For example, some simple signal processing (either electronic or optical) may be present in the physical layers of today’s networks, making them “opaque” to a greater or lesser degree. Conversely, as optical technology for signal processing matures, it is beginning to make its way into the logical layers. Nevertheless, the somewhat simplified view of a transparent (linear) optical layer underlying an electronic (nonlinear) logical layer is very helpful in providing a generic model for most multiwavelength networks. It will be used throughout this book, with exceptions duly noted as they appear. To provide a proper framework for the discussion that follows, we start in Section 2.1 with a description of layers and sublayers of the multiwavelength network architecture. The functional characteristics of the optical resources – the network links, the optical network nodes (ONNs), and the network access stations (NASs) – are discussed in Sections 2.2, 2.3, and 2.4, respectively. They provide the basic functionality for setting up and routing optical connections through the network. We refer to a purely optical network as a wavelength- or waveband-routed network (WRN). However, as pointed out in Section 1.4, the additional functionality provided by an electronic overlay is often required to satisfy network requirements. Thus, in Sections 2.5 and 2.6 we consider hybrid networks that contain a purely optical network infrastructure complemented by an electronic overlay. Section 2.5 deals with special-purpose overlay processors (OLPs) for enhancing the performance of the optical network while maintaining many of the desirable features of transparency. In Section 2.6 we add logical switching nodes (LSNs) to create logically routed networks (LRNs). The LRN relies on the resources of the physical layer to provide high

Layered Architecture

29

throughput, and the processing power and routing capabilities in the LSN to provide high connectivity.

2.1

Layers and Sublayers In Figure 2.1(a) the layered view of the optical network, first introduced in Figure 1.3, is expanded into the sublayers of the multiwavelength network architecture. To fix ideas, think of the higher layers as representing a packet/cell-switched client logical network (LN), including a virtual connection layer, which might correspond to the IP or ATM logical network, shown in Figure 1.1. The characteristics of each logical layer and its sublayers are determined by the specific structure of that client network (e.g., SONET, ATM, and IP). The physical layer acts as a server for one or more LNs. For example, in the case of Figure 1.1 the optical layer has two LN clients: the IP network and the stacked IP over ATM over SONET network. The multiwavelength network architecture to be described is representative of a WAN operating with dedicated and/or demand-assigned optical connections providing a

Virtual Connection Logical Path Logical Connection

Logical Layer

Physical Layer

Optical Layer

Transmission Channel Optical Connection λ -Channel

Fiber Layer

Optical/Waveband Path Fiber Link Fiber Section Sublayers (a)

NAS eo TP OT RP OR

Access Link

NAS oe

Network Link OA

ONN

ONN Fiber Section Fiber Link Optical/Waveband Path λ-Channel Optical Connection Transmission Channel Logical Connection (b)

Figure 2.1 Layered view of optical network connections.

OR RP OT TP

30

Multiwavelength Optical Networks

common infrastructure for its client LNs. In smaller or simpler networks (e.g., optical access networks) some of the functions described may be nonexistent. In describing the architecture we start in the optical layer of Figure 2.1(a) and work down from the interface between the logical and physical layers to the fiber layer. In terms of hardware, the logical–physical layer interface is located at the external (electronic) ports of the NASs. A detailed look at a typical point-to-point connection is presented in Figure 2.1(b). As indicated, the access stations terminate the optical connections and serve to interface electronic end systems (typically user equipment) or electronic switches (typically communications carrier equipment) to the optical network.1 (As mentioned, the access station acts as the boundary between the electronic and optical domain, moving information between its external ports on one side of the electro-optic interface and optical ports on the other.) On its optical side, each NAS connects to an ONN through an access link, which consists of one or more pairs of access fibers.2 Optical signals are exchanged between the station and the network by optical transceivers in the station. Each node is connected to neighboring nodes by pairs of fibers, which constitute internodal network links. We call the graph that describes the interconnections of the network links and network nodes the physical topology of the network. A long link may contain one or more optical amplifiers (OAs) to compensate for attenuation in the fiber, so that the link becomes a series connection of several fiber sections between amplifiers. For simplicity we usually assume that fibers are used in unidirectional pairs to support bidirectional transmission. (In practice, single fibers are sometimes used for bidirectional transmission to reduce cost.) The physical layer of our multiwavelength network architecture is designed to capture the essential generic features of optical networks for purposes of design, modeling, performance evaluation, and control. It is divided into an optical layer, encompassing the various aspects of the optical connection: signal generation and transmission, routing, survivability, and so on, and a fiber layer, comprising the aspects of the supporting fiber infrastructure. The optical connection provides an end-to-end optical communication channel terminating on an optical transmitter/receiver pair. The characteristics of the optical layer in Figure 2.1 are a function of the way in which the optical spectrum is partitioned. The smallest entities in the partition are λ-channels, each of which is assigned a distinct wavelength (or optical frequency). These are the basic information carriers in the physical layer. To keep things simple at this point, the λ-channels are assumed to be routed (switched) independently by the ONNs. (This is the way most multiwavelength networks are configured at present.) A more efficient two-tiered partitioning involving wavebands is introduced in Section 2.2. Each point-to-point optical connection (OC) is carried on a λ-channel and is created by (1) assigning a λ-channel to the source transmitter and destination receiver on a 1

2

In cases when the switch technology is electronic but it has little more signal processing functionality than a purely optical switch, we sometimes make an exception and lump the switch into the optical layer (with an appropriate explanation). For survivability, a station might connect to two (or more) network nodes, a configuration called dual homing in current telecommunication networks. In dual homing a facility, such as a telephone Central Office, is connected to two hubs (nodes) of a backbone network.

Layered Architecture

31

selected wavelength and (2) by establishing an optical path (OP) through a sequence of network nodes to carry that wavelength from source to destination. These actions are normally under the control of a network manager, the details of which we leave for later. We call a unidirectional connection between external ports on a pair of source and destination NASs a logical connection (LC). This connection carries a logical signal in some agreed-on electronic format (e.g., ATM cells, IP packets, SONET digital bit streams, or analog video). Each logical signal format is tailored to the needs of a particular client LN or end system. All LCs draw on the resources of the physical layer, as shown in Figure 1.1. Each LC is carried on an optical connection through the intermediary of a transmission channel. The transmission channel performs an adaptation function, converting the logical signal to a transmission signal. This conversion (as well as certain multiplexing functions) takes place in the transmission processor (TP), with the reverse operations implemented in the reception processor (RP). Just as the logical signal format must be adapted to the LN that it serves, the transmission signal format must be matched to the requirements of the optical equipment that carries it. For example, the transmission signal must be chosen such that it, and the optical signal that carries it, satisfies the bandwidth constraints of the optical transmitter (OT) and the optical receiver (OR), as well as the permissible spectrum occupancy of the optical channel carrying it. The optical connections are established through the coordinated actions of the nodes and stations. It is the function of the terminating NASs to tune their transceivers to the assigned wavelength of the λ-channel, thereby establishing the optical connection. Similarly, it is the role of the nodes to create an optical path on that wavelength, thereby establishing the support for a potential connection. Activation of a connection on the OP only occurs when the transceivers are properly tuned and the path is “lit.” The main consideration in breaking down the optical layer into sublayers is to account for multiplexing, multiple access (at several layers), and switching. Using multiplexing, several logical channels may be combined on a single λ-channel originating from one station; using multiple access, λ-channels originating from several stations and operating on the same wavelength may carry multiple LCs to the same destination; and through switching, many distinct optical paths may be created on different fibers in the network, using (and reusing) λ-channels on the same wavelength.3 In the fiber layer, the optical path is carried by a succession of fiber links that connect the network nodes. The fiber layer is subdivided further into the fiber link sublayer supported by the fiber section sublayer. Amplification may occur within the fiber layer either in lumped or distributed form (see Section 4.4). Each of the internodal connections along an optical path constitutes an optical hop. A typical connection illustrating these concepts is shown in Figure 2.2. A virtual connection (VC) between a pair of end systems is carried on a logical path (LP) that consists of two LCs (two logical hops) terminated by LSNs. The first and second LCs are carried on two- and three-hop OCs, respectively. (The access links are not counted as hops.) Although this discussion has been limited to point-to-point connections to simplify the exposition, the terminology also applies to multipoint connections. As a matter of 3

Optical spectrum reuse is a recurring theme throughout the book because it is the key to scalability of optical networks to large sizes.

32

Multiwavelength Optical Networks

VC LP

VC LP

LC

LC

OC

OC

Virtual Connection Logical Path Optical Amplifier Logical Switching Node

Network Access Station Optical Network Node End System

Figure 2.2 A typical connection.

fact, optical multipoint connections play a unique and important role in the network architectures to be discussed later. At this point, a brief comparison with another layered view of the optical network is in order. Figure 2.3 is a schematic portrayal of ITU-T recommendation G.872 [ITU-T01] for optical transport networks. It shows three layers: the optical channel (OCh) layer, the optical multiplex section (OMS) layer, and the optical transmission section (OTS) layer. (Another layer, the physical media layer [PM] dealing with the properties of fibers, not shown in the figure, is the server for the OTS layer.) As in all ITU-T network standards, the focus is on establishing agreed-on requirements for operational networks. This is in contrast to the objectives of our layered architecture, which is designed not as a standard but rather as a way of thinking about networks. Because the G.872 standard is directed toward facilitating interworking of operational networks, it pays special attention to means for monitoring and ensuring integrity of the connections in each layer as well as supervisory functions for network management. To decode the figure, some explanation of the nomenclature used in ITU networking standards is needed [ITU-T00]. In each layer, an adaption processing function (e.g., OCh A Source) accepts information from the client layer above it, and processes it to allow transfer over a trail in its layer network, in this case the OCh layer network. (The OCh A Sink executes the reverse processing function.) The trail is a bidirectional “transport entity,” an abstraction that represents transfer of information between its termination points in the indicated (OCh) layer. The access points (AP) are the reference points marking the borderline between a trail and its associated adaptation functions. The next processing function encountered in descending the OCh layer is the

Layered Architecture

OCh_A source

OCh_A sink

OCh Trail

OCh AP OCh_TT source

OCh layer network

OCh AP OCh_TT sink

OCh_NC

OCh TCP

33

OMS/OCh_A source

OCh TCP OMS/OCh_A sink

OMS Trail OMS AP OMS layer network

OMS AP OMS_TT source

OMS_TT sink

OMS_NC OMS TCP

OMS TCP

OTS/OMS_A source

OTS/OMS_A sink

OTS Trail

OTS AP

OTS AP OMS_TT source

OTS layer network

OMS_TT sink

OTS_NC OTS TCP

OTS TCP

Figure 2.3 Client server associations in an optical transport network (From [ITU-T01, Figure 1]. c 2001 ITU. Reproduced with the kind permission of The International TelecommuCopyright
nications Union.)

OCh trail termination (TT) processing function, which completes the processing necessary to present information (including added overhead) to the layer below, in this case the OMS layer network. The OCh network connection (NC) represents a series of links joining the two ends of the OCh trail, and the OCh terminal connection points (TCP) bind the TT source and sink to the bidirectional connection. The OCh, OMS, and OTS layers combined correspond roughly to the optical layer in Figure 2.1(a). (The PM layer corresponds to the fiber layer.) They provide for an end-to-end optical connection, including routing (OCh layer), λ-channel multiplexing (OMS layer), transmission parameters (OTS), and the various overhead and supervisory functions required for network operations and management. Except for the position of the routing and transmission functions, there is an approximate correspondence between these layers and individual layers in Figure 2.1(a): OTS/OCh, OMS, and PM correspond to transmission channel/optical connection/λ-channel, optical/waveband path, and fiber link, respectively. The reason for the different positioning of the routing and transmission functions in the two architectures is that the G.872 standard is focused on an opaque network, where optical connections are terminated at the ends of each transmission link, meaning that routing is a higher layer function and transmission is a lower layer function than in Figure 2.1(a). In a transparent network represented by the physical layer of Figure 2.1(a), those positions are reversed.

34

Multiwavelength Optical Networks

A layered architecture is important because it helps to deal with the complexity of a network by breaking it down into separate parts. However, it is nothing more than a framework, which often has to be bent into shape and changed to adapt to specific features that continue to evolve in optical networks. Now let us examine each of the key components of the network in more detail.

2.2

Network Links: Spectrum Partitioning A large number of concurrent connections can be supported on each network link through successive levels of multiplexing as discussed earlier. A schematic view of the fiber resources illustrating this concept appears in Figure 2.4. A typical network link consists of a cable containing several (sometimes more than 100) fibers, which are used as bidirectional pairs. (Given the cost of the right-of-way and labor in laying a cable, it would be very inefficient to deploy a cable that consists of only a single fiber pair.) This bundling of fibers into one cable is an example of space division multiplexing in the fiber layer. The usable optical spectrum on a single fiber is wide enough to carry many high-speed optical connections through several additional levels of multiplexing. In our discussions, wavelength division is the basic multiplexing technique, with each fiber carrying connections on many distinct wavelengths (λ-channels). This is indicated by a coarse and fine division of the optical fiber spectrum within the wavelength dimension in Figure 2.4. (An explanation of the coarse/fine spectrum division is given later.) First, the spectrum is divided into wavebands, and these are divided further into λ-channels. The assigned wavelengths of the λ-channels must be spaced sufficiently far apart to keep neighboring signal spectra from overlapping. (Overlapping signal spectra cause interchannel cross-talk; in other words, interference among the signals at the

Network Link Space

Wavelength

Time

{ Transmission Channels Out

λ-Channels Out Cable

Fibers

Wavebands

λ-Channels In { Transmission Channels In Figure 2.4 Fiber resources.

Layered Architecture

λ1

λ2

Usable spectrum

35

λm

f λ GHz/nm

10 GHz/ .08 nm

(a) λ-channel Spacing for Separability at Receivers λ1

λ2

λm

f λ

100 GHz/0.8 nm

(b) λ-channel Spacing for Separability at Network Nodes 10 GHz λ1,1

λ2,1

λ10,1

100 GHz/0.8 nm

w1

100 GHz/ 0.8 nm

w2

100 GHz/ .08 nm

wm

f λ

(c) Wavebands Figure 2.5 Wavelength and waveband partitioning of the optical spectrum.

optical receiver.) Figure 2.5(a) shows an example of a WDM wavelength assignment, in which the λ-channels are spaced 10 GHz apart in optical frequency. Note that an optical wavelength scale is also shown, because wavelength is often used in specifying optical components. Because wavelength λ and frequency f are related by f λ = c, where c is the velocity of light in the medium, we have the relation (cλ) (2.1) f ≈ − 2 λ between small changes in frequency  f and wavelength λ. This gives a correspondence between wavelength and frequency intervals of 100 GHz ≈ 0.8 nm in the range of 1550 nm, where most modern lightwave networks operate.4 One of the facilitating forces behind commercial development of WDM has been standardization by the ITU of an optical wavelength grid and a set of transmission bands covering a range of 1260–1675 nm (see Section 4.3). A grid laying out alternatives for close wavelength/frequency spacing, dubbed dense wavelength division multiplexing was established in 2002 [ITU-T02a]. The DWDM grid is centered at 193.1 THz (1552.52 nm) and specifies a variety of regular spacings from 100 to 12.5 GHz. Recognizing the need for much larger wavelength spacing to accommodate wavelength drift when inexpensive components are used, the coarse wavelength division multiplexing standard was adopted for metropolitan area 4

This is the region of minimum attenuation in an optical fiber.

36

Multiwavelength Optical Networks

networks. It specifies 18 wavelengths with a 20-nm spacing over the 1270–1610 nm wavelength range [ITU-T02b]. Returning to our example of wavelength assignment, the total bandwidth of each channel (i.e., the optical bandwidth occupied by the modulated laser signal) is shown (see Figure 2.5, shaded areas) to be somewhat less than the frequency spacing to leave sufficient “guard bands” to allow for imprecision and drift in laser transmitter tuning and to make it possible to separate adjacent signals at the receivers with reasonably simple receiving equipment (see Chapter 4). The 10-GHz channel spacing shown in this example is sufficient to easily accommodate λ-channels carrying aggregate digital bit rates on the order of 1 Gbps or occupying analog bandwidths on the order of 1 GHz.5 Efficient use of channels with this much capacity frequently requires sharing one λ-channel among several lower rate transmission channels. A common way of doing this is by time division multiplexing/multiple access techniques, indicated by the time dimension in Figure 2.4 (see Section 3.2.1). As mentioned in Chapter 1, the grooming operation of multiplexing several lower rate transmission channels onto a higher capacity λ-channel for achieving efficient use of the channel plays an important role in network management (see Section 7.1). The 10-GHz channel spacing shown in Figure 2.5(a) is very dense by current standards, but it would be feasible for accommodating many highcapacity λ-channels on a single network link, given the characteristics of typical optical transceivers. However, we must also anticipate the limitations of the ONNs because they have the job of routing each optical connection on a specified path through the network. Recall that in transparent networks containing optical switching nodes, several λ-channels may be multiplexed on each fiber, with each routed selectively on a different path based on its wavelength. To accomplish this the nodes must have the capability to “recognize” each λ-channel on an incoming fiber independently and to direct it to a specified output fiber. This is referred to as wavelength-selective routing or simply wavelength routing. Now the characteristics of current optical node technology (see Chapter 4) suggest that the relatively close channel spacings that can be resolved by optical receivers (e.g., 10 GHz) are too close to permit independent wavelength routing of each channel at the nodes of a transparent optical network.6 In fact, in today’s switched multiwavelength networks the λ-channel spacings are typically more than 100 GHz. Thus, there is an order of magnitude mismatch between the spacings required in the λ-channel layer and the optical path layer. A wider spacing of the λ-channels to 100 GHz, permitting individual wavelengths to be routed independently, is shown in Figure 2.5(b). Note that this reduces the channel density and hence the fiber throughput by a factor of 10. Such a loss of throughput is certainly to be avoided if possible. A brute force (and costly) way of solving this problem would be to refine the switching node technology to produce an order of magnitude improvement in its resolution in the transparent case, 5 6

In the digital case, this represents a modulation efficiency of 0.1 bps/Hz, which is poor for electronic systems, but typical for optical systems. One reason for this is that many nodes may be traversed on a long optical path. At each node the imperfections in signal resolution (optical filtering) for closely spaced channels result in signal attenuation, distortion, and switch cross-talk, which accumulate along the path in a transparent network.

Layered Architecture

37

or to drop transparency altogether, using an “opaque” switch (see Section 2.3.2.4). A less costly approach, and one that partially circumvents the problem, is shown in the two-tiered spectrum partition in Figure 2.5(c). Here we define a waveband as the smallest segment of the spectrum that is optically resolvable in the node (i.e., in the optical path), in contrast to the λ-channel, which is the smallest unit resolvable in the access station by a tunable receiver.7 Below the plot of λ-channels in Figure 2.5(c) is another plot showing a coarser partition of the spectrum into wavebands. In this example, we show wavebands with a width of 100 GHz placed at intervals of 200 GHz (i.e., separated by guard bands of 100 GHz). Wavebands with this width and separation are easily resolvable by waveband-selective optical nodes without pushing the current state of the art; that is, each waveband can be recognized and switched (routed) independently at each network node without introducing excessive signal impairment. Each of these wavebands has enough capacity to support many individual λ-channels. (Ten are shown in the figure.) This second partition of the spectrum requires positioning of wavebands in the optical layer as shown in Figure 2.1(a). Now the waveband carries an optical path and the optical/waveband path may carry several λ-channels. Several wavebands may now be multiplexed on each fiber (waveband multiplexing) and several λ-channels may be multiplexed on each waveband (wavelength multiplexing). The resultant space waveband and wavelength multiplexing is indicated schematically in Figure 2.4. In subsequent chapters, we discuss both the one-tiered and two-tiered approach to spectrum partitioning. Clearly, the partitions of Figure 2.5(a) and 2.5(b) can be considered special cases of Figure 2.5(c). If the sparse spacing of Figure 2.5(b) is used to allow each λ-channel to be switched individually, this corresponds to an m-waveband system with one λ-channel per waveband. If the dense spacing of Figure 2.5(a) is used to achieve higher throughput, with channels spaced so close that they cannot be switched individually, then we have a single-waveband network. (Of course, the distinction between the two cases depends on the characteristics of the network nodes. If the nodes have no waveband selectivity then any spectrum spacing is equivalent to a single-waveband partition.) To simplify the terminology, from now on we refer to networks using the spacing of Figure 2.5(b) as wavelength-routed networks, and those using the spacing of Figure 2.5(c) as waveband-routed networks. A more graphic depiction of the three alternatives in spectrum partitioning and their influence on network operation is shown in Figure 2.6. A multiwavelength network with λ-channels on m wavelengths, λ1 , λ2 , . . . , λm , spaced far enough apart to be switched independently (a wavelength-routed network), can be envisioned as m copies of one network, each with the same physical topology, as shown in Figure 2.6(b). An optical connection between a pair of stations runs on an optical path laid out on the copy of the network corresponding to the connection’s assigned λ-channel. The layers of the figure illustrate nicely the concept of wavelength continuity: A signal generated at a given wavelength in a transparent network must remain on that 7

We assume a simple, direct detection receiver; in other words, a photodetector possibly preceded by a tunable optical filter, which can select and detect the instantaneous power in any desired λ-channel (see Section 2.4.2).

38

Multiwavelength Optical Networks

1, 2,

m

(a) Single Waveband

1,1

10,1

w1

w1

2

w2

w2 1,2

10,2

1,m

m

10,m

wm

(b) Wavelength-routed

wm

(c) Waveband-routed

Figure 2.6 Network picture based on spectrum partitioning.

wavelength (i.e., on the corresponding copy of the network) from source to destination.8 If the wavelength spacing is so close that wavelength-selective switching is impossible (the single-waveband case), all λ-channels are forced to share a single copy of the network; see Figure 2.6(a). Finally, if a two-tiered partitioning is used, then we have a waveband-routed network, in which there are as many copies of the network as there are wavebands, and several (in this case 10) λ-channels share each copy of the network, as in Figure 2.6(c). How do these approaches compare? Using the numbers in the example of Figure 2.5, we find that the wavelength-routed case has a fiber throughput reduced by a factor of 10 compared with the single-waveband case. This is the price paid for being able to route each λ-channel independently. The waveband-routed case is a compromise between the two extremes of complete wavelength routing and none at all. The example of Figure 2.5(c) uses 10 λ-channels in each waveband for an aggregate capacity five times that of the wavelength-routed case and one half of the single-waveband case. (The same effect would be achieved if the 10 λ-channels were replaced by a single very-high-speed λ-channel operating at 10 times the speed of the original channels.) As shown in Section 3.4, waveband routing imposes some special routing constraints not present in wavelength-routed networks. The constraints are due to the fact that all λ-channels sharing a common waveband are forced to “stick together” on the same optical path.9 These partially offset the throughput gains produced by waveband routing. 8 9

An exception to this occurs in networks containing wavelength interchangers (see Section 2.3.3). The constraints on waveband routing of groups of λ-channels within the same waveband are identical to those affecting wavelength routing of groups of time division-multiplexed channels groomed onto the same wavelength.

Layered Architecture

39

The coarse/fine (waveband/λ-channel) partition has the following advantages: 1. Regardless of the wavelength density on the fiber, it is less costly to switch/route the optical signals as a small number of aggregated groups of channels, with each group contained in a continguous waveband, rather than as a large number of individual λ-channels. (The cost of a wavelength-selective switch typically increases at least linearly with the number of segments of the spectrum it switches independently.) 2. In many cases (e.g., multipoint optical connections), it is actually desired to route a “bundle” of λ-channels on a common path through the network – something that is especially easy to do if these channels lie within a common waveband (see Section 3.4.2). 3. Network management at the level of the optical nodes is simplified if there are a small number wavebands to keep track of instead of a large number of λ-channels. Given these considerations, a fundamental issue in network design is how a network deals with the question of waveband versus wavelength routing. As pointed out in Section 1.6, the first optical networks were broadcast-and-select, so that routing was wavelength independent; i.e., the case of Figure 2.6(a). In the mid-1990s, a number of multiwavelength network testbeds were developed, based on wavelength routing (Figure 2.6[b]), which typically used λ-channel spacings of several nanometers. However, these testbeds were driven more by proof-of-concept considerations rather than economic ones. As commercial deployment began at the turn of the millennium, economic considerations pointed toward waveband routing (Figure 2.6[c]), and at this writing commercial products exist that implement waveband routing, wavelength routing, and hybrid waveband/wavelength routing. It should be noted that whenever λ-channels are bundled together for the purpose of waveband routing, it is important from the point of view of efficiency to “fill up” each waveband with the maximum number of constituent λ-channels; this constitutes grooming at the wavelength/waveband level.

2.3

Optical Network Nodes: Routing, Switching, and Wavelength Conversion The functions of the optical path layer (establishing transparent paths for the optical signals) are implemented in the ONNs. Therefore, the more functionality these nodes have, the more flexible is the network in reacting to fluctuating user demand, changing loads, and equipment problems, and the better it will perform under all types of conditions. One way of viewing functionality is in terms of the way various degrees of freedom (or dimensions) are exploited. Most networks make use of the space dimension by assigning different signals to different fibers. Through multiplexing, multiwavelength networks make use of the wavelength dimension and perhaps the time dimension if time division multiplexing is used. However, the benefit obtained from the various dimensions depends on what happens to the multiplexed signals as they pass through the network nodes. In this section, we examine the basic types of node functionality in transparent networks, classified in increasing order of complexity as follows:

40

Multiwavelength Optical Networks

r Static nodes – Directional couplers (including fixed signal combiners, splitters, and star couplers) – Static routers (including waveband multiplexers and demultiplexers) r Dynamic (switching) nodes – Permutation switches – Generalized switches – Linear divider-combiners (LDCs) – Waveband-space switches – Wavelength-interchanging switches Given the multiplicity of options and terminology in currently deployed networks, we need to simplify our language in discussing generic network concepts. Therefore, we will usually use the term waveband when discussing components operating on either a waveband or wavelength basis (because a wavelength is a special case of a waveband, which carries one λ-channel). However, there are times when our generic terminology may conflict with terms widely accepted in the commercial world, in which case we shall revert to the more commonly accepted term; for example, our waveband-space switch is commonly known as a wavelength-selective cross-connect, whether it switches wavelengths or wavebands or both. It should be recalled that ONNs, as defined here, perform only transparent (linear) operations on the optical signals. In networks possessing some degree of opacity, additional nontransparent nodes may be present with nonlinearities (usually electronic) in the signal path. These include network elements where optical signals are converted to the electrical domain to facilitate switching, wavelength conversion, or signal regeneration. These devices are also treated here and in Sections 2.5 and 2.6. The network nodes as well as other network elements discussed in this chapter are presented in terms of idealized generic mathematical models. This is the approach that is most useful in network analysis and design. Detailed discussions of practical realizations, physical characteristics, and performance impairments in these devices appear in Chapter 4.

2.3.1

Static Nodes Figure 2.7 illustrates three simple optical network topologies with nodes (shown as circles) that are comprised solely of static devices. (These structures are suitable mainly for LANs and MANs.) In each, the physical topology is in the form of a tree. This means that no alternate paths are available for optical path routing, so that the role of the nodes is relatively limited. In each case we assume that the node serves to combine and/or divide (split) signal power without any wavelength selectivity, so that signals propagate through the network in the direction of the arrows. (In the figure, each unidirectional fiber is shown individually with an arrow that indicates its direction of propagation.) Thus, in the broadcast star network of Figure 2.7(a), n access stations are joined by access links to a single node, which is an n × n star coupler, combining all n inbound signals and broadcasting them on each outbound access fiber.

Layered Architecture

41

1

n

2

Star Coupler

4

3

(a) Broadcast Star Network

Bus

1

2

Head End

n

3

(b) Folded Bus

A

Head End B

T R

T R

1

2 R T 3

R T

R T

4 5 Trees (Forest)

R T 6

(c) Tree Topology Figure 2.7 Tree physical topologies.

In the folded bus shown in Figure 2.7(b), optical power from each station is coupled onto the upper bus through a 2 × 1 combiner. The transmitted signals from the n access stations propagate to the right, so that a combination of all signals appears at the head end and is then distributed from the lower end of the bus, through 1 × 2 dividers (optical taps) to the receivers in the stations. The network of Figure 2.7(c) contains six stations joined in a tree topology. Signals from the set {1, 2} are combined and then multicast to the set {3, 4, 5, 6} and those from the set {3, 4, 5, 6} are combined and multicast to the set {1, 2}. Because there are two fiber-disjoint trees here carrying signals in opposite directions, it is possible to run signals in both directions reusing the same wavelengths without causing any interference between them. This is a rudimentary illustration of how the fiber topology can help or hinder the reuse of the optical spectrum. (Spectrum reuse is not possible in the star and bus topologies of Figures 2.7[a] and 2.7[b].) The star and bus will be of special interest to us later. Note that if there is no wavelength

42

Multiwavelength Optical Networks

selectivity in the nodes, the two networks are functionally the same. Each is a broadcast network that can potentially support n(n − 1) unidirectional point-to-point connections among the n stations (not counting connections from a station back to itself). (However, the star is more efficient than the bus in conserving signal power – see Problem 4.)

2.3.1.1

Directional Couplers Each of the nodes in the previous examples can be built using one or more 2 × 2 directional couplers. A 2 × 2 directional coupler, is an optical four-port device, which we represent as shown in Figure 2.8, with ports 1 and 2 designated as input ports and 1 and 2 designated as output ports.10 Optical power enters the coupler through fibers attached to the input ports, is combined and divided linearly, and leaves via fibers attached to the two output ports. Provided that the signals entering each input port originate at distinct optical sources,11 the action of the coupler can be expressed in terms of input signal powers P1 and P2 and output powers P1 and P2 . The power relations are given by P1 = a11 P1 + a12 P2 P2 = a21 P1 + a22 P2 .

(2.2)

For ideal symmetric couplers, the power transfer matrix A = [ai j ] is of the form
 (1 − α) α A= , (2.3) α (1 − α) where the parameter α may take on any value between 0 and 1. As defined in Equation (2.3), this is a passive and lossless device, meaning that power is conserved for the signals passing through it. If the parameter α is fixed, we call it a static device. If α can be varied through some external control (for example, electronic, thermal, or mechanical), 1

1′

2

2′

0< –α< –1 Figure 2.8 Directional coupler. 10

11

By designating certain ports as inputs and others as outputs, we imply that the power propagates in the indicated directions only, a condition that depends on the internal characteristics of the coupler as well as the external network to which it is connected. (See Section 4.10.2 for a more complete description of multiport devices in terms of incident and reflected waves.) This ensures that the optical fields of signals interacting with each other in the coupler are not coherently related, so their relative phases and polarizations can be ignored. In this case, the operation of the device can be expressed as an input/output power relation, which does not depend on the specific relations among the fields.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

43

Outputs

Inputs

Layered Architecture

Figure 2.9 A 16 × 16 star coupler.

then the device is dynamic or controllable. Now suppose, for example, that α = 1/2 (fixed). If signals are present at both inputs, the device acts as a 2 × 2 star coupler – see the n = 2 case of Figure 2.7(a). If we set P2 = 0, it acts as a power divider – as used on the lower bus in Figure 2.7(b), and if we use only output port 1 , terminating port 2 with an (absorbing) dummy load, it acts as a combiner – as used on the upper bus in Figure 2.7(b). In the latter case it is important to observe that even if the coupler is lossless, there is an inevitable combining loss (dissipated in the dummy load). In reality, all physical devices are lossy. In a lossy 2 × 2 directional coupler, for example, a11 + a21 < 1 and a12 + a22 < 1, indicating the presence of excess losses due to device imperfections. Excess losses are to be distinguished from combining losses, which are inevitable, even in ideal devices. Other more elaborate types of static nodes can be constructed by interconnecting 2 × 2 couplers. For example, Figure 2.9 shows a 16 × 16 star coupler built using an array of 32 2 × 2 star couplers arranged in a Banyan structure [Hui90, p. 99]. A setting of α = 1/2 is used for each coupler. (If each 2 × 2 is lossless, then the complete 16 × 16 system will be lossless.) In the dynamic case, an ideal directional coupler would be fully controllable through the range α = 0 (the bar state) to α = 1 (the cross state). Controllable directional couplers are used commonly as building blocks for optical switches, using only the bar and cross states and thus acting as binary switching devices [Hinton93] (see Section 2.3.2.1). If both input/output ports are used, they act to permute the two inputs between the two output ports. If only a single input/output port is used, the device acts as a simple on/off switch. But the directional coupler is also useful as a building block for more complex network nodes, in which α is used as a continuously variable parameter. For example, using only ports 1 and 1 , the device becomes a variable attenuator. The power transfer relations introduced in Equation (2.2) generalize naturally to larger numbers of ports and to waveband selective configurations. For an r × n multiport device

44

Multiwavelength Optical Networks

operating on m wavebands, they can be written in the form P  (wk ) = A(wk )P(wk ),

k = 1, 2, . . . , m,

(2.4)

where A is an n × r power transfer matrix, P(wk ) is a column vector of r input powers, and P  (wk ) is a column vector of n output powers. The waveband argument indicates that power is specified as a function of waveband. The element, ai j (wk ), of A(wk ) represents the power transfer ratio from input port j to output port i valid for waveband wk .12 As in the 2 × 2 case, the elements of a general power transfer matrix may be either static or controllable. In the latter case, the matrix represents a waveband-selective switch (see Section 2.3.2).

2.3.1.2

Static Routers A serious defect of the broadcast star in Figure 2.7(a) is that without waveband or wavelength selectivity, all signals entering the star coupler are combined on all outbound fibers. This limits the total throughput to the usable capacity of a single fiber, thereby reducing the capacity available to any one station by a factor of 1/n. One way of improving the throughput of this network is by replacing the star coupler with a static routing node. Figure 2.10 shows a static router with n input and n output fibers, each carrying up to n distinct λ-channels, with wavelengths chosen from a set  = {λ0 , λ1 , . . . , λn−1 }. (For simplicity we discuss this device in the context of wavelength routing, but the discussion can be transposed to the waveband routing domain by replacing wavelength with waveband throughout. In spectral partitions with one λ-channel per waveband, the two terms are equivalent.) Each input fiber is connected to a 1 × n wavelength demultiplexer (WDMUX), which spatially separates the wavelengths on the fiber. Similarly, each output fiber is connected to an n × 1 wavelength multiplexer (WMUX), which is identical to the demultiplexer but is used in the opposite direction of signal flow, that combines the different wavelengths onto that fiber. We assume that a fiber has a capacity of n λ-channels. The wavelengths are assumed to be individually recognizable by the WDMUX 1

λ0 λ1 λn

λ0 λn 1

WMUX 1

1

λ1

2

2

n

n

Figure 2.10 Static routing node. 12

By using a discrete set of wavebands, it is implied that the multiport operates uniformly on all signal frequencies within each waveband.

Layered Architecture

45

demultiplexer, so that this is a wavelength-routed network. (We consider the whole “fabric” of WDMUXs, WMUXs, and their interconnecting fibers as contained in a single “black box”; that is, they act as a single node.) There are n 2 fibers between the input and output stages, connected in a way that prevents identical wavelengths from different input ports from being combined on the same output port, thus avoiding interference among the different channels. The physical path that a signal takes through the node is determined uniquely by its wavelength and port number, with the routing rule as follows: A signal on input port j, carried on wavelength λk , is routed to output port i, where k = (i − j) mod n. For example, in a 4 × 4 network, a signal entering on port 2, to be routed to output port 1, would use λ3 .13 Using the matrix notation of Equation (2.4), the power transfer ratios for the n × n system using the preceding “arithmetic” routing rule are ai j (wk ) = 1 when k = (i − j) mod n and ai j (wk ) = 0 otherwise. Note that if input and output ports with the same label correspond to fiber pairs carrying signals in opposite directions in the same link, then the wavelength λ0 produces loopback connections. Thus, if loopback connections are excluded, only n − 1 wavelengths are required for routing. Multiports with similar but more general wavelength permutation connectivity have been termed Latin routers [Barry+93], after the Latin square, an n × n array, where each entry contains one of n numbers with no number appearing in a row or column more than once. The Latin square is equivalent to a power transfer matrix, A , for a Latin router, where all entries for some number, k, correspond to the 1s in ai j (wk ). The reference [Barry+93] describes some multistage fabrics for general Latin routers. If it is used in conjunction with tunable transceivers, the static wavelength router functions as a permutation switch (see Section 2.3.2.1). For example, consider the router of Figure 2.10 placed at the center of the directed star network of Figure 2.11, replacing the star coupler. (The directed star is similar to the undirected case in Figure 2.7[a] except that the transmitters and receivers have been separated to reflect the fact that they may belong to different access stations.) A connection is established between any transmitter/receiver pair by tuning both transmitter and receiver to the unique wavelength that is routed on a path between the desired input/output ports.14 Transmitting Stations 1

Receiving Stations 1

2

2

n

n

Figure 2.11 Directed star. 13 14

Although Figure 2.10 shows a node constructed with bulk components, integrated optic static routers have also been fabricated in the form of arrayed waveguide gratings (see Section 4.10.4). Another way of making the router function as a permutation switch is by placing wavelength interchangers (see Section 2.3.3) in the input/output ports, as indicated by the rectangles in Figure 2.10.

46

Multiwavelength Optical Networks

More generally, up to n signals, each at a distinct wavelength in the set , can be routed from each input fiber to each output fiber in this manner. Thus, suppose each transmitting station is equipped with an array of n transmitters each tuned to one of the n wavelengths in , with a similar arrangement of tuned receiver arrays in the receiving stations. Using all transmitters and receivers simultaneously would create full connectivity among all transmitting and receiving stations at maximum throughput (n 2 fully utilized channels). In this arrangement, the use of the more complex (but static) node architecture together with the transceiver structures that can exploit it allows for an n-fold reuse of the optical spectrum. Compare this with the broadcast star, in which the throughput is limited to n fully utilized channels – the available capacity of a single fiber.

Dynamic Nodes The simplest dynamic optical node is a space switch with no waveband selectivity. It routes the signals on each input port to one or more selected output ports. Space switches come in two flavors: permutation and generalized. In an n × n permutation space switch, connections between input and output ports are point-to-point; neither one-to-many nor many-to-one connections are allowed. Thus, the admissible connection patterns are in the form of permutations, as illustrated by the connection matrix in Figure 2.12(a).15 (A permutation switch is square, and its connection matrix has a single 1 in each row and each column.) The specific connection pattern is selected under external control (i.e., switches are active devices in the sense that an external control agent is present).16 An n × n nonblocking permutation switch can create any one-to-one connection pattern, so that it has n! possible input/output patterns or connection states. This seems like a lot, but relatively speaking it is not.

1

1

3 4

1 1 (a) Permutation

Input Port

2

1

Output Port 3 4 2

5

1

1

1

1

2

1

Output Port 2 3 4 1

1 Input Port

2.3.2

1

1

1

3 1

4

1

1

(b) Generalized

Figure 2.12 Space switch connection matrices.

15 16

Note that the connection matrices in the figure are transposes of the corresponding power transfer matrices. The terms active and passive are tricky. Unless amplification is present in the signal path, an optical node, or any other device in the network for that matter, is “seen” by the optical signals flowing through it as a passive device because no energy is supplied to the signal as it traverses the device. We generally use the term passive in that sense. Nevertheless, energy is required for control, so a power source must be present for controllable devices. Hence, controllable (dynamic) devices are often termed active in the literature, and static devices are called passive.

Layered Architecture

47

A generalized space switch can create any input/output pattern, including one-to-many and many-to-one connections. A connection pattern for a generalized 4 × 5 switch is illustrated in Figure 2.12(b). (Connections are shown by 1s.) Counting all the possibilities in the case of an r × n generalized switch, we find 2nr connection states. Because they are capable of creating multipoint as well as point-to-point connections, two additional control possibilities exist in generalized switches. For each one-to-many connection, it is possible to specify how the input power splits among the various outputs, and for each many-to-one connection, the combining ratios among the various input powers can be specified. We call generalized switches that control these power-dividing and -combining ratios linear divider-combiners. Because combining and dividing ratios can take on a continuum of values, LDCs have an infinite number of connection states. It is important to note that generalized switches have the ability to terminate a connection, thus “dumping” its signal internally (by setting all elements of a row of the power transfer matrix equal to zero), whereas permutation switches do not. Similarly, they have the ability to set the power to zero on any output port (by setting all elements of a column equal to zero), whereas permutation switches do not. The connection termination property has important practical implications, because in a network built around permutation switches, unwanted closed paths may be created inadvertently, causing degradation in network operation. To illustrate, consider the network in Figure 2.13, which consists of n access stations each containing a pair of OTs and ORs connected to a unidirectional ring via a network node composed of a 2 × 2 coupler operating as an add/drop switch. With its coupler in the cross state (the add/drop state), a station is coupled into the ring so that it transmits to the next station on its right, which is in the add/drop state, and it receives from the station to its left, which is also in the add/drop state. With its coupler in the bar state, the station is bypassed. Figure 2.13 shows a condition in which station 1 transmits to station n and n transmits to 1. A problem arises if all couplers are in the bar state. In this case a closed loop exists. At first glance, this would not appear to create a problem. However, in a network with long fiber spans, there would normally be amplifiers in the links. Assuming that the gains are adjusted to compensate for the losses over the links and nodes, we have a ring

OR

OT

OT

1

OR

2

n

OT OR Figure 2.13 Unidirectional ring.

48

Multiwavelength Optical Networks

with a loop gain of unity. Because each amplifier generates some spontaneous emission noise, the gain in the ring will cause the noise power to grow without bound (or at least until the amplifiers saturate). Problems of this type have been observed in WDM ring networks, in which closed paths may exist for certain wavelengths, with the amplified noise robbing power from and interfering with signals at other wavelengths. Permutation switches therefore must be interconnected with care!

2.3.2.1

Permutation Switches: Characterization and Complexity There is a vast amount of literature on switches dating from the early days of telephony.17 Although technology has progressed from electromechanical to electronic to photonic (optical), the basic structure of the switch “fabric” has remained largely unchanged. One of the simplest structures that is adapted to both permutation and generalized switch operation is the crossbar. As shown in Figure 2.14, an r × n crossbar switch consists of r input lines, n output lines, and r n cross-points located at the intersections of the lines. The implementation of the cross-points has progressed from electromechanical relays through electronic gates to controllable optical couplers and micromirrors. In an n × n crossbar switch, a permutation connection is made by closing one crosspoint in each row and each column. (Multicast connections can be made by closing more than one cross-point in a row.) The main problem with the crossbar, used as an n × n permutation switch, is that the number of cross-points grows as n 2 , which is far more than necessary to create all possible permutation connections. Because optical switching devices are generally costly, switch fabric realizations with a minimum of these devices are desirable. A common method of constructing large switch fabrics that are more economical regarding cross-points is through a multistage fabric. A popular configuration is a three-stage arrangement, which can be configured to produce many different types of switches. As we shall see, multistage configurations usually can be realized with far fewer cross-points than the crossbar, especially for large-size switches. Figure 2.15 shows an n × n symmetric three-stage Clos network (see [Hinton93, p. 93]) composed 1 2

r 1

2

n

Figure 2.14 Crossbar switch. 17

For a good view of developments from classical theory to early developments in photonic switching, see [Benes65, Hui90, Hinton93].

Layered Architecture

49

k k 1 p m

m p

1

1

n

n

k

k

m Figure 2.15 Clos switch.

of k p × m switches in the first stage, mk × k switches in the middle stage, and km × p switches in the third stage, where n = kp. One possible way of realizing each of the smaller switches is as a crossbar. For n × n permutation switches with n a power of 2, the general Clos structure of Figure 2.15 can be “factored” recursively to produce a Benes switch fabric, which is one of the most economical permutation switches in terms of cross-points. The idea is shown in Figure 2.16. In the first factorization, we take p = m = 2 so that there are two (n/2) × (n/2) switches in the middle stage, and all first and third stage switches are 2 × 2. Now, each middle-stage switch can be factored in the same way, and the process repeated until the middle-stage switches are 2 × 2. The result is an array consisting only of 2 × 2 switches. The Benes switch for the case n = 8 is shown in Figure 2.17. It is

n × n 2 2 n n × n 2 2

Figure 2.16 Recursion for Benes switch.

50

Multiwavelength Optical Networks

1

1

2

2

8

8

Figure 2.17 An 8 × 8 Benes switch.

left as an exercise for the reader to show that a general n × n Benes switch can be built using (n log2 n − n/2) 2 × 2 elements. Multistage switch fabrics such as the Benes, composed of 2 × 2 elements, are particularly attractive in the optical domain. The reason is that each element (a 2 × 2 permutation switch) is operated in only two states: the bar and cross states defined in Section 2.3.1.1. Therefore, it can be implemented optically using a controllable directional coupler, with a power transfer matrix of the form of Equation (2.3). Systems composed of arrays of 2 × 2 directional couplers, operated in various modes, appear frequently in this book. We have already seen an example in the Banyan network of Figure 2.9, in which each element was operated as a (static) star coupler. The minimum number of binary switching elements required for a permutation switch can be deduced by equating connection states to switch states. A switch having S connection states requires at least log2 S binary switching elements (giving S switch states). Because a permutation switch has S = n! connection states, we find that the minimum number of switching elements for any realization using any type of binary devices is log2 (n!), which (using Sterling’s formula) is approximately (n log2 n − 1.44n) for large n. Comparing this number with the number of elements required in the Benes switch, we find that for large n the Benes fabric is very close to optimal in its use of hardware.

Characterization of Blocking Permutation switches are classified in terms of their blocking characteristics. In discussing blocking performance, it is assumed that we are considering sequences of demand-assigned connections between input and output ports on the switch, with requests for connection establishment and termination occurring at random points in time. An n × n switch is “rearrangeable” or rearrangeably nonblocking if there exists a set of paths through the switch fabric (i.e., a set of switch states) that realizes each of the n! connection states. The term rearrangeable comes from the fact that it may be necessary to rearrange currently active connections to support a request for a new connection between a pair of idle input and output ports. Rearrangeable switches have two problems: 1. For any given set of desired connections (any given permutation of inputs to outputs), the determination of the required device settings to route the connections generally requires complex computation.

Layered Architecture

51

2. Connections in progress may have to be interrupted momentarily while rerouting is taking place to accommodate new connections. The good news about rearrangeable switches (of which the Benes is one) is that they can be realized with a minimum amount of device hardware. The price of hardware simplicity is control complexity. A control algorithm for the Benes switch, known as the looping algorithm, is described in Appendix G. It has a complexity of the order of (nlogn)2 for an n × n switch. Wide-sense nonblocking networks are those that can realize any connection pattern without rearranging active connections provided that the correct rule is used for routing each new connection through the switch fabric. These require more hardware than rearrangeable networks, and still need intelligent routing algorithms, but have the advantage that active connections need not be interrupted. Algorithms for wide-sense nonblocking networks are not necessarily complex. As an example, it is left as an exercise for the reader to devise a connection algorithm for the path-independent loss crossbar in Figure 4.74, which is wide-sense nonblocking. Strict-sense nonblocking networks, sometimes called simply nonblocking, require no rearrangement and no complex routing algorithm. New connection requests are allowed to use any free path in the switch. As might be expected, these networks require still more hardware than the wide-sense nonblocking fabrics – this cost is traded for the lack of connection disruption and simplicity of routing. The Clos network of Figure 2.15 can be constructed to be either wide sense or strictly nonblocking. A necessary condition on the number of middle-stage switches to make the network wide-sense nonblocking is m = 2 p − p/k, where x denotes the largest integer equal to or less than x [Smith+76]. If k = 2, then a necessary and sufficient condition to make it wide-sense nonblocking is m = 3n/2 [Benes65]. A necessary and sufficient condition to make the Clos network strict-sense nonblocking is m = 2 p − 1. For our purposes, strict-sense nonblocking switches are the simplest and most practical but not always cost-effective. If the sizes of the switches in its three stages are optimized, it can be√ shown that a strict-sense nonblocking Clos switch can be realized with approximately 4 2n 3/2 cross-points. Another way of realizing a strictly nonblocking switch is through space dilation of the original Benes fabric. An m-fold space dilation of an n × n Benes switch is created in the general three-stage form of Figure 2.15. The dilated switch is built using n 1 × m switches in the input stage, n m × 1 switches at the output, and m Benes networks in the middle stage. With m = log2 n, it turns out that this system, called a Cantor network, is strictly nonblocking because a free path through the fabric can always be found for any pair of idle input/output ports via one of the m middle switches. This switch has O(n[log2 n]2 ) cross-points, which is an improvement over the strictly nonblocking version of the Clos switch for sufficiently large n. In terms of the number of cross-points required for strictly nonblocking switches, multistage fabrics do not start to become worthwhile until the switch size is fairly large (n > 32). However, for rearrangeable or wide-sense nonblocking switches, multiple stages are definitely more economical.

52

Multiwavelength Optical Networks

At this point it is worth noting the contrasting features of the traditional and optical network switching requirements. In traditional networks, switches often have very large numbers of ports, and cross-points (realized electronically) are very inexpensive. Conversely, optical switches are orders of magnitude smaller and their cross-points are orders of magnitude more expensive. Now, in large switches it is possible to take advantage of laws of large numbers to reduce dramatically the hardware complexity of the switch in return for accepting some small blocking probability (meaning that not all possible connection patterns can be realized). Typical telephone switches with n on the order of 105 operate with blocking probabilities on the order of 10−3 or less. This is not particularly relevant to optical switches, however, because their size is more likely to be on the order of 10. In traditional networks, rearrangeable and wide-sense nonblocking switches are rarely used because the computational complexity involved in finding and executing an appropriate routing algorithm is overwhelming for large-size switches and is not worth the savings in switch hardware. In the optical domain, however, switches are small, so routing complexity for rearrangeable and wide-sense nonblocking switches is reduced. Furthermore, in transparent WDM networks the routing algorithm is used only in the space dimension, so algorithmic complexity is that required for a single waveband only, whereas the hardware savings are roughly proportional to the number of wavebands being switched. Also, computational speed can be increased by doing computations for all wavebands in parallel. These considerations suggest that it is worth taking a second look at the rearrangeable and wide-sense nonblocking category for optical applications. Another issue that is important in optical switches is cross-talk. In electromechanical or electronic switches, the cross-points are close to ideal; a switch is either definitely open or closed. In optical switches, the binary elements (controllable couplers) typically used as cross-points may have as much as a 5% cross-state “leakage” when they are set to the bar state and vice versa. This necessitates using switch fabrics with special designs involving extra binary elements to reduce cross-talk (see Section 4.12.1). Thus, a minimum cross-point realization is not always the best.

2.3.2.2

Generalized Switches Because the number of connection states of a generalized r × n switch is 2nr, it is clear that we need at least nr binary elements to realize it. This is the number of cross-points used in the traditional crossbar switch, and one way of realizing generalized optical switches is through an optical version of the crossbar. Figure 2.18 shows one possible realization [Sharony94]. It consists of three stages, the first and third of which are static and the middle stage is controllable. Stage 1 consists of an array of r 1 × n signal dividers (splitters), and stage 3 is a similar array of n r × 1 signal combiners (which could be realized with the same type of hardware, used in the opposite direction of signal flow). The middle stage consists of r n binary on/off switches, which could be realized, for example, using the 2 × 2 coupler of Figure 2.8 or the Y-branch switch of Figure 4.46. Any generalized connection pattern of the type shown in Figure 2.12(b) can be created by turning on a middle-stage switch for each 1 entry in the connection matrix. Assuming that the splitting (combining) stages produce equal

Layered Architecture

53

r 1 Combiner 1

1r

12

11

2

2r

22

21 1 n Splitter

n

n2

n1 1

2

nr

r

Figure 2.18 Generalized optical switch.

splitting (combining) ratios for each signal, this switch will produce an input/output power relation in the form P  = A P,

(2.5)

where P and P  are vectors of input and output powers, respectively, and the elements of the n × r power transfer matrix A are  1/nr if switch i j is on ai j = (2.6) 0 otherwise.

2.3.2.3

Linear Divider-Combiners Although the crossbar is an extremely simple configuration, it has the disadvantage that the two passive stages produce a combined signal attenuation (splitting loss followed by combining loss) of 1/r n. A different approach, which has less inherent loss and at the same time offers complete generality in setting combining and dividing ratios, is shown in Figure 2.19. We call this a δ–σ LDC. It consists of a power-dividing stage followed by a power-combining stage, both of which are controllable. A 4 × 4 example is shown in Figure 2.19(a). It has a two-stage power-dividing network fed by four input fibers followed by a combining network (the reverse of the dividing network) connected to four output fibers. Each box in the figure represents a 2 × 2 controllable coupler operated as a continuously adjustable power divider or combiner. This structure generalizes in a straightforward way to an r × n LDC – as shown schematically

54

Multiwavelength Optical Networks

11

1

. .

11 21

12 . .

n1

1r 21

12

2

. .

. . .

n2

r

22

22 . .

2′ 2r

1r

n1 . .

2r

n2 . .

nr

(a)

1′

. . .

n′ nr

(b)

Figure 2.19 δ–σ linear divider-combiner.

in Figure 2.19(b) – that requires a total of [r (n − 1) + n(r − 1)] couplers. If we denote by δi j the fraction of power from input port j directed by its divider to the combiner for output port i  and denote by σi j the fraction of power received from the divider serving input port j and combined onto output port i  , then the elements of the power transfer matrix for this LDC are given by ai j = δi j σi j .

(2.7)

The dividing and combining ratios are varied by setting the coupler states appropriately. In the ideal case of lossless couplers, the ratios are subject to the physical constraints  δi j = 1 (2.8) i

and



σi j = 1.

(2.9)

i

Within these constraints, arbitrary combinations of inbound signals at the node can be directed to each outbound fiber. For example, if the coupling state for each 2 × 2 coupler in Figure 2.19(a) is set at 0.5, the LDC operates as a 4 × 4 star coupler, with ai j = 1/16 for all i and j. If each 2 × 2 coupler is used only in a binary (bar/cross) state, the system operates as a permutation switch. Note that any input signal can be “dumped” (i.e., absorbed in the switch) by directing it to an output port with a combining ratio that is set to zero. For example, a signal at input port i can be terminated by setting δik = 1 and σik = 0. Similarly, the power at output port j  can be set to zero by letting δk j = 0 and σk j = 1. Note that the switch of Figure 2.18 can be converted to an LDC by making the dividing and combining ratios in Equation (2.6) more general. If the on/off switches are replaced by continuously variable attenuators, the elements of the power transfer matrix

Layered Architecture

55

Figure 2.20 A node without loopback connections.

become ai j = αi j /nr , where αi j is the transmission constant for the i j th attenuator in the switch. The only functional difference between this and the δ–σ configuration is that the constraints on ai j given in Equations (2.7), (2.8), and (2.9) are replaced by 0 ≤ ai j ≤ 1/nr.

(2.10)

These switch fabrics are representative of a very large class of configurations. Many variants are possible for adapting to special situations or for reducing cost. For example, less hardware is needed if full connectivity is not required. Figure 2.20 illustrates a case of a switching node of degree three (connected to three bidirectional links), where loopback connections along the same link are not required. A δ–σ LDC for this node can be realized with 6 (instead of 12) 2 × 2 couplers as shown. As another example, the large controllable component count in the LDC of Figure 2.19 can be reduced, at the cost of additional splitting (combining) loss, by replacing the controllable dividing (combining) stage with a fixed dividing (combining) stage, as is used in the switch of Figure 2.18. The elements of the power transfer matrix of an r × n LDC are then given by ai j = δi j /r

(2.11)

ai j = σi j /n

(2.12)

for a fixed combining stage and

for a fixed dividing stage. Different switch architectures and switch configurations produce different effects on signal power levels at the output ports. For example, considering square switches built from ideal lossless couplers, the Benes switch and the δ–σ LDC are lossless when used as permutation switches, whereas the optical crossbar incurs a loss of 1/n 2 for any switch configuration. When configured as a broadcast star, however, both the crossbar and the δ–σ LDC produce a 1/n 2 combining/splitting loss, whereas the Banyan realization

56

Multiwavelength Optical Networks

(Figure 2.9) produces the minimum possible loss of 1/n. (The Banyan structure does not, however, contain enough couplers to be used as a nonblocking switch.)

2.3.2.4

Waveband-Space Switches By adding waveband and/or wavelength selectivity in various ways, the basic switch fabrics described earlier can be converted to waveband/wavelength-selective switches (WSSs). In this section we begin the discussion with transparent switches without wavelength interchange. In keeping with the idea that any spectral selectivity in a transparent optical switch is assumed to be limited to wavebands – the “coarse” subdivisions of the optical spectrum defined in Section 2.2, we focus on the waveband as the elementary spectral unit being switched. Later in the section some opaque architectures are introduced. In an m-waveband WSS, signals carried on m wavebands in the set W = {w1 , w2 , . . . , wm } are multiplexed on each input fiber. An r × n WSS directs these signals in a waveband-selective manner to the n output ports. Thus, the switch operates on mr inputs. (Although there may be several λ-channels grouped on each waveband, as far as the switch is concerned the waveband is the smallest recognizable entity.) The switching pattern is independently controllable for each waveband. Figure 2.21 shows one way of realizing an n × n WSS using a three-stage configuration, with the middle stage composed of m switching layers, each one operating on signals in one waveband. The m wavebands on each input fiber are separated spatially into m layers by WDMUXs in the first stage. Each layer is a waveband-independent n × n space switch, which operates on all signals in one waveband. The space switching can be of any type, from permutation to LDC. The switched signals are recombined on the output fibers by WMUXs in the third stage. This realization of a WSS used a combination of fixed (passive) waveband-selective devices (WMUXs and WDMUXs) and dynamic but waveband-insensitive switching devices. A more compact way of realizing the system would be to use multiwaveband WDMUX

WMUX n

1

wm

w1

n w1

1 n

2

1

n

2

2 . . .

. . .

wm

. . .

n

n n

n m

Figure 2.21 Three-stage realization of a waveband-space switch.

Layered Architecture

P1(w1), … P1(wm )

P1(w1), … P2(wi ), P2(wj ), … P1(wm )

P2(w1), … P2(wm )

P2(w1), … P1(wi ), P1(wj ), … P2(wm )

57

Switch Control Figure 2.22 Multiwaveband directional coupler.

switches (MWSs) where switching and waveband- or wavelength-selective functionality are integrated into the same device. Waveband-selective directional couplers (i.e., 2 × 2 MWSs) have been constructed using a variety of technologies, with a capability of independently and simultaneously switching several wavebands under external control (see Section 4.10.5). The power transfer matrix for a lossless symmetric 2 × 2 MWS is of the form of Equation (2.3), where the parameter α is now a function of waveband w. Figure 2.22 shows a 2 × 2 MWS operating on m wavebands. The switch is set in the cross state for wi and w j and in the bar state for all other wavebands. Each of the space-switching fabrics of Figures 2.17, 2.18, and 2.19 can be converted to an m-waveband WSS simply by replacing each switching device by an MWS operating on m wavebands. Of course, the distinction between a WSS, which we show as combining many individual components performing separate functions, and an MWS, shown as an integrated device, is largely arbitrary. If we draw a black box around the collection of elements in Figure 2.21 and do not look inside, it might be considered to be an MWS. There are, however, many optical switching devices that are integrated to the point that they cannot be subdivided even conceptually into components performing separate functions; these are unquestionably MWSs (see Section 4.10.5). The power relations in an WSS can be described using the power transfer matrix of Equation (2.4). In a permutation switch, A(wk ) is a permutation matrix for each wk in the set W, whereas in a waveband-selective LDC, each element of A may take on a continuum of values. The elements of A will, of course, change in response to external control. Thus, each switch configuration is defined by m independently controllable matrices A(w1 ), A(w2 ), . . . , A(wm ) – one for each waveband. For example, in the realization of Figure 2.21, the setting of the kth layer of the switch would be identified with A(wk ). The WSS is one of a family of switching devices commonly called optical crossconnects (OXCs). The most commonly used WSSs operate as wavelength-selective or waveband-selective permutation switches, which are also called wavelength-selective cross-connects (WSXCs). For n × n m-waveband WSSs, there are (n!)m connection 2 states for the permutation case and 2n m connection states in the generalized case. Because mn entities are being switched when the fibers carry m wavebands, one might first think that the permutation or generalized switch should have (nm)! and 2 2(nm) connection states, respectively. This is not so because of the wavelength continuity constraint: Wavelengths of signals do not change in passing through a WSS,

58

Multiwavelength Optical Networks

so less switching alternatives exist in the wavelength dimension than in the space dimension. To avoid a plethora of details and special cases, we have restricted the discussion here to fairly general models of switches. Of course, in practice there are many variants of the basic switch, and many different realizations adapted to particular sets of needs. We summarize them briefly now, directing the reader to Section 4.12 for more details on switch architectures and Section 4.10 for the enabling switch technologies. First, there is the issue of hardware realization, and the related question of transparency versus opacity. Transparent switches require purely optical realization using photonic components of the type described in Section 4.10. An example of a transparent OXC, sometimes called a photonic cross-connect (PXC), is shown in Figure 4.86. It is based on the three-stage architecture of Figure 2.21, with signals remaining in optical form throughout the switch. This arrangement is termed an O-O-O switch because signals enter and leave the switch in optical form, and the fabric is also optical. Local users can access the switch on add/drop ports via NASs whose access links are connected directly to switch ports. This corresponds to our model of a transparent ONN with an associated NAS. The optical connections terminating on the OXC must use the same wavelengths as those carried on the internodal links connected to the switch ports. Figure 2.23 shows a special case of the architecture of Figure 4.86 with a single input and output port and multiple access fibers linking it to a colocated NAS. Known variously as a wavelength add/drop multiplexer (WADM), an optical add/drop multiplexer (OADM) or a reconfigurable optical add/drop multiplexer (ROADM) it can be realized in many different forms, and is the workhorse of current metropolitan area networks (see Section 4.12.2). We have already seen an add/drop switch, in the form of a 2 × 2 coupler, in Figure 2.13. The WADM is a direct generalization of that to the multiwavelength case, where it independently adds and drops each wavelength on a pair of inbound and outbound unidirectional internodal links. In the realization shown in W A DM W D MUX λ1

WM U X

λ2 λm

λ1

λ2

OT OR OT OR

...

λm OT OR

TP/R P

... Figure 2.23 WADM–NAS combination.

NAS

Layered Architecture

59

Figure 2.23, a controllable directional coupler is associated with each wavelength to be added/dropped at the local NAS. Thus, a separate pair of access fibers is required for each wavelength. With the coupler in the bar state the wavelength is passed through the WADM, and in the cross state the wavelength is dropped to the local receiver, and a signal on the same wavelength can be added to the signal set exiting on the output port.18 This is an especially useful and cost effective network element for accessing a ring or an intermediate point on a long link in a mesh network (see Figure 4.81). Although transparency has many advantages, most currently deployed networks use opaque switching nodes. Figure 4.87 shows an opaque OXC using a three-stage O-E-O arrangement, wherein the switch fabric is electronic, requiring the demultiplexed wavelengths to be converted to electrical form before entering the switch and then back to optical form at the output ports of the switch. For various practical reasons, the conversion between λ-channels and electrical signals is implemented as a two-stage process, first shifting the wavelengths of the λ-channels typically to the 1.3-µm band using a transponder and then converting the resultant 1.3-µm optical signals to electrical form. (A transponder accepts a signal in optical form, converts it to electrical form, provides some degree of regeneration, and then converts it back to optical form on a different wavelength.) The devices for converting the 1.3-µm signal to and from electrical form at the electrical switch fabric ports are called transceivers; they are composed of an optical transmitter and receiver pair operating in the 1.3-µm band. They perform the electrical-to-optical and optical-to-electrical signal conversion, respectively. Note that this architecture offers the possibility of connecting electrical signals from client logical networks (e.g., SONET, ATM, or IP) directly to add/drop ports on the electrical switch fabric. Electronics creates a bottleneck, limiting the bit rates of the signals passing through the fabric. Furthermore, because electronics is generally neither bit-rate nor format independent, changes in the signal bit rate and format require a complete change of the switch fabric. Finally, Figure 4.88 shows an opaque O-O-O architecture similar to the O-E-O case, except that the switch fabric is optical, eliminating the need for transceivers. The opaque O-O-O differs from the transparent case in Figure 4.86 because electronics is present in the form of transponders translating the optical signals to the 1.3 µm band as previously noted. This destroys the transparency of the connections. (Signals from client networks again access the switch fabric directly, but in this case they are in optical form at 1.3 µm.) In the opaque O-O-O and O-E-O architectures, the transponders are costly and, again, create an electronic bottleneck. However, on the positive side they clean up the signals, avoiding the propagation of signal impairments, and as a by-product they offer the possibility of wavelength conversion in the switching node. However, conversion is possible only if the switch fabric is configured as a nonblocking permutation switch interconnecting all ports. Thus, an m wavelength n × n OXC requires a single mn × mn switch fabric rather than m n × n fabrics (as in Figure 2.21) if it is to perform wavelength

18

In cases where the add/drop connections remain fixed for long periods of time, a more economical static version of the WADM can be constructed by replacing the controllable couplers by fixed connections.

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Multiwavelength Optical Networks

conversion. The former is more costly than the latter, especially when m is large. Finally, an advantage of the two opaque configurations is the fact that the signal is converted to electrical form at some point, meaning that there is easy access to control and management overhead carried in the bit stream. This allows for realization of various signal monitoring, control, and recovery functions. In any of the three switch architectures described above there remains the issue of whether they should be operated on a waveband or wavelength basis. The opaque architectures can switch wavebands and wavelengths equally well once they are demultiplexed, and can perform wavelength conversion. In either case they must operate on each wavelength individually, so that they cannot take advantage of the cost advantages of bundling λ-channels into wavebands: demultiplexing is difficult and costly for many closely spaced channels as compared to fewer widely spaced bands. Furthermore, the complexity of the switch fabric grows rapidly with the number channels being switched. As DWDM systems move to large numbers of wavelengths on a fiber this becomes a serious issue. The transparent OXC offers simplicity and generally lower cost than the opaque case but it is better suited to waveband than wavelength operation and cannot perform wavelength conversion. Recall that a disadvantage of waveband-switching mentioned earlier is that all λ-channels in a common waveband stick together in the optical path layer. But in a purely waveband-oriented network architecture not all of the optical connections grouped into a common waveband will terminate at the same source and destination ONNs. When at least one connection terminates, its waveband must be multiplexed (demultiplexed); i.e., its optical path must terminate. Optical path termination involves additional processing and equipment at the nodes and therefore should be reduced to a minimum. This problem can be eliminated or at least reduced if all λ-channels for connections terminating at a common node are grouped into a single waveband that terminates at that node. The remaining through traffic is carried on wavebands routed through the node. The hybrid switch of Figure 4.89, which has a waveband-switching layer (WSXC), and a wavelength-switching layer (λSXC), is adapted to this approach. Through traffic is carried on wavebands that are routed through the switch without being passed to the λSXC layer. Waveband w3 , carrying terminating traffic is passed to the λSXC layer for demultiplexing and further processing in an NAS. To achieve the efficiencies that are attainable with a hybrid switch, traffic terminating at common nodes must be grouped into common wavebands wherever possible, a grooming process mentioned earlier in Section 2.2.

2.3.2.5

Wavelength-Space-Time Switches: Optical Packet Switching When used as a node in a large circuit-switched network, the WSS described earlier would normally be reconfigured on a relatively slow timescale. For example, dedicated connections used to provision one of the specialized LNs shown in Figure 1.1 might be held for very long periods of time, much like connections through digital cross-connects in traditional networks. For demand-assigned connections, the timescale would be faster – on the order of minutes or hours. For efficient operation in a circuit-switched mode, the time required to establish and terminate each connection (including routing computations, signaling, and switch reconfiguration) should be small compared with the

Layered Architecture

61

holding time (for example, on the order of seconds for demand-assigned connections). This is no problem using the WSS structures discussed earlier. However, there are other cases when much faster switch reconfiguration timescales are required. For example, consider a wavelength-routed network (one λ-channel per waveband), in which each λ-channel entering a switch is carrying several logical channels time divisionmultiplexed into slots (of microsecond duration) in a fixed periodic time frame (see Section 3.2.1). In this case each time slot is equivalent to a subchannel of the λ-channel. Now suppose we want to switch the subchannels independently among different space ports on a time slot-by-time slot basis. This requires rapid switching (on a microsecond time scale) to route each time slot independently to a different output port. With proper time slot synchronization among all the entering signals, the switch could be reconfigured to create a different space connection pattern for each wavelength in each time slot, producing switching in three dimensions: a wavelength-space-time switch. Assuming a frame with l time slots, the total number of connection states for an m-wavelength n × n (permutation) switch would be (n!)lm. In a wavelength-space-time switch, the time dimension behaves much like the wavelength dimension: The number of connection states does not grow as rapidly with l (or m) as it does with n. This reflects time slot continuity through the switch; in other words, information entering in a particular time slot leaves in the same time slot. As the central node in a star network, this kind of switch is an interesting alternative to the passive star. Using time and wavelength, a set of lm logical connections (one for each time division subchannel and each λ-channel) can occupy each input port, resulting in a total of lmn independently switched connections using m wavelengths. Extending this idea to WANs is much more difficult. Because of time slot continuity, the optical wavelength-space-time switch is not as versatile as its electronic counterpart. Traditional electronic space–time switches in wide area networks use time slot interchange (TSI) in addition to space switching. With TSI, the order of the time slots in the frame is permuted as they pass through the switch. This produces more connection states but requires buffering, which is much more difficult in the optical domain than the electrical domain. With or without buffering, time domain switching is not a transparent process because of the necessity of aligning the slot boundaries with the switching times. Once transparency is abandoned, the temptation is to continue in this direction to fully optical packet switching (OPS). In a WDM network, OPS is characterized by two features: (1) time division multiplexing of information packets on each wavelength and (2) routing packets through ONNs based on control information in the packet headers. As mentioned in Section 1.3 the use of this in-band control information for routing provides a high degree of virtual connectivity in a packet-switched network. However, implementation in the optical domain requires optical buffering, because (1) packets are typically of variable length (as in IP networks) or are separated by idle periods, so that they are multiplexed asynchronously, (2) packets arriving at a node from different sources are not synchronized in time, and (3) packets contending for transmission on a given output line at a node may require buffering to resolve contention. As optical technology progresses the feasibility of optical buffering and signal processing improves. Thus, although

62

Multiwavelength Optical Networks

speculative today, OPS may be a viable option in the near future and the wavelength-space-time switch will come into its own (see Chapter 10).

2.3.2.6

Cross-Talk So far we have avoided consideration of device imperfections and their effect on the performance of switching devices; these are largely relegated to Chapter 4. However, these play an important role in the evaluation of cost–performance trade-offs in each of the switch architectures described earlier. One important performance consideration in a switch is cross-talk, which results when some of the power from an input signal leaks through to an unintended output port. (This was one of the transmission impairments listed in Section 1.3 as a downside to transparency.) For example, the directional coupler defined in Equation (2.3) is in the bar state when its control parameter α = 0. However, for any real device α can be made close to but not exactly equal to zero. If α =  > 0 when the switch is meant to be in the bar state, we have on port 1 P1 = (1 − )P1 +  P2 ,

(2.13)

where the presence of P2 (at the same nominal wavelength as P1 ) represents undesired leakage of the signal entering on input port 2 to output port 1 . This leakage constitutes cochannel heterodyne cross-talk between the signals entering on the two input ports (see Section 4.9). When a switch is composed of a multistage fabric, as in Figure 2.17, there are many sources of cross-talk on a signal path within one ONN, compounding this problem. Furthermore, when an optical path consists of several hops, the cross-talk accumulates from each optical node along the path. The accumulated cross-talk causes interference at the optical receiver, with a consequent deterioration of the bit error rate (BER) or SNR. In addition to cochannel heterodyne cross-talk, leakage paths can also cause cochannel multipath cross-talk, which occurs when a portion of the signal power leaks through an unintended path within a switch fabric and recombines with the original signal. The recombination may be “downstream” on a parallel path or “upstream” on a feedback path. Multipath cross-talk is more troublesome than heterodyne cross-talk because the recombining optical fields originate from the same source so that phase and polarization relations influence the magnitude of the resultant interference. In waveband-space switches interchannel cross-talk may also be present, either due to imperfect waveband demultiplexing or imperfect operation of multiwaveband switches. These phenomena and means of combatting them are explored in more detail in Chapter 4. The main point to be observed here is that cross-talk effects increase with the size of a switch, the number and packing density of the wavebands, and the imperfections in the basic building blocks. As shown in Chapter 4, it is possible to offset some of the crosstalk effects due to imperfect components by using more complex switch architectures. However, these require a higher component count and therefore an increased cost. Herein lies a basic three-way cost–performance trade-off: Large-size switches that handle many densely packed wavebands are desirable for increasing optical network connectivity and throughput but require high-quality (and hence expensive) component technology and/or a high-component count for satisfactory performance.

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63

Another cost–performance issue stems from the relation between component count and switch functionality (closely related to switch states). The component count is O(n log n) for n × n rearrangeable switches and increases as we go to wide-sense nonblocking, strictly nonblocking, and generalized switches, and LDCs. The more switch states, the larger the required component count and/or the more complex the components (i.e., the higher the cost). But, as mentioned at the beginning of this section, switches with a large number of states offer better network performance in terms of throughput, connectivity, flexibility, and survivability. These trade-offs between network performance and switch functionality are discussed in more detail in later chapters.

2.3.3

Wavelength Converters The basic optical transmission channel considered here has been a λ-channel, which was generally assumed to remain on a fixed wavelength from end-to-end in transparent networks – a condition called wavelength continuity. Here for the first time we introduce the operation of wavelength conversion, which violates the wavelength continuity condition. Wavelength conversion can be implemented electronically, electro-optically, or optically and it can be executed as a (nearly) transparent or opaque operation. We have already seen instances of wavelength conversion, performed as a by-product of operations in the electronic domain in opaque OXCs. Transponders operating on single demultiplexed optical signals at input ports translate the input signal frequencies to a common band, the resultant signals are switched either optically or electronically and then converted to optical output signals at a different wavelength. As a result of this wavelength conversion capability the opaque O-O-O and O-E-O OXCs take on the functionality of a wavelength-interchanging switch, also known as a wavelength-interchanging crossconnect (WIXC). A signal entering the switch at any port and any wavelength on a designated grid can be routed to any output port on any other wavelength on the grid – with the constraint that signals routed to the same output port are carried on different wavelengths to avoid interference. Of course, a transponder is itself an opaque wavelength converter, executing the conversion operation by using an optical-to-electrical followed by an electrical-to-optical signal conversion. The transponder is an example of a converter based on “strongly” nonlinear devices, in this case the optical detector and transmitter. Other opaque wavelength converters based on strong nonlinearities are discussed in Section 4.11.2. We turn our attention now to a class of wavelength converters using wavelength mixing, called coherent converters. These are based on “weak” nonlinearities, and for all practical purposes they are transparent. An ideal (transparent) wavelength converter is a single input/output device that converts the wavelength of a λ-channel appearing on its input port to a different value at its output port but otherwise leaves the optical signal(s) unchanged. Converters based on strong nonlinearities typically operate on a single input signal and are not transparent to bit rate and modulation format, whereas wavelengthmixing converters operate transparently on multiple signals within a broad band of wavelengths. Within this category, devices based on four-wave mixing and difference

64

Multiwavelength Optical Networks

Signal

Wavelength Converter

s

Output c

p

s

Pump p

(a )

0

p

c

2 (b)

Signal

Low-Pass Filter

s

cos2

p

s

Output c

p

s

pt

(c )

Figure 2.24 Wavelength conversion as a linear operation.

frequency generation (also called three-wave mixing) have been demonstrated. Our discussion here is confined to difference frequency generation. Section 4.11.2 discusses both types in more detail. An illustration of the operation of a difference frequency-generating parametric wavelength converter or difference frequency converter (DFC) [Yoo+95] is shown in Figure 2.24(a). A signal at optical frequency νs is applied to the input port and mixed with a pump signal at frequency ν p . The DFC contains an optical medium that possesses a second-order (square-law) nonlinearity, so that various sum and difference frequencies may be present at the output. If the proper phase-matching conditions are satisfied within the converter, only the difference frequency νc = ν p − νs is present. Expressing this in terms of wavelengths, we have 1/λc = 1/λ p − 1/λs . Typical spectral relations are shown in Figure 2.24(b). Here, three input signals are shown in a band at nominal frequency νs , and the pump frequency is chosen at slightly less than double the signal frequency. For these frequency relations, the output spectrum is an attenuated mirror image of the input spectrum, reflected about the “mirror frequency” ν p /2.19 Although the DFC depends on nonlinearity to produce frequency conversion, it can actually be modeled mathematically as a (time-varying) linear device, as shown in Figure 2.24(c). In the model, the input signal is multiplied (modulated) by a sinusoid at the pump frequency, and the product is low-pass filtered to retain only the difference 19

This means that the spectrum of each input signal is inverted. The inversion is removed by an even number of passes through DFCs. Spectrum inversion can be used to an advantage in compensating for fiber dispersion (see Section 4.3.2.3).

Layered Architecture

p

s2

2 (a)

3

s1

2

1

p

1

2

65

3

2 (b)

Figure 2.25 Wavelength interchanger.

frequency terms. Thus, for all practical purposes, it produces wavelength conversion while exhibiting the transparency we expect of any linear device. The conversion is controllable by varying the pump frequency. If only two input signals are present, and it is required to interchange their optical frequencies, this can be done by placing the mirror frequency midway between the signal frequencies, producing a wavelength interchanger as shown in Figure 2.25(a). Because the DFC acts as a linear device, it obeys the same conversion rules for superimposed signals as for individual ones; that is, it is a “bulk” conversion device. For example, suppose a set of signals at equally spaced optical frequencies is present at the input of a DFC. Then, by placing the mirror frequency midway between two of the signal frequencies, as shown in Figure 2.25(b), the DFC interchanges signal frequencies in pairs: ν1 → ν−1 , ν−1 → ν1 , ν2 → ν−2 , ν−2 → ν2 , and so on. It is also possible to use two pump frequencies to produce a more elaborate interchange pattern. (More than two pump frequencies make the device impractical, introducing unwanted converted frequencies that cannot be filtered out.)

Wavelength-Interchanging Switches An example of a 2 × 2 WIXC is shown in Figure 2.26. Three optical signals are active on the input ports. S1 (λ1 ) and S2 (λ2 ) appear on port 1 at the indicated wavelengths, and S3 (λ1 ) enters on port 2. The switch is shown in the bar state for S2 and S3 and in the cross state for S1 . In addition, the wavelength of S1 has been changed to prevent a wavelength conflict at output port 2 . This example generalizes in a natural way to switches of higher dimension. It can be realized as an opaque switch using the OXC architectures of Figures 4.87 and 4.88. Another approach using transparent components is shown in Figure 2.27. It uses a combination of 2 × 2 WSXCs and difference frequency wavelength interchangers to realize a general WIXC [Antoniades+96]. The architecture is based conceptually

1

S1( λ 1), S2(λ2)

S2(λ2)

1

WIX C 2

S3(λ 1)

Figure 2.26 Wavelength-interchanging switch.

S1(λ 2), S3(λ 1)

2

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Multiwavelength Optical Networks

S1(λ1)

S2(λ2)

S3(λ1)

S3(λ1)

S3(λ2) WSXC WSXC

WSXC

S2(λ2) WSXC

S1(λ2), S3(λ1)

S1(λ1)

S1(λ2) DFC

DFC

λ1 X

=

X

λ2 =

=

=

Figure 2.27 WIXC implementation.

on a modified Benes structure called a twisted Benes switch fabric, with DFCs placed between the stages. The figure shows a realization of the 2 × 2 two-wavelength WIXC in the example of Figure 2.26, implemented using this approach. It requires three WSXCs and two DFCs used as single-wavelength interchangers. The settings of the WSXCs (bar or cross for each wavelength) to produce the signal configuration shown in Figure 2.26 are indicated in Figure 2.27. The realization procedure yields a general structure for any n × n m-wavelength WIXC, provided that n and m are powers of 2. For example, a 2 × 2 four-wavelength WIXC can be built with five 2 × 2 wavelength selective switches and four fourwavelength interchangers. The general structure, based on the twisted Benes fabric, requires 2 log2 (nm) − 1 multiwavelength switching stages and 2 log2 (nm) − 2 wavelength interchanger stages. Because of their relation to the Benes switch, these structures are rearrangeably nonblocking. To add another connection on an idle pair of ports and wavelengths, it may be necessary to rearrange active connections. One application of WIXCs is to circumvent connection blocking problems associated with the wavelength continuity constraint. Recall that all optical signals multiplexed on the same fiber in a multiwavelength network must be assigned distinct wavelengths so that they can be distinguished at the receivers without interchannel interference. But the wavelength continuity condition sometimes makes it impossible to satisfy the distinct wavelength requirement, resulting in blocked connections. In these cases a WIXC can be used to resolve the conflict. To illustrate, consider the wavelength-routed network of Figure 2.28(a), operating with two independently routed wavelengths λ1 and λ2 . Two optical connections are active, with optical signal S2 (λ2 ) carrying a connection from station 2 to 4 on wavelength λ2 , and signal S3 (λ1 ) connecting station 3 to 6 on wavelength λ1 . Suppose a new connection is required from 1 to 5. If a wavelength-continuous λ-channel is used for the new connection, neither of the two wavelengths can be assigned to it without causing a conflict. A solution to the problem is shown in Figure 2.28(b). Here, the new connection is carried on signal S1 , with its wavelength changed from λ1 on the first part of its path to λ2 on the last part. The central node of the network is the 2 × 2 WIXC, which was shown previously in isolation in Figure 2.26. The port labels in that figure match the corresponding points in Figure 2.28(b). The applications of WIXCs in more general network settings are discussed in Chapter 6.

Layered Architecture

67

3 WIXC 1

S3(λ 1) 5

W

S2(λ 2) 2

4

6

( a)

3

1

1

S3( λ1)

2

S1( λ 1)

W

S2(λ 2) 2

S1(λ 2)

2

5

1 WIXC 4

6

( b)

Figure 2.28 Wavelength-routed network.

2.4

Network Access Stations The functions in the logical connection, transmission channel, and λ-channel layers of our network architecture, shown in Figure 2.1, are implemented in the NASs. The NAS uses the services of the optical path layer to provide LC services to end systems or electronic switching equipment attached to its external ports. These devices will generally belong to a client network. Thus, the access station is involved in two functions. First, it interfaces the external LC ports to the optical transceivers. Second, it implements (in the transceivers) the functions necessary to move signals between the electronic and optical domains. These functions can become quite complex, especially when the connections are multipoint. In this section we discuss the structure of the NAS, focusing on the functionality required for supporting point-to-point LCs and optical layer signals. Section 4.7 goes into more detail concerning the signal transmission and reception functions of the NAS, and Section 3.2.1 deals with the functions specific to multipoint connections: multiplexing, multicast, multiple access, and demultiplexing. A typical access station is presented in Figure 2.29. The transmitting side consists of a transmission processor (TP) with a number of LC input ports and transmission channel output ports. The output ports of the TP are connected to optical transmitters (OTs), and each transmitter is in turn connected to an outbound access fiber (using a signal combiner or WMUX if there are several transmitters multiplexed on one fiber). The function of the TP is to convert each logical signal to a transmission signal in a form suitable for modulating a laser in the OT. In some cases, for example, on-off keying (OOK), where a binary digital signal turns the laser on and off, there is little or no processing. In other cases, the conversion may itself involve a modulation process at a subcarrier level – for example, subcarrier phase-shift keying (PSK) and frequency-shift keying (FSK) for digital signals and AM or FM for analog signals (see Section 5.2.2). These represent

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Multiwavelength Optical Networks

a

b OT

WMUX

eo

TP a

b OT

Logical Connection Ports

c ONN e

d WDMUX

OR RP

Access Fiber pair

eo d

e OR

Figure 2.29 Network access station.

a second level of modulation preceding the laser modulation operation. Transmission signals are present at points a in Figure 2.29, and optical signals are generated at points b, with the multiplexed optical signals appearing at c. The TP may also perform coding and multiplexing functions. For example, redundancy might be added for forward error correction (FEC) on poor-quality optical connections. Also, the data streams from several incoming LCs may be multiplexed onto a common λ-channel for purposes of grooming or for distribution to several destinations via an optical multicast connection. Assuming that it is tunable, each OT transmits an optical signal on a wavelength assigned by the network manager. The OTs may be fabricated in the form of an integrated array of fixed tuned lasers multiplexed onto the access fiber. If only one laser in the array is active at a time, this combination acts as a single transmitter, tunable over the set of wavelengths generated by the array. Alternatively, assuming that all wavelengths are distinct, the system might transmit simultaneously on several fixed wavelengths, each carrying its own LC. On the receiving side, the optical signal arriving on an inbound access fiber is split or wavelength demultiplexed and the resultant signals are passed on to the optical receivers (ORs). If an OR is not preceded by a WDMUX, it will generally need its own optical filter, which may be tunable to select an assigned λ-channel. The ORs convert the optical power to electrical transmission signals, which are versions of the original transmitted signals corrupted to some degree by noise and other transmission impairments. Additional electronic operations are required in the reception processor (RP) to convert the corrupted transmission signal to one or more logical signals. For example, digital signals may be regenerated to produce a “clean” logical signal (possibly with some bit errors), and analog signals may be filtered to remove noise, distortion, and interference accumulated in the transmission process. Also, the inverse of the various operations executed in the source TP must be performed in the destination RP, including error correction, demultiplexing, electrical demodulation, and so on. In addition, when several incoming signals from different sources are multiplexed on the same λ-channel on

Layered Architecture

Logical Connection [A, B]

NAS

69

NAS B

A TP

RP

a Electrical

Optical Connection (A, B) 1

OT

Optical

e

Transmission Channel

b

1

m

OR

-channel

m

1

WMUX

d WDMUX

c

Optical Path A, B w1

ONN

ONN

w1 w2 ONN

Figure 2.30 Example of a logical connection between two NASs.

the same access fiber (as in many-to-one connections), the RPs and ORs may implement the receiving side of a multiple access protocol (see Sections 3.2.1, 5.2, and 5.7). Finally, signals received and processed in the RP may be reinserted into the network via a TP in the same NAS instead of exiting into the logical layer. In this case the RP/TP pair form a unit that we call an overlay processor. These devices are useful for signal regeneration, wavelength conversion, grooming, and other functions in the optical layer (see Section 2.5). An example of the functions performed by a pair of NASs, A and B, in creating a point-to-point LC is shown in Figure 2.30. An LC, denoted [ A, B] in the figure, originates at one of the external input ports of station A. It may be multiplexed in the TP with other LCs entering at other input ports. The LC is carried on a transmission channel (point a), which modulates a laser in the OT, producing an optical signal carried on a λ-channel at wavelength λ1 , appearing at point b. The OT is the point of origin of the optical connection, denoted ( A, B)λ1 , which carries LC [ A, B] to station B. If there are other OTs in the station, their signals may be multiplexed together, with the combined signals appearing on the outbound access fiber at point c. This point is the origin of an optical path, denoted A, B w1 to indicate that the path is carried on waveband w1 . As shown

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Multiwavelength Optical Networks

in Figure 2.30, the optical path traverses three ONNs and terminates on the inbound access fiber of station B. (Another optical path on waveband w2 , with a different source and destination, is shown traversing the same network nodes.) At the entrance to the station, optical connection ( A, B)λ1 is demultiplexed from any other optical connections that might have entered the station on the same access fiber. The resultant demultiplexed signal (point d) is converted to a transmission channel in the OR and finally undergoes any required additional electronic operations (e.g., regeneration and demultiplexing) in the RP. The logical connection terminates at one of the output ports of station B. In the previous example, a single bidirectional pair of access fibers served to connect the access station to a port on an ONN. (Only the transmitting side of station A and the receiving side of station B were shown.) Other configurations might use multiple pairs of access fibers, each one connecting a transmitter–receiver pair to a separate port on a network node or to separate network nodes. Many variants are possible. An example is the WADM of Figure 2.23 attached to an NAS through several fiber pairs. (This is a natural arrangement when the NAS and WADM are colocated.) One advantage of this configuration is that there is no need for receiver tuning or wavelength demultiplexing because only one wavelength reaches each receiver. Other access arrangements adapted to special network applications appear in Section 3.3.2. In the next two subsections, we discuss the basic functions of the OT and OR. More detailed discussions appear in Sections 4.5, 4.6, and 4.8.

2.4.1

Transmitting Side Figure 2.31 shows the details of the modulation process. (The points a and b correspond to similarly labeled points in Figure 2.29.) Two possible OT configurations are shown. In a directly modulated laser diode (see Figure 2.31[a]), the laser drive current is modulated by the transmission signal s(t) appearing at point a, whereas in an externally modulated transmitter, an electro-optic modulator is driven by the modulating waveform s(t) to vary the optical signal emitted by the laser diode. (The mechanisms, advantages, and disadvantages of direct and external modulation are discussed in Section 4.5.3.) Laser Diode a

Ss(f )

b

s(t) (a) Direct Modulation Laser Diode a s(t)

f

0 Bs Se( ) External Modulation

b 0

s

Be (b) External Modulation

Figure 2.31 Optical transmitter.

(c) Spectrum

Layered Architecture

71

The optical field E(t) of a laser tuned to an optical frequency νs can be represented as the real part of a complex signal:   (2.14) E(t) = Re E(t)e j2πνs t . The complex envelope E(t) can be written in the form  E(t) = 2I (t)e jϕ(t) ,

(2.15)

where I (t) = |E(t)|2 /2 is the instantaneous optical signal intensity (proportional to power P) and ϕ(t) is the instantaneous phase. If the laser is intensity modulated by s(t), we have I (t) = I0 [1 + ms(t)] 0 < m ≤ 1, |s| ≤ 1,

(2.16)

where I0 is the intensity of the unmodulated signal and m is the modulation index. In analog or subcarrier modulation, s(t) is a continuously varying signal, and in digital modulation (e.g., OOK), s(t) is in the form of a sequence of pulses. The phase ϕ contains components due to the complexities of the modulation process (pure intensity modulation is not achievable in practice) as well as random phase fluctuations due to the laser itself.20 If the modulating signal s(t) is restricted to a bandwidth Bs , then s and E have power spectral densities Ss ( f ) and Se (ν), respectively, as shown in Figure 2.31(c). Note that only the positive frequencies of Se (ν) are shown. Recall that in amplitude modulation (as opposed to intensity modulation), the bandwidth of the modulated signal is just twice that of the baseband modulating signal. However, in intensity modulation of a laser, the bandwidth Be of the optical field is considerably more than this because of the square root in Equation (2.15) as well as the extraneous phase and frequency modulation represented by ϕ(t).

2.4.2

Receiving Side There are several common OR structures. The simplest is the direct detection receiver shown in Figure 2.32. In the tunable version shown in Figure 2.32(a), the optical signal is first passed through an optical filter and is then detected by a photodetector (PD) to produce a photocurrent i(t) at point e. (Points d and e in Figure 2.32 correspond to the same points in Figure 2.29.) The spectrum Se (ν) of the input optical field E(t) and the spectrum Si ( f ) of the output photocurrent are shown in Figure 2.32(b). Note that two λ-channels are shown on the access fiber, at optical frequencies ν1 and ν2 .21 The receiver is tuned to select the former by appropriately positioning its optical filter transfer function HOF . (Of course, this selection is possible only if the two optical signal spectra do not overlap.) 20 21

The randomness in the lasing process also produces fluctuations in intensity, called relative intensity noise (RIN; see Section 4.6.4). The effect of RIN is omitted here. The set of optical frequencies that reaches the input of a particular receiver depends on the selectivity (if any) of the splitting/WDMUX device in front of it, as well as the waveband selectivity and setting of the network node. Recall that a waveband-selective switching node directs all signal power in a selected waveband to each desired output port. The waveband may contain a set of several closely spaced λ-channels, from which the desired channels must be selected by the receiver.

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Multiwavelength Optical Networks

PD Signal

d

OF

e

i(t)

(a) Tunable Direct Detection Receiver

Se( ) HOF

0

2

1

Si(f )

f (b) Spectra

PD1 OF1

e i(t)

Signal

Switch

d OFm

e

e PDm

(c) Arrayed Receiver

Figure 2.32 Optical receivers.

Ideally the photodetector, acting as a photon counter, produces a photocurrent that is an exact replica of the instantaneous optical intensity impinging on it; in other words, it acts as a square-law detector of the optical field. More precisely, i(t) = R I (t) =

R |E(t)|2 , 2

(2.17)

where R is the responsivity of the photodetector. Thus, if intensity modulation is used to generate the optical signal E(t), as in Equation (2.16), this receiver will recover the transmission signal s(t). It is important to note, however, that a direct detection receiver recovers no phase information, so it cannot be used to recover optical phase- or frequency-modulated signals.22 22

By appropriately shaping the optical filter transfer function HOF , it is possible to introduce phase and frequency discrimination in front of the photodetector, thereby making it possible to detect optically phaseand frequency-modulated signals; e.g., optical frequency-shift keying and differential phase-shift keying (see Section 4.7.1).

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73

PD

(t) d

i(t)

Coupler LO

x(t) BPF

ED

e

LO (a)

Si (f) HI f fI

0

2

LO

(b)

Sx(f)

f (c)

Figure 2.33 Heterodyne receiver and spectra.

Precision tunable filters (especially widely and rapidly tunable filters) are difficult to fabricate and hence are costly. Another way of producing the equivalent of receiver tunability is to use an array of m photodetectors, each preceded by a fixed optical filter tuned to a different frequency, as in Figure 2.32(c). Using an m × 1 (electronic) switch, an equivalent receiver tunable over the m filter frequencies is obtained (point e ). Without the switch, the resultant system receives m simultaneous signals (point e). As indicated, the direct detection receiver normally cannot recover frequency and phase information. Furthermore, it has less-than-ideal noise discrimination properties. These problems are essentially absent in the coherent heterodyne receiver (see Section 4.8 for noise performance). In principle, heterodyne detection gives the best possible reception and is the most versatile. Modeled after the ubiquitous heterodyne radio receiver, the heterodyne approach is shown in Figure 2.33. The incoming optical signal E(t) at frequency νs is first combined in a passive coupler with a signal at optical frequency νLO , generated by a local oscillator, LO (a tunable laser). The combined signal is detected by the photodetector. Because the photodetector behaves as a square-law device, the photocurrent i(t) contains a component at intermediate frequency f I = νs − νLO , where the local oscillator frequency νLO is chosen very close to the signal frequency, giving an f I at a point in the radio frequency spectrum that is chosen for convenience in subsequent electronic signal processing. The signal i(t) is passed through a bandpass filter (BPF) with transfer function HI centered at f I , and the resultant filtered signal is detected again by an electrical detector to produce the desired output signal x(t).

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Next we show that the heterodyne receiver eliminates the need for a tunable optical filter. Consider the case when the λ-channel at optical frequency ν1 is to be selected from the pair of incoming optical signals at neighboring frequencies ν1 and ν2 . Assuming that the polarizations of the two input signals and the local oscillator signal are all aligned, the combined optical field at the input to the photodetector will be   (2.18) E(t) = Re E 1 e j2πν1 t + E 2 e j2πν2 t + E L e j2πνL O t , where E i (t) = |E i (t)|e jϕi (t)

(2.19)

Ii (t) = |E i (t)|2/2.

(2.20)

and

Now, assuming that the local oscillator is tuned to frequency νLO = ν1 − f I and its power is much greater than the incoming signal power, the photocurrent at the output of the detector is approximately   i(t) ∼ I L + I1 I L cos(2π f I t + ϕ1 − ϕ L ) + I2 I L cos(2π [ν2 − ν L O ]t + ϕ2 − ϕ L ), (2.21) where low-power terms have been neglected.23 The positive frequency spectrum Si ( f ) of the photocurrent is shown in Figure 2.33(b). Note that the phase of the desired signal ϕ1 is present in the photocurrent of Equation (2.21), so that phase- and frequency-modulated information can be recovered at the receiver. The bandpass filter selects the signal centered at frequency f I and rejects the other signal. The final electrical direct detection stage recovers the desired signal x(t) ∼ E 1 (t) at point e, with the spectrum Sx ( f ), as shown in Figure 2.33(c). The heterodyne receiver has three advantages over the direct detection receiver: 1. It has inherently better noise reduction properties, producing better SNRs or, equivalently, BERs. 2. It can demodulate all types of optical signals, including those using frequency and phase modulation. 3. It is tunable without requiring a tunable optical filter. The primary (and significant) disadvantage of this receiver is that it is complex and expensive to build.

2.5

Overlay Processors As indicated in previous sections, an optical WAN eventually “runs out of steam” if all connections are required to be optically transparent end to end. One reason is 23

Equation (2.21) is an idealized expression that ignores the effects of inexact polarization alignment, local oscillator phase noise, and other problems in physical realization.

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75

that the “reach” of an optical connection is limited by transmission impairments such as noise, dispersion, nonlinear distortion, and optical node cross-talk, all of which accumulate along a transparent path. Even when reach is not the issue, there are many cases when some additional functionality in the optical layer serves to enhance network performance. An example is the use of an opaque switch in an otherwise transparent network to achieve the combined benefits of signal regeneration, wavelength conversion, and switching, all in one network element. In all of these situations, some additional (nonlinear) functionality must be inserted at the upper edge of the physical layer (in the transmission channel sublayer of Figure 2.1). We use the term overlay processor (OLP) as a catchall for devices that perform any of these functions. Typically, the OLP interfaces to the optical transceivers but not always to the logical layers. Overlay processing is generally format dependent, so transparency is lost when signals traverse OLPs. A typical network access station containing an OLP is shown as station U in Figure 2.34. Note that the OLP replaces the TPs and RPs, and in the case shown it has no interface with the outside world. (For simplicity only one OT and OR are shown, without the usual WMUX and WDMUX.) The purpose of the OLP is merely to process signals in electronic form at the outputs of the optical receivers and reinsert the processed signals into the network via the optical transmitters. There are many possible functions for such devices. We provide two examples.

Y

λj

U OT OLP OR

λi

X

Figure 2.34 Overlay processor.

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Multiwavelength Optical Networks

2.5.1

Regeneration Consider establishing a logical connection between stations X and Y in Figure 2.34, in which the stations are beyond each others’ reach in the sense that the quality of the signal would be unacceptable if it were transmitted purely optically from X to Y. A solution to the problem is to relay the signal through station U, which contains an OLP whose function is to regenerate the signal so that a clean version of it is reinserted into the network and retransmitted from U to Y. The resultant LC from X to Y is thus carried on two concatenated optical connections. The OLP accepts the signal in electrical form from the OR, detects (reshapes) the bit stream, retimes it, and passes the regenerated version of the signal on to the OT. These are the functions typically performed by a regenerative repeater (see Section 4.11). Of course, this type of regeneration is a highly nonlinear operation, and it is generally confined to signals operating with a well-defined format, say a 1-Gbps digital bit stream carried on a binary intensity-modulated transmission channel. As we have seen, network elements of this type, which combine an optical receiver, some degree of regeneration, and an optical transmitter, are called transponders. If a transponder transmits on a different wavelength than that of the received signal, it is also performing a wavelength interchange operation. Although there may be some finite BER accompanying this process, the degradation due to bit errors is far less than what would be experienced by trying to extend the purely optical path beyond its reach. More elaborate variants of the regenerator OLP might include processors that support format conversion, error correction, and other signal transformations at the transmission channel level.

2.5.2

Wavelength Interchange In routing optical connections through a transparent optical layer containing no wavelength interchangers, wavelength continuity is required end to end. Thus, even if two stations are within reach of each other optically, it may be impossible to make a connection between them if no wavelength-continuous optical path is free to support it without interfering with other connections. (An example of this problem was shown in Figure 2.28, for which optical wavelength interchange was presented as a possible solution.) The equivalent result may be obtained by using a wavelength-interchanging OLP. Referring again to Figure 2.34, suppose a connection is required from station X to Y, but no wavelength-continuous path exists. If paths can be found from X to U and from U to Y, say on wavelengths λi and λ j , respectively, then an end-to-end path can be relayed through U using the appropriate wavelengths. First, an optical connection is established from X to U using λi and from U to Y using λ j . This carries an optical signal from X to U, which is converted to electronic form in the receiver of station U and is passed through the OLP to the OT in the station. The transmitter, tuned to λ j , places the resultant signal on the optical path to station Y. The function being performed in the station is equivalent to wavelength conversion, but it is accomplished in the

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77

electronic domain. Although the operation of the OLP in this case is minimal24 – simply connecting the receiver and transmitter back to back – the effect on network operation may be substantial. As is shown in Section 6.4, significant enhancement of network performance through reduced connection blocking is sometimes achieved by using wavelength interchange in an otherwise transparent optical network. As in the case of regenerative repeaters, wavelength-interchanging overlay processors are format dependent, but in return for this dependence they can provide signal regeneration functions as a by-product of the conversion function. (We have already seen that signal regenerators can perform a wavelength interchange function as a by-product of regeneration.) In large, geographically dispersed transparent networks, enhancement of optical layer performance can be realized by placing a number of NASs equipped with versatile OLPs at strategic points throughout the network. If these stations contain multiple transceivers, they can act as optical layer “servers” available to provide regeneration, wavelength conversion, and possibly other optical layer enhancement functions to several simultaneous connections. The term translucent has been coined to describe networks of this type. These functions are especially important in cases when two or more isolated and independently managed optical networks are to be concatenated by extending connections from one network to the other. In this case the OLP performing network concatenation would have separate access links to the different networks and would be controlled jointly by them both.

2.6

Logical Network Overlays Moving up to the logical layer in the multiwavelength network architecture, there are many possible logical network overlays that can be used for implementing various user services over an optical network. These overlays are typically client networks that will themselves have layered architectures. Without entering into the details of the logical network structure, a generic arrangement for a logically routed network is shown in Figure 2.35. (The rationale for LRNs was explained in Section 1.4.) The hexagons in the figure are logical switching nodes, which communicate with each other through logical links, forming a logical topology superimposed on the physical layer. By showing the logical topology as a collection of nodes and links in the logical layer, we simplify the representation of the logical network, concealing unnecessary detail in the supporting optical network. The end-to-end connections in an LN constitute the virtual connections in the top layer of the architecture of Figure 2.1. A blown-up view of a typical LSN appears in Figure 2.35, which shows a logical switch (LS) interfaced to an ONN in the optical network through an NAS. The LS may also be interfaced to end systems through ports external to the network. An LS without interfaces to end systems performs a transport (essentially routing) function only, whereas one with external interfaces provides user access as well. For 24

The OLP may also perform some linear filtering in the case of analog signals or regenerating in the case of digital signals.

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Multiwavelength Optical Networks

Logical Switching Node

LS NAS

ONN

LSN Logical Layer

Physical Layer

LSN Including NAS

Figure 2.35 Logically routed network.

example, the LN overlay may be an IP network whose LSs (IP routers) are a mix of core routers (without an external interface) or edge routers (with an external interface). In Section 2.3.2.4, we described several versions of optical cross-connects (illustrated in Figures 4.86 through 4.88). Each of these can be used as an implementation of the ONN/NAS combination supporting the LSN of Figure 2.35. Each performs “passthrough” functions, routing optical channels through it, and add/drop functions shunting “local” traffic to and from the attached LS. However, there are differences among the three switches in terms of the granularity of the switching operations and the functions performed on the transiting traffic. The transparent switch performs the routing function at a coarse level of granularity: either wavebands or λ-channels. The O-O-O switch operates at a λ-channel level, and the O-E-O switch may operate at a sub-λ-channel level if it demultiplexes time division-multiplexed transmission channels within a λ-channel before switching them, a common feature of this type of switch. (In this case the O-E-O switch is capable of rearranging these subchannels as part of the routing function for purposes of grooming.) As noted, the opaque switches also perform regeneration and wavelength translation functions. Thus, as far as the transiting traffic is concerned the switches operate as either transparent ONNs or overlay processors, as described in Section 2.5. With respect to the add/drop traffic, each can operate as a full-fledged LSN

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79

belonging to a SONET, ATM , IP, or other LN, as suggested in Figures 4.87 and 4.88. At the logical layer, the LS may perform “edge” functions, terminating virtual connections to end systems, transit functions such as routing virtual connections over logical paths, or other functions such as logical layer grooming, fault recovery, traffic engineering, and so on. By showing the LS interfaced to the physical layer via an NAS we have implicitly assumed that it is electronic. In future networks the LS may be an optical packet or burst switch, in which case it would access the physical layer directly through the ONN (see Chapter 10). Effectively, this moves the boundary between optics and electronics upward to the top of the logical layer of the architecture in Figure 2.1. Each logical link in the LN overlay is realized as a unidirectional or bidirectional logical connection (see Figure 2.2) between a pair of source and destination access stations, and each LC is in turn carried on an optical connection, routed through the fibers on an optical path. (Typically, the optical path may involve several optical hops.) The logical and physical topologies are independent of each other, with each logical link being embedded in a dedicated path in the physical topology. A logical path supporting a virtual connection is reconfigurable by changing the LSN settings in the LN. Similarly, the logical topology is reconfigurable by changing the ONN settings in the physical layer. Thus, if network reconfiguration is required because of changing traffic patterns or equipment failures, it can take place in the logical or physical layer or both. The generic LRN shown in Figure 2.35 may take many specific forms, depending on the structure of the LS. Consistent with current and evolving standards, we now briefly describe four important types of LRNs in use today: SONET networks, ATM networks, IP networks, and MPLS networks.

2.6.1

SONET Networks SONET networks are based on transmission links carrying digital synchronous transport signals (STS) at standard rates that range from STS-1 (51.48 Mbps) to STS-192 (9.95324 Gbps) and higher.25 These signals are supported by corresponding optical carriers, OC-1 to OC-192, that are constructed by scrambling the STS-n signal and converting it to optical form. The basic STS-1 is carried in a 125-µs frame containing transport overhead plus the information payload. Higher rate STS-n signals are formed by byte-interleaving STS-1s. The frame structure is fairly complex, incorporating overhead for both communications and maintenance functions.26 In terms of our generic layered architecture the STS-n (electrical) signal is associated with a logical connection and the OC-n (optical signal) is carried on an optical connection. The logical switch in a SONET network is a SONET digital cross-connect system (DCS) or add/drop multiplexer (ADM). The DCS and ADM play the same roles in the 25 26

In Europe and Asia the North American SONET standard is replaced by a more or less equivalent SDH (synchronous digital hierarchy) standard. One of the most important features of the SONET standard is a comprehensive set of maintenance and protection switching functions implemented using the transport overhead; see Chapter 8, Appendix F, and [Bernstein+04].

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Multiwavelength Optical Networks

logical layer that a WSXC and WADM play in the physical layer. In current SONET networks, the DCSs are interconnected through internodal point-to-point fiber links carrying OC-n signals and are connected to local equipment through electronic add/drop ports. The function of a DCS is to demultiplex, switch (route), and remultiplex the signals with which it interfaces. An ADM performs operations similar to a DCS except that the ADM has only two internodal ports, like the WADM of Figure 2.23. The inter-DCS connections (corresponding to the logical links in Figure 2.35) are operated as dedicated connections, and the signals carried on the internodal links range from STS-1 to STS-48 and higher. At its external local equipment interface, the DCS or ADM typically exchanges digital signals with end systems at DS-1 (1.544 Mbps) or DS-3 (44.736 Mbps) rates. The end systems using these signals may be located in telecommunications carrier Central Offices or on the premises of large users. A DCS or ADM demultiplexes signals on its inbound internodal links down to a low rate (e.g., STS-1 or STS-3), sorts and cross-connects these signals (dropping some signals to end systems), and remultiplexes the cross-connected signals (including signals added from end systems) onto the outbound internodal links. Thus each internodal logical link operating at a high bit rate typically carries many time division multiplexed lower rate signals. The DCS performs a grooming function on these signals as well as a routing function. An end-to-end connection terminating at externally attached end systems will typically traverse several SONET DCSs, resulting in a multiple-hop source-destination path. The SONET network typically provisions and holds these connections over long time periods. In currently operational telecommunication networks the SONET network uses pointto-point optical fiber transmission links for its inter-DCS connections, without the intervention of a reconfigurable physical layer. This is shown in Figure 2.36(a), in which the fibers are interfaced directly to the DCS through its own optical transceivers. In the first implementations these transmission links were single wavelength, carrying only one bidirectional logical link. They were therefore grossly underutilized. However, with the trend toward WDM transmission, the transmission links are now being operated on multiple wavelengths, so they can carry several parallel logical connections either from the same or different electronic equipment. In this case, the link terminating equipment, which interfaces the DCS (or similar equipment) to the fiber transmission link, is called a WDM terminal, wavelength terminal multiplexer, or WDM transport system. Typically, the interconnections between the link terminating equipment and the electronic switching equipment are optical. For example, a SONET DCS might transmit and receive OC-n optical signals in the 1300-nm band on its input/output ports. The output signals from the DCS are connected to WDM terminals via a short-reach fiber. Each terminal then converts these signals to electrical form and remodulates them onto another optical carrier at a wavelength (normally in the 1550-nm range) suitable for longdistance WDM transmission. The reverse operations are carried out at the receiving end of the link transmission link. This arrangement is similar to the O-E-O cross-connect in Figure 4.87. The intermediate connection between the DCS and the short-reach fiber is made via a 1300-nm transceiver, and the connection from the short-reach fiber to the long-haul transmission link requires a transponder of the form previously described in Section 2.3.

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81

DCS

DCS

DCS

NAS

(a)

(b)

WDM Terminal SONET DCS

Figure 2.36 SONET DCS.

One difficulty with this arrangement is that there is no possibility for optically reconfiguring the SONET network, either for load redistribution or for fault recovery. Furthermore, if the equipment interfaced to the WDM terminal does not generate a throughput comparable with the capacity of the transmission link, much of the fiber capacity will be wasted. Finally, each WDM terminal can only connect the DCS to one other switching node. A more flexible and efficient arrangement is shown in Figure 2.36(b). An NAS, together with an ONN (e.g., a wavelength-selective cross-connect), replaces the WDM terminals on the three links in Figure 2.36(a). Now, the logical connections from the DCS can be routed to different destinations under the control of the optical node. By superimposing the DCS on a purely optical network, additional flexibility is introduced, with optical multiplexing and logical network reconfiguration made possible by control in the optical layer. It is interesting to view the functions of the SONET switch in terms of granularity. The signals it exchanges with the optical network are highly aggregated and well adapted to the capacity of the optical transmission channels. Those exchanged with the end systems are typically of a much finer granularity, adapted to the needs of the attached telecommunications equipment. In the DCS, information channels of relatively fine granularity are sorted individually and then reaggregrated for transport on the logical links. This illustrates the use of electronics (in the DCS) to provide the fine-granularity grooming needed for high connectivity and transport efficiency while using optics to carry coarse-granularity traffic at high throughputs.

2.6.2

ATM Networks An ATM logical network is very similar to a SONET network except that the basic entities processed by the LSNs are cells rather than synchronous bit streams. The logical switch in these networks is an ATM (cell) switch. ATM was developed to support broadband services that require a wide range of bandwidths and quality of service.

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Multiwavelength Optical Networks

VPI/VCI,

Data

Header (5 bytes)

Payload (48 bytes)

Figure 2.37 ATM cell format.

It uses the ATM cell as the basic information unit: a fixed-length structure that contains 53 bytes, of which 5 constitute the header and the rest are data (Figure 2.37). The header contains a virtual channel identifier (VCI) and virtual path identifier (VPI), assigned as part of a call setup process, as well as header error control, flow control, and priority and other information. Typically an ATM network is configured in the form of a virtual topology that consists of dedicated virtual paths (VPs), acting as virtual internodal links joining pairs of ATM switches. (Each VP may traverse several ATM switches between its end points.) End users request demand-assigned virtual channels (VCs) to support each application, with the bandwidth and other characteristics of the VC designed to provide the required quality of service for that application. The VPs are sized to carry many asynchronously multiplexed VCs, which are the entities carrying cells end to end in an ATM network. Because the cells are multiplexed asynchronously on the internodal links, the VC and VP information is necessary for sorting and routing individual cells within the ATM switches. Each cell header is read by the switch, and the cell is directed to the output port specified by the VCI, the VPI, and the routing table stored in the switch. As shown in Figure 2.38, the ATM switches are joined together through internodal logical links and are interfaced to end systems through external ports. An end system could be a workstation, a gateway to an access network, a supercomputer, a switch in a client network carried over ATM (e.g., an IP router), or any other device equipped with an ATM interface. As in the SONET case, a typical connection between end systems (carried on a VC) will traverse several ATM switches between source and destination, resulting in a multihop logical path. The internodal links connecting the ATM switches can be realized in various ways. Figure 2.38 shows three possibilities. In Figure 2.38(a), the internodal ports on the switch are connected directly to point-topoint digital transmission links, which might be fiber links terminated with the necessary optical transceivers. These links form a logical topology that cannot be reconfigured without physically changing the connections. (This is analogous to the SONET DCS network connection of Figure 2.36[a].) An alternative is to use a hybrid ATM over SONET structure as shown in Figure 2.38(b). Now the ATM switch acts as an end system attached to the external ports of a SONET DCS, so the ATM network is a client network of the SONET network and each ATM internodal logical connection is carried on a SONET STS routed through the DCS to another ATM switch. (The network of ATM switches may represent just one

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83

ATM ATM

ATM

NAS

S

(a)

(b)

(c)

ATM or ATM ATM switch S

SONET DCS including optical transceivers

Figure 2.38 ATM switch connections.

of many client networks served by the SONET infrastructure, and the SONET network may in turn be one client LN supported by a larger optical network infrastructure.) In the latter case reconfiguration is possible at the SONET level by changing the routing within the DCS, and in the physical layer of the optical network by reconfiguring the OXCs. Figure 2.38(c) shows a third alternative, in which the ATM switch accesses an optical network directly through its own NAS. This is analogous to the SONET DCS configuration shown in Figure 2.36(b). As in the SONET case, a logical topology for the ATM network is constructed by creating the desired logical connections among the NASs, and reconfiguration is possible through control in the optical layer.

2.6.3

IP Networks An IP logical network is a packet-switched LN where the information units are IP packets and the LSNs are IP routers (packet switches). IP packets are of variable length and generally much longer than an ATM cell. The function of an IP router is to examine incoming packets, determine destination addresses, compare them with the contents of a routing table stored in the router, and then forward the packets to the appropriate output interface. As mentioned, logical networks typically have their own layered architectures. The Internet protocol stack, called TCP/IP, is shown in Figure 2.39,27 for comparison with the Open Systems Interconnection (OSI) Reference Model. Layers 2 and 3 in these architectures concern us the most. Layer 2, called the network interface layer in TCP/IP 27

The Transmission Control Protocol (TCP) is at level 4 in the figure and is used for reliable transport over an end-to-end path that may consist of multiple links, and the Internet Protocol (IP) comprises layers 2 and 3.

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Multiwavelength Optical Networks

7 Application 6 Presentation

Application

5 Session

4 Transport

4 Transport

3 Network

3 Internet

2 Data link control

2 Network interface

1 Physical OSI

TCP/IP

Figure 2.39 TCP/IP and OSI.

and the data link control (DLC) layer in OSI, has the function of ensuring reliable and ordered delivery of data frames over a point-to-point link. It uses the services of the physical layer, which provides for transmission of the underlying bit stream on a pointto-point physical medium such as a wire or fiber. Layer 3, the Internet layer in TCP/IP and the network layer in OSI, is concerned with routing – in our case, routing in the logical layer of Figure 2.35. As we consider aspects of network control in later chapters, these lower layers of the protocol architectures will reappear in the discussions. The Internet is a highly dynamic structure composed of many individual interconnected IP networks with frequently changing topologies. Thus, one of the fundamental capabilities of IP routers is the discovery of the current network topology and the creation of routing tables that adjust themselves automatically to changes in network topologies caused by link and equipment modifications and failures and by addition or deletion of nodes, fibers, or wavelengths between routers. Networks of IP routers can be configured over optical infrastructures using the various options suggested previously for SONET and ATM networks, with the additional option of running the IP network as a client on top of ATM switches and/or SONET DCSs.

2.6.4

MPLS and Its Extensions As the number of users, the traffic volume, and the variety of high-speed real-time services increase on IP networks, efforts are expanding to make the IP protocols and other packet- and cell-switched systems more effective. In IP networks in particular, the processing requirements together with the growing demand for service differentiation present a challenge. A development that attacks these problems is Multiprotocol Label Switching (MPLS). It is a method of streamlining the processing of packet- and

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85

cell-switched protocols within the logical network, enforcing specialized treatment of differentiated services by implementing CoS and QoS requirements and providing a tool for optimizing network performance. MPLS manages flows by appending simple fixed length labels to packets, which are used in label-switching routers (LSRs) for expediting packet forwarding. The labels are assigned during a call setup process much like that used for ATM. A label on a packet inbound to an LSR is compared to a routing table that makes a forwarding decision and at the same time swaps the inbound label for an outbound label. Thus, the sequence of labels and the LSR routing tables define virtual paths for the packets. This label-switching procedure, reminiscent of those used in the earliest packet-switched data networks of the 1970s [Tymes81], facilitates efficient packet processing and generalizes to many other applications. One of these is GMPLS, which adapts the control functions in MPLS to other types of networks, including MWNs (see Chapter 9 for more details). Label switching is also being tried in optical packet-switching networks (see Chapter 10). We explore logically-routed network (LRN) structures in considerable detail in Chapter 7. As shown there, LRNs can be constructed with logical connectivity that is more general than a collection of point-to-point logical connections. Through the use of optical multicast connections in the underlying physical layer, logical hypernets can be built with logical topologies that consist of multipoint logical connections. Hypernets are ideal structures for supporting multicast virtual connections, which are becoming increasingly important in many Internet and multimedia applications.

2.7

Summary Having presented the essential resources of the networks being explored in this book, we can now summarize their relationship to network functionality. Figure 2.40 shows a taxonomy of multiwavelength networks, classified according to r r r r r

Physical topology Optical connectivity Optical node functionality Station and overlay functionality Network architecture

The physical topologies break down into trees (no alternate paths exist) or a general topology (providing alternate paths and hence better response to changing loads and faults). The optical connectivity may be static or controllable, depending on the functionality of the optical network nodes. In the simplest case, the nodes are static coupling devices with no spectral selectivity. Better performance, including wavelength reuse, is achieved with static wavelength- and waveband-selective nodes, and the most versatile networks contain controllable nodes, ranging from space switches to waveband-spacetime switches, wavelength-interchanging cross-connects, and optical packet switches. The space and waveband switches may operate in a simple permutation mode or may provide generalized switching functions to support multipoint optical connectivity.

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Multiwavelength Optical Networks

Physical Topology

Tree

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Optical Connectivity

Static

Controllable

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Static Coupler

Station and Overlay Functionality

Network Architecture Broadcast/ Select

Wavelength Router

Space Switch

w-s, w-s-t Switch

Permutation

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LDC

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Overlay Processor Regeneration, Wavelength Conversion

TP/RP MUX/MA MAC/PS

Transceiver Fixed/Tunable/ Arrayed

WIXC

OPS

Logical Switch

Logically Routed Linear Lightwave

Multihop

Hypernet

Optically Packet, Burst, Label-Switched

Figure 2.40 Taxonomy of multiwavelength networks.

The station and overlay level in the taxonomy of Figure 2.40 includes equipment ranging from optoelectronic to purely electronic. The functionality of this equipment – the NASs, overlay processors, and logical switches – complements the capabilities of the resources in the optical network nodes. Optoelectronic station equipment ranges from a single fixed-tuned optical transceiver to multiwavelength transmitter and receiver arrays, with the latter configuration providing multiple simultaneous optical connections. In addition, the electronic TPs and RPs in a station may be equipped to execute a wide variety of functions that increase the communication efficiency, fidelity, and connectivity of the optical connection. Electronic multiplexing/demultiplexing in the TP/RPs can be used for grooming low-granularity channels onto higher rate transmission channels. FEC can be implemented in the TP/RPs to improve the bit error rate on a logical channel. In a network with a controllable optical path layer, a multipoint connection can be constructed through the combined efforts of the TP/RPs residing in the NASs and generalized switches residing in the ONNs. First, a multipoint optical path is set up by using splitting/combining functions in the generalized switches; then a multipoint optical connection is created on the optical path through appropriate multiplexing/multiaccess functions in the TP/RPs. A particularly flexible means of operating multipoint connections is through packet switching in the optical layer, implemented through a media access control (MAC) protocol executed within the TPs and RPs. Network access stations might also contain overlay processors executing signal regeneration to extend optical reach and wavelength interchange to improve network

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performance. They might also contain some simple opaque switching equipment to achieve regeneration, wavelength interchange, and grooming in a single integrated subsystem. The OLP may be viewed as a “shim” slid in between the physical layer and the logical layer of Figure 2.1(a). It has neither the transparency of the lower optical layers nor the functionality of the upper logical layers. Full logical layer functionality is achieved by the (electronic) logical switch (e.g., DCS, ATM switch, or IP router) included at this level of the network equipment. (Opaque optical switches, which have not been explicitly shown in the taxonomy, would lie somewhere between the optical node level and the overlay level.) Network architectures built on these components may be purely optical and transparent, containing only the transparent physical layer in Figure 2.1(a), they may be opaque, containing one or more opaque OXCs, or they may be hybrid, containing one or more logically routed network overlays. The transparent networks range from the simple broadcast star, with a tree topology and containing no control in the ONNs, to wavelength-routed and linear lightwave networks, with general topologies and substantial controllability. In wavelength-routed networks, each optical connection is point-to-point and is routed independently through the network on its own λ-channel. In the linear lightwave network, multicast optical connections that provide the necessary optical infrastructure for hypernet LRNs are supported. In optical networks containing opaque OXCs signals are regenerated in the ONNs, which means that wavelength translation is possible, and the network is no longer transparent to speed and formatting of optical signals. Furthermore, overhead information in the data streams can be used for control and management. Logical switching in hybrid networks equipped with electronic overlays provides additional intelligence, greatly increasing the potential connectivity of the network. These networks may be based on point-to-point logical connection topologies, in which case they are called multihop networks, or they may have hypernet structures, which achieve greater connectivity by combining logical switching in an electronic overlay with multipoint optical connections in the physical layer. As discussed in Section 2.6, it is also possible to implement logical switching in a purely optical but nontransparent architecture using optical packet, burst, or label-switching techniques. In this way, the connectivity benefits of logical switching are combined with the throughput advantage of optical processing. Subsequent chapters explore each of these major classes of networks in detail.

2.8

Problems 1 List and discuss the factors that limit channel spacing in wavelength-routed networks and waveband-routed networks. 2 (a) (b) (c)

What are the factors limiting throughput in the following types of networks: A star coupler-based LAN A purely optical wavelength-routed WAN A WAN consisting of electronic switches joined by point-to-point WDM links

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3 For the 8 × 8 Benes switch: (a) If all 2 × 2 switching elements are in the bar state, determine the connection state of the switch. (b) Find a switch setting (a set of device states) to produce the following connection state: Input

Output

1 2 3 4 5 6 7 8

7 5 4 8 1 2 6 3

4 Compare the power distribution rule for the star coupler with that of the folded bus. (Assume in both cases that there are no excess losses in the devices or attenuation in the fibers.) (a) Show that in the 16 × 16 star coupler of Figure 2.9 each output port receives the sum of the powers entering at all input ports attenuated by a factor of 16. (b) Show that in a folded bus with 16 stations, configured to act as a star coupler, the best one can achieve is an attenuation factor of 256. (Hint: See Section 4.10.2.) (c) Generalize this to the n × n case. 5

Prove that an n × n Benes switch uses n log2 n − n/2 binary elements.

6 Compare the number of cross-points (binary switching elements) for the n × n crossbar and Benes switches with log2 (n!) for n = 4, 8, 16. 7 Compare the number of cross-points in a 16 × 16 strictly nonblocking Clos switch to those in the same size Benes switch. Assume that the parameters in the Clos switch are p = k = 4 and that each smaller switch is realized as a crossbar. 8 Consider the 4 × 4 P-I crossbar switch in Figure 4.73. The switch fabric consists of waveguides placed at 45◦ angles to the horizontal. Each 2 × 2 switching element can be in either the cross state, where no connection is made between the two waveguides crossing it, or the bar state where the waveguide connection is made whose direction is indicated by the arrow on the switching element. Show by example that this is a wide-sense nonblocking switch; i.e., find a set of switch states that will block the switch. Then, give an algorithm for routing connections through the switch in a nonblocking manner. Generalize this to the n × n case. 9 This problem concerns cost trade-offs in WSS realization using either the threestage architecture of Figure 2.21 or using MWSs as the elementary switching devices. Assume that the device costs are Ccc for a 2 × 2 controllable coupler (or on/off device),

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mCmux for an m-wavelength MUX or DMUX, nC p for an n-fold passive splitter or combiner, and mCmws for an m-wavelength 2 × 2 MWS. (a) For a four-waveband 8 × 8 permutation switch using a Benes fabric, find the range of values of Cmws over which the MWS realization is more economical than the three-stage realization. Express your answer in terms of the other cost parameters. (b) Repeat the previous part for a generalized switch fabric of the type shown in Figure 2.18. (c) Generalize the previous results to m-waveband n × n switches. 10 Consider a 48 × 48 permutation switch connecting three cables, each containing 16 bidirectional fiber pairs. (a) Indicate how many connection states are required, propose a possible strictly nonblocking switch design based on binary switching elements, and indicate how many binary elements it uses. (b) Now, suppose that the switch connections are required to be made only on a fiber-tocable basis without loopback connections. That is, each inbound fiber in one cable must be connected to an outbound fiber in a prescribed (different) cable, but it does not matter which outbound fiber is used. Again, indicate how many connection states are required, propose a possible strictly nonblocking switch design, and indicate how many binary elements it uses (hopefully fewer than in the previous case). In counting connection states, do not distinguish between different fibers in the output cables. 11 Show how the physical constraints of the various switch fabrics lead to Equations (2.8), (2.9), (2.10), (2.11), and (2.12). 12 In the WIXC of Figure 2.27, assume that the inputs are as shown in the figure. Find the device states (i.e., the settings of the three WSXCs) to send S 2 (λ1 ) and S 3 (λ2 ) to the upper output port and S 1 (λ1 ) to the lower output port. 13 Show a design for a 2 × 2 four-wavelength WIXC using five 2 × 2 WSSs and four wavelength interchangers. 14

List as many functions as you can for OLPs, and suggest realizations.

15

Invent some new forms of LRN overlays on optical networks.

16 Discuss some of the problems of managing and controlling a network that is independently reconfigurable at several different layers: virtual, logical, and physical. For example, how should fault recovery be managed? How should reconfiguration be managed in the face of congestion?

Bibliography [Antoniades+96] N. Antoniades, K. Bala, S. J. B. Yoo, and G. Ellinas. A parametric wavelength interchanging cross-connect (WIXC) architecture. IEEE Photon. Technol. Lett., 8(10):1382– 1384, 1996.

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[Barry+93] R. A. Barry and P. A. Humblet. Latin routers: Design and implementation. IEEE/OSA J. Lightwave Technol., 11(5/6):891–899, 1993. [Benes65] V. E. Benes. Mathematical Theory of Connecting Networks and Telephone Traffic. New York: Academic Press 1965. [Bernstein+04] G. Bernstein, B. Rajagopalan, and D. Saha. Optical Network Control: Architecture, Protocols, and Standards. Boston, MA: Addison-Wesley 2004. [Hinton93] H. S. Hinton. An Introduction to Photonic Switching Fabrics. New York: Plenum Press 1993. [Hui90] J. Y. Hui. Switching and Traffic Theory for Integrated Broadband Networks. Norwell, MA: Kluwer Academic Publishers 1990. [ITU-T00] ITU-T. Generic Functional Architecture of Transport Networks. ITU-T Recommendation G.805, 2000. [ITU-T01] ITU-T. Architecture of Optical Transport Networks. ITU-T Recommendation G.872, 2001. [ITU-T02a] ITU-T. Spectral Grids for WDM Applications: DWDM Frequency Grid. ITU-T Recommendation G.694.1, 2002. [ITU-T02b] ITU-T. Spectral Grids for WDM Applications: CWDM Frequency Grid. ITU-T Recommendation G.694.2, 2002. [Sharony94] J. Sharony. Architectures of Dynamically Reconfigurable Wavelength Routing/ Switching Networks. Ph.D. thesis. Department of Electrical Engineering, Columbia University, New York, 1994. [Smith+76] D. G. Smith and M. M. Rahmnekhan. Wide-sense nonblocking networks, and some packing algorithms. In Int. Teletraffic Cong., pp. 542-1–542-4, Melbourne, Australia, 1976. [Tymes81] L. R. Tymes. Routing and flow control in TYMNET. IEEE Trans. Commun., 29(4):392–398, 1981. [Wu92] T.-H. Wu. Fiber Network Service Survivability. Norwood, MA: Artech House 1992. [Yoo+95] S. J. B. Yoo, C. Caneau, R. Bhat, and M. A. Koza. Wavelength conversion by quasiphase-matched difference frequency generation in AlGaAs waveguides. In Proceedings of the IEEE/OSA Optical Fiber Commun. Conf., Paper PD14-2, San Diego, CA, February 1995.

3

Network Connections

The multiwavelength network architecture described in Section 2.1 contains several layers of connections. By exploiting the various alternatives in each layer, it is possible to produce a rich set of transport network configurations. This chapter explores how a desired connectivity pattern can be established using the combined functionality contained in the various layers. The approach is to examine the properties of different classes of networks through a sequence of simple illustrative examples. The design objective in each example is to provide a prescribed connectivity to a set of end systems. Each of the network classes illustrated in this chapter is discussed in more detail in later chapters, as is the issue of optical network control. Our first example is shown in Figure 3.1. Five geographically dispersed end systems are to be fully interconnected by a transport network, which is to be specified. The end systems might correspond to physical devices such as supercomputers that interact with each other, or they may be gateways (interfaces) to local access subnets (LASs) serving industrial sites, university campuses, or residential neighborhoods.1 In all of these cases, a dedicated set of connections is desired (shown as dashed lines in the figure), providing full connectivity among all the sites. Figure 3.2(a) shows one possible transport network, whose physical topology (PT) is a star, in which the central node is a star coupler of the type shown in Figure 2.7(a). Each end system is connected to the star through its own network access station. Full connectivity requires 20 unidirectional logical connections. The logical topology (LT) for the network is shown in the logical connection graph (LCG) in Figure 3.2(a), where each link represents a pair of unidirectional logical connections carrying signals in opposite directions. A bipartite representation of the LCG is also shown, in which transmitting and receiving stations are separated, so an arc from vertex i to j represents an LC originating at station i (the left vertex) and terminating at station j (the right vertex). Each network access station interfaces four pairs of logical connections to its attached end system to produce the fully connected logical topology shown in the figure. The LT shows the connectivity realized in the transport network as “seen” by the end systems but suppresses the details of the underlying physical layer. When a set of LCs is realized over a given transparent optical infrastructure, the logical topology is said to be embedded in the supporting PT. In the case at hand, the LT is a set of links that connects directly all pairs of end systems. The burden of supporting full logical connectivity is borne completely 1

A common form of LAS is the passive optical network (PON), whose gateway to the larger network (the end system in Figure 3.1) is known as an optical line terminal (OLT). See Section 5.7 for details of PONs.

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Figure 3.1 End systems: full connectivity.

1 1

2 3

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3 (a)

1 1

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1 PS

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Figure 3.2 Star physical topology.

1

1

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1

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2

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LCG

Figure 3.3 Bidirectional ring physical topology.

by the optical network. This requires multiplexing of many optical connections on each fiber, together with replication of optical and/or electronic equipment within the NASs to support multiple connections. If WDM is used on the star, each LC requires a distinct wavelength and its own transmitter/receiver pair. (No spectrum reuse is possible; see Section 3.2.) Thus a fully connected LT requires four optical transceivers in each station, with each transmitter operating on a different wavelength, for a total of 20 wavelengths. Another possible realization, in the form of a transparent optical bidirectional ring containing wavelength-routing optical network nodes, is shown in Figure 3.3. Each end system is connected through its own NAS to an ONN. (The bidirectional NAS-ONN combination can be implemented in the form of a pair of unidirectional WADM-NAS structures of the form shown in Figure 2.23.) This time if we use WDM it turns out that only three or four wavelengths are needed to support the 20 logical connections,2 indicating that a substantial degree of spectrum reuse is possible if wavelength routing is employed. As a third option, returning to the simpler star network, we can reduce the connectivity burden on the physical layer by inserting a logical switching node (LSN) between each end system and its NAS as shown in Figure 3.2(b). This provides a switching function at the electronic level, which will share the task of providing full connectivity. (Another alternative for reducing the connectivity burden uses packet switching in the optical layer, as illustrated in Figure 3.2(c). It will be discussed in Section 3.2.2.) To fix ideas, let us assume that the LSNs are IP edge routers as described in Section 2.6. Their function is to route data in the form of IP packets among the attached end systems. I/O ports on the routers are connected to NASs on the network side and to the end systems on the user side. A possible LCG and its bipartite representation for this case is shown in Figure 3.2(b). Only five LCs are present now, requiring a total of five wavelengths and one optical transceiver per station and producing a logical topology in the form of a unidirectional ring connecting the five LSNs. This is a simple example of an IP over WDM network. It is now the LSNs and their interconnecting logical links that are seen by the end systems as their transport network, in contrast to the cases of Figure 3.2(a) or Figure 3.3, in which the transport network was purely optical. So far we 2

The exact number of required wavelengths depends on the stations’ access connections (see Section 3.3).

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do not have complete connectivity among the end systems. This is provided by moving up to the virtual connection layer of Figure 2.1(a) and superimposing a set of 20 virtual connections on the logical topology of Figure 3.2(b) to form a fully connected virtual topology that corresponds to the dashed lines in Figure 3.1. Note that the virtual connections must be supported by logical paths consisting, in most cases, of more than one logical hop. The network of LSNs realizes the virtual connections by sorting and forwarding the cells according to their destinations. If the LSNs are IP routers, these operations are executed by reading the packet headers and directing the packets to the outbound port on the router specified in its routing table. This is a simple example of a transparent optical network with a logically routed or multihop electronic overlay. In this case the physical layer is required to support only five logical links, with the remaining connectivity supplied electronically in the overlay. In comparison with the purely optical star, the wavelength requirement has been reduced by a factor of 4. However, many virtual connections must be now multiplexed on each logical link. This logical layer multiplexing (and switching) is possible because of the intelligence in the LSNs. Each logical connection is now shared by 10 multiplexed virtual connections (why?), so that the total capacity of the network is reduced to 10% of the capacity available in the case of the bidirectional ring (see Section 3.5). It is important to note that in each of these examples, the logical topology is independent of the physical topology into which it is embedded. Furthermore, it is generally reconfigurable and therefore adaptable to changing conditions. For comparison, consider traditional (opaque) networks made up of point-to-point transmission links (optical or otherwise) that interconnect LSNs. The logical topologies of these networks are identical with their physical topologies and therefore are frozen once the transmission links are in place. A commonly used physical configuration is a bidirectional ring, which superficially resembles the transparent optical ring of Figure 3.3.3 The nodes in the traditional ring are logical switches, so the logical topology resembles that of Figure 3.2(b) (except that the logical ring is generally bidirectional). In contrast, the transparent optical ring of Figure 3.3 is configured to produce a fully connected LT, a topology that could be reconfigured by changing settings in the ONNs and NASs without reconnecting fiber links. It should now be clear that there are many possibilities for realizing connectivity in multiwavelength optical networks. The resultant logical and virtual topologies are generally reconfigurable and offer many opportunities for trading cost and performance. The remainder of this chapter illustrates how connectivity is created in each layer, focusing on relations among physical constraints, device functionality, connectivity, and throughput. To avoid unnecessary complications, the exposition proceeds from simple to complex classes of networks based on the level of controllability in the network nodes: r r r r 3

Static (broadcast-and-select) networks Wavelength-routed networks Linear lightwave networks (LLNs) Logically-routed (hybrid) networks (LRNs) Bidirectional rings based on point-to-point optical fiber links are used commonly in many networking applications, the most common being SONET rings.

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These four categories correspond to the network classes shown from left to right in the taxonomy of Figure 2.40. The static category comprises transparent optical networks based on static network nodes with no wavelength selectivity; i.e., single-waveband networks. (We are arbitrarily excluding wavelength selectivity of the type present in static wavelength routers to simplify our network categorizations.) Any signal introduced into such a network propagates along all possible paths, tending to “flood” the network, and eliminates possibilities for reusing the optical spectrum on different fibers. For all practical purposes, this limits the PT to a tree (or a collection of separate trees) and eliminates the function of the optical path sublayer in Figure 2.1(a). Hence the size, throughput, and flexibility of static networks are limited. (They are typically used as LANs.) The only way to support many simultaneous connections in these networks is through optical multiplexing, multicast, and multiaccess techniques. The basic features of this approach are discussed in Section 3.2. By adding either static or dynamic wavelength selectivity to the network nodes, we obtain wavelength-routed networks (WRNs), wherein spectrum reuse and hence improved performance becomes possible through the use of appropriate connection management. These networks normally have more general physical topologies. Wavelength-routed networks are discussed in Section 3.3. Consistent with the way in which typical WRNs are operated, the discussion there is limited to point-to-point optical connections, which can be realized in networks equipped with wavelengthselective cross-connects. In Section 3.4 the discussion is extended to LLNs, which are a generalization of wavelength-routed networks, characterized by the fact that their optical nodes are waveband-selective linear divider-combiners (LDCs) rather than wavelengthselective permutation switches. The properties of LLNs are more general than those of wavelength-routed networks, in two respects: 1. They are waveband rather than wavelength routed. In other words, their spectrum partitioning can be visualized as shown in Figure 2.5(c) rather than Figure 2.5(b). 2. They support multipoint optical connections in addition to point-to-point connections. The multipoint optical switching capability of an LLN offers a wealth of connection alternatives. Among them is the possibility of tying subsets of NASs together into fully connected multipoint subnets (MPSs). This is a particularly effective way of creating high connectivity in large networks, as well as providing a transparent optical support adapted naturally to logical multicast connections. Section 3.5 introduces LRNs, constructed using electronically switched overlays supported by a transparent optical substructure. Networks with two types of logical topologies are described: those in which the basic logical link is (fixed capacity) point-topoint and those with multipoint logical links. We call the latter category hypernets. Although point-to-point logical topologies (multihop networks) can be implemented over wavelength-routed optical networks, hypernets require an LLN physical layer to support their multipoint links. The shared-capacity multicast connections in the logical layer are supported by MPSs in the physical layer. The chapter concludes with a summary and comparison of the different approaches to realizing network connectivity.

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To set the stage for the discussion of these different network structures, we must have at least a rudimentary idea of how connections can be managed. Management and control issues in large networks encompass an enormous range of critical functions, some of the most important being: r Fault management (including fault detection and service recovery) r Performance management (monitoring the performance of the various network elements and connections) r Configuration management (provisioning and reconfiguration of network connections) Associated with these are signaling and many other operations, administration, and maintenance functions. These topics are still in their embryonic form for optical networks and must evolve as the networks themselves evolve (see [Bernstein+04, Maeda98, Wei+98]). The international standards bodies break down the generic architectural issues in traditional networks into three parts by defining a transport plane, a control plane, and a management plane, and this approach is now being applied to optical networks. The transport plane encompasses equipment associated with data transfer, the control plane is concerned with the operations required for connection provisioning and reconfiguration, and the management plane deals with the myriad other operations administration and maintenance functions required to ensure satisfactory network performance, including performance monitoring, fault diagnosis, statistics gathering, and so on. In the next section, we briefly address the issue of connection management and control; that is, we focus on the functions of the control plane. The objective is to provide some understanding of what operations must take place to provision connections in an optical network, and how these operations might be implemented.

3.1

Connection Management and Control In any large network, connections do not remain static. In the case of demand-assigned connections, users become active, request specific connections, and then disconnect. In the case of dedicated connections, even though the intention is to retain a fixed connection configuration, changes in traffic conditions or network faults may dictate changes in routing, wavelength assignment, and bit rates while the connection is in progress. Our repeated use of the term connection implies that information flow is normally connection oriented in the networks we are discussing. Any connection-oriented network requires a system for connection management. Our layered view of the network allows us to look at optical, logical, and virtual connection management as three (almost) separate issues. At each layer, the protocols for connection management consist of three phases: connection establishment, information transfer, and connection release. In the most elaborate case, exemplified by the virtual, logical, and physical layers in the network of Figure 3.2(b), these phases must occur within each layer. Typically, the higher the layer, the more frequent are the connection changes. Thus, the optical connections would normally be provisioned on a dedicated basis, with changes

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made only for fault recovery. The logical connections might also be fixed perhaps with some modifications to support changing load distributions. On the other hand, most of the virtual layer connections would be demand assigned. Looking at the connectivity from the top down, suppose Figure 3.2(b) represents an ATM layer that serves as a logical network acting as a transport network for an IP network with its own virtual topology. (This is a case of two stacked logical layers over the physical layer: IP over ATM over WDM.) The logical nodes in Figure 3.2(b) are ATM switches, and the end systems are IP routers. The ATM logical topology is configured to act as a server for the IP client network, whose virtual topology is carried on ATM virtual connections. A connection between IP routers is realized on a virtual channel carried on a specified virtual path in the ATM network. To support the expected traffic demand, the ATM network in this example is configured on a dedicated LT (in this case a unidirectional ring) capable of carrying the anticipated load. This is set up when the ATM network is initialized, at which time the appropriate LCs are created using the available resources in the physical layer. (In larger networks, additional NASs providing services to other client layers might access the same optical infrastructure.) If traditional point-to-point transmission links were used between the ATM switches, the logical and physical topologies of the ATM network would be identical, so initialization of the logical network would be straightforward. In the point-to-point case, once the logical topology has been chosen, ports on the switches (LSNs) are interconnected accordingly using point-to-point links that are either owned by the network operator or leased from another carrier. However, creation of an arbitrary target LT over a transparent optical network is a more complex operation – an inevitable consequence of the many connection alternatives available in the physical layer.4 Now let us focus on connections within the physical layer of a WRN. Optical connection establishment consists of setting up a path through a sequence of optical network nodes for an assigned wavelength and then tuning source transmitter and destination receiver(s) to the chosen wavelength.5 These are the operations occuring in the optical path and λ-channel sublayers, respectively. Thus, connectivity at this level is realized through the combined action of the optical transceivers and the network nodes. For demand-assigned connections, connection establishment and release involves signaling between the party initiating the action and a network manager, followed by issuance of routing and wavelength assignment commands from the network manager to the network elements (stations and network nodes) involved in the connection. (In the case of 4

5

When transmission links are leased from another carrier, our optical network becomes a client network and the carrier’s network is the server network. Now, there are similar connection alternatives in the physical layer, but they are handled by the carrier providing the links. The operator of the logical network is normally shielded from this part of the problem, in effect isolating management of the logical layer from that of the physical layer. At this point, it is assumed that transceiver tuning remains fixed for the duration of a connection. In later sections, we consider multipoint connections involving rapid retuning of transceivers during the information transfer phase of the connection.

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Control Plane

Control domains Control agent Client network node (LSN) Optical network node (ONN)

Figure 3.4 The control plane in an optical network.

dedicated connections, the various connection provisioning operations would usually be executed manually off-line.) As networks get more complex, with multivendor equipment and with multiple optical network providers interacting with each other, a high-level view of the network as viewed from the control plane has become essential. Figure 3.4 shows an abstract view of the control plane in an optical network consisting of two control domains. The two domains might represent networks owned by different providers or subnets owned by the same provider but controlled separately. The optical network, consisting of ONNs and optical links, is a server for a higher layer logical network, represented by the three client nodes (e.g., IP routers and/or ATM switches), which we represent using our generic LSN symbols. The control plane is shown in the form of interconnected control plane agents, which are abstractions of the control functions that take place in both the client (logical) and server (optical) networks. The realization of these agents may take many forms, from controllers physically associated with each network element (a distributed control approach) to a single controller remotely linked to several network elements (centralized control approach). The associations between the control plane agents and the networks and network elements they control are indicated by dashed lines in Figure 3.4, and the various interactions between control agents are represented by solid lines. Network control requires message exchange among control plane agents through a dedicated Data Communications Network (DCN) using some standard signaling protocol. The DCN may be supported by the optical network being controlled using separate

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Control Processor ONC

Controller Access Station

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Switch Control Interface Optical Switch

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B

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Figure 3.5 Connection management system.

control channels or by an independent infrastructure, and it may be used to support communication with other network management systems as well. We relegate a more comprehensive treatment of general network control to Chapter 9 and continue here with a discussion of the specific control functions that must be performed in the optical network elements. The exact form of the connection management system (e.g., either centralized or distributed) is not important at this point, nor are the details of the messaging system (e.g., either supported by the information-bearing fibers or realized as a separate data network). To fix ideas, Figure 3.5 shows one way the connection management system might operate. The connection management function is divided into local ONN control functions and an overall network connection management function. Each optical network node contains an optical switch (e.g., a WSXC) together with a control agent: an optical node controller (ONC) that executes the local control functions. An optical network manager (ONM) disseminates commands for switch reconfiguration. In this example, the ONM is attached through an access link to node C of the optical network. It consists of a network management processor accessing the optical network through an NAS. The ONM might be part of a larger network management system (NMS) encompassing logical and virtual layers as well as the optical layer, in which case it would interact

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with the NASs, the ONNs, and possibly higher layer NMSs to receive optical connection requests and supervise their execution. In this example, we assume that the ONM, the various ONCs, and all NASs intercommunicate via a DCN multiplexed onto the information-bearing fibers.6 For simplicity we assume here that a single dedicated wavelength shared among all network elements is used for supporting control signals. As shown in Figure 3.5, the ONC is composed of an access station connected optically to a port on its switch through an access fiber pair and a control processor connected electrically to the switch control interface. Commands received from the ONM are interpreted in the control processor of the ONC, which issues electrical commands to configure the switch as required. (The ONC might also send information back to the ONM regarding performance monitoring. Parameters to be monitored might include switch status, power levels of signals transiting the switch, faults on the links, and so on.)

3.1.1

Optical Connections Suppose station 1 in Figure 3.5 wishes to set up an optical connection to station 3. Using the signaling network it sends a connection establishment request to the ONM, which determines an optical path, say the one-hop path from node A to B, and selects an available wavelength. The ONM then signals nodes A and B to set up the correct path for the assigned wavelength and signals stations 1 and 3 to tune to that wavelength. The information transfer phase can then begin. Connection release would normally be initiated by a signal from one of the communicating stations to the ONM, which would then signal all participating network entities to release the connection. Note that if the network nodes contain generalized switches, then optical multicast paths can be set up in the form of trees, wherein the optical signal power is split, with a portion reaching each destination station. For example, station 1 can multicast to 2 and 3 through a tree rooted at the source node ( A in this case). Furthermore, many-to-one optical paths such as a path from 1 and 2 to 3 can be set up in a similar fashion using a tree rooted at node B, which combines the signals from the transmitting nodes. The total time required for establishing and releasing an optical connection includes signaling time (including propagation delays), routing and wavelength assignment computation time, optical node switching time, and transmitter/receiver tuning time. Because this total can be substantial (possibly on the order of 100 ms for propagation times alone), optical connections should generally be held for much longer periods of time (seconds or more) for efficient operation.

3.1.2

Logical Connections Once an optical connection exists, one or more logical connections can be carried on it through the assignment of suitable transmission channels, which serve to adapt each 6

If there are optical line amplifiers along the fiber links, they would normally include their own control agents for surveillance and reconfiguration purposes.

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LC to its supporting optical connection. Referring to Figure 2.29, a point-to-point LC originates at an external port on the transmission processor (TP) of the source station, which converts the logical signal to a suitable transmission signal, appearing at point a in the figure. This is in turn converted to an optical signal, appearing at point b, which may be multiplexed with other optical signals before it leaves the NAS. The process is reversed at points d and e in the receiving side of the destination station. Because all operations concerning LCs and their associated transmission channels take place within NASs, logical connection management does not require the intervention of the ONCs once the supporting optical paths are in place. As a result, connection reconfiguration within a logical network can often take place through distributed connection management protocols executed cooperatively among the participating NASs without calling on the functions of lower layer network entities (optical path layer and below). (Examples of this appear in subsequent sections.) This can greatly simplify the various network management and control functions and provides further justification for viewing the multiwavelength network in terms of independent logical and physical layers and sublayers.

3.1.2.1

Stream and Packet Traffic During the information transfer phase, it is useful to distinguish between two types of data flow on a logical connection: stream type and asynchronous (packet or cell) traffic. Stream traffic, exemplified by SONET STSs described in Section 2.6, typically supports voice, fixed bit-rate video, and similar applications. A synchronous optical network infrastructure such as SONET or SDH can also be used as an underlying server for a packetor cell-switched client network, as in ATM or IP over SONET. In the case of stream traffic, the information entering and exiting a logical port on an NAS is in the form of a continuous, synchronous bit stream. To support this type of traffic, the transmission channel carrying the LC must act as a dedicated, fixed-capacity “pipe.” In the asynchronous case, the information flow is in the form of random bursts carried in data packets or cells. In this case, each packet or cell carries explicit addressing information in a header, processed by various network entities (e.g., IP routers, ATM switches, or MPLS LSRs) involved in routing the data along a path to its destination. In the case where a synchronous network serves as the infrastructure for a packetor cell-switched network the distinction between stream type and asynchronous traffic is not always obvious. Thus, in packet-based systems, a random sequence of packets is typically embedded into a synchronous bit stream by encapsulating the packets into data frames separated by “idle” characters. Similarly, when a sequence of ATM cells is carried on a SONET connection, the data cells are carried within an STS bit stream interspersed with idle cells. In this case, the useful information is “bursty” or asynchronous, but the bit stream carried by the optical network is steady or synchronous. For our purposes, the traffic type is characterized by the way in which the NASs operate on the bit stream. If the bits are processed transparently, without distinguishing the individual informationbearing packets/cells from the idle bits, it is considered to be stream type. If the individual packets/cells are extracted from the bit stream when it enters the transmission processor, and headers are read for the purposes of multiplexing, scheduling, routing, and so forth,

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the traffic is considered to be asynchronous. The latter mode of operation corresponds to packet- or cell-switching within the optical layer. Typically, this requires that transmission and reception processors execute a media access control (MAC) protocol designed for sharing one or more λ-channels dynamically among many logical connections. Although synchronous traffic is always connection oriented, asynchronous traffic may be either connection oriented or connectionless. For example, traffic in IP networks is usually connectionless: Packets are launched into the network as individual entities without establishing a virtual connection.

3.1.2.2

Multipoint Logical Connections The efficient realization of one-to-many and many-to-one logical connectivity among stations is an important issue in optical networks. A station maintains one-to-many logical connectivity if it is the source of more than one simultaneous logical connection. Similarly it maintains many-to-one logical connectivity if it is the destination for more than one LC. For a station accessing a network node through a single pair of fibers, oneto-many logical connectivity requires multiplexing several transmission channels on the outbound access fiber, and many-to-one connectivity requires that transmission channels from several source stations share the inbound access fiber using some multiple access procedure. These multipoint connections are crucial for achieving high connectivity in optical networks. When multiplexing and multiple access are used for multipoint connections, the issue of addressing arises. Addressing methods are linked to the particular multiplexing techniques being used. For example, with synchronous time-division multiplexing (TDM), addressing is implicit in the position of a data segment in a periodic time frame, and thus very little processing is required for the multiplexing/multiaccess operations in the NASs (see Section 3.2). However, in the asynchronous case, each packet/cell header must be processed individually in the station’s transmission processor (reception processor) to execute the multiplexing (demultiplexing) function on an outbound (inbound) transmission channel. Thus, packet switching in the optical layer typically carries with it considerable computational overhead within the NASs.

3.2

Static Networks As we have seen, in static networks the physical layer is stripped down to its bare essentials: passive splitting/combining nodes interconnected by fibers to provide static fiber connectivity among some or all pairs of optical transmitters and receivers (OTs and ORs). This was illustrated in the examples of Figure 2.7, which show full potential connectivity from each transmitting to each receiving station in the case of a star or bus, and partial connectivity in the case of a tree. Because the optical paths cannot be modified, connection control in static networks is confined to the λ-channel and transmission channel sublayers. This means that if the NASs have a means of signaling among themselves, connection management can be executed in the NASs without the intervention of a separate optical network manager. Consider the broadcast star example

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of Figure 3.6, which consists of a star coupler connecting transceivers in three stations. This is an example of a static shared medium network. As described in Section 2.3.1, the star coupler combines signals from all transmitting stations and broadcasts them to all receiving stations.7 Thus a receiver can make a logical connection with any transmitting station by selecting the information destined for it and discarding the rest. This is the essence of the broadcast-and-select method of operating shared-medium optical networks. Its most attractive feature is that the broadcast function permits complete optical connectivity among all pairs of transmitting and receiving stations because every receiver “sees” every transmitter. This has a downside, however. Two transmitters emitting optical signals at the same time must transmit on distinct wavelengths. If they use the same wavelength, they will interfere with each other at all receivers, destroying any useful information flow. This distinct channel assignment (DCA) constraint prevents any reuse of the optical spectrum in broadcast networks. It should be noted that broadcast-and-select is not limited to networks based on star couplers. For example, any of the tree networks in Figure 2.7 can be operated in a broadcast-and-select mode. Each of them combines signals from a set of transmitters and broadcasts them to a set of receivers. (In the case of Figure 2.7[b], all transmitted signals are combined at the head end of the bus and broadcast to all receivers, and in the case of Figure 2.7[c], each of the two unidirectional trees provides signal combining and broadcasting for the subsets of transmitters and receivers which it interconnects.) We now examine the operation of static networks using the broadcast star as an illustrative example.

3.2.1

Point-to-Point and Multipoint Connections Although the discussion thus far has focused on point-to-point connections, the broadcast star is a natural medium for creating multipoint connections for enhanced connectivity, and these play an important role in the discussion that follows. To keep track of what is happening, we use different notation for connections in each layer, as introduced first in Section 2.4: r A unidirectional point-to-point optical path from station a to station b is denoted as a, b , and a multicast path from station a to the receiving set {b, c, . . .} is denoted as a, {b, c, . . .} . r A unidirectional point-to-point optical connection from station a to station b is denoted as (a, b). To show the wavelength of a λ-channel carrying the connection, say λk , we use the notation (a, b)k . A multicast optical connection from station a to the receiving set {b, c, . . .} is denoted as (a, {b, c, . . .}).8 r A unidirectional point-to-point logical connection from an external port on station a to one on station b is denoted as [a, b], and a multicast LC from a to a set {b, c, . . .} 7

8

If transmitter–receiver pairs are shown with corresponding numbers, as in this case, they are assumed to belong to the same station, and the network is called an undirected star; otherwise, it is referred to as a directed star. An optical path from a station a to a receiving set B can carry any optical connection from a to a receiving set C ⊆ B.

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is denoted as [a, {b, c, . . .}]. In a multicast LC, the transmitting station sends the same information to all receiving stations. (If station a transmits different information streams to a set of destination stations, the connections would be designated [a, b], [a, c], . . . .). As we shall see, there is a significant difference between the realization of a set of one-to-many LCs composed of several point-to-point connections (say, [a, b] and [a, c]) and the related multicast connection [a, {b, c}]. The former requires multiplexing of two distinct information streams at the transmitting station, whereas the latter does not. This means, for example, that a multicast LC can be supported by creating a multicast optical path in the form of a directed tree that delivers the same signal to several destinations.9 Summing up the possibilities at the logical level, there are three possible types of logical connectivity on the transmitting side: 1. One-to-one: A source station maintains a point-to-point logical connection with a single destination. 2. Multicast: A source station maintains a multicast logical connection with several destinations. 3. One-to-many: A source station maintains several point-to-point logical connections to different destinations. Note that an n-fold multicast connection can always be realized as a set of n point-topoint connections, but it is generally wasteful of communication resources to do so. On the receiving side the possibilities are one-to-one, in which a destination station maintains a logical connection with a single source, and many-to-one, in which a destination station maintains several point-to-point logical connections from different sources. The term multipoint is used generically to designate an arrangement including one-to-many, manyto-one, and/or multicast logical connections. Now let us illustrate the layers of connectivity using the example of Figure 3.6. The star coupler provides a permanent broadcast optical path of the form i, {1, 2, 3} from each transmitting station to the set of all receiving stations (including loopback paths from each station to itself) independent of the wavelength being used. Thus, suppose station 1 transmits on wavelength λ 1 . Then a point-to-point optical connection (1, 2) 1 is set up by tuning a receiver in station 2 to λ 1 , and a multicast optical connection, say (1, {2, 3}) 1 , is created by tuning receivers 2 and 3 to λ 1 . Because the signal power originating from station 1 is split among all receiving stations, exactly the same information reaches both stations 2 and 3. It is important to note that the optical path produced by the broadcast star offers the potential for various types of point-to-point and multicast optical connections, but the actual connections are realized through the actions of the access stations (transmitter/receiver tuning). Furthermore, the optical power from all transmitters is always delivered to all receivers whether they use it or not. 9

Multicast connections can also be created in the logical path layer of Figure 2.1(a) by forming a directed tree in the electrical domain.

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NAS 1 [1, 2] [1, 3]

[2, 3]

TP1

TP2

TP3

1a

OT1

2a

3a

OT2

OT3

1b

1d

2b

2d

3b

OR1

3d

OR2

OR3

1e

2e

3e

RP1

RP2

RP3

[1, 2]

[1, 3] [2, 3]

(a) RP3 [1, 3] 3d

OR3

3e

(1, {2, 3}) (2, {2, 3}) 1

1

2

2

3

3 (b)

[2, 3]

Synchronized Gate or Packet Filter and Buffer (c)

Figure 3.6 Star coupler example.

Moving up to the logical layer, suppose the point-to-point LCs [1, 2], [1, 3], and [2, 3] are to be realized, as shown in the bipartite LCG in Figure 3.6(b). Both one-to-many and many-to-one LCs are present. Depending on the throughput requirements and available network resources, these connections can be realized in various ways. The simplest approach is by “brute force,” using WDM and wavelength division multiple access (WDMA). If three wavelengths – λ 1 , λ 2 , and λ 3 – are available, then three optical connections – (1, 2) 1 , (1, 3) 2 , and (2, 3) 3 – are set up, which in turn carry the corresponding LCs. Note that this requires two optical transmitters in station 1 and two receivers in station 3 and provides the full capacity of a λ-channel to each LC. (The NASs in Figure 3.6[a] are assumed to contain the necessary number of transceivers, even though they are not shown explicitly.) One-to-two logical connectivity from station 1 is realized by multiplexing the two wavelengths λ 1 and λ 2 onto its outbound fiber (WDM), and two-to-one connectivity at station 3 is realized by accessing the station with λ-channels on wavelengths λ 2 and λ 3 superimposed on its inbound fiber (WDMA).

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Now suppose the two point-to-point connections [1, 2] and [1, 3] are carrying the same information. They can be replaced by a multicast LC [1, {2, 3}]. The multicast connection could be supported by the same optical connections as used for the pointto-point case simply by modulating the two optical transmitters in station 1 by the same (electrical) transmission signal. However, a more economical way of achieving the same result is to use a single multicast optical connection (1, {2, 3}) 1 , eliminating one transmitter in station 1 and using only two wavelengths instead of three while achieving the same throughput. Because the multicast connection [1, {2, 3}] is still just one connection, it can be realized without multiplexing. This simple example shows that there are often several alternatives for realizing connections at the logical and optical levels, so judicious choices are important to obtain cost-effective results. For larger networks, a high degree of multipoint connectivity is often required, as suggested in the example of Section 1.5. However, the WDM/WDMA approach of dedicating a λ-channel to each LC becomes prohibitively expensive and quickly exhausts the available optical spectrum as the number of connections increases. Thus, other methods of achieving high logical connectivity while using less network resources (wavelengths and transceivers) must be employed. These typically involve combinations of wavelength division and other multiplexing and multiple access techniques. Two additional cases: time division and time-wavelength division will suffice for illustration.

3.2.1.1

Time-Division Techniques TDM and time division multiple access (TDMA) are methods of realizing multipoint connectivity that are particularly cost-effective in terms of optical resources. At any one time, a large network will be carrying many optical connections, some dedicated and some demand assigned. All of these are sharing the network resources subject to certain admission control rules, executed by the ONM to ensure satisfactory operation of the network. The dedicated connections can be considered to be fixed for the purposes of the current discussion. With demand-assigned (circuit-switched) traffic, the connection pattern evolves with time as active connections are released and new ones are established. These events generally occur on long time scales (seconds or more) for reasons explained at the beginning of this chapter. In addition to this “slow” time sharing of network resources, when TDM/TDMA is used sharing also occurs on a much faster (microsecond/nanosecond) time scale by interleaving concurrent connections in time. To illustrate the different time scales, Figure 3.7 shows the signals appearing at one of the receivers when the LCs in the example of Figure 3.6 are realized with one transmitter or receiver per station and only one wavelength λ 1 by time sharing the optical medium. Is this a sequence of circuit-switched connections or a set of simultaneous time-multiplexed multipoint connections? It all depends! If the time scale is on the order of seconds or more, then the figure shows circuit-switched connections, with the idle times between the [1, 2]

[1, 3]

[2, 3]

Figure 3.7 Time-shared medium.

t

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information flows used for connection establishment and release. However, if the time scale is microseconds, there is no time for connection changes between information flows, and the picture must represent TDM/TDMA. In this case, the two LCs [1, 2] and [1, 3] are time division multiplexed onto a transmission channel in station 1, and the optical transmissions from stations 1 and 2 each access the same λ-channel during different time intervals – an example of TDMA. The time interleaving might be on a packet, cell, byte, or bit basis. The essential differences between the circuit-switched and multiplexed cases concern how the connections are managed. In the circuit-switched case, each connection is released before the next is established; in the multiplexed case, both connections are established before the information transfer phase and are maintained simultaneously in time. (Because the optical path is static in this example, all connection management can occur via signaling among the access stations.) Note that in either case the total amount of time required to transmit a given number of bits (the throughput in bits per second, neglecting idle periods) is the same and is limited by the capacity of the shared λ-channel. (In the WDM/WDMA case, in which three λ-channels were used, the maximum throughput was three times as much as in TDM/TDMA, illustrating the fact that there is no free lunch. If less optical resources are used, less transmission capacity is available.) Now let us examine the multipoint case in more detail. Observe that because all information is transmitted on wavelength λ 1 , all transmitters and receivers remain tuned to λ 1 at all times, creating the multicast optical connections (1, {2, 3}) 1 and (2, {2, 3}) 1 .10 Using TDM at the transmission channel level, the information streams for connections [1, 2] and [1, 3] arriving at the two input ports on station 1 in Figure 3.6(a) are multiplexed onto a single transmission signal (point 1a) that modulates the transmitter OT 1 . The TP contains buffers and synchronization equipment for interleaving the two information streams. The transmission channel supporting these multiplexed LCs is carried on the optical connection (1, {2, 3}). The LC [2, 3] is carried on optical connection (2, {2, 3}) without any multiplexing. Because both optical connections are carried on the same wavelength, an optical multiple access scheme is necessary to keep the two signals from interfering with each other at the receivers and to distinguish the different information streams. In this example, we use TDMA, in which the two optical signals are transmitted in nonoverlapping “bursts” by switching the transmitters on and off intermittently. The overall connection arrangement is therefore a combination of TDM at the logical (transmission channel) level and TDMA at the optical (λ-channel) level. At each receiver the optical signal is first converted to electronic form (at points 2e and 3e in Figure 3.6[a]) and then demultiplexed to extract the information destined for it. The rest is discarded. Demultiplexing requires synchronized gates or “packet filters” in the RP, as shown in Figure 3.6(c) for station 3. The packet filters are required if the data stream interleaving is done on a packet/cell basis (see Section 3.2.2). 10

Note that the loopback connection from 2 to itself is not needed but is an unavoidable result of the broadcast topology.

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OT1

[1, 2]1 [1, 2]2 [1, 3]1

[1, 2]3 [1, 2]4 [1, 3]2 [2, 3]1

OT2

Coupler

From 1

[2, 3]2

From 2

From 1

F1 OR2

F2

From 1

From 2

From 1

F1 OR3

From 2

From 1

F2 From 2

F1

From 2

From 1

From 2 F2

Figure 3.8 A TDM/TDMA schedule.

A typical schedule of transmissions for this example is shown in Figure 3.8. In the figure, transmissions occur in synchronized time slots of equal length, where the notation [a, b]n in a slot means that it is used for the LC [a, b], with n indicating that this is the nth segment of the data stream transmitted on that connection. The first two time traces in the figure show the information streams as they appear at the transmitters OT 1 and OT 2 , and the third shows the streams interleaved as they appear at the coupler. Note that the different propagation delays between the stations and the coupler indicate that station 1 is farther away from the coupler than station 2. The last two traces represent the streams as they appear at receivers OR 2 and OR 3 , showing that station 3 is closer to the coupler than station 2. Multiple access synchronization is required at the transmitters to produce nonoverlapping optical signals at the coupler (and hence at each receiver), taking into account different propagation delays from each transmitter to the star coupler. Although the multiplexed stream from one transmitter can be contiguous in time, small guard times are required between signals from different transmitters to provide tolerance for small errors in synchronization. The schedule shown has a fixed periodic frame pattern, with F 1 and F 2 being two successive frames containing four slots per frame. In this case, the destination address of each data segment is implicit in its position in the frame, so no explicit addressing need be carried with the information itself to indicate to which logical channel it belongs. This is typical of a fixed (as opposed to a dynamic) capacity allocation. (The fraction of the frame time allocated to each logical channel is proportional to its allocated capacity.) Several operations that require exchange of control information are involved in initializing the connections. A frame schedule must be determined by a connection manager and must be made known to all participating stations. Also, each station must acquire frame, slot, and bit synchronization. These functions take place during the connection establishment phase. The only control information needed during the information transfer phase is that required for maintaining timing synchronization – a process known

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as ranging. This is generally an operation that requires very little overhead.11 To keep latency times and buffer sizes small, the slot and frame sizes should be small. However, for efficient operation the slot size should be much larger than the guard time. These two considerations dictate schedules with a small number of slots per frame, with slot and frame sizes typically on the order of microseconds for transmission channel bit rates on the order of gigabits per second. These aspects of connection control and synchronization are typical of all fixed frame (synchronous) TDM/TDMA systems regardless of the underlying technology. The previous TDM/TDMA example illustrated how time division techniques can be used to create multipoint logical connectivity with a minimum of optical transmitting and receiving hardware, and minimal use of the optical spectrum. In realizing multipoint connections, four successive operations were involved: 1. 2. 3. 4.

Logical channel multiplexing in the transmitting stations Optical multicast, enabling each transmitter to reach several receivers Optical multiple access, enabling several transmitters to access each receiver Logical channel demultiplexing in the receiving stations

All multipoint connections in optical networks generally rely on the same fundamental operations, as shown in subsequent chapters.

3.2.1.2

Time-Wavelength-Division Techniques The previous illustrations showed how either TDM/TDMA or WDM/WDMA could be used to achieve multipoint connectivity. The former was economical in the usage of optical hardware and the optical spectrum but had limited throughput due to the fact that transceivers were idle part of the time and only one λ-channel was used. The latter had high throughput but required more optical transceivers and more spectrum. Is there a way of obtaining the best features of both? Figure 3.9 shows another example using the broadcast star network of Figure 3.6. This time, suppose full logical connectivity is required (without loopback connections) as shown in the LCG of Figure 3.9(a). The logical channels are now created using multiple wavelengths as well as multiple time slots. In contrast to the WDM/WDMA case, each station contains only a single transceiver. However, to achieve the necessary multipoint connectivity, some tunability of the transceivers during the information transfer phase of the connections is needed. Because optical connectivity between a given transmitter/receiver pair is created by tuning both to a common λ-channel, a number of tuning options are available: Use fixed transmitters with tunable receivers (FT-TR), tunable transmitters with fixed receivers (TT-FR), or tunable transmitters with tunable receivers (TT-TR). In this simple example, we use the FT-TR option, in which case each transmitter is tuned permanently to a distinct 11

In systems based on undirected stars or their equivalent, as in the current example, all stations see all transmissions, including their own. This makes it relatively easy to execute mutual synchronization. The RP in each station can observe the arrival times of all signals, and the station’s TP can use this information to adjust the transmitted bursts to fall correctly into their allocated time slots. In cases in which stations do not see their own transmissions (for example, directed stars, or stars without loopback connections), some feedback for synchronization purposes must be provided from the receiving stations to the transmitting stations via a separate control channel or a return data channel. See Section 5.7.

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T

R

1

1

2

2

3

3 (a) LCG

Channel 1 Channel 2 Channel 3 Slot

[1, 2]

[1, 3]

[1, 2]

[2, 3]

[2, 1]

[2, 3]

[3, 1]

[3, 2]

[3, 1]

1

2

3

F1

t

(b) Time-wavelength schedule Figure 3.9 TDM/T-WDMA.

wavelength (that is, a unique λ-channel is “owned” by each transmitter). Each transmitted signal fans out through the star coupler, reaching all three receivers (including its own). If only one-to-one logical connectivity were desired, then all connections could be realized by equipping each destination station with a single receiver that remains tuned to the desired transmitter’s wavelength for the duration of a connection. To rearrange the connections on a circuit-switched basis, the receivers would be retuned, but because receiver tuning occurs as part of connection establishment, low tuning speeds (milliseconds) would be acceptable. Here we are seeking multipoint logical connectivity, which can be achieved by using a single, rapidly tunable OR at each station and using TDM in the transmitters. This arrangement potentially provides full, simultaneous logical connectivity among all sources and destinations. Any receiver wishing to select information from a given transmitter tunes to that transmitter’s wavelength whenever the transmission signal contains information for it, resulting in a time-wavelength-division multiple access (T-WDMA) scheme. To create a prescribed set of source-destination LCs, appropriate TDM schedules must be set up for each transmitter so that the desired information can be extracted by each receiver from the various transmitted information streams using a suitable tuning rule. This creates a TDM/T-WDMA system. In contrast to the TDM/TDMA case, in which transmitters operated in nonoverlapping bursts to avoid interference, the TDM/ T-WDMA system can operate with all transceivers active at all times, providing more efficient usage of the optical equipment. A possible time-wavelength schedule using a periodic frame with two equal size time slots is shown in Figure 3.9(b), which displays the interleaved transmissions on each channel (wavelength) as seen at the star coupler. Each transmitter multiplexes the logical channels destined to the other two receivers into a fixed TDM frame, with all stations transmitting simultaneously on different

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wavelengths. The transmitters must be synchronized with each other at the star coupler, as shown in Figure 3.9(b), so that the TDM frames for each wavelength are aligned to allow conflict-free receiver tuning schedules.12 A receiver conflict occurs if information from two different transmitters, destined for the same receiver, is transmitted during overlapping time intervals, making it impossible for the receiver to pick up both transmissions. In a conflict-free schedule, no destination station may appear more than once in each time slot because this would require the station’s receiver to tune to two different wavelengths at the same time. Note that the conflict-free condition is satisfied in Figure 3.9(b). In the first slot, stations 1, 2, and 3 tune to wavelengths 3, 1, and 2, respectively, and in the second slot, they tune to 2, 3, and 1, respectively. Because all channels and all transceivers are active at all times, the network resources are utilized to maximum efficiency. Note that the four ingredients of multipoint connectivity are present in the TDM/ T-WDMA case: 1. 2. 3. 4.

Logical channel multiplexing at the transmitters using TDM Optical multicast via the star coupler Optical multiple access using T-WDMA Logical channel demultiplexing by tuning the receivers to the right wavelength at the right time

Multipoint connections can be realized using many types and combinations of multiplexing and multiple access. These are discussed in more depth in Chapter 5.

3.2.2

Packet Switching in the Optical Layer: The MAC Sublayer The fixed-capacity allocation produced by the periodic frame structure in the previous examples is well adapted to stream-type traffic. However, in the case of asynchronous packet traffic13 this approach may produce very poor performance. To see why, consider two basic characteristics of applications typically carried using packet switching: 1. Burstiness: The “bursty” nature of the information flow tends to produce poor transmission efficiency if fixed-capacity allocation is used. 2. Need for high logical connectivity: In typical packet-switched applications, it is common for a station to require simultaneous logical connections with a large number of other stations. (This is one of the primary advantages of packet-switched transport.) Although the aggregate traffic may be large, each logical connection between a pair of stations typically carries only a very small fraction of the total. These considerations suggest that dedicating a fixed capacity to each LC is difficult, inefficient, and offers poor quality of service. For example, imagine extending the 12

13

As in the TDM/TDMA case of Figure 3.8, this may require accounting for unequal propagation delays to the coupler. In certain extreme cases involving long distances and/or large fiber dispersion, synchronization at the coupler is insufficient to avoid conflicts in TDM/T-WDMA. These occur when the differences in propagation times for different wavelengths may be great enough so that different λ-channels are aligned at the coupler but are misaligned by the time they reach the receivers. Henceforth, we usually employ the term packet generically to refer either to fixed-length cell-based or variable-length packet-based data flow.

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broadcast star example of Figure 3.6 to 100 stations, each supporting an end system operating as a gateway to an LAS and requiring an LC to each of the 99 other gateways. Full connectivity using TDM/TDMA would require 9900 time slots (one for each LC), which would lead to poor optical transceiver utilization and large delays. The poor transceiver utilization results from the fact that each transmitter or receiver is idle 99% of the time. The delay is due to the fact that when a user in one LAS has traffic ready to be transmitted to another LAS (say, a file to be transferred), it needs a largecapacity allocation immediately. Instead, it has only one slot available out of 9900 in a long frame. For example, with a 1-Gbps channel bit rate, the effective capacity allocated to one connection is only 101 Kbps. The inferior performance is a consequence of the fact that we are treating packet traffic exactly as if it were stream traffic and assigning a fixed-capacity pipe to each LC. However, by implementing a packet-switching function that reaches down into the optical layer it is possible to maintain a very large number of LCs simultaneously using dynamic capacity allocation. This is accomplished by exploiting the essential advantage of the packet approach: packet-by-packet addressing and control. In packet traffic the identity of the LC to which a packet belongs (that is, addressing information), together with other control information, is carried in each packet’s header. Traditional packet-switched transport networks (e.g., IP networks) process the packet headers in their logical switching nodes (packet switches), where they perform packet sorting and routing and other control functions. They typically have a sparsely connected logical topology. For example, if the network of Figure 3.2(b) is used to support a packet-switched logical overlay it appears to be an ordinary packet-switched network with a logical topology in the form of a unidirectional ring. Packets delivered to the ring by the end systems are forwarded to the next LSN until they reach the destination node, at which point they exit to the end system. The binary decision forward/exit is made by reading the packet header. However, if the packets are processed in the transmission/reception processors of the NASs instead of in the LSNs, the packet-switching functions are executed in the transmission channel sublayer of the optical layer. This is especially advantageous in the case of broadcast networks for three reasons: 1. Full logical connectivity for packet streams is supported easily by the fully connected optical infrastructure already in place. 2. Optical broadcast together with packet processing in the NASs amounts to parallel packet processing using the resources of all stations simultaneously instead of sequential processing of all packets in each LSN they traverse. 3. An optical broadcast path is the most efficient support for logical multicast connections. The reader should note the difference between packet switching in the optical layer (which is accomplished electronically in the NASs) and optical packet switching, in which most packet-switching functions, including some header processing, routing, and possibly packet buffering are executed optically in the ONNs. This is discussed in detail in Chapter 10.

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A schematic representation of a network executing packet switching in the optical layer is shown in Figure 3.2(c). On the left we show an underlying structure in the form of our ubiquitous broadcast star, the same physical topology that supported the fully connected logical topology in Figure 3.2(a) and the logical ring in Figure 3.2(b). Now the NASs are labeled PS to indicate the presence of an optical layer packet-switching function replacing the electronic switching executed in the LSNs of Figure 3.2(b). The logical topology is now shown as a “cloud” containing the five NASs. This is meant to convey the idea that packet routing is effected using the NASs and the shared broadcast medium, as opposed to the case of Figure 3.2(a), in which five stations are joined by 20 individual point-to-point LCs. To achieve full connectivity on the broadcast star without resorting to logical switching, we use the broadcast-and-select property of the star, in which information in the form of packets is broadcast to all receivers. Each one selects those destined for it and discards the others. Although broadcast-and-select with fixed-capacity assignments for stream traffic was described in Section 3.2.1, we now have the potential for dynamic capacity assignment. The TPs can schedule packets based on instantaneous demand, priorities, and quality of service requirements for different traffic classes. Comparing the logical topologies in Figures 3.3, 3.2(a), and 3.2(c), they all show full connectivity among the stations over a single logical hop. Their difference lies in how capacity is assigned. In the case of Figure 3.2(c), the capacity of the broadcast star is shared dynamically among the five stations, whereas in the other cases, a fixed capacity is assigned to each logical link. In all cases, the actual physical topology – a ring or star – is hidden from view and is largely irrelevant. To delve further into implementation of packet switching in the optical layer, it is useful to recall its antecedents. Packet switching over a shared medium existed well before the deployment of optical fibers. Examples of conventional shared media are terrestrial radio and satellite, in which the medium is free space (the “ether”), and wired buses and rings, in which the medium is a copper cable. When these were introduced into data networks, special MAC protocols were developed to adapt the higher layers of the network architecture to the unique features of the shared medium. The role of the MAC protocol is to replace the standard data link control protocols designed for a pointto-point link by procedures for efficient utilization of a medium accessed simultaneously by many users. Most MAC protocols are designed for LANs and MANs, and the basic problem they must address is contention resolution; that is, how to control the scheduling of packet transmissions among many contending stations so that the system performs as efficiently as possible. The most commonly used shared optical medium is the broadcast star. Control of optical media requires a MAC protocol that has many of the same features as the earlier MAC protocols yet is adapted to the unique properties of optical networks. A good way to understand how an optical MAC protocol should operate is to examine a LAN protocol designed for a wired bus: carrier-sense multiple access with collision detection (CSMA/CD). Employed in Ethernet, this is probably the most widely used MAC protocol today. To illustrate, consider the system of Figure 3.10(a). Four stations are shown connected to the bus, which propagates signals in both directions. The station access

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1

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Interface T/R tap (b)

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...

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Figure 3.10 CSMA/CD.

arrangement appears in Figure 3.10(b), indicating that the end system (for example, a data terminal, personal computer, or workstation) is interfaced to a transceiver coupled into the cable. (Typically the interface is located in the end system, and the transceiver is located at the cable tap.) A packet to be transmitted from the end system is passed to the interface, which encapsulates it into a format suitable for transmission on the cable and buffers it to await transmission. A simplified picture of the encapsulated format is shown in Figure 3.10(c). The information from the end system is first enveloped in a variable-length logical link control (LLC) frame to which is added a preamble to allow the receiver to synchronize and recognize the beginning of the frame. A header including destination address (DA) and source address (SA)14 also precedes the LLC frame, which is followed by a group of cyclic redundancy check bits (CRC) used for error detection (but not for error correction). The procedure for transmitting the frames is as follows: On receipt of a data frame from the end system, the interface sends the encapsulated frame to the transceiver for transmission on the cable as soon as the transceiver senses the cable to be idle 14

If the LAN is part of a larger network (for example, a subnet of an internet), then the LAN addresses are local addresses, which differ from global internet addresses. In some of our subsequent discussions of packet switching in subnets of large optical networks, this local/global distinction is needed.

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[1, 2] t

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Figure 3.11 CSMA/CD collision.

(the “carrier sense” part of the protocol). All receivers listen to all transmissions and read their destination addresses, selecting those destined for their station and dropping the others. This is equivalent to the broadcast-and-select procedure used in a star network. If, during transmission of a frame, the transceiver detects another frame on the cable, that is, a collision (the “collision detection” part), it aborts its transmission and schedules the frame for retransmission at a later time. A characteristic of this and many other MAC protocols is the possibility that a frame will be lost, thereby requiring retransmission. The CSMA/CD protocol is a refined version of random access, which uses neither carrier sense nor collision detection. However, the two protocols share the following three properties: (1) there is no coordination among transmissions from different stations, (2) collisions can occur, and (3) if collisions occur, retransmissions are used to recover lost packets. The refinements in CSMA/CD over random access simply reduce the probability of collisions and mitigate their effects when they occur. Figure 3.11 shows the events associated with a collision. It is assumed that packet transmission times are long compared with τ – the end-to-end propagation time on the bus. A worst case is shown in which stations 1 and 4, at opposite ends of the bus in Figure 3.10(a), are transmitting to stations 2 and 3, respectively. Time traces as seen at transmitters T 1 and T4 and receivers R 1 and R4 are shown in Figure 3.11. Observe that T 1 begins transmission of a frame [1, 2] first, and it takes a time τ to propagate to station 4. Just before it reaches the other station, T4 , thinking the cable is idle, begins to transmit a frame [4, 3], which propagates to the left, toward station 1. Both of these transmissions begin to overlap on the bus, garbling the information received at stations 2 and 3. But shortly after beginning its transmission, R4 detects the collision with [1, 2], which has now reached station 4. This causes T4 to abort its transmission as shown. None of this is known to station 1 until approximately τ seconds later, when the initial portion of [4, 3] reaches the other end of the cable, at which time R 1 detects the collision and aborts its transmission [1, 2]. The total time occupied on the cable by this event is slightly more than 2τ . This represents time lost for useful transmissions and limits the maximum throughput of the system. Furthermore,

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each collision and retransmission causes an additional delay until the frame is finally transmitted successfully. It should be clear from the figure that the smaller the end-to-end propagation time τ , the less time is lost in collisions. In fact, it turns out that the throughput efficiency of CSMA/CD under high loads is approximately 1/(1 + 5a), where a = τ/m and m is the frame transmission time. Thus, perfect efficiency (no time wasted in collisions) is approached as propagation time gets small (i.e., a short cable) and/or frame transmission times get large (i.e, many bits per packet and/or low transmission bit rates). For example, typical parameters for Ethernet might be a cable of length 2.5 km, a transmission rate of 10 Mbps, and a frame length of 1000 bits. Taking the speed of light in the cable as 1.5 × 108 m/s, this gives an end-to-end propagation time of approximately 10.9 µs, a = 0.109, and a throughput efficiency of approximately 0.65. The short propagation time means that the overall delay for successful packet transmission, even after several retries, is quite small. Unfortunately, all of this changes radically in the context of optical networks. For MANs and WANs the distances and bit rates make CSMA/CD impossible. This still leaves local access subnets. In a star-based LAS, a common form of access network, the star coupler creates a shared medium much like the copper Ethernet bus. However, in this case, the fibers are typically using unidirectional rather than bidirectional transmission. Thus each station accesses the star via an OT/OR pair, using a pair of unidirectional access fibers. In this configuration, the typical distances (of the order of kilometers) and bit rates on a single λ-channel (of the order of gigabits per second) still yield a 1 for typical network parameters. Thus, CSMA/CD is impractical except in some special cases. What are the alternatives? To suggest some approaches, let us extend the TDM/TDMA and TDM/T-WDMA examples of Figures 3.8 and 3.9 to packet switching using the star coupler of Figure 3.6 operating in a broadcast-and-select mode. In these two cases, we created three and six fixed-capacity LCs, respectively, by using a periodic slot assignment. Now, one of our objectives is to achieve dynamic capacity allocation; that is, to implement complete and instantaneous sharing of the broadcast medium on demand. The role of

Data Frames

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NAS Figure 3.12 NAS equipped for packet switching.

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the MAC protocol is indicated schematically in Figure 3.12, which shows an NAS with packet-switching functionality. We assume now that the data belonging to LCs [1, 2] and [1, 3] is delivered from the end system to the NAS over either two separate external ports or over a single port in the form of randomly interleaved packets. It is then up to the TP to recognize the individual packets and process them accordingly, encapsulating them into a properly addressed format (similar to the CSMA/CD format of Figure 3.10). The encapsulated packets must then be scheduled and transmitted under the supervision of a suitable MAC protocol (indicated by the MAC block in the station). How should this protocol operate? It all depends on the objectives to be achieved and the information and resources available to the participating stations. For simplicity, let us begin with the assumption of a single, shared channel using only time division techniques. Suppose the packets are of fixed length, so they can be inserted into slots of equal length. Now the time traces in Figure 3.8 can be reinterpreted in a packet-switched context. The time slots are no longer grouped into a periodic frame schedule; instead, packets are transmitted on a slot-by-slot basis. Ignoring the fixed periodic schedule in Figure 3.8, the data segments can now be interpreted as packets, where, for example, the notation [1, 2] 2 represents a packet with source and destination addresses 1 and 2, respectively, and with sequence number 2. The reception processors in the stations must now contain suitable packet filters, which have the functions of stripping the encapsulation, reading local destination addresses, capturing all packets destined for the station, routing them to the correct external ports, dropping the unwanted packets, and performing error checking. Thus, in this example the reception processor in NAS 2 must filter out all packets belonging to LCs [1, 3] and [2, 3], retaining only those on [1, 2]. Note that dynamic scheduling allows for the capacity to be allocated based on instantaneous demand: When a station has a packet to transmit, it schedules it in the next available slot. It is this feature of packet switching that allows us to treat the broadcast star as a shared-capacity rather than fixed-capacity medium. The difficult part of dynamic scheduling is that, in the absence of a periodic frame structure, some sort of coordination and synchronization among the transmitters is needed to resolve contention among the stations for the available channel capacity. It is the function of the MAC protocol to arbitrate contention, attempting to keep collisions to a minimum and rescheduling transmissions in cases of lost packets. The simplest protocols (e.g., random access) have no coordination and minimize collisions by keeping traffic levels low. More complex protocols rely on the exchange of coordination and scheduling information, which reduces collisions or avoids them altogether. (The example in Figure 3.8 shows perfect scheduling with no collisions.) Although the contention problem is the bad news in multiwavelength broadcast networks, the good news is that multiple wavelengths are available to use for concurrent transmissions. To illustrate, consider a packet-switched generalization of the TDM/ T-WDMA case of Figure 3.9, as shown in Figure 3.13. We continue with the assumption of a slotted system with six active logical channels. Figure 3.13(a) shows four slots in a possible sequence of packet transmissions using three λ-channels and FT-TR. Note that the schedule is now assigned dynamically according to the demand of each station, with

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Figure 3.13 Packet switching in the optical layer.

connection [1, 3] carrying more traffic than the others. Note also that there is a receiver conflict in the third and fourth slots, with receiver 1 required to tune to two different channels in slot 3 and receiver 3 required to tune to two different channels in slot 4. In these cases each receiver must make a choice, picking up one transmission and losing the other. Figure 3.13(b) shows a variation of the previous case, using three channels and TT-FR. The same sequence of packet transmissions appears, but in this case they are carried on different channels because the channel assignment is determined by the destination address rather than the source address. As a result, the conflicts that appeared in the third and fourth time slots in the previous example are now replaced by collisions in

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those slots. This is a more serious problem because a collision mutilates both packets, resulting in double the loss of the FT-TR case. Figure 3.13(c) shows another version of the same example, using only two channels and requiring tuning of both transmitters and receivers. Now a properly coordinated channel–slot schedule is shown, in which all of the packets are transmitted in six time slots (the minimum possible number with two channels) without losses. Note that the channel–slot assignment is done on a packet-by-packet basis and results in a different interleaving of packets than in the previous cases. However, it can be seen from the sequence numbers that the packets are delivered in the proper order. Although the previous examples focused on point-to-point LCs, multicast connections come “free” simply by including the multicast destination addresses in the packet headers. (Of course, packet scheduling must be arranged to accommodate the multicast addresses without conflicts.) In comparing the previous examples, it can be seen that each is characterized by different constraints on capacity sharing. In TDM/TDMA, the medium (one λ-channel) is fully shared among all logical channels. In FT-TR, TT-FR, and TT-TR, partial sharing takes place in which all logical channels originating at a given station may share a capacity not exceeding one λ-channel, with the same type of sharing occurring for all channels terminating at a given station.15 The conflicts and collisions (and concomitant packet loss) in the FT-TR and TT-FR cases resulted from a lack of coordination among the stations. To avoid losses, two conditions must exist: (1) the transmitting stations must agree among themselves on a conflict- and collision-free set of channel–slot assignments and (2) the receiving stations must know these assignments so they can tune properly to pick up the packets destined for them. Both of these conditions require the following actions: (1) each station must request permission to transmit to a specified destination, (2) a control algorithm must be executed (either centralized or distributed among all stations), and (3) a scheduling decision must be communicated to the concerned parties. Depending on the network configuration, these actions require exchanges of varying amounts of control information among the stations. This might be done on one or more wavelengths or time slots reserved for that purpose, which act as separate control channels, and in some cases it might require additional optical transceivers. Control information can also be piggybacked on information packets. In the FT-TR and TT-TR cases, both transmitting and receiving stations must exchange control information, but in TT-FR, it is only the transmitters that need to agree on a schedule to avoid collisions because no conflicts are possible. In typical packet-switched broadcast-and-select systems, there is usually a compromise between ideal lossless communication (requiring extensive exchange of control information together with possibly complex scheduling computations) and uncoordinated lossy transmissions. The compromise involves a trade-off between cost (in terms of NAS complexity and spectrum utilization) and performance (in terms of throughput, 15

All remarks here on conflicts and collisions in shared channel systems are predicated on the assumption of a single transceiver per station. In cases where stations may have arrays of transmitters and/or receivers, the rules governing conflicts and collisions are more complex (see Section 5.4).

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loss, and delay). The performance-to-cost ratio for most MAC protocols diminishes as the number of stations increases. The procedures governing exchange of control information, conflict resolution, and recovery from losses are the distinguishing features of MAC protocols. Figure 3.12 indicates as dashed lines the various control paths associated with media access control. In the most general case, the MAC protocol coordinates many activities executed in the other components of the NAS, including scheduling, transceiver tuning, and signaling to and from other stations. Because the specifics of these exchanges vary from one protocol to another, we defer further remarks to Section 5.6, where a more detailed treatment of packet switching in the optical layer is presented, and to Section 5.7, where applications to access networks are discussed. It is instructive to place the optical layer MAC protocols in the context of the multiwavelength network architecture. The relations among the layers are most easily understood by comparing them with MAC protocols in a conventional layered network architecture: the OSI Reference Model (see [Schwartz87], p. 15). The second layer of the OSI model is the DLC layer, whose function is to ensure reliable and ordered delivery of data frames over a point-to-point link (see Figure 3.14). The DLC layer uses the services of the physical layer, which provides for transmission of the underlying bit stream on a point-to-point physical medium such as a wire or fiber. (Layer 2 in the IP protocol stack, which corresponds to the DLC layer here, is called the network interface layer.) As shown in Figure 3.14, in the case of shared media these two layers are replaced by three: the LLC and MAC sublayers and a new physical layer adapted to multiaccess on the shared medium. For example, in CSMA/CD, the higher level features specific to this protocol, including encapsulation and decapsulation, scheduling transmissions and retransmissions, and error detection, are performed in the MAC sublayer. Lower level functions associated with the physical medium, including sensing the idle channel

Application Presentation Session Transport

OSI MWNA Logical Path

Network

LLC

Logical Connection

DLC

MAC

Transmission Channel

Physical

Physical

LLC MAC

λ-Channel Optical Path Fiber

Figure 3.14 MAC protocol in the layered architecture.

Physical

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and detecting collisions, as well as bit transmission and reception, are performed in the physical layer. The remaining high-level functions associated with error-free delivery of frames in the correct sequence take place in the LLC sublayer. Comparing this layered view with the hardware implementation of CSMA/CD shown in Figure 3.10(b), the physical layer would normally be implemented in the transceiver, the MAC sublayer in the interface, and the LLC sublayer in the interface and/or the end system. Now let us turn to the optical network case. The position of optical layer packetswitching functions in the multiwavelength network architecture is shown in Figure 3.14 for comparison with the OSI protocol stack. In this case, similar LLC, MAC, and physical layers can be defined, which take the place of the logical connection, transmission channel, and λ-channel sublayers as shown. The location of the MAC and physical layers at a level corresponding to the transmission channel and λ-channel layers reflects the fact that medium access generally requires special operations on the bit stream (frame encapsulation and decapsulation, packet filtering, parity check computation, and so forth) and may require control of the optical transceivers (rapid tuning for WDMA operation and/or on/off switching for TDMA operation). As in the case of conventional packet-switched networks, an LLC frame enveloping the useful data is prepared in the LLC sublayer and is passed down to the MAC sublayer together with the local address (i.e., the identity of the NAS) to which it is destined. The MAC sublayer encapsulates it for transmission on the optical medium in a format similar to Figure 3.10(c). Included in the encapsulation is the source address (the address of the transmitting NAS) and the destination address (the destination NAS specified by the LLC sublayer). The encapsulated frame is then passed to the physical layer, which does the actual transmission on the optical medium, executing the necessary slot synchronization and transmitter tuning. Encapsulated frames received by an NAS are passed up from the physical layer to the MAC sublayer, which performs error detection and packet filtering (i.e., address checking), strips away the encapsulation, and passes the successfully received frames up to the LLC sublayer. In the case of a purely optical transport network (e.g., end systems attached directly to NASs), as in Figure 3.2(a), the logical link layer would normally reside in the end system, with the MAC and physical layers in the NAS. However, the transport network may consist of several packet-switched optical subnets joined together through an overlay that consists of logical switching nodes (e.g., IP routers). In this case, the subnets function as multipoint links between the LSNs, and we have two layers of packet switching. Now the LLC sublayer would be located in the LSNs, where a routing table would determine the next (optical layer) destination address. In this case the destination address may be the local subnet address of an intermediate station on a logical multihop path to the ultimate destination (see the hypernet example in Section 3.5.2).

3.2.3

Additional Comments on Broadcast-and-Select We have seen that static networks can be operated in a broadcast-and-select mode with either fixed-capacity or dynamically scheduled (packet-switched) LCs. Fundamental to broadcast-and-select is the fact that LCs can be created and reconfigured solely by the actions of the access stations, with logical connectivity ranging from a few point-to-point

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connections to complete multipoint connectivity. The multiplexing and multiple access procedures necessary for realizing high connectivity rely in important ways on resource sharing: in the stations and on the fibers. This is a recurring theme throughout the book. The broadcast-and-select principle based on a star, bus, or tree physical topology cannot be scaled to large networks for three reasons: 1. Spectrum use: Because all transmissions share the same fibers, there is no possibility of optical spectrum reuse, so the required spectrum typically grows at least proportionately to the number of transmitting stations. 2. Protocol complexity: Synchronization problems, signaling overhead, time delays, and processing complexity all increase rapidly with the number of stations and the number of LCs. 3. Survivability: There are no alternate routes in case of a failure. Furthermore, a failure at a critical point (either the star coupler or the head end of a folded bus) can bring the whole network down. For these reasons, a practical limit on the number of stations in a broadcast star is approximately 100. Nevertheless, we show in Section 3.4.2 how larger networks can be constructed using broadcast-and-select subnets as basic building blocks. Using this technique, it is possible to construct networks that circumvent the limits of geographical reach imposed by the optical technology, as well as the limits of connectivity and throughput imposed by economics.

3.3

Wavelength-Routed Networks In the previous section, we limited consideration to connections over networks with fixed optical paths and no wavelength selectivity in the nodes. Much more flexibility is achieved when wavelength selectivity and/or controllability is introduced in the optical path sublayer producing a wavelength-routed network. These networks have the property that each λ-channel can be recognized in the ONNs and routed individually. We assume throughout that the stations contain transceivers tunable over the full range of usable wavelengths, and the network nodes are either static (e.g., the static router of Figure 2.10) or controllable (e.g., wavelength-selective cross-connects). We limit consideration to transparent networks where all optical paths are point-to-point (either fixed or reconfigurable), so the only way multipoint logical connectivity can be achieved is by using several point-to-point optical connections (WDM/WDMA). This normally requires one optical transceiver for each connection.16 (Multipoint connections supported 16

In theory, a multipoint connection can be set up in a wavelength-routed network using time and wavelength division techniques. Thus, if a single transmitting station is connected to several receiving stations, with each connection realized on a point-to-point optical path using a distinct wavelength, then a one-tomany connection can be realized by tuning the transmitter rapidly over all wavelengths in the set. A similar arrangement can be constructed for many-to-one connectivity using receiver tuning. In principle, many-to-many connectivity can also be achieved by tuning both transmitters and receivers rapidly. However, this approach becomes exceedingly complicated because each path has a different propagation delay. Therefore, we do not pursue it further here.

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by multipoint optical paths are discussed in the context of linear lightwave networks in Section 3.4 and in Chapter 6.) To keep things simple, we also assume in this chapter that there is no wavelength conversion in the network. (Wavelength conversion in WRNs is considered in Chapter 6.) As indicated in Figure 2.6(b), a wavelength-routed network with m λ-channels can be viewed as m independently controlled copies of the original network. Because all connections are point-to-point, the distinctions between connectivity at the optical path, optical connection, and LC levels disappear, so it is sufficient to deal with optical connections only, with the understanding that an optical connection (a, b)i supports a corresponding LC [a, b] and is supported by a corresponding optical path a, b . In moving from the static networks of Section 3.2 to the WRNs in this section, we shift our focus from logical connections created by the actions of the NASs alone to optical connections created by the coordinated actions of the NASs and the optical network nodes. Wavelength selectivity in the ONNs offers considerable advantages in terms of spectrum reuse, even without control of the optical paths. Suppose, for example, that we reconsider the example of Figure 3.2(a), where a star network is used to provide full interconnection of five end systems. To change this from a broadcast-and-select network, we replace the star coupler with a static wavelength router. Figure 3.15 shows the resultant network, in which each NAS contains four transceivers tuned to route optical

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connections to the other four NASs and to receive signals from the others. Using the wavelength assignment rule described in Section 2.3.1.2, these connections can be realized using four wavelengths for a fivefold reuse of the optical spectrum. The wavelength assignments for the optical connections are shown in Figure 3.15(b). Comparing the static router with the broadcast star of Figure 3.2(a), we find that five times as many wavelengths are necessary to support full connectivity in the broadcast star, reflecting the lack of wavelength reuse in the broadcast network.

3.3.1

Routing and Channel Assignment In establishing an optical connection, we must deal with both channel assignment and routing. Channel assignment (executed in the λ-channel sublayer) involves allocating an available wavelength to the connection and tuning the transmitting and receiving station to the assigned wavelength. Routing (executed in the optical path sublayer) involves determining a suitable optical path for the assigned λ-channel and setting the network nodes to establish that path. Because an optical path in a WRN is associated with a particular wavelength, a path cannot be set up until a wavelength is allocated.

3.3.1.1

Dedicated and Switched Connections: Rearrangeability Implementation of routing and channel assignment (RCA) is a quite different problem depending on whether the optical connections are dedicated or switched. Dedicated connections are assumed to be held for a relatively long period of time. This would be the case, for example, for connections provisioned to support a fixed logical network overlay riding on top of the optical infrastructure. These connections would be chosen at the design stage of the LN and perhaps modified occasionally in response to changing load conditions or equipment failure. Switched (demand-assigned) connections, however, are established and released on demand, with holding times that might be as short as a minute or less. Thus, a network supporting only dedicated connections can be built using just static ONNs. For example, a WRN in the form of a ring can be constructed using static WADMs as network nodes. In the infrequent cases where connections are modified, the WADMs can be reconfigured manually. For our purposes the characteristic that distinguishes dedicated from switched connections is that requests for the former occur in the form of a prescribed set, whereas switched connection requests occur as a random sequence. Thus, the complete set of dedicated connections for an LN is known and established when that network is initialized. However, requests for switched connections are signaled to an optical network manager as they occur, and the ONM has the responsibility of (1) deciding whether to accept or to block the request and (2) determining a routing and channel assignment. In practice, the admission control decision may be based on various considerations, including the current network load, fairness, priorities, and so forth. However, for simplicity we assume here that the only factors that govern acceptance of a connection are the physical constraints on RCA that either permit or block the connection. It should be

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clear that the ability of a network to accept a switched connection request depends on its current state; that is, the pattern of connections that are currently active in the network. As we shall see, rearrangement of active connections can sometimes create free paths for new requests that would otherwise be blocked.

3.3.1.2

Channel Assignment Constraints We illustrate the constraints on channel assignment in WRNs using the example of Figure 3.16. The figure shows a network containing seven access stations and three ONNs. Each internodal link consists of a single pair of unidirectional fibers. Because the physical topology is a tree, there is only one possible optical path between any pair of stations, so there are no routing decisions to be made. Because there is no wavelength conversion in this network, each optical connection must obey the wavelength continuity condition already invoked in Section 2.2. Suppose that the available optical spectrum contains two λ-channels on wavelengths λ 1 and λ 2 . The unidirectional optical connections (2, 4), (1, 6), and (5, 7) are to be established. Assume first that they are dedicated connections. By inspecting the complete set, it is clear that the wavelength assignment (2, 4) 1 , (1, 6) 2 , and (5, 7) 1 is a feasible choice because connections superimposed on common fibers are always assigned distinct λ-channels. We encountered this distinct channel assignment condition earlier in the context of star networks. The DCA condition is one of the basic optical network routing constraints. It ensures that the optical signals sharing a fiber do not interfere with each other at the receivers. Now suppose the same set occurs as part of a sequence of switched connections as follows. The first request is for connection (2, 4), which is assigned λ 1 . This is followed by (3, 6) and (5, 7), which are assigned λ 1 and λ 2 , respectively. Then (3, 6) is terminated and a request for (1, 6) is received. Because λ 1 is now in use on link a and λ 2 on link b, the current state of the network makes it impossible to add the new connection without a DCA violation, and it is blocked. Note that blocking was caused by an unfortunate evolution of the state of activity in the network. However, if it were possible to rearrange the active wavelength assignments to (2, 4) 1 and (5, 7) 1 , then the new request could be

1

3

a

2

6

b

4

5

Figure 3.16 Channel assignment example.

7

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Multiwavelength Optical Networks

accepted using λ 2 .17 We have encountered an analogous rearrangeability problem before in the context of permutation switches (see Section 2.3.2.1). In the switch, however, the entity being rearranged is a path rather than a wavelength assignment. In a larger WRN, connection rearrangement might involve changing both optical paths (rerouting) and channel assignments. This is generally impractical for two reasons: 1. It requires momentary interruption of active connections, which may be unacceptable to users. 2. Finding a rearrangement that “opens up” the network to the new connection request typically requires solving an extremely complex combinatorial problem. We therefore rule out the possibility of rearranging switched connections. Note, however, that given a prescribed set of dedicated connections, knowledge of all connections on the list can often be used to accommodate them without conflicts. Thus, we see that blocking is more likely to occur when assigning a sequence of switched connections rather than a set of dedicated ones, and it is more likely to occur without wavelength converters than with them. As one more variant of this example, suppose we change things by using only a single wavelength, but putting two fiber pairs in each internodal link; that is, we trade a twofold reduction in spectrum usage for a twofold increase in fibers. How does network performance change? In all the cases cited earlier, there were never more than two connections active on an internodal fiber (and only one connection active on each access fiber). This implies that all connections can be accommodated on the modified network while using (and reusing) only one wavelength. There is an important observation here: Networks with multifiber links perform better than those with multiple wavelengths. The rule is that a network operating on F wavelengths with one fiber pair per link can never perform better (and generally will perform worse) than the same network with its links increased to F fiber pairs and its spectrum decreased to a single wavelength. However, a network operating on F wavelengths with one fiber pair per link and all of its nodes equipped with wavelength converters is equivalent to the same network with its links increased to F fiber pairs and its spectrum decreased to a single wavelength.18 The reason for these relations is that transparent multiwavelength networks without wavelength converters operate under the constraint of wavelength continuity, but there is no equivalent constraint in multifiber networks, nor in networks using wavelength conversion. To summarize the previous discussion as it applies to wavelength-routed networks without wavelength converters, we see that routing and channel assignments are subject to the following two constraints:

17 18

If wavelength conversion was allowed, the new connection could be accommodated without rearrangement by assigning it λ 2 on link a and λ 1 on link b. This assumes that each ONN is capable of permuting connections among all fibers on a link.

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1. Wavelength continuity: The wavelength of each optical connection remains the same on all links it traverses from source to destination. 2. Distinct channel assignment: All connections sharing a common fiber must be assigned distinct channels. (This applies to access links as well as internodal links.) Note that the first is a constraint imposed by the laws of physics, whereas the second is a design constraint required for proper network operation. At this point, it is important to clarify the meaning of the phrase distinct channel. In the context of this section, channel means λ-channel, so distinct channel assignment means distinct wavelength assignment. However, we have already seen a case, in Figure 3.8, in which signals on the same wavelength were superimposed on the same fibers (the outbound fibers on a star) without interfering. This was possible because they were nonoverlapping in time. In that example, the noninterfering channels corresponded to optical signals sharing a common wavelength but confined to different time slots using TDMA. More generally, we shall define channels to be distinct if they can be distinguished at a receiver when superimposed on its access fiber. Distinct channels are normally orthogonal, typically nonoverlapping in either optical frequency or time. As we consider more complex multiplexing and multiple access schemes in later chapters, the meaning of distinct channels is reexamined. Although the DCA condition is necessary to ensure distinguishability of signals on the same fiber, it is possible (and generally advantageous) to reuse the same wavelength on fiber-disjoint paths. This includes paths that may be carried on different fibers contained in the same (multifiber) link. Channel assignment constraints equivalent to the DCA condition exist in all types of communication networks. However, the wavelength continuity condition is unique to transparent optical networks, making routing and wavelength assignment a more challenging task than the related problem in conventional networks. The performance of a network depends not only on its physical resources, but also on how it is controlled. Routing and channel assignment is the fundamental control problem in large optical networks. Generally, the RCA problem for dedicated connections can be treated off-line, so computationally intensive optimization techniques are appropriate. However, routing and channel assignment decisions for switched traffic must be made rapidly, and hence suboptimal heuristics must normally be used. The RCA problem is made even more complex when fault recovery is considered. In this case, simultaneous assignments are needed for both working and backup paths. (See Chapter 8.) The objective of an RCA algorithm is to achieve the best possible network performance within the limits of the physical constraints. The RCA problem can be cast in numerous forms. For example, in the case of dedicated connections we may wish to accommodate a prescribed set of optical connections using a minimum number of wavelengths or using the shortest optical paths for each connection consistent with a limit on the number of available wavelengths. In the case of switched connections, the objective might be to maximize the offered traffic given a fixed number of wavelengths and a specified limit on blocking probability. The next section includes a few simple examples, confined to dedicated connections only. A more comprehensive look at RCA for both dedicated and switched connections appears in Chapter 6.

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3.3.2

Routing and Channel Assignment Examples Several examples of RCA are presented in this section to illustrate various possible tradeoffs among network resources and performance. All examples deal with finding optical connections that provide full logical connectivity among five stations; that is, embedding a fully connected five-station logical topology on the given physical topology while observing the constraints of the network. An RCA solution that satisfies the constraints is given in each case. Although this is presented as a dedicated connection problem, its solution gives an RCA rule for demand-assigned connections as well. If the five stations make point-to-point connection establishment/release requests in a random sequence, the requests can always be accepted without blocking as long as the RCA rule found for dedicated full connectivity is used. In these and other examples to follow in later sections, the wavelength assignments are dependent on the manner in which the NASs are connected to their ONNs. A typical arrangement, in which a single fiber pair access link connects an NAS to an ONN, appears in Figure 2.29. In the case at hand, the ONN contains a wavelength-selective cross-connect, and the two access fibers connect to an input and output port on the WSXC. In the NAS, several optical transmitters are multiplexed onto the outbound access fiber, and several optical receivers are fed by the signals demultiplexed from the inbound access fiber. The transmission/reception processors interface the transceivers to the external ports and may include electronic multiplexing/demultiplexing and switching equipment for connecting specified data streams from the transceivers to designated logical input/output ports. Although all transmitters and receivers may be tunable, the DCA condition requires that all inbound (outbound) channels (wavelengths) must be distinct in this case, because they share common fibers. Figure 3.17 shows a different version of the NAS–ONN connection. Now n, the number of fiber pairs in the access link, is the same as the number of internodal links incident on the node (assuming one fiber pair per internodal link). In the most general case, each access fiber directed outbound from the NAS may multiplex signals from several fixed or tunable optical transmitters, and each access fiber directed inbound to the NAS may feed signals to several fixed or tunable optical receivers. Thus, the contents of each of the n boxes labeled OTs/ORs in Figure 3.17 corresponds to the optical portion of the NAS in Figure 2.29. Again, the transceivers are interfaced to external input/output ports through TPs/RPs that may be used for cross-connecting data streams in an arbitrary fashion between the transceivers and end system equipment. In this case, there are no limitations on channel assignment due to the access fibers, because the access fibers can accommodate as many wavelengths as the internodal fibers incident on the network node. We shall refer to these multifiber pair access connections as nonblocking access links. Carrying the nonblocking idea one step further, suppose that the usable optical spectrum contains  λ-channels and that each OTs/ORs box contains an array of  fixed tuned transceivers covering the complete set of channels. Now the NAS (and its access fibers) can support as many optical connections as the internodal links incident on its ONN can carry. We shall refer to this arrangement as a nonblocking NAS, meaning that no

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129

Internodal Links 1

...

2

n

ONN

... 1 2

OTs/ORs

OTs/ORs

n

... ...

Access Fiber Pairs

OTs/ORs

NAS TPs/RPs

... Input/Output Ports

Figure 3.17 Nonblocking access link.

potential optical connection terminating on that NAS will be blocked due to limitations in the station or its access link.19

3.3.2.1

The Bidirectional Ring Our first example is shown in Figure 3.3, wherein a fully connected logical topology containing five NASs is to be embedded on a ring. Because each LC requires a point-topoint optical connection in this case, four optical transceivers are needed in each NAS. The objective now is to specify RCAs using a minimum number of wavelengths. As mentioned earlier, the feasible wavelength assignments depend on how each NAS is connected to the network. The simplest approach is the configuration of Figure 3.18(a), which uses the single fiber pair access connection of Figure 2.29. In the ring of Figure 3.18(a), the network nodes are WSXCs with three input and output ports, routing each wavelength around a ring or between a ring and an NAS. The rings are labeled L(R) for left (right) propagation past the NASs. Each NAS has four optical transmitters multiplexed onto its outbound access fiber and four receivers fed from its inbound access fiber. Because this is a two-access-fiber connection, the optical connections are subject to the DCA condition on the access links; each inbound (outbound) connection to an NAS must be on a distinct wavelength. 19

The nonblocking condition does not apply to connections from other NASs attached to the same ONN.

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Multiwavelength Optical Networks

4 3

5 2

L R 1

NAS (a) Ring Destination 1 T

R

1

1

1

2

3

4

5

1L

2L

3R

4R

3L

4L

2R

2L

1L

2

1R

3

3R

4R

4

4L

2R

1R

5

2L

3L

4R

Source

5

5 (b) OCG

(c) Routing Table

Figure 3.18 Bidirectional ring: single access fiber pair.

3L 1R

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131

A bipartite optical connection graph (OCG) for this example is shown in Figure 3.18(b). The DCA requirements on the access links translate into a classic edge-coloring problem on the OCG. Selecting distinct wavelengths is equivalent to coloring the edges of the OCG so that all edges incident on the same vertex have different colors. It is well known that this problem can always be solved in bipartite graphs using a number of colors equal to the maximum degree of the vertices, which in this case is four. This is only a necessary condition, however, because DCA must also be observed on each internodal fiber in the ring. A solution to the RCA problem, in this case using shortest path routing, is shown in an RCA table in Figure 3.18(c), which indicates the path and wavelength assigned to each optical connection. (An entry in the table of the form [nL] means that the corresponding connection is routed on ring L using wavelength λn .) Shortest path routing means that each connection is routed on the ring (L or R) yielding the least number of optical hops: in this case, no more than two. Note that the problem is solved using four wavelengths, for a spectrum reuse factor of 5. Another approach uses a nonblocking two fiber pair access link following the interconnection structure depicted in Figure 3.17. In this case, the ONN–NAS combination can be realized as shown in Figure 3.19(a). Now the NAS consists of two separate pairs of transceivers, with each pair multiplexed onto a separate access link L or R, connected to one of the rings. Using two access fiber pairs instead of one removes the DCA constraint on the access links, so fewer wavelengths are required. Because the allowable interconnections are limited, the WSXC can be realized using two 2 × 2 multiwavelength switches, one for each ring, operating as wavelength add/drop multiplexers. Each WADM is similar to that shown in Figure 2.23 except that each access fiber carries multiplexed add/drop wavelengths instead of a single wavelength. In effect there are two separate unidirectional ring/WADM–NAS combinations grouped together at each node. The external input/output ports are connected to the transceivers through the TPs/RPs. A possible set of wavelength assignments using shortest path routing is shown in the RCA table in Figure 3.19(b). For example, the source (destination) assignments in row (column) 2 correspond to the transmitter (receiver) tuning for NAS 2, as shown in Figure 3.19(a). This RCA requires only three wavelengths instead of four for full connectivity, because wavelengths can be reused on the same access link (but not on the same access fiber). It is optimal in the sense that three is the minimum number of wavelengths required for full connectivity irrespective of how the NAS–ONN connection is realized. (A more general treatment of RCA on rings appears in Section 6.3.4.2.)

3.3.2.2

A Mesh Network Now consider the five stations connected to ONNs in the mesh network of Figure 3.20. Our objective is the same as it was in the case of the ring: Find an RCA using a minimum number of wavelengths to realize the fully connected OCG shown in Figure 3.18(b). Again, each NAS must have a total of four transceivers. In this case a lower bound on the number of required wavelengths is two, no matter what routing is used and no matter what access connections are used (why?). Now let us first assume single fiber

132

Multiwavelength Optical Networks

ONN 2

L

R MWS MWS

L

1

3

R

1

3

1

1 1

3

3

NAS 2 TPs/TRs

Input/Output Ports (a) ONN–NAS Combination Destination 1 1

2

3

4

5

1L

2L

2R

1R

1L

3L

3R

2L

1L

2

1R

3

2R

1R

4

2L

3R

2R

5

1L

3L

1R

Source

3L 3R

(b) Routing Table

Figure 3.19 Bidirectional ring: two access fiber pairs.

pair access connections, which means that the lower bound is raised to four because four connections must share each access fiber. It is left as an exercise for the reader to show that four wavelengths are sufficient using shortest path routing. Unfortunately, this result provides no improvement in spectrum utilization over the bidirectional ring, despite the richer physical topology of the mesh. This is because the access fibers are the bottleneck. To remove this bottleneck, suppose a nonblocking multifiber pair access link is used for each station. (No increase in the number of transceivers is required.) In this case all connections can be realized using two

Network Connections

4

3

D

C

133

E

5 A

B

1

2

Figure 3.20 A mesh network.

wavelengths, representing a ten-fold reuse of the spectrum.20 The exact wavelength assignment is left as an exercise for the reader. These RCA examples illustrate some key issues in the design and control of wavelength-routed networks. In particular, they show a trade-off between the extent of the fiber topology and the optical spectrum requirement: The more fibers there are and the more “densely” they are interconnected, the fewer wavelengths are needed. In this trade-off, the functionality of the network node is a limiting parameter because the WSXC permits only point-to-point connections. The networks described in the next section broaden our view to include multipoint optical connectivity, thereby expanding greatly the options in implementing desired connectivity patterns.

3.4

Linear Lightwave Networks: Waveband Routing The three classes of purely optical networks discussed in this chapter all have the same basic structure: NASs and optical nodes interconnected by fiber links. Their differences reside in the functionality of the nodes. The static networks described in Section 3.2 and the wavelength-routed networks of Section 3.3 represent two extremes of ONN functionality: the former having no wavelength selectivity and no control and the latter having independent control of each wavelength. In other words, the static case comprises static single-waveband networks, whereas the wavelength-routed case corresponds to dynamic single-wavelength-per-waveband networks. The LLNs described in this section subsume both of the other two classes as special cases. We use the term LLN to designate any network with nodes that perform strictly linear operations on optical signals. The most general type of LLN has nodes that function 20

The minimum required number of access fibers varies from station to station, depending on the routing and wavelength assignments.

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Multiwavelength Optical Networks

as waveband-selective linear divider-combiners (LDCs) (that is, generalized optical switches). Recall from Section 2.3.2.3 that an LDC performs controllable optical signal routing, combining, and splitting. The combining function allows for the multiplexing of channels from several inbound fibers onto a single outbound fiber, and the splitting function provides for multicasting of optical power from a single inbound fiber to several outbound fibers. These functions are required to support multipoint optical connectivity. As was the case for WRNs, we exclude wavelength conversion from the LLNs and assume that switching within an optical network node occurs on a slow timescale, with switches changing their settings only during optical path establishment and release. Waveband selectivity in the nodes means that the optical path layer routes signals as bundles that contain all λ-channels within one waveband. One can conceive of many special cases of LLNs. First, there are the “limiting” cases mentioned before. The static networks treated in Section 3.2 are examples of networks having no wavelength selectivity within their ONNs (single-waveband networks), in which the nodes are fixed but still have combining and splitting functionality. At the other extreme, the wavelength-routed case of Section 3.3 has no combining and splitting functions, but spectral resolution is refined to the point where the bundles recognized by the nodes consist of only one λ-channel. Another important category is a single-wavelength-per-waveband network possessing combining and splitting properties (i.e., an “enhanced” WRN, containing LDCs in its nodes rather than permutation switches). Such a network is capable of establishing optical multicast connections as well as optical many-to-one connections on a wavelength-bywavelength basis. A simple application of optical multicast is a “drop-and-continue” function at the optical level, providing an efficient way of distributing information on a common λ-channel to multiple destinations. Optical many-to-one connections are useful for time division multiplexing of LCs originating at different access stations onto a common λ-channel. Neither one of these functions can be realized using an ordinary WRN operating with permutation switches. Because ordinary wavelength-routed networks are limited to point-to-point optical paths, there is no need to distinguish among a logical connection, an optical connection, and an optical path in those networks. However, with the potential for multipoint paths in LLNs, all layers of connectivity and their interrelations must be examined more carefully. We already saw an example of this in the broadcast star. For example, in Figure 3.6, using WDM/WDMA the LCs [1, 2] and [1, 3] are carried on individual optical connections (1, 2) 1 and (1, 3) 2 , which are in turn carried by the multicast optical path 1, {1, 2, 3} . However, in the TDM/TDMA case, the same LCs are time division multiplexed on a single multicast optical connection (1, {2, 3}) carried by the OP 1, {1, 2, 3} . In each case the optical path reaches all destinations, intended or not, because there is no controllability or selectivity in the star node. How are the LCs sorted out in the receiving stations? In the WDM/WDMA case the optical connections (and the LCs they carry) reach only their intended destinations because the ORs are assumed to tune selectively to the assigned wavelengths. In the TDM/TDMA case, all optical connections are carried on the same wavelength, so selecting the proper LC at the receiving stations must be accomplished at the transmission channel level through time division demultiplexing.

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Last, in the TDM/T-WDMA case, LCs must be sorted out by combined actions at the λ-channel level (receiver tuning) and at the transmission channel level (time division demultiplexing). These connectivity alternatives reappear, coupled with the added possibilities of switching optical paths, as we examine the properties of LLNs.

3.4.1

Routing and Channel Assignment The two constraints on optical connections applicable to wavelength-routed networks (wavelength continuity and distinct channel assignment) also apply to LLNs. But when each waveband contains multiple channels (a condition unique to LLNs), additional constraints apply within each waveband. These are r Inseparability: Channels combined on a single fiber and situated within the same waveband cannot be separated within the network. r Distinct source combining (DSC): Only signals from distinct sources are allowed to be combined on the same fiber. Inseparability is a consequence of the fact that the LDCs operate on the aggregate power carried within each waveband without distinguishing among signals on different channels within the band; that is, inseparability is a condition imposed by the architecture of the optical switch combined with the choice of channel spacing. Figure 3.21(a) illustrates inseparability. It shows two point-to-point optical connections (1, 1∗ ) and (2, 2∗ ) assumed to be within the same waveband. Connection (1, 1∗ ) is routed via the minimum-hop path 1, 1∗ comprising nodes A–B–C–F–G, and connection (2, 2∗ ) is routed along its own minimum-hop path via nodes A–B–D–E. The label Si on a link in Figure 3.21 denotes a signal generated at a source station i. Observe that power from both sources is combined on fiber a. Thus, the DCA condition requires them to be on distinct channels. For example, they might be carried on different wavelengths within the same waveband (WDMA) or on the same wavelength but in different time slots (TDMA). Because the two signals are in the same waveband, they cannot be separated at node B. Thus, to route them both to their destinations, node B must multicast both signals to nodes C and D, expanding the intended point-to-point paths to multicast optical paths 1, {1∗ , 2∗ } and 2, {1∗ , 2∗ } – an unavoidable result of inseparability. Note the appearance of unintended destinations (underlined). We call these unintended destinations fortuitous destinations and refer to the unintended paths as fortuitous paths. In Figure 3.21, intended paths are shown as solid lines and fortuitous paths are indicated with dashed lines. These extra paths may potentially cause interference at the receivers. However, as long as the DCA condition is observed, interference among superimposed channels can be avoided by tuning in the desired channel and tuning out the undesired one. For example, in the case of WDMA this would mean tuning the optical receiver to select the correct wavelength and reject any others, implying that the selectivity is implemented in the λ-channel layer. However, if TDMA is used to distinguish the superimposed channels, the receiving station would select the intended

136

Multiwavelength Optical Networks

1*

1

S1

C A

a

B

F D

H

2

E

S2

3*

S1 S2

3

G

2*

(a) 1*

1 S1 A

2

S2

C a

S1

B

F

H

D

G

E 3* S2

3

2*

(b) Figure 3.21 Inseparability.

channel in its RP by capturing information in the proper time slots. In this case, selectivity is implemented in the transmission channel layer.21 Inseparability causes connections that share a common fiber on a common waveband to branch out of their original paths, fanning out onto an optical path in the form of a directed tree, with new, fortuitous destinations added to the tree as new paths are activated within the same waveband. The set of destinations (both intended and fortuitous) for each source consists of all destinations downstream on the tree from that source. Fortuitous destinations tend to waste fiber resources and power and are therefore to be avoided if possible. In this case, the fortuitous destinations could have been avoided by rerouting (2, 2∗ ) on a longer path, via node H , as shown in Figure 3.21(b). Inseparability does not apply to connections in different wavebands, which are routed independently of one another. Thus, fortuitous paths associated with inseparability do not exist in wavelengthrouted networks. In WRNs all optical paths are point-to-point so connections from different source stations on the same wavelength are never combined on the same fiber. Now let us examine the distinct source combining constraint.22 The DSC condition forbids a signal from splitting, taking multiple paths in the network, and then recombining with itself. Figure 3.22 shows two ways a source may combine with itself. In Figure 3.22(a), the signal power is split into two parallel paths and then later recombined 21

22

Fortuitous paths occurred in the broadcast star example of Figure 3.2 without referring to them as such. For example, using WDM/WDMA, all signals reach all three destination stations, but each signal is only intended for one of them. Without calling it by that name, the DSC requirement was invoked in Section 2.3.1.1 in discussing power relations in directional couplers.

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137

C

A

B (a)

C

A

B (b)

Figure 3.22 Two violations of DSC.

downstream, whereas in Figure 3.22(b), a portion of the signal is fed back onto the upstream portion of its path. In both cases, the combined signals interfere with each other, garbling their information.23 Even if routing decisions are made correctly to avoid DSC violations, some low-level DSC violations may still occur because of imperfections in hardware. For example, DSC violations in Figure 3.22 could be created because of small leakage paths through the optical switches in nodes A and B. These effects, which are unavoidable in any purely optical network (either wavelength or waveband routed), produce cross-talk, which is tolerable as long as it is kept sufficiently small (see Section 4.9.1). From now on, when we speak of DSC violations, effects due to nonideal hardware are ignored. Figure 3.23 illustrates how a correct but poor routing decision may produce an inadvertent violation of the DSC condition. With connections (1, 1∗ ) and (2, 2∗ ) in progress, 1

1*

S1 A S2

S1 + S2

C B

F

H S1 + S2

D

d

E

G f S1 + S2 + S3 3*

2 3

S3

S1 + S2 + S3 S1 + S2 + S3

2*

Figure 3.23 Inadvertent violation of DSC. 23

Garbling is due to the fact that the signal is combined with a delayed replica of itself, creating interference at the receiver. Long delays cause interference in the demodulated signal, which translates into noise and distortion for analog signals and bit errors for digital signals. Short delays (on the order of the optical signal’s coherence time) produce interference at the level of the optical fields, analogous to multipath effects in radio transmission.

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1*

1 S1

C

A

2

S2

S1

B

G

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H

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S2 + S3

E

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(a)

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c a

B

F b

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S1 + S2 + S3

C

h

D

d

f

G

E S1 + S2 + S3

S3 3

S1 + S2

1*

3*

2*

(b)

Figure 3.24 Avoidance of DSC violations.

as shown in Figure 3.21(a), a new connection (3, 3∗ ) is routed via path D–E–F–G. All three connections are assumed to be in the same waveband but on distinct channels. The signals as they appear with the new connection in place are shown in Figure 3.23. Due to inseparability, signal S 3 carries with it (fortuitously) portions of signals S 1 and S 2 after combining with them on fiber d. This causes S 1 (which split at node B) to recombine with itself on fiber f , violating the DSC condition. (The same holds true for S 2 .) The new connection therefore must not be routed along the path shown. It could, however, use that path if it was assigned to a different waveband, because inseparability would not apply in that case. Two other solutions to this problem are shown in Figure 3.24. Figure 3.24(a) solves the problem by rerouting connection (2, 2∗ ) via node H , and in Figure 3.24(b) the problem is solved by routing (3, 3∗ ) via the path D–B–C–F–G. Inseparability can also indirectly cause violations of the DCA condition. We call these color clashes. This may occur when activation of a new connection results in combining onto the same fiber two or more connections (on the same waveband) that were already in progress on previously disjoint paths. If these connections use the same channel (for example, the same wavelength), a color clash occurs. To illustrate, consider the example in Figure 3.25. In Figure 3.25(a) two connections (1, 1∗ ) and (2, 2∗ ) are shown occupying disjoint paths. Wavelength λ 1 is assigned to both of them. Now a new connection (3, 3∗ ) is assigned to path D–E–F–G using the same waveband. Because S 3 shares link d with S 2 , it is assigned a different wavelength, λ 2 . The effect of adding connection (3, 3∗ ) is shown in Figure 3.25(b). Note that, because of inseparability, all three signals are now carried on fiber f . Because S 1 and S 2 are now on the same fiber, using the same

Network Connections

1*

1 S1

(1,

1*)1

A

2

139

S2

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B H

G

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S2

S1 F

B H

D

3

S3

d S2 (3,

3*)

f S2 + S3

E 2

G

3* 2*

(b)

Figure 3.25 Color clash.

λ-channel, they produce a color clash. This violation could have been avoided if all three connections had been assigned distinct wavelengths. Among the four constraints discussed earlier, wavelength continuity and inseparability are both consequences of physical laws and technological constraints; the others – DCA and DSC – are routing requirements imposed to ensure satisfactory operation of the network. When we discuss routing and channel assignment later, the role of the DCA and DSC constraints is of paramount importance. The examples used to illustrate the constraints bring out once again the difficulties associated with demand-assigned as opposed to dedicated connections. If the state of the network happens to evolve in a way that tends to “clog up” the fibers as connections are assigned in sequence (dynamically), then it may be difficult or impossible to assign new connections without violating the routing and channel assignment constraints. With rearrangeability, however, the original routing and channel allocations are not irrevocable. Thus in the example of Figure 3.21, changing the path of (2, 2∗ ) to pass through node H , as in Figure 3.21(b), would have avoided violation of the DSC condition. Similarly, changing the wavelength assignment of (1, 1∗ ) to λ 3 in Figure 3.25(b) would have avoided a color clash. Because we have decided that rearrangeability (resulting in service disruption) is not a reasonable option, it is especially important that the algorithms used for dynamic routing and channel assignment in these networks have a certain amount of foresight built into them. Even though future connection requests may not be predictable, it is still possible to make optical path and channel allocations in a way that tends to reduce the probability of blocking for new connections. For example, the color clash example of Figure 3.25 suggests that λ-channel assignments within a

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common waveband should be made in a way that minimizes the reuse of each wavelength; an idea that is counterintuitive because wavelength reuse is desirable for conserving the optical spectrum. We study these questions in more detail in Chapter 6.

3.4.2

Multipoint Subnets in LLNs In the previous examples, attempts to set up several point-to-point optical connections within a common waveband led to the unintentional creation of multipoint optical paths, and consequent extra complications in routing and channel assignment. In view of these additional complications in waveband-routed networks (LLNs), one might ask what is to be gained by adding this extra complexity? One answer is that waveband routing leads to more efficient use of the optical spectrum, given the technological constraints of the ONNs (see Section 2.2). Another answer is that the multipoint optical path capability, which showed up accidentally earlier, can be turned to our advantage in creating intentional multipoint optical connections. Because of this property, LLNs can deliver a high degree of logical connectivity with minimal optical hardware in the access stations. This is done by creating controllable multicast and multiaccess optical paths that support both one-to-many and many-to-one optical connections. This multipoint optical connectivity property is one of the fundamental advantages of LLNs over point-to-point wavelength-routed networks. We shall illustrate its power through a few examples in Section 3.4.3, which compare the WRN and LLN approaches. Figure 3.26(a) shows a network containing seven stations interconnected by two-fiber bidirectional links on an LLN with a mesh physical topology. As will be seen in the examples that follow, it is often useful to create full logical connectivity among specified “clusters” of stations within a larger network. We call such clusters multipoint subnets (MPSs). Suppose, for example, that the set of stations {2, 3, 4} is to be interconnected in this way. The LCG defining full connectivity in this subset (without loopback connections) is shown in Figure 3.26(b), together with the resultant logical topology (Figure 3.26[c]). Note that the unconnected stations are shown isolated from each other, and the stations contained in the MPS are shown enclosed in a “cloud,” indicating that there is full logical connectivity among all of them. Recall that this type of representation was used earlier in Figure 3.2(c) to symbolize stations sharing a common broadcast medium. We shall see that these two interpretations are closely related. How is full logical connectivity realized on the MPS? One possibility is to create an optical path on a single waveband in the form of a tree joining the stations, where the path is “carved out of ” (embedded in) the larger network by setting the LDCs appropriately. Figure 3.27(a) shows a shortest path tree connecting the three stations. The actual signal paths on the tree can be chosen in various ways. One convenient approach is to define a “root node” as the basis for an embedded broadcast star; that is, we route all signals from the stations to the root (now playing the role of a star coupler), combine them there, and broadcast the combined signals back to all stations. Figure 3.27(b) illustrates the equivalence between the tree, rooted arbitrarily at node C, and a broadcast star. The fibers and nodes along paths taken by the signals to and from

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4 4

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1 1

b 7

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T 2

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7 (c) Logical Topology

Figure 3.26 Seven stations on a mesh.

the root are indicated in Figure 3.27(b), where fibers sharing the same bidirectional link in the network of Figure 3.26(a) are now shown separately, and nodes traversed twice are duplicated. (Signal flow is from left to right.) In this and subsequent discussions, we adopt a standard convention for labeling fibers in two-fiber bidirectional links. Each link is shown with a reference arrow and a label, where the label designates the fiber carrying signals in the direction of the reference arrow, and a corresponding primed label denotes the fiber carrying signals in the opposite direction. For example, in Figure 3.26(a), fiber a carries signals from node A to E, and a  carries signals in the reverse direction. If a δ-σ LDC is used in node C, then it would be set as shown in Figure 3.27(c). The solid lines indicate the combining and dividing paths if the node is operated as a star coupler without loopback paths (a nonreflecting star), and the dashed lines indicate added paths for loopback operation (a reflecting star). In each case the combining/dividing ratios are equal for all ports: one-half for the case without loopback and one-third with loopback, so as to distribute power equally to all stations. (Although the specified LCG does not include loopback connections in this case, they are useful for control purposes – for example, synchronization in a TDMA system – and for determining propagation delays to the root, knowledge of which may be required in a MAC protocol.)

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Figure 3.27 Tree embedded in mesh.

Once an optical path is established to emulate a broadcast star, that is exactly what is “seen” by the stations. In other words, the desired MPS has been created by embedding its LCG into the physical topology of a mesh network in the form of a broadcast star. The embedded star can now be operated exactly like any other shared broadcast medium. Any multiplexing and multiple-access channel-sharing methods applicable to broadcast stars can be used, including fixed-capacity allocation, dynamic capacity allocation, TDM/TDMA, and TDM/T-WDMA. For example, a single λ-channel might be allocated to the MPS, in which case all stations would share the channel in a TDM/TDMA mode. If several λ-channels are available within the waveband supporting the MPS, then a TDM/T-WDMA implementation might be used. In each case, the full capacity of all allocated λ-channels would be available for sharing among all LCs. It is for this reason that we represent the MPS as a multipoint link that contains a set of stations sharing a common capacity.

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After the optical path supporting the MPS has been created by fixing the settings of the participating ONNs, there are no further control functions required within the optical path layer. Thus, control of the MPS, once it is configured, is completely in the hands of the NASs. The choice of multiplexing and multiple-access techniques, establishment and release of logical connections, apportionment of shared capacity, and so forth, can all be implemented through suitable signaling among the stations in the MPS without any interaction with the ONNs. The concept of the MPS is very general. From a top-down point of view, the stations contained in an MPS (e.g., the stations in the “cloud” in Figure 3.26[c]) appear at the logical level as a fully connected subset allocated some total capacity, which is shared among all LCs. Logical connections within an MPS may be multicast as well as pointto-point. As conditions evolve, the logical aspects of the MPS might be reconfigured by adding or deleting stations or by modifying capacity allotments. The details of how the connections are implemented in the physical layer are concealed and are unimportant from a logical layer point of view. However, from a bottom-up physical layer view, there are many possible ways of implementing a given MPS, including the choice of waveband, channel, and optical path; the use of single or multiple transceivers in the stations; single or multiple fiber pairs in the access links; different multiplexing and multiple-access schemes; fixed or dynamic capacity assignment (packet switching); and so forth. Furthermore, it is possible to reconfigure these physical layer aspects of an MPS in response to changes in load (e.g., add or subtract channel allocations) or in response to network faults (e.g., route the supporting optical path around failed links or nodes). Last, many variants of the fully connected MPS are possible. For example, if we remove the transmitting connections from stations 3 and 4 in Figure 3.27(b), the result is a multicast optical path from station 2 to stations 3 and 4. Similarly, removing the receiving connections from stations 2 and 3 results in a multiaccess path from transmitting stations 2 and 3 to receiving station 4. The MPS reappears in the next section as a basic building block of a hypernet.

3.4.3

A Seven-Station Example Now let us test the LLN concept against the wavelength-routed network approach. The test case will be the implementation of full logical connectivity among a set of seven end systems. We assume throughout that each end system is connected to an NAS and then to a network node through a nonblocking access link. The optical transceivers in the stations are assumed to be tunable over the full spectrum needed to support the connections, where each transmitter runs at a bit rate of R 0 bits per second, which may be taken to be the full capacity of a λ-channel. Implementation on three physical topologies – (1) the bidirectional ring, (2) mesh, and (3) multistar – is compared. Implementation on a bidirectional ring is considered a point of reference. We then compare approaches using the mesh of Figure 3.26 and the multistar configuration of Figure 3.28(a). The topology of the mesh was chosen arbitrarily and has no relation to the logical topology being implemented. In contrast to the mesh, the multistar structure was custom designed to support the seven-station example. The different physical topologies

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Figure 3.28 Multistar network.

were chosen to illustrate their effect on station and optical spectrum resource utilization. This is the first (but not the last) time we encounter a multistar topology. Until now, the underlying fiber placements were assumed to be given; that is, the network implementer was assumed to have no control over the physical topology, which is not necessarily adapted to the needs of the end systems. When using a multistar arrangement, the

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assumption is exactly the opposite. The fiber topology is now put in place expressly to serve the network being implemented; that is, the design of the physical topology is part of the overall network design. At the logical level, three logical topologies are considered: point-to-point (42 unidirectional LCs), fully shared multipoint, and hypernet (partial sharing). The first is realized using wavelength routing, and the others are realized using multicast optical connections in an LLN.

3.4.3.1

Wavelength Routing In the case of wavelength routing, the nodes in all physical topologies considered are wavelength-selective permutation switches. Because individual point-to-point optical connections are required for each LC in this case, six transceivers are needed in each station. For the bidirectional ring, it can be shown24 that full interconnection of seven stations requires six wavelengths, which results in a wavelength reuse factor of seven for the 42 LCs. For the mesh, it is left as an exercise for the reader to show that the connections can be realized with four wavelengths (a wavelength reuse factor of 10.5), which is the minimum possible number. In the multistar topology, each station accesses three stars, using two transceivers on each star. The two transceivers accessing one star are multiplexed onto a single access fiber pair. The central node of each star, a WSXC, is set to operate as a permutation wavelength router of the type shown in Figure 2.10, where the routing rule is as follows: A signal on input fiber i, carried on wavelength λk , is routed to output fiber j, where k = j − i mod 3. For example, the star joining stations {1, 2, 6} is shown in Figure 3.28(b). With the fibers numbered as in the figure, the wavelength assignments for the six optical connections on the star are (1, 2) 1 , (1, 6) 2 , (2, 1) 2 , (2, 6) 1 , (6, 1) 1 , and (6, 2) 2 . Because two wavelengths are used on each star, and these can be reused on all stars, the reuse factor for the 42 connections is 21. For all three physical topologies, the network has a total capacity of 42R 0 bps, and each LC has a fixed-capacity allotment of R 0 bits per second.

3.4.3.2

LLN Realization: Fully Shared Logical Topology In each wavelength-routed network realization, full connectivity was realized necessarily using point-to-point LCs, with a fixed capacity assigned to each. However, in an LLN realization, we have the option of replacing point-to-point logical connections with MPSs; that is, we may group stations together into fully connected subsets, in which the LCs joining the stations in the subsets share a common capacity. Extending the idea of the MPS to its ultimate limit in this example we may include all seven stations in a single subnet. Irrespective of the underlying physical topology, this can always be accomplished by choosing any convenient tree that links all seven stations to form an optical path that supports all optical connections among the stations. Following Section 3.4.2, the tree can be used to emulate a broadcast star. In the case of the bidirectional ring of Figure 3.29(a), any node can be chosen as the root node for 24

See Section 6.3.4.2.

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Figure 3.29 Embedded star on a bidirectional ring.

the star, with its LDC set to do full combining and broadcasting for all λ-channels on a specified waveband. All other nodes serve to complete the tree configuration, joining the NASs to the root node. In this case, node A has been chosen as the root node so that the optical paths can be redrawn in the form of a directed tree as shown in Figure 3.29(b). Depending on the selectivity of the LDCs, the waveband allocated to the star may be “thin,” holding only a single λ-channel, or “thick,” containing the full usable optical spectrum. (The latter case would hold if the LDCs had no waveband selectivity.) In either case, the combined capacity of all allocated λ-channels is available for sharing among the seven stations. (A negative feature of the embedded star is that there is no spectrum reuse, so the total throughput can never be more than the combined capacity of all allocated channels.) If only one λ-channel is used, all stations can share the

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channel using TDM/TDMA, and this requires only a single transceiver per station. This is the most economical realization in terms of station cost. In this case, the total network capacity is R 0 . If more than one λ-channel is used, full utilization of the channels requires either multiple transceivers or rapidly tunable transceivers in the stations. For example, if seven λ-channels are available, then the TDM/T-WDMA approach of Section 3.2 could be employed in an FT-TR arrangement with one fixed transmitter and one tunable receiver in each station. This would yield a total network capacity of 7R 0 bps. The same approach can be applied to the mesh network of Figure 3.26. For example, node E in the figure can be taken as the root of an embedded star. Except for the different physical topology, all aspects of the discussion of the bidirectional ring apply here as well. (In fact, the seven stations accessing the embedded star have no way of knowing in what physical topology their MPS is embedded.)

3.4.3.3

LLN Realization: Hypernet Logical Topology The point-to-point and fully shared logical topologies represent two extremes for realizing full connectivity among seven stations. Compromise solutions are also possible. Figure 3.30(a) shows a seven-node hypernet logical topology in the form of a hypergraph25 with seven hyperedges. Each hyperedge represents an MPS that contains three stations. Note that all stations in a common hyperedge can communicate with each other because they are members of the same MPS. Because there is full logical connectivity among all stations (vertices) within an MPS (hyperedge), and all LCs share a common capacity, there are in effect six LCs “hidden” in each hyperedge, all sharing the same capacity. In this example the logical topology is arranged so that each station can reach any other station via one of the three hyperedges to which it is connected. This is a way of providing full logical connectivity among all stations without placing them in the same MPS. Just as the point-to-point links comprising an LCG are represented conveniently as a bipartite graph linking source and destination stations, the MPSs (multivertex hyperedges) comprising a logical connection hypergraph (LCH) can be represented by a tripartite graph, as shown in Figure 3.30(b). Here each hyperedge of the LCH is represented as a central vertex connecting a transmitting set on the left to a receiving set on the right. For example, hyperedge E 1 connects the transmitting set {2, 5, 7} to itself. When each hyperedge’s transmitting and receiving sets are identical, the hypergraph is called undirected, which is analogous to an undirected graph. So far we have a top-down view of the network. Now, suppose this logical topology is to be realized in the mesh network of Figure 3.26 as a set of embedded multicast stars. A routing tree must be chosen for each of the seven MPSs, and wavebands must be assigned to the trees so that the routing constraints are satisfied. A solution to this problem is shown in Table 3.1. The tree for each hyperedge is given, together with its waveband assignment. (The details of tree realization were discussed in Section 3.4.2. The tree for hyperedge E 3 is displayed as an illustration in Figure 3.27.)

25

See Appendix A for the definition of a hypergraph.

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Note that the assignments are made so that no waveband is used more than once on a link, ensuring that the DCA condition holds on all links. The routing and waveband assignments were accomplished with a total of two wavebands, which is the minimum number possible. As can be seen, it was possible to achieve a minimum waveband assignment using shortest path routing of all trees except that supporting E 7 , which required three links. As in the previous example, this realization can be implemented with various numbers of λ-channels per waveband and various numbers of transceivers per station.

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Table 3.1 Tree routing on mesh. Hyperedge

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Tree

Waveband

1 2 3 4 5 6 7

2, 5, 7 1, 2, 6 2, 3, 4 1, 4, 5 1, 3, 7 4, 6, 7 3, 5, 6

b h f, g a, d a, c d, e b, g, h

1 1 1 1 2 2 2

Suppose, for example, that we allocate three λ-channels per waveband with the channels accessed using TDM/T-WDMA operating in FT-TR mode. This could be implemented using a single transceiver in each station to access each MPS, requiring three transceivers per station, one for each MPS to which it is connected. The total network capacity is then 21R 0 , with a capacity of 3R 0 shared among six logical connections in each MPS.

3.4.3.4

Multistar Realization In the previous example, we chose a logical topology (a seven-hyperedge hypergraph) and embedded it on a given physical topology, in this case a mesh. There was no relation between the chosen LT and the prescribed PT. However, as mentioned, one is sometimes free to design the PT to suit the chosen LT. In the case of hypernets, a multistar realization is a natural design approach, in which the tripartite graph representation of the LCH can be viewed as defining a multistar physical topology. For example, in the LCH of Figure 3.30(b), suppose each hyperedge corresponds to a broadcast star, in which the central vertex represents the star coupler, directing signals from the transmitting set to the receiving set. In this example, each station is connected to three stars, allowing it to reach all other stations. As seen from the stations, this multistar realization has the same characteristics as the case of multiple broadcast stars embedded on the mesh physical topology of Figure 3.26. The only difference is that all stars are realized on separate fibers, so we no longer need two wavebands to support the MPSs. To conform to the previous example, a single waveband containing three λ-channels can be reused on each star, with TDM/T-WDMA operation on the star using FT-TR. This would require three transceivers per station and would give a total network capacity of 21R 0 . By this time the reader should have realized that the multistar topology used in the earlier WRN example was derived from the hypernet development presented here. (Compare the multistar WRN of Figure 3.28[a] with the hypernet LCH of Figure 3.30[b].) The only difference is that the nodes in the WRN case perform wavelength permutation instead of simple combining and broadcasting in the hypernet.

3.4.3.5

Comparisons Table 3.2 summarizes the main features of the various seven station realizations presented earlier. It compares them based on spectrum utilization, reuse factor, transceivers

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Table 3.2 Seven-station comparisons.

Type of network

Physical topology

Logical topology

Reuse factor

No. of transceivers per station

Spectrum

Network capacity

WRN WRN WRN LLN LLN LLN

Ring Mesh Multistar Any Mesh Multistar

Pt.-Pt. Pt.-Pt. Pt.-Pt. Fully shared Hypernet Hypernet

7 10.5 21 1 3.5 7

6 6 6 1 3 3

1200 800 400 200 400 200

42R 0 42R 0 42R 0 5R 0 21R 0 21R 0

Pt.-Pt. = point-to-point.

per station, and total network capacity. Because there are so many variables in these examples, we must be careful not to compare apples with oranges! For the purpose of comparing spectrum usage, some assumptions must be made about wavelength and waveband spacing. Figure 3.31 shows the assumed values. For the wavelength-routed case, Figure 3.31(a) shows a 200-GHz spacing between centers of wavebands, with a guard band of 75 GHz between wavebands and a single λ-channel in each waveband. Figure 3.31(b) shows the case of the LLN, in which the 125-GHz usable bandwidth of a waveband supports up to five λ-channels on 25-GHz spacings. This λ-channel spacing could easily accommodate optical signals running at OC-48 speed (2.5 Gbps) on each channel. Thus, for example, the hypernet realization on a mesh requires two wavebands and uses three λ-channels in each waveband. Assuming that a full waveband is dedicated to the hypernet regardless of whether all of its channels are used, we assign a spectrum allocation of 400 GHz to this case. 200 GHz

λ-channel

125 GHz

75 GHz

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125 GHz

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f 25 GHz (b)

Figure 3.31 Assumed channel spacings.

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How does the spectrum utilization compare? In going from the ring to the mesh to the multistar, spectrum utilization tends to decrease and the reuse factor increases, as is to be expected with the improved physical topologies. The LLN realizations are more economical of spectrum and transceiver usage than the WRN case, with the fully shared LLN realization requiring the least resources. (Reuse factors in WRNs and LLNs are difficult to compare because WRNs use point-to-point optical connections, whereas LLNs use multicast connections.) Because network capacity is directly proportional to the number of transceivers, the WRN realizations have the most capacity. Of course, there are other factors to take into account, such as rapid tuning requirements in multichannel LLN realizations and lack of channel sharing in WRNs. Channel sharing has some significant advantages that are not immediately apparent. When a pooled capacity is shared among many logical connections, that capacity can be allocated to match unbalanced traffic distributions and can be reapportioned as traffic demand changes. Furthermore, in a shared medium it is possible to provide dynamic capacity allocation by implementing packet switching in the optical layer (see Section 3.2.2). For example, in the case of a fully shared logical topology, all 42 LCs could be operated in a packet-switched mode so that each connection receives the capacity it needs just when it needs it. In the WRN case, the fixed capacity allocations make it impossible to adapt to unbalanced traffic. Consequently, traffic imbalances result in reduced network throughput. Finally, as mentioned at the beginning of this chapter, shared-medium multipoint optical connections act as a natural support for multicast logical connections.

3.5

Logically-Routed Networks The examples in the previous sections illustrate various alternatives for implementing high logical connectivity in transparent optical networks while keeping resource utilization under control, limiting station complexity to a few optical transceivers, and using a few hundred GHz of optical bandwidth. This was possible because the networks were small, containing at most seven stations. When we move to larger networks, the transparent optical approach soon reaches its limit. For example, we found that six wavelengths and six optical transceivers per station were necessary and sufficient to achieve full logical connectivity among seven stations on a bidirectional ring using wavelength-routed point-to-point optical connections. But suppose we increase the number of stations to 22. Then the number of wavelengths increases more than ten-fold to 61, requiring many terahertz of optical bandwidth at the assumed wavelength spacings, and the number of transceivers per station increases to 21 (see Section 6.3.4.2). Economically and technologically, this begins to push the limits of the current state of the art. When the limits of optics are exceeded we must turn to electronics; i.e., logically-routed networks. In Section 2.6 the concept of a logical network overlay was presented, wherein logical switching nodes interconnect a set of logical links, forming an electronically switched logical topology (i.e., an LRN). The example of Figure 3.2(b), which consists of a unidirectional ring logical topology embedded in a star physical topology, is a very simple

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illustration of how switching in the logical layer can remove some of the connectivity burden from the purely optical portion of the network. We shall now extend that approach to larger networks. To fix ideas, in the discussion that follows it is useful to focus on a specific type of logical network overlay, such as an IP network. In this case the LSNs are IP routers, and the links are carrying asynchronous (packet) traffic. In this case the associations between end systems are virtual connections, and the logical layer is supported directly by the multiwavelength optical layer. This is IP over WDM. However, the development applies equally well to synchronous traffic, in which the LSNs might be SONET digital cross-connect systems or similar devices, and the endto-end connections would consist of synchronous bit streams in the form of dedicated circuits running at DS1 or DS3 bit rates (1.544 or 44.736 Mbps, respectively) or higher. These would typically be used as supports for higher layers of virtual connections, which might be made through telephone Central Offices (for POTS) or IP routers (for Internet data). Often several of these layers are stacked. An example is IP over ATM over SONET over WDM. This constitutes a division of the virtual connection layer in Figure 2.1(a) into several virtual sublayers. The essential feature of the LSN is that it performs a sorting and routing function for traffic delivered to it from an NAS accessing an underlying optical network. The traffic arriving on the input ports interfaced to the NAS (usually in the form of a continuous bit stream) is demultiplexed into small units (for example, IP packets, ATM cells, or DS3 circuits) routed to the appropriate output ports and remultiplexed for delivery to an NAS or an associated end system. This is illustrated in Figure 3.32, which shows an LSN interposed between an NAS (accessing a node in an optical network) and two end systems. The difference between logical connections in a purely optical network and an LRN is illustrated nicely by Figure 3.2. In a purely optical network, the end systems connect directly to the external ports of the NASs, as exemplified by Figure 3.2(a), so transport End systems

LSN

NAS

ONN

Figure 3.32 A logical switching node in an optical network.

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between a pair of end systems is supported by the logical connections originating and terminating at the corresponding NAS ports. Assuming that all logical connections are point-to-point, this means that the transport network seen by the end systems can be described by a logical connection graph as shown in Figure 3.2(a), in which the vertices represent the NASs terminating those connections. In the case of an LRN, however, the LSNs create an extra layer of connectivity between the end systems and the NASs. Thus the end systems access the logical network through the LSNs, the LSNs access the transparent optical network through the NASs, and the NASs are interconnected through optical network nodes. The vertices of the connection graph describing an LRN – for example, that of Figure 3.2(b) – are the LSNs instead of the NASs. If the underlying optical infrastructure is a wavelength-routed network, each link in the logical topology is supported by its own point-to-point optical connection. But each optical connection requires one optical transceiver, so the number of logical connections incident on each LSN (a vertex of the logical connection graph), called its degree, is limited by the number of optical transceivers in the NASs serving the corresponding LSN. Because this number is generally small for reasons of cost, the connectivity among the LSNs is sparse. Because of this sparse logical connectivity, most paths between pairs of end systems require multiple logical hops, traversing several LSNs. At each LSN the traffic is sorted and routed on the next logical link toward its destination. A key performance measure for any LRN is the average number of logical hops experienced by its traffic. Because each unit of traffic uses a network link on each hop, the same unit of traffic reuses the network a number of times equal to its hop count. Therefore, the average link load is proportional to the injected traffic multiplied by the average hop count. Because links have limited capacities, the maximum possible injected traffic, or network capacity, is inversely proportional to the average hop count. In addition to improving network capacity, lower hop counts improve many other measures of network performance. In the case of packet/cell traffic, fewer hops means less processing in the switches, less delay, less probability of packet/cell loss, and better survivability, because each connection is less likely to encounter a link or node fault. Similar remarks apply to circuit-switched traffic except that delay and cell loss are replaced by call-blocking probability. In this section we explore some of the properties of LRNs embedded on transparent optical infrastructures. Point-to-point logical topologies are considered first, followed by hypernets. The issue of the design of the logical topology is not considered here. However, this is an important factor in achieving high performance. The logical topology must be “matched” to the expected traffic distribution to keep the average hop count low, and at the same time it must be matched to the underlying optical infrastructure so that cost-effective embedding is possible. The design problem for LRNs is discussed in more detail in Chapter 7.

3.5.1

Point-to-Point Logical Topologies As mentioned earlier, one of the objectives of using logical switching on top of a transparent optical network is to reduce the cost of the station equipment (particularly

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(c) Folded Bus Figure 3.33 Eight-node ShuffleNet.

by reducing the complexity of the optics) while maintaining high network performance. Thus, we are interested in logical topologies that achieve a small average number of logical hops at a low cost (meaning small node degree and simple optical components). An interesting class of regular logical topologies called multihop networks was proposed with this objective in mind. An example is ShuffleNet. The idea of the ShuffleNet design was to use simple station equipment (a small number of transceivers requiring no rapid tuning) and yet achieve good performance in networks scalable to large sizes. Figure 3.33(a) shows an eight-node ShuffleNet logical topology assumed to be operated in a packet-switched mode. It consists of two stages with four LSNs each, joined by unidirectional links. There are 16 links, compared with a requirement of 56 for full logical connectivity. To clarify the flow of traffic, the first stage is repeated, and connections from the second stage back to the first are indicated by dashed lines, showing that the

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logical topology forms a cylinder. As in the end system interconnection example at the beginning of this chapter, each LSN performs two functions: It exchanges traffic with externally connected end systems, and it sorts and routes traffic in transit through the network to the destination end systems. A connection between any pair of end systems can be established over a suitable logical path, and it is easily seen that any node can communicate with any other node using a number of logical hops not exceeding three. It turns out that with uniform traffic and shortest path routing, the average number of hops in this network is two. Figure 3.33(b) presents the LCG in our standard bipartite form. These networks are scalable to large sizes by adding stages and/or by increasing the degree of the nodes (see Section 7.2.1). Up to this point nothing has been said about the underlying optical network. As originally proposed, a suggested implementation of ShuffleNet was on a folded bus, as shown in Figure 3.33(c). This PT is equivalent to a broadcast star in the sense that it allows no spectrum reuse, and therefore if a WDM/WDMA system is used, 16 wavelengths are required for the 16 point-to-point LCs. To improve spectrum utilization, let us now assume that the ShuffleNet is to be embedded in a wavelength-routed bidirectional ring using single fiber pair access links and two transceivers in each station. Figure 3.34 shows the locations of nodes on the ring. (The arrows on each link simply define a reference

d

4

e 8

3

f

c

6

7

g

b

5

2 a

Figure 3.34 ShuffleNet embedding.

1

h

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Table 3.3 Wavelength assignments for ShuffleNet on a ring. Source node

Destination node

1

2

3

4

5

6

7

8

1

—

—

—

—

1

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—

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6

—

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—

7

2

1

—

—

—

—

—

—

8

—

—

2

1

—

—

—

—

direction for the fibers according to the labeling convention introduced in Section 3.4.2. Each link consists of a pair of bidirectional fibers.) Note that the node/station placements are not arranged in numerical order. They were, in fact, chosen to minimize the required number of wavelengths. A possible wavelength assignment is shown in Table 3.3, which indicates that the 16 LCs can be realized using only two wavelengths, for a reuse factor of 8 (routing is always on the shortest path on the ring). Figure 3.35 shows the details of node 1 in this realization. As shown in Figure 3.35(a), node 1 receives traffic from nodes 5 and 7 (in addition to injected traffic) and forwards nonexiting traffic to nodes 5 and 6. Packets from node 5 (7) are received on wavelength λ 1 (λ 2 ) and delivered to the LSN, where they are buffered and sorted as indicated in Figure 3.35(b). Those destined for the local end systems exit the network at this point, and the others are routed to the two transmitters. Packets whose next node is 5 (6) are delivered to O T 1 (O T 2 ). The total capacity of this network is 8R 0 bps, where R 0 is the transmitter bit rate. This represents one half of the combined capacity of all transmitters, with the factor of 12 appearing because the average hop count is 2. For comparison, if full logical connectivity (56 LCs) was to be realized purely optically on the ring, this would require eight wavelengths with seven transceivers in each station and would give a network capacity of 56R 0 .

3.5.2

Multipoint Logical Topologies: Hypernets The seven-station example in Section 3.4.3 illustrated how high connectivity may be maintained in transparent optical networks while economizing on optical resource utilization through the use of multipoint connections and hypernet logical topologies. But that example was a special case of a hypernet wherein each station can reach any other station in one logical hop, so that no logical switching was required. These ideas are even more potent in larger networks, when multipoint connections are combined with logical switching. We now extend them to this more general setting while at the same time outlining a general approach for embedding logical topologies on prescribed

Network Connections

5

1

157

5

1

1 2

2

7

6 (a)

1

2

1

1

2

OT1

OT2

OR1

OR2

TP

TP

RP

RP

(b)

Figure 3.35 Details of ShuffleNet node.

physical topologies. Our illustrative example in Figure 3.36(a) shows a logical connection hypergraph that consists of 22 vertices (logical network nodes) and 10 undirected hyperedges, with each hyperedge of size four (i.e., containing four vertices). To provide more generality to this example, two types of nodes are included: LSNs (shown as hexagons) and logical terminal nodes (LTNs) (shown as triangles). The terminal nodes have no logical switching function and serve only as access points to interface end systems to the LRN. For example, in an ATM network the terminal nodes might execute ATM end node functions (such as call setup and admission control), and the LSNs would execute end node functions as well as call-routing functions (VP switching). The degree of a vertex in an undirected hypergraph is defined as the number of hyperedges to which it belongs. In this example all LSN vertices are of degree two, whereas the terminal vertices are of degree one. Vertices within the same hyperedge can communicate with each other over a single logical hop, whereas those in different hyperedges require several hops in tandem. For example, a packet moves between nodes U and C in one hop but requires three hops between nodes U and K . The sorting and routing functions in the LSNs

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permit full connectivity among all pairs of nodes in this network, albeit over multiple logical hops in most cases. Using shortest path routing, the longest logical path between a pair of nodes is four hops (e.g., from U to S via G–J–M). This is called the diameter of the hypergraph. (The seven-node LCH in Figure 3.26 is an example of a hypergraph in which every vertex has degree three, every hyperedge is of size three, and any vertex can reach any other in a single logical hop. Thus the diameter is one, implying that no logical switching is required.) We assume that the hypergraph has been chosen to match the traffic requirements of end systems attached to the nodes. However, it bears no relation to the physical topology that will support it. Figure 3.36(b) shows the location of the logical nodes on a prescribed physical topology to be called Atlantis. It is made up of 29 links and 18 ONNs. The ONNs contain LDCs; thus the optical infrastructure is an LLN. Each LSN or LTN accesses the optical network through its own NAS, connected to an ONN by a single fiber pair access link. We wish to embed our hypergraph into Atlantis, making economical use of optical resources. The general approach is shown in Figure 3.37(a), which shows several intersecting hyperedges of an arbitrary hypernet topology. They are to be embedded in the physical topology shown below them. As in our previous examples, each hyperedge represents a set of logical network nodes that are to be fully connected via an MPS realized in the purely optical network. For example, hyperedge E 1 in Figure 3.37(a) contains four LSNs and a terminal node, all of which must be embedded as an MPS. The dashed vertical lines indicate access links connecting the logical nodes to ONNs, where, for example, the NAS supporting logical node A is connected via link a to optical node a. Once more using the embedded broadcast star as a vehicle for realizing the MPS, we select a tree joining the logical nodes in the MPS (bold lines in Figure 3.37[a]), designating one ONN, say b, as its root. Because this is an undirected embedded star, the LDCs at the various nodes are set to route all optical signals from the transmitters in the five NASs to node b and then multicast the combined signals back to the five NASs. In this way, all 20 logical connections within the MPS can be arranged to share the common multicast medium. (As usual, this can be done with or without loopback connections.) The resultant routing tree is shown in detail in Figure 3.37(b), indicating the fibers used for each connection. The general embedding procedure now consists of two parts: (1) find a set of trees for the MPSs and (2) assign wavebands to the trees to satisfy the DCA constraint. The tree selection might be made to minimize tree size (shortest path routing), to avoid links loaded heavily with traffic from other users of the same infrastructure, or to satisfy any other criteria. If minimization of optical spectrum utilization (i.e., assigned wavebands) is the priority, the trees should be routed to minimize their intersections. This may lead, at times, to using some circuitous routes. Because the degree of each LSN is two, there will always be a pair of trees that intersects on each access link. With single fiber pair access links, this means that a lower bound on the number of required wavebands is two. It turns out that it is possible in this case to find trees and waveband assignments to support the 10 hyperedges using a total of three distinct wavebands, for a reuse

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159

A N

B

E3

E1

P

O

E2

E4

L

C

Q

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E E9

K

E7 T

F

U

E10 J

E8

G

I

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(a )

L l j 9

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q 8

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r

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4 d

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D (b)

Figure 3.36 Twenty-two node hypernet.

c C

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Multiwavelength Optical Networks

E4

B

A C

F

E D

E2 E1

E3 b

a

e

d

c

2

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(a) LSN including NAS LTN including NAS

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1 a A

a

a

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B d

3 d

d

D

D 4

E

e

e (b)

Figure 3.37 Hypernet embedding.

e E

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factor of 3.33. For example, a possible choice of trees for E 1 , E 2 , E 5 , and E 6 is shown as bold links in Figure 3.36(b). Note that they are nonintersecting on all links, including their access links, and hence the same waveband can be reused on all of them. It is left as an exercise for the reader to find routing trees and waveband assignments for the remaining MPSs. As in our other examples, either thin or thick wavebands can be used to implement the 12 required logical connections within each MPS. For example, if a single λ-channel is used in each waveband, the LCs can be realized using TDM/TDMA, with a single transceiver needed for an NAS to access one MPS. Because each LSN accesses two hyperedges, its supporting NAS needs two transceivers. The terminal node NASs only require one transceiver. However, if higher network capacity is desired, a TDM/T-WDMA system can be used over thick wavebands. Using 4 λ-channels in each waveband, for a total of 12 channels, and operating in FT-TR mode, each MPS now supports four times as much traffic as in the TDM/TDMA case, without increasing the number of optical transceivers. However, rapid receiver tuning is now required. Again using the wavelength-routed bidirectional ring as a baseline for comparison, we recall from the beginning of this section that 61 wavelengths are required for purely optical full connectivity among 22 stations, and 21 transceivers are required in each NAS. The improvement in spectrum usage in Atlantis compared with the ring is only partly attributable to a richer physical topology in this case. (Using a wavelength-routed network with single fiber pair access links, the least possible number of wavelengths and transceivers for full connectivity with any physical topology is 21.) The key to optical resource conservation in this case is the hypernet logical topology. The number of transceivers is kept small by the combination of multipoint optical paths and logical switching, which means that each LSN’s transmissions “fan out” to six other logical nodes using only two transmitters, with the same “fan in” advantage for the receivers. Spectrum usage is kept small by using a single routing tree to support 12 LCs in each MPS. The hypernet model is not limited to LRNs realized on an optical infrastructure. Any large network characterized by highly connected clusters of nodes, with intercluster communication realized through specially equipped relay points, has the basic structure of a hypernet. For example, a cellular radio network is conveniently modeled as a hypergraph in which each hyperedge represents a cell, the LSNs correspond to base stations, and the terminal nodes correspond to mobile users. In another context, an internet can be modeled as a hypernet. In this case, the hyperedges represent constituent networks (subnets of the internet), the LSNs represent gateways between the subnets, and the terminal nodes represent hosts accessing the subnets. What these networks have in common is that each is made up of constituent parts wherein the capacity of each part (a wireless cell or a subnet) is shared among its included nodes.26 End-to-end connectivity is maintained in any of these hypernets by finding a “good” logical path through the various relay points. 26

In the case of an internet, complete sharing within a subnet may not always be a good model of what is actually happening.

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3.6

Summary This chapter has given a largely descriptive picture of the four basic classes of multiwavelength networks in order of increasing complexity: uncontrolled or static, wavelength routed, waveband routed, and logically routed. In each case the underlying physical layer was a transparent optical network. The objective was comparative analysis. In drawing comparisons, realistic, specific parameter values were used as much as possible so that quantitative comparisons could be made. We found that the larger the network, the more complex the supporting architecture must be. The hardest part of scaling to large sizes is connectivity, because the number of possible end-to-end connections in a network grows as the square of the number of end systems. Connectivity in uncontrolled networks is limited by the inability to reuse the optical spectrum. Connectivity in wavelength-routed networks is limited by the fact that an optical transceiver and a λ-channel are required for each logical connection. This limit is circumvented in waveband-routed networks (LLNs) by using multipoint optical paths sharing the capacity of a common waveband. Multipoint paths are in turn limited by the fact that their combined throughput cannot exceed the capacity limit of the waveband they share. LRNs break the connectivity bottleneck but do it by implementing high connectivity electronically, incurring concomitant high processing loads in electronic switching equipment. The easiest aspect of scaling networks to large sizes is the throughput or network capacity requirement because optical fibers have so much capacity to start with and because each time we add an NAS to the network we add transceiver capacity. In moving from simpler to more complex architectures, we find that the efficiency of optical resource utilization improves, with more optical spectrum reuse and less optical transceivers required per station. The illustrative examples in this chapter, even the 22-node case, were chosen intentionally as “toy” problems – small enough and simple enough to be solved by inspection, by trial and error, or by a small amount of hand calculation. They are to be considered as points of departure for a deeper exploration of the various issues introduced here: topological considerations at the physical and logical levels; routing, channel, and waveband assignment; methodologies for network design; and the relations among performance, control, and resource utilization. Each section points to a later chapter: The static networks discussed in Section 3.2 are studied in more depth in Chapter 5, wavelength-/waveband-routed networks discussed in Sections 3.3 and 3.4 are described again in Chapter 6, and LRNs (discussed in Section 3.5) are described in detail in Chapter 7. The most important concept to retain from this chapter is the view of the overall network as many superimposed layers of connections. These connections can be configured to be one-to-one, one-to-many, many-to-one, and multicast. The end systems typically see a set of virtual connections that provide end-to-end transport on demand. The virtual connections are in turn supported by a network of logical connections matched to the needs of a community of end users. The links in the logical layer are realized

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as transparent optical connections, which are in turn supported by optical paths laid out on a fixed fiber topology. Except for the fibers, everything else in the network is reconfigurable, with each layer (to a large extent) independent of the others. Virtual connections can be routed over many alternate logical paths, and logical topologies together with the optical connections/paths that support them can be reconfigured independently. A large, purely optical network may serve as a common infrastructure for several independently managed logical networks, each one tailored to the needs of a different user community. Conversely, a logical network covering a geographic expanse extending beyond the reach of purely optical connections may be supported by several concatenated, independent optical networks, in which the end-to-end logical paths are formed as a sequence of separate parts joined together at logical gateways between networks. This extraordinary degree of flexibility produces opportunities and challenges that are explored in the rest of the book.

3.7

Problems 1 Show that if full virtual connectivity is required among the five stations in Figure 3.2(b), and if shortest path routing is used, there will be 10 virtual connections multiplexed on each LC. Generalize this result to the case of n stations. 2 The network of Figure 3.20 is operated as a WRN with each internodal link equipped with a single fiber pair. (a) Explain why at least two wavelengths are required to support full optical connectivity among the five stations. (b) Assuming single fiber pair access links, find a possible RCA for the 20 optical connections using a minimum number of wavelengths. (c) Repeat part (b) using nonblocking access links and only two wavelengths. 3 The network of Figure 3.26 is operated as a WRN with each internodal link equipped with a single fiber pair and each station having a nonblocking access link. (a) Explain why at least four wavelengths are required to support full optical connectivity among the seven stations. (b) Find a possible RCA for the optical connections required for full connectivity using a minimum number of wavelengths. 4 Modify the seven-station example of Section 3.4.3 and Table 3.2 as follows. Let guard bands between wavebands be increased to 100 GHz, with the usable bandwidth maintained at 125 GHz. Assume that in the case of the WRN the optical signals can run at twice the bit rates of the LLN (e.g., 5 Gbps instead of 2.5 Gbps) because of their increased wavelength spacing. Recalculate the spectrum utilization and total network capacity in this case for each row in the table. 5 Complete the routing and waveband assignment for the hypernet of Figure 3.36. (Check your answer against Table 7.6.)

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Bibliography [Bernstein+04] G. Bernstein, B. Rajagopalan, and D. Saha. Optical Network Control: Architecture, Protocols, and Standards. Boston, MA: Addison-Wesley 2004. [Maeda98] M. W. Maeda. Management and control of transparent optical networks. IEEE J. Select. Areas Commun., 16(7):1008–1023, 1998. [Schwartz87] M. Schwartz. Telecommunication Networks: Protocols, Modeling, and Analysis. Reading, MA: Addison-Wesley 1987. [Wei+98] J. Y. Wei, C.-C. Shen, B. J. Wilson, M. J. Post, and Y. Tsai. Connection management for multiwavelength optical networking. IEEE J. Select. Areas Commun., 16(7):1097–1108, 1998.

4

Enabling Technology Coauthored by Neophytos Antoniades The City University of New York/College of Staten Island

Throughout this book the approaches taken to system design and performance evaluation are based on the constraints of the enabling technology. Available fiber capacity is assumed to be limited by the constraints and imperfections of optical transceivers, amplifiers, and cross-connects. These constraints affect maximum available spectrum, wavelength spacing, and maximum bit rates per channel. Optical connections are assumed to have limited reach, both geographically and in terms of the number of optical cross-connects they may traverse. Sizes of switches as well as their speed, complexity, and functionality are also assumed to be limited by cost and performance constraints, ultimately going back to the limits of the underlying technologies. Trade-offs between optical and electronic methods of implementing connectivity and routing are suggested, in which the optimal design point depends again on relative cost and performance of the enabling technologies. Although emphasizing that these technological constraints are paramount, we purposely keep as much of a separation as possible between the architectures discussed in the book and the limitations of any specific technology. The reason is obvious: Today’s technology is likely to be obsolete tomorrow. After more than a decade of gestation in the laboratory, photonic and optoelectronic technology has matured to the point where a wide range of technological choices are available for implementing each function in a network, so that cost-effectiveness and viability in the field are the primary issues now rather than proof of concept, which was the issue in the network testbeds just a few years ago. After a period of explosive growth at the turn of the millennium followed by consolidation as the economy contracted and then stabilized, a broad industrial base now exists worldwide to respond to the need for components for the optical communications infrastructure. Keeping this background in mind, we present in this chapter the essential features of representative devices and systems used in the implementation of multiwavelength optical networks. The objective is to convey an understanding of the principles that underlie the functioning of the basic components of these networks, some notion of the performance of typical current devices, and a methodology for evaluating the performance of devices in network applications. It is impossible in a few pages to give more than a glimpse of the state of the art of current photonic and optoelectronic technology as it applies to optical networking. For

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a more complete treatment the reader is referred to books completely devoted to the subject (e.g., [Kaminow+02a, Kaminow+02b]).

4.1

Evolution of Transmission and Switching Technology Advances in optical networking thus far have been largely in the area of point-to-point optical transmission. Transmission system performance is typically measured in terms of bit-rate-distance products, where distance is measured from the source to the point at which a signal must be regenerated (i.e., “cleaned up”).1 The evolution of telecommunications from the early days of the telephone to current optical fiber systems shows about three orders of magnitude of increase in bitrate-distance product each time a new technology is introduced – from megabits per second-kilometers for microwave relays to gigabits per second-kilometers for unamplified fibers to terabits per second-kilometers for single-wavelength amplified links. Interest in single-wavelength optical transmission was soon replaced by a focus on multiwavelength transmission in the mid 1990s. Progress in the field can be judged anecdotally by comparing “hero” experiments reported in the first edition of this book to current record performance. A 360-Tbps-km experiment was performed in 1997 at KDD using 16 2.5-Gbps channels over 9000 km with wavelength separations of 0.5 nm [Otani+97]. Results presented the following year by the same group increased the bitrate-distance product to approximately 530 Tbps-km using 60 channels at 5.3 Gbps over 1650 km [Murashige+98]. Recently, a bit-rate-distance product of approximately 36,000 Tbps-km has been obtained using 149 channels running at 40 Gbps with 50GHz spacing over 6120 km [Charlet+05]. The resultant optical spectral efficiency of 0.8 bps/Hz is more than an order of magnitude higher than the previous results. For some time there has been nothing left to prove as far as transmission distance is concerned. Soliton demonstrations both in the laboratory and in the field suggest that the reach of the amplified, but unregenerated, optical fiber transmission link is for all practical purposes unlimited (see Section 4.3.3). Advances in optical switching technology have followed the scenario traced by earlier electronic networks. Developments in digital transmission (e.g., the T1 carrier system) made it cost-effective to move from analog to digital (i.e., time division) switching and therefore drove the development of digital switching technology. In a similar way, developments in multiwavelength transmission are a driving force in the development of multiwavelength switching technology. Because current activity is centered on network deployment in the metropolitan and regional area sectors, switching systems are being developed most actively for those applications. However, large switches for long-haul networks have also been successfully deployed. 1

Digital signal regeneration involves reshaping and retiming the pulses, operations that are almost always done electronically. However, all-optical approaches are now being demonstrated and have the potential to replace electronic techniques, especially at high bit rates.

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167

This chapter is organized as follows. Sections 4.2 to 4.9 focus on the point-to-point transmission link. We begin with a brief overview of the components of the communication path, followed by a discussion of optical fiber transmission principles and transmission impairments. Amplifiers, transmitters, and receivers are then considered in conjunction with related technologies: modulation formats, equalization, and forward error correction. A discussion of performance impairments in a network environment completes the transmission link picture. Sections 4.10 to 4.12 deal with the devices and subsystems necessary to combine transmission links into networks: coupling and switching devices, filters, multiplexers, wavelength converters, signal regenerators, and optical cross-connects. We conclude in Section 4.13 with discussion of physical layer performance evaluation methodology, applying it to some case studies of current interest. The objective is to show how simulation can be used to obtain insight and quantitative results based on large complex network models.

4.2

Overview of the Optical Connection A point-to-point optical connection (OC) in a multiwavelength network consists of an optical transmitter (OT) and receiver (OR) joined by an optical path consisting of fiber links traversing one or more optical network (switching) nodes (ONNs), as shown in Figure 4.1. In the example shown, a laser is modulated externally by an information source and is followed by a power amplifier that raises the power level launched on the fiber. Additional line amplifiers between fiber sections boost the power to compensate for attenuation along the line, and a preamplifier raises the power level at the input of the receiver to improve the signal-to-noise ratio (SNR) at the input to the electrical detector. A tunable optical bandpass filter is also shown, making this a tunable receiver. In digital systems, performance is measured in terms of bit error rates (BERs), whereas in analog systems the criterion is electrical SNR at the output of the receiver. The performance is affected by imperfections in the optical transmitter (for example, laser drift, linewidth, and modulator performance), transmission impairments in

Information Source Line Amplifier

Preamplifier

PD

Optical Modulator Power Amplifier

Laser

Electrical Detector ONN

OT

Figure 4.1 A point-to-point optical connection.

Bandpass Filter OR

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the fibers (attenuation, dispersion, and nonlinear effects such as four-wave mixing), amplifier characteristics (nonflat gain profile, amplified spontaneous emission noise), switch imperfections (cross-talk), and receiver imperfections (nonideal filtering and tuning, noise figure). Our discussion is limited for the most part to intensity-modulated direct-detection (IM/DD) systems. That is, the laser is intensity modulated either directly or externally, and the receiver uses direct detection of optical intensity.

4.3

Optical Fibers This section begins with an exposition of how light propagates through optical fibers under ideal conditions. This is followed by a survey of the principal impairments in fiber propagation: attenuation, dispersion, and fiber nonlinearities. We conclude with a brief description of soliton propagation, which provides a promising approach to very long-distance optical transmission.

4.3.1

Principles of Guided-Wave Propagation The end-to-end optical path in a communication system consists of a series of guidedwave structures: the fibers themselves, as well as the various components used for coupling signals in and out of the fibers and switching them from one fiber to another. Thus, we begin with a brief discussion of how light propagates through waveguides. In cases when geometries have large dimensions compared with a wavelength (for example, in multimode fibers or in free space), geometric optics is sufficient to explain the phenomena of interest. In other cases (for example, single-mode fibers), a wave picture is necessary.

4.3.1.1

Rays: Geometric Optics The wave-guiding properties of an optical fiber are easily understood in the context of a multimode step-index fiber. (However, for reasons to be explained later, multimode fibers are rarely used in high-speed long-distance communications.) As shown in the cross-section in Figure 4.2(a), a step-index fiber has a core of radius a with a constant refractive index n 1 , and a surrounding glass cladding of outside radius b with a slightly lower index n 2 .2 The air surrounding the cladding has a refractive index n 0 . Single-mode fibers typically have cores with diameters of 8 to 12 µm and a cladding diameter of 125 µm, whereas multimode fibers have cores of approximately 50 µm in diameter. Reflection and refraction at the boundary of two media are illustrated in Figure 4.3, which shows the case n 1 > n 2 . The relations between the angle of incidence θi , the angle of reflection θr , and the angle of the transmitted ray (angle of refraction) θt are θr = θi 2

(4.1)

The refractive index of a medium is the ratio of the speed of light in free space to the speed of light in the medium.

Enabling Technology

Step-Index Fiber

169

Graded-Index Fiber Jacket

b

b

Cladding a

a Core

n1

n2

a Index

Index

n1

n2

a b

b

n0

n0

Radial Distance (a)

Radial Distance (b)

c Figure 4.2 Refractive index profiles for fibers. (From [Agrawal97, Figure 2.1]. Copyright
1997. Reprinted by permission of John Wiley & Sons, Inc.)

and n 1 sin θi = n 2 sin θt ,

(4.2)

where the latter is called Snell’s law. What makes fiber optics work is total internal reflection, which was first demonstrated by John Tyndall in 1854. Equation (4.2) shows that there is a critical angle

Incident Ray

n1

n2 < n 1

θi

θr

θt

Reflected Ray Refracted Ray

Figure 4.3 Snell’s law.

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Unguided Ray

Acceptance Cone

θa

Guided Ray

θc

θc

Figure 4.4 Ray propagation in a step-index fiber. (From [Saleh+91, Figure 8.1-3a]. Copyright

c 1991. Reprinted by permission of John Wiley & Sons, Inc.)


θc = sin−1 n 2 /n 1 , where the transmitted ray lies right on the boundary. At angles of incidence greater than θc , all energy is totally reflected, resulting in a guided ray. It is these rays, guided within the core of the fiber, that carry our optical signals. Figure 4.4 illustrates guided and unguided rays. A ray entering the fiber at a sufficiently small angle of incidence (shown as the acceptance cone in the figure) is totally reflected, bouncing back and forth between the internal walls of the fiber as it propagates, whereas a ray incident outside the acceptance cone is partially refracted at each bounce. For rays entering the fiber from air, the largest possible angle of incidence for guided rays is the acceptance angle θ a , which equals 

n 21 − n 22 . sin−1 The numerical aperture of the fiber is defined as 

N A = sin θ a = n 21 − n 22 . Note that numerical aperture increases with the fractional refractive index change,  = (n 1 − n 2 )/n 1 . In a typical cladded fiber,   1; thus, only a narrow cone of light is accepted as a guided ray. However, in an uncladded fiber, both  and NA are large. (For an uncladded silica glass fiber, NA > 1, meaning that rays from all directions are accepted.) It is important to note that guided rays entering the fiber at slightly different angles of incidence take shorter or longer paths from end to end. Thus, if a pulse of energy incident on the fiber is spread throughout the acceptance cone, the arriving energy is dispersed in time due to the different path lengths so that the pulse is “smeared out” in time. This multipath or intermodal dispersion becomes worse for large numerical apertures. Thus, a high light-gathering ability does not make the fiber a good communication medium. To reduce the effects of intermodal dispersion, fibers are also manufactured with a graded-index profile. The ideal graded index for reducing dispersion turns out to be one in which n decreases parabolically from the center of the core to the cladding; see Figure 4.2(b). With a graded index, the rays are bent as they approach the cladding, as shown in Figure 4.5. This bending, together with the fact that rays farther from the core travel faster (due to a lower n), can reduce intermodal dispersion by several orders of magnitude.

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n2 a n1

Figure 4.5 Ray propagation in a graded-index fiber. (From [Agrawal97, Figure 2.3]. Copyright

c 1997. Reprinted by permission of John Wiley & Sons, Inc.)


4.3.1.2

Modes: The Wave Picture Geometric optics fails to predict the behavior of light when the dimensions of the confining medium are comparable with the wavelength, which is the case in singlemode fibers and in most photonic devices. In this case, the electromagnetic wave picture is required. For this we must start with Maxwell’s equations, the fundamental equations governing all electromagnetic phenomena. An electromagnetic wave is defined at any point in space and time by its electric and magnetic field vectors E(r, t) and H(r, t) respectively, where r is a position vector in some arbitrary coordinate system. These quantities are related by ∂D (4.3) ∇ ×H = ∂t ∂B (4.4) ∇ ×E = − ∂t ∇ ·D = 0 (4.5) ∇ · B = 0.

(4.6)

These are Maxwell’s equations for a charge- and current-free medium, where the quantities D and B are the electric and magnetic flux densities, respectively. In free space, the flux densities are directly proportional to the corresponding fields. However, in general we have D = 0 E + P

(4.7)

B = µ0 H + M,

(4.8)

where 0 and µ0 are the electric permittivity and the magnetic permeability of free space, respectively, and P and M are the polarization and magnetization densities, respectively. In almost all situations encountered in optical transmission, the medium is nonmagnetic (M = 0) – a condition that is assumed henceforth. The P vector can be related in complex ways to the corresponding E field, and it is this complex relationship in the glass medium that is responsible for most of the impairments in fiber transmission. Furthermore, by modifying this relationship in special ways through external control, it

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is possible to construct the many devices that are the building blocks of optical networks: switches, isolators, filters, and so forth. A wave analysis of fiber propagation normally begins with an assumption of an “ideal” fiber, after which nonideal conditions are added as small perturbations on the ideal model. Let us assume the following ideal conditions. The medium is r Linear: P is a linear function of E. r Nondispersive: The medium is nondispersive in time if P at any point in time is a function of E at the same point in time (in other words, the medium is memoryless). It is spatially nondispersive if P at any point in space is a function of E at the same point in space. r Homogeneous: The relation P(E) is independent of r. r Isotropic: The relation P(E) is independent of the direction of E. In isotropic media, P and E are parallel. Under these assumptions, Equation (4.7) can be written in the simpler form D = E,

(4.9)

where the scalar constant  is now the permittivity of the medium. Maxwell’s equations can then be combined to yield the vector wave equation ∇ 2u −

1 ∂ 2u = 0, c2 ∂ 2 t

(4.10)

√ where c = 1/ µ0 is the speed of light in the medium. The wave equation is satisfied by each component of both E and H. Because of linearity, any solution of the wave equation can be represented as a linear combination of other solutions. This suggests representing the optical fields that satisfy that equation as sums of sinusoids; that is, moving from the time domain to the optical frequency domain. Let E(r, t) = Re{E(r)e jωt } and H(r, t) = Re{H(r)e jωt }, where E and H are the complex envelopes of E, and H, respectively, and ω is the optical frequency in radians per second. Each of these functions represents a monochromatic signal. Substituting either of these into the wave equation, we obtain the Helmholtz equation ∇ 2 U + k 2 U = 0,

(4.11)

where k = ω/c is called the wavenumber. The story becomes much more involved from now on. Solutions of the Helmholtz equation must be found that match the boundary conditions of the fiber for the core and the cladding. Working in the cylindrical coordinate system of the fiber (Figure 4.6), the spatial variables are radius r , axial distance z, and angle φ. The classic approach uses separation of variables, in which case we seek solutions in the form U (r, φ, z) = u(r )e− jlφ e− jβz

(4.12)

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x

Er a

Cladding r

φ

Ez Core y

z

Eφ

c 1991. Reprinted Figure 4.6 Cylindrical coordinates. (From [Saleh+91, Figure 8.1-4]. Copyright
by permission of John Wiley & Sons, Inc.)

for an integer value of l.3 These represent traveling waves along the axis z of the fiber, with propagation constant β. Solutions of Equation (4.11) that satisfy the required boundary conditions exist only for certain permissible (characteristic) values of β. These values must satisfy the characteristic equation associated with Equation (4.11), which embodies the constraints of the fiber geometry. Each characteristic value of β yields a solution for u(r ) – a Bessel function in the case of cylindrical geometry – and for each value of the index l in Equation (4.12), the characteristic equation has a discrete set of solutions: βlm , for m, a positive integer. A solution of Equation (4.11) for each value of βlm represents a mode in the fiber. Note that there is a dependence on optical frequency (or wavelength) in Equation (4.11), through the wavenumber k = ω/c, so that the propagating (guided) modes and the constants βlm depend on the frequency of the optical signal. The function connecting each propagation constant βlm to frequency ω is called a dispersion relation. It is this relation that determines the speed at which power in the mode propagates: its group −1 = dβlm /dω. velocity vlm . The group velocity for a given mode is related to β by vlm When vlm is frequency dependent, monochromatic waves of different frequencies propagate at different speeds. This phenomenon is known as waveguide dispersion. Because a pulse of light contains energy distributed over a range of frequencies, each of these frequency components propagates at a different speed, resulting in a deformation (typically spreading) of the pulse as it propagates down the fiber (see Section 4.3.2.3). There are only a finite number of guided modes in the fiber at any given frequency, corresponding to a finite number of values of β satisfying the boundary conditions. Each mode except the fundamental mode (l = 0, m = 1) has a cutoff frequency below which it cannot be sustained in the fiber. Thus, as the optical frequency is decreased (wavelength is increased), a point is eventually reached when only one guided mode exists. A fiber is single mode when it operates over the range of wavelengths sustaining only the fundamental mode. These concepts have a more intuitive interpretation in the case   1, which corresponds to typical fibers. In this case, the wave propagating down the fiber has only small 3

Each component of the electric and magnetic fields is in this form.

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field components in the z direction, so that it approximates a transverse electromagnetic (TEM) wave (i.e., a plane wave propagating along the axis of the fiber). The condition for single-mode operation of a fiber of radius a is given by this equation:4 2π(a/λ0 )NA < 2.405,

(4.13)

where λ0 is the free space wavelength of the signal. Equation (4.13) indicates that for single-mode operation, a fiber must have a radius on the order of its wavelength of operation and/or a small numerical aperture. Like plane waves, modes in waveguides have polarization states. For each mode (i.e., each value of βlm ), there are two linearly independent solutions of the wave equation, which correspond to two orthogonal polarization states. For example, linear polarization in the two Cartesian directions x and y represents two orthogonal states. The polarization states are actually two “degenerate” modes with the same propagation constant. In a fiber with a circular cross-section, the polarization state (i.e., the relative amplitudes of each polarization mode) tends to change randomly along the fiber due to minor defects in the fiber. This is because the symmetry of the fiber and the fact that the two polarization modes propagate down the fiber at the same speed make it easy for power to be transferred from one mode to another. Sometimes it is desirable to eliminate this random drift in polarization, which can be done by forcing the two polarization states to have different propagation constants. In practice this is accomplished in polarizationmaintaining fibers by making the cross section of the fiber noncircular or by making the medium anisotropic.5

4.3.2

Optical Fiber Technology: Transmission Impairments As demands for high performance increase, it becomes more and more important to minimize the effects of transmission impairments in fibers, either through improvement and redesign of the fiber itself or by compensating for deleterious effects. This section explores the limits imposed on transmission distance and bandwidth imposed by attenuation, dispersion, and certain nonlinear effects in fibers.

4.3.2.1

Fiber Geometry and Fiber Cables Optical fibers for communications are normally made of silica and are manufactured with a variety of geometries. Certain dopants (e.g., GeO 2 and P2 O5 ) are used in the core to increase the refractive index, and others (e.g., B2 O3 ) are used in the cladding to decrease the refractive index. The refractive index profile influences the waveguide dispersion and can be designed to manage the overall dispersion characteristics of the fiber (see Section 4.3.2.3). The cables containing the fibers must be designed for mechanical and environmental protection as well as strength. Large cables may contain well over 100 individual fibers. Figure 4.7 shows some typical cable designs. One illustration depicts a ribbon cable, in which as many as 12 ribbons containing 12 fibers each are packed in one cable. 4 5

See [Saleh+91, p. 286]. Anisotropy can be induced by stressing the fiber in a particular direction.

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Kevlar

Mylar Tape

Polyurethane Polyethylene

Fiber

Fiberglass

Steel Core

Polyethylene

Fiber Ribbons Fiber

175

Steel Rod

Polyethylene Tube

c 1997. Reprinted Figure 4.7 Commercial fiber cables. (From [Agrawal97, Figure 2.20]. Copyright
by permission of John Wiley & Sons, Inc.)

4.3.2.2

Attenuation Although the ideal fiber discussed earlier is a lossless medium, real fibers have losses due to a number of mechanisms. The simplest way to express these losses is through a relation of the form d P/dz = −α P,

(4.14)

where P is the optical power propagating down the fiber at some point z, and α is a positive attenuation coefficient. Integrating Equation (4.14) gives PR = e−αL PT ,

(4.15)

where PT is the power launched into the fiber and PR is the power received at the end of a fiber of length L. The attenuation coefficient is generally expressed in units of decibels per kilometer; that is, αdB = −

PR 10 , log10 L PT

(4.16)

where L is in units of kilometers. In this case, the exponent in Equation (4.15) becomes −0.23αdB L. The coefficient α is a function of wavelength, as shown in Figure 4.8. As can be seen, the attenuation minimum occurs near 1550 nm and is approximately 0.2 dB/km in currently used fibers. A secondary minimum of approximately 0.5 dB/km occurs near 1300 nm. Most current systems operate near 1550 nm. As indicated in Figure 4.8, losses are due primarily to three effects: material absorption, Rayleigh scattering, and waveguide imperfections. r Material absorption occurs because of resonances of the silica molecules as well as the impurities in the fiber. Resonance absorption is indicated in the figure as ultraviolet (UV) absorption (due to electronic resonances at the short wavelengths) and infrared absorption (due to vibrational resonances at the longer wavelengths). The most serious impurity effect is due to OH ions (from traces of water trapped in the fiber), which cause the major peak at 1390 nm as well as several minor peaks.

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αdB (dB/km)

3

1 OH Absorption

Rayleigh Scattering 0.3 UV Absorption Band Tail

0.1 0.6

0.8

1.0

Infrared Absorption

1.2

1.4

1.6

1.8

Wavelength λ (µm)

Figure 4.8 Attenuation as a function of wavelength. (From [Saleh+91, Figure 8.3-2]. Copyright c 1991. Reprinted by permission of John Wiley & Sons, Inc.)


r Rayleigh scattering occurs because the medium is not absolutely uniform, which causes small fluctuations in the refractive index. This causes the light to be scattered, attenuating the propagating wave. Because the scattering is proportional to λ−4 , Rayleigh scattering is the dominant loss factor at the short wavelengths. This, together with infrared absorption at the long wavelengths, limits the usable optical spectrum to a range of approximately 800 to 1700 nm (excluding the OH attenuation peaks), which represents approximately 20 THz of bandwidth. r Waveguide imperfections are caused by nonideal fiber geometries, which occur due to manufacturing imperfections and small bends and distortions in the fibers. Normally these contribute a relatively small additional component to the loss. As can be seen from Figure 4.8, the various loss mechanisms limit the low loss transmission window to a neighborhood of 1550 nm. Most current high-speed and medium- to long-haul transmission systems operate near the center of this window: 1530–1565 nm, designated as C- (conventional) band by the International Telecommunications Union (ITU). As demand has grown for more usable bandwidth, and technology has been developed to exploit a wider transmission window, additional bands have been defined: the longer wavelength bands, the L- (long) band, 1565–1625 nm, and the U- (ultra long) band, 1625–1675 nm; and the shorter wavelength bands, the S- (short) band, 1460– 1530 nm, the E- (extended) band, 1360–1460 nm (where the OH peak occurs) and the O- (original) band, 1260–1310 nm, where the first generation of relatively short-range lightwave systems operated. In 1998 Lucent Technologies introduced ALLWave fiber, in which the “water peak” loss is effectively eliminated. Other manufacturers followed with similar products.

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Dense wavelength division multiplexing (DWDM) with wavelength spacings down to 12.5 nm was standardized by the ITU in 2002 for long-haul applications. However, it was clear that the economics of metropolitan area optical networks were quite different from long-haul networks (link lengths of 100s or 1000s of kilometers). Metro networks (diameters of about 100 km or less) can tolerate more severe fiber transmission impairments than long-haul networks, which means they can operate outside of the C band. However, they cannot justify the expensive equipment necessary to maintain the tight requirements for wavelength stability in DWDM. These considerations, together with the fact that the problem of the water peak had been eliminated led to the coarse wavelength division multiplexing (CWDM) standard designed for Metro networks. It specifies a grid at 20-nm spacing providing 18 channels covering a range from approximately 1300 to 1600 nm. Although fiber loss was the major limiting factor in optical fiber transmission until the 1990s, this limitation was eliminated to a large degree in C-band systems, virtually overnight, with the introduction of the EDFA in 1989 (see Section 4.4). More recently, improvements on the EDFA, Raman amplifiers (Section 4.4.2), and other types of fiber amplifiers have extended the usable long-haul transmission window into the longer and shorter wavelength bands.

4.3.2.3

Dispersion With optical amplification greatly mitigating the effects of attenuation, the effects of fiber dispersion become more important. A narrow pulse launched on a fiber tends to smear out as it propagates along the fiber, with its width broadening as it progresses. When a pulse broadens to the extent that it overlaps neighboring pulses, the resultant intersymbol interference (ISI) sharply increases the BER. This fiber dispersion phenomenon imposes a limit on the bit rate that can be supported on a dispersive fiber of a given length. There are two basic dispersive effects in a fiber: intermodal dispersion and chromatic dispersion.

Intermodal Dispersion The most serious form of dispersion, intermodal dispersion, occurs in multimode fibers because different modes have different group velocities. (This was illustrated in the geometric optics picture of propagation, in which different rays traveled different distances.) Because the pulse power is distributed (generally unequally) over different modes, replicas of a pulse, one for each mode, arrive at the destination with different propagation delays, spreading out the received energy. Figure 4.9 illustrates pulse-broadening effects in different types of fibers. As shown in Figure 4.9(a), the pulse arrivals for different modes in a step-index fiber are spread out in time, with different energy in each. Figure 4.9(b) depicts how received pulses are bunched closer together in a graded-index fiber. In fibers greater than a certain critical length, mode coupling due to material imperfections can cause a reduction of the intermodal dispersion, as shown in Figure 4.9(c). Because of intermodal dispersion, multimode fibers are limited to very low bit-rate-distance products (on the order of tens of megabits per second-kilometer for step-index fibers and a few gigabits per second-kilometer for graded-index fibers).

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0

t

0

t

0 (a) Multimode Step-Index Fiber

t

0

t

0

t

(b) Graded-Index Fiber

0

t

(c) Multimode Step-Index Fiber (coupled modes) Figure 4.9 Broadening of pulses due to dispersion. (From [Saleh+91, Figure 8.3-8]. Copyright

c 1991. Reprinted by permission of John Wiley & Sons, Inc.)


Chromatic Dispersion Although intermodal dispersion is absent in single-mode fibers, there is another effect – chromatic dispersion, also called group velocity dispersion (GVD) – that occurs due to the frequency dependence of the group velocity. There are two sources of chromatic dispersion: (1) waveguide dispersion and (2) material dispersion. A simple way of understanding chromatic dispersion without going into the exact details of pulse shapes is to work in the frequency domain. Assuming that the transmitted pulse has a spectral width σω in optical (radian) frequency, (or, equivalently, σλ = λ2 σω /2πc in wavelength), each spectral component of the pulse arrives with delay T = L/vg , where vg is the group velocity for that component. The received pulse width is then broadened by an amount    d −1   (4.17) σT ≈ L  vg  σω = |D|Lσλ , dω where the dispersion parameter D (given in units of picoseconds per kilometer– nanometer) is defined as6 2π c d 2 β . (4.18) λ2 dω2 As indicated in Section 4.3.1.2, the propagation constant β in a single-mode fiber depends on frequency through the dispersion relations, which in turn depend on the D=−

6

When operating at a zero-dispersion wavelength (D = 0), a more exact expression for pulse broadening, taking higher derivatives of β into account, is necessary.

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Cladding

a

Core

a 0

0

n

n

Dispersion Coefficient

179

0

0

λ (a) Dispersion-Shifted Fibers

λ (b) Dispersion-Flattened Fibers

Figure 4.10 Dispersion coefficients as a function of frequency. (From [Saleh+91, Figure 8.3-6].

c 1991. Reprinted by permission of John Wiley & Sons, Inc.) Copyright


geometry of the fiber. This is the source of waveguide dispersion. In addition, the material in fibers is time dispersive; that is, the polarization density P is not a memoryless function of E in the fiber medium. This time-dependent effect can be modeled as a frequency-dependent refractive index n(ω), which produces an additional, materialdependent frequency variation in β, so that D = D M + DW ,

(4.19)

where D M is the material dispersion and DW is the waveguide dispersion. In general, both components are frequency dependent. The material dispersion in standard silica fibers is negative at short wavelengths and positive at the longer wavelengths, passing through zero at approximately 1300 nm. Because DW is a function of fiber geometry, it is possible to produce fiber designs, called dispersion-shifted fibers, in which the contribution of DW causes the zero-dispersion point to shift to the 1550-nm range, thereby making minimum dispersion occur at the same wavelength as minimum loss. In dispersion-flattened fibers, the design produces a dispersion profile that is close to zero over a wide spectral range. These two cases are illustrated in Figure 4.10. A typical refractive index profile for a dispersion-shifted fiber is shown in Figure 4.10(a), together with the dispersion coefficient as a function of wavelength. The material dispersion is indicated by a dashed line and the combined material and waveguide dispersion is shown by a solid line. Figure 4.10(b) shows the same quantities for dispersion-flattened fibers.

Polarization Mode Dispersion As mentioned earlier, the two polarization states of the fundamental mode may propagate at slightly different group velocities due to asymmetries in the fiber. This is known as polarization mode dispersion (PMD), which also produces pulse broadening. The effects of PMD are considered to be negligible on a digital transmission link if the average time differential, τPMD , between the two modes is less than one-tenth of

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the bit period of the signal. For example, this gives an allowable value of average time differential of 40 ps for a 2.5-Gbps OC-48 signal or 10 ps for a 10-Gbps OC-192 signal. As opposed to chromatic dispersion, a phenomenon that can be calculated in a deterministic fashion, PMD is typically produced by independent randomly distributed effects in a fiber. Because of this the PMD effects accumulate in a “square root” fashion. Thus, for a link of length L km, we have √ τPMD = D PMD L, (4.20) √ where D PMD is the PMD dispersion coefficient in ps/ km. In a normal (not polarizationmaintaining) fiber, this effect is usually negligible compared to chromatic dispersion. However, because of its deterministic nature chromatic dispersion is relatively easy √ to manage, whereas PMD is not. Typical values of PMD are less than√ D PMD = 0.2 ps/ km 7 for newly installed standard single-mode fiber √ and D PMD = 2 ps/ km for a dispersionshifted fiber but can be as low as 0.15 ps/ km. However, PMD can be more serious in polarization-sensitive switching devices. For example, for a lithium niobate switch (see Section 4.10.1), a typical value of average time differential might be τPMD = 2 ps. Thus, cascading of a large number of these can present dispersion problems at high bit rates. See Section 4.9 for a more detailed discussion.

Polarization-Dependent Loss and Gain The variation of the insertion loss/gain with the different input signal polarization states is called polarization-dependent loss/gain (PDL/PDG). Due to these effects, a polarized signal applied to a polarization-sensitive component may be attenuated or amplified differently than the unpolarized noise, and the signal-to-noise ratio will be modified accordingly. Such SNR modifications lead to performance degradation. PDL appears in optical components such as directional couplers, isolators, filters, and band-splitters, whereas PDG is an effect found in optical amplifiers and certain types of switches. The system impact of PDL is discussed in Section 4.9.2.

Effects of Dispersion on Maximum Bit Rates As stated in the beginning of this section, it is the ISI due to pulse broadening that limits bit rates in dispersive fibers. A good estimate of the maximum permissible bit rate is obtained by assuming that the bit rate Rt cannot be allowed to exceed a value Rmax = k/σT = k/|D|Lσλ , where a reasonable value of k is 0.25, indicating a pulse overlap into the next bit interval of approximately 25%. It remains to connect pulse spectral width σλ to other system parameters. Unfortunately, σλ depends in a complex way on the laser characteristics, the way in which it is modulated (direct or external), and the waveform of the modulating pulse. However, some simplified relations can be obtained in two limiting cases: (1) wide laser linewidth and (2) narrow laser linewidth. In the first case, the laser linewidth (in optical frequency) is assumed to be much larger than the width of the Fourier transform of the pulse itself. The latter is on the order of 7

√ Although there is still older fiber in the ground with PMD values of D PMD = 0.5 ps/ km.

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T p−1 , where T p is the pulse width. This is typical of directly modulated or multimode lasers modulated at relatively low bit rates. In this case, σλ ≈ σl , where σl is the laser linewidth (in wavelength units), so that 0.25 . (4.21) Rmax ≈ |D|Lσl In the second case, the laser linewidth is assumed to be small compared with the pulse spectral width. This is called the transform-limited case. It applies, for example, to externally modulated single-mode lasers running at high bit rates (gigabits per second). We now have σλ ≈ λ2 /cT p . Assuming that Rt ≈ T p−1 , we have 1 0.25c . (4.22) Rmax ≈ λ |D|L

Analog Modulation: Subcarriers Effects of dispersion manifest themselves significantly but differently in analogmodulated systems. For example, consider an optical carrier, intensity modulated by a sinusoidal signal. This produces upper and lower sidebands with phases relative to the carrier that are modified by dispersion as the signal propagates down a fiber. This phase change can produce a situation in which the intensity modulation is almost completely converted to phase modulation. Using direct detection, the phase modulation will not be reproduced at the output of the photodetector, so that the modulating signal will be lost. These phase shifts due to dispersion limit the maximum possible modulation frequency. In an example of this effect, analyzed in [Meslener84], it is shown that a total dispersion of 2000 ps/nm limits the modulation frequency to a maximum of approximately 5 GHz. (Dispersion of this magnitude might typically occur over 100 to 200 km of standard single-mode fiber [SMF].) This is a serious limitation in subcarrier systems, in which the subcarrier frequencies can be in the 10-GHz range (see Section 5.2.2).

Dispersion Management The history of fiber deployment throughout the world closely reflects the development of the related transmission system technology. Until the late 1990s, most of the fiber in the ground in the United States and Europe was standard SMF, with properties that are consistent with the state of the transmission art in the mid-1980s. The dominant transmission systems at that time were “second-generation” systems (using the terminology in [Henry+88]). These systems, operating in the 1300-nm band, became feasible when suitable laser and detector technology became available in that band. (At that time, the technology for the 1550-nm band had not yet been developed.) Dispersion was a particularly serious impairment in second-generation systems because single-mode lasers were not available at that time. Thus, the fact that 1300 nm is close to the zero-dispersion point in an SMF made it a logical band in which to work even though attenuation is considerably higher than at 1550 nm. With the development of single-mode lasers (see Section 4.5.1) – the current “thirdgeneration” systems – operating in the 1550-nm band largely replaced the second generation. However, to deal with the nonzero dispersion in this band, the fiber deployed to

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optimize transmission performance was dispersion-shifted fiber (DSF). Because Japanese deployment followed that in the United States, DSF is the dominant fiber deployed in Japan today. The advent of the EDFA, which operates in the 1550-nm band, extended enormously the reach of these third-generation systems. And with that extension came the appearance of nonlinear transmission impairments; in particular, four-wave mixing (FWM). Nonlinear effects had been negligible before the EDFA because the maximum transmission distances between signal regenerators were on the order of hundreds of kilometers, compared with thousands of kilometers for amplified systems. This was not long enough for nonlinear impairments to become significant. It was quickly realized that minimization of dispersion in the fiber made the four-wave mixing problem worse. Thus, and fortuitously, the older SMF turned out to be better for combatting nonlinearities (see Section 4.3.2.4). It remained, however, to deal with dispersion. It can be seen from Equation (4.22) that in the best possible case – the transformlimited regime – the dispersion-limited reach of a connection decreases with the square of the bit rate. Thus, to extend this limit, something must be done about the dispersion coefficient D. In attacking this problem, the most important observation is that it is a linear phenomenon. Thus, its effects are reversible by cascading suitable compensating linear components. One of the first approaches to dispersion management was to use lengths of dispersion-compensating fibers (DCFs) in cascade with a dispersive transmission link. The DCFs are designed to have a large negative dispersion in the 1550-nm band to compensate for the 10 ps/nm-km to 20 ps/nm-km positive dispersion in SMFs. Another approach uses a nonzero dispersion fiber (NDF) with small dispersion (positive or negative), tailored to reduce the effects of nonlinearities without significant pulse broadening. By using fiber spans with alternating positive and negative dispersion, the net dispersion coefficient can be made almost zero over a long span. At the same time, the nonzero dispersion over each span tends to reduce the effects of FWM [Chraplyvy+95]. In one experiment a 400-Tbps-km bit-rate-distance product was achieved using combinations of positive-dispersion NDF and DCF [Srivastava+98]. Spectral inversion is still another approach to dispersion compensation [Gnauk+97]. In midsystem spectral inversion, the signal spectrum is inverted in the middle of a long span of dispersive fiber. In this way, the frequency dependence of the group velocity (which produces dispersion), experienced by the signal over the second segment of the span, is just the opposite of that experienced over the first half of the span. This is because the frequency components of the inverted signal spectrum “see” relative phase shifts that are just the opposite of those seen by the uninverted spectrum. The net effect is to cancel out the dispersion over the total span. Several processes have been used for implementing spectrum inversion, including four-wave mixing and difference-frequency conversion (see Sections 4.3.2.4 and 2.3.3). Finally, in a fourth approach, chirped fiber Bragg gratings (FBGs) are used for dispersion compensation. In the work reported in [Garrett+98], six long (1-m) gratings were cascaded over 480 km of conventional fiber to achieve eight-channel transmission with a 38.4-Tbps-km bit-rate-distance product. We leave the details of FBGs to Section 4.10.4.3.

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With the development of various means of controlling dispersion the next issue that appeared was dispersion slope. In a WDM system carrying many wavelengths, the slope of the dispersion curve causes different wavelengths to experience different dispersion. To improve performance, compensation is now applied to slope as well as dispersion itself. A monolithically integrated AWG-based slope equalizer has been demonstrated with 8- and 16-channel operation at 40 and 20 Gbps, respectively [Takiguchi+03]. At this writing, dispersion compensation is a rapidly developing activity. It is clear that as systems and networks move to higher bit rates (OC-768 and beyond) dispersion management becomes a critical task for the network designer. Dispersion slope compensation also becomes necessary for long-haul systems. Several commercial dispersion compensation modules have been announced that have a high dispersion slope compensation rate (80–100%) that is suitable for standard single-mode fiber. At the above high data rates, small changes in dispersion across the system can be very deleterious. These changes may come from various factors such as temperature changes in the environment, network reconfiguration, and/or fiber imperfections. Tunable optical dispersion compensators are an attractive solution to this problem although their economics are not yet justified. (A MEMS-based tunable dispersion compensator is described in [Madsen+99].) A more economical approach is settable optical dispersion compensation modules, which offer the designer the ability to change their dispersion value on the day of system installation but remain fixed thereafter. In addition to compensation at the level of fibers, there are several new and promising approaches for pre- and postcompensation within the transmitter and receiver, respectively. Examples are prechirp techniques (Sections 4.5.3.1 and 4.9.3) and electronic equalization (Section 4.7.3). We are close to the day when dispersion will be managed at will, and the dispersion problem will be largely vanquished.

4.3.2.4

Nonlinear Effects Just as dispersion effects became significant when the attenuation problem was solved, nonlinear effects became dominant when dispersion was reduced. Although the nonlinear effects in fibers are very small, they can become important over long, amplified but unregenerated transmission spans. The following nonlinear effects are of concern in multiwavelength systems: r r r r

Stimulated Raman scattering (SRS) Stimulated Brillouin scattering (SBS) Four-wave mixing (FWM) Self- and cross-phase modulation (SPM/XPM)

Stimulated Raman Scattering In each of the two scattering effects just listed, light interacts with the fiber medium producing inelastic collisions, during which the wavelength of the scattered photon is longer than that of the incident photon, meaning that energy is lost in the process. In each case, a signal present at the wavelength of the scattered photons can produce stimulated emission of another photon at the same wavelength and hence amplification.

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Stimulated Raman scattering involves energy loss to vibrational waves in the medium, with all waves interacting with each other as they propagate in the same direction in the fiber. In the case of a single-channel system, the threshold power at which SRS becomes important is very high: approximately 500 mW. Thus, as opposed to Rayleigh scattering, it is unimportant as a loss mechanism in communication systems. It does have important consequences in introducing cross-talk (intermodulation) in multiwavelength systems. Because SRS involves the transfer of power from the wave at the higher frequency to that at a lower frequency, it can be used to advantage for optical amplification, in which the higher frequency light serves as a pump, amplifying the lower frequency wave carrying the signal to be amplified. This is a very broadband process (approximately 200 nm), which was proposed for fiber amplification as long ago as 1973 [Stolen+73]. Although it has been largely eclipsed by the EDFA, there has recently been renewed interest in Raman amplification in both the 1300- and 1550-nm bands. (See Section 4.4.2.)

Stimulated Brillouin Scattering In SBS, the power lost in the scattering process is transferred to an acoustic wave. In this case the scattered wave and the acoustic wave both propagate in the backward direction. As opposed to SRS, SBS is a very narrowband process: The downshift in frequency for systems operating in the 1550-nm band is approximately 11 GHz. For this reason, there is no significant cross-talk effect, but the threshold for SBS is much lower than that for SRS – a few milliwatts for fibers longer than 20 km.

Four-Wave Mixing FWM and phase-modulation effects can be explained in terms of a nonlinear relation between P and E, producing nonlinear refraction. In silica glass, there is a small cubic term in this nonlinearity, related to a third-order nonlinear susceptibility χ (3) (see [Agrawal95, p. 16]). The cubic relation between P and E can be expressed as a square-law nonlinearity in the refractive index: n(ω, |E|2 ) = n 0 + n 2 |E|2 ,

(4.23)

where the first term represents the linear, frequency-dependent part of n, accounting for material dispersion, and the second term accounts for various nonlinear effects. The relation in Equation (4.23) produces intensity-dependent and lossless8 effects on the fiber, leading to pulse distortion when only a single signal is present and cross-talk when several signals are present, as in multiwave systems. Although these effects are small, they can accumulate substantially on long, amplified fiber links. These effects are not always deleterious. They can sometimes be used to advantage (see Sections 2.3.3 and 4.3.3). Four-wave mixing can occur if three signals are present at neighboring optical frequencies ω1 , ω2 , and ω3 . The cubic nonlinearity then potentially produces additional signals at the sum and difference frequencies – for example, ω4 = ω1 + ω2 − ω3 . (Another possibility exists with only two signals present at frequencies ω1 and ω2 , 8

Energy may be exchanged between propagating waves, but no energy is absorbed by the medium, as it is in SRS and SBS.

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giving ω4 = 2ω1 − ω2 .) These signals occur only if certain “phase-matching” conditions are satisfied. Phase matching of two propagating waves occurs when their propagation constants are equal. In this case, approximate phase matching happens when the frequencies of all four signals are sufficiently close together. This condition occurs in the case of dense WDM systems running at moderate power levels. We then have ω1 ≈ ω2 ≈ ω3 ≈ ω4 , where ω4 = ω1 + ω2 − ω3 . With phase matching, FWM accumulates along a long fiber link, imposing a severe distance limit on multiwave transmission. For example, a 32-channel system running at a power of 0.5 mW per channel with 50-GHz channel spacing on DSF is limited to approximately 100 km. The problem of FWM on a DSF is exacerbated by the fact that the near absence of dispersion causes different frequency waves to propagate at nearly the same group velocity, producing approximate phase matching. A way to avoid FWM is to break the phase-matching condition by intentionally introducing dispersion. For example, in an amplified SMF, the system described in the previous paragraph has negligible FWM for as long as 5000 km! Thus, it is possible to solve both the dispersion problem and the FWM problem simultaneously by introducing controlled alternating (positive and negative) dispersion throughout a long fiber link to disrupt phase matching while maintaining the net end-to-end dispersion near zero [Chraplyvy+95].

Self- and Cross-Phase Modulation Refractive index nonlinearity can also produce small changes in the propagation constant as a function of intensity. Over a length of fiber, this produces accumulated phase shifts, which vary with intensity. The phase shifts represent SPM, producing a corresponding broadening of an optical pulse as it propagates down the fiber. To a first approximation, this can be explained by the fact that the pulse is phase shifted by a different amount at each point in time, depending on its intensity at that time. When more than one signal is present, the nonlinear interactions between the signals can produce a related phenomenon – XPM – that depends on the aggregate power in all signals. This becomes more troublesome as more signals are superimposed on each other in multiwavelength systems.

Effective Interaction Length Because the various nonlinear effects depend on intensity, they become less significant as the intensity of a wave decays. Thus, the nonlinear effects tend to disappear as the wave progresses down the fiber. This results in a limited effective length over which the fiber nonlinearity influences the wave. For a fiber with attenuation coefficient α, this effective interaction length is defined as L eff =

1 − e−αL , α

(4.24)

where L is the actual fiber length. For long fibers, we have L eff ≈ 1/α, which gives L eff ≈ 20 km in the 1550-nm window. It is important to note that the concept of effective interaction length applies to unamplified systems. In systems containing cascaded line amplifiers, each amplifier on

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the fiber typically restores the signal intensity to the value originally launched on the fiber, so that an additional length L eff must be associated with each amplified span.

Nonlinear Effects in Multiwavelength Systems As indicated, most of the nonlinear impairments in fibers increase with increasing numbers of simultaneous channels. Thus, it is of interest to study these effects in systems with many closely spaced channels. Figure 4.11 shows relations between maximum permissible power per channel and number of channels in a typical system, considering the various nonlinear effects individually. The case considered is a DSF of length 22 km operating in the 1550-nm band with 10-GHz channel spacing. The criterion for determining upper limits is a 1-dB power penalty; that is, at the limiting power a typical channel will have its SNR degraded by 1 dB at the receiver due to the nonlinearity being examined. In the case of SRS with many closely spaced channels, power is transferred from the higher frequency channels to the lower frequency ones; that is, the higher frequency channels act as pumps, amplifying the lower frequency channels. Because this pumping

1000 Raman

100 Maximum Power per Channel (mW)

186

XPM

10

Brillouin

FWM

1

0.1

0.01 1

10

100

1000

Number of Channels Figure 4.11 Limitations due to nonlinear effects in multiwavelength systems. (From [Chraplyvy90,

c 1990 IEEE. Used by permission of The Institute of Electrical and Figure 10]. Copyright
Electronics Engineers, Inc.)

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depends on the intensity at a particular point in time, the power transfer depends on the bit patterns of the various waves, thus introducing a cross-talk effect. The Raman power limit in a multiwavelength system is illustrated in Figure 4.11. Another curve in the figure shows the power limit imposed by XPM, as a function of the number of channels. In contrast to SRS and XPM, Figure 4.11 shows that the power limits imposed by FWM and SBS are independent of the number of channels. However, they are the critical ones for systems with a small number of channels.

Large Effective Area Fibers Much effort has gone into the design of fibers with a large “effective core area.” The objective is to increase the area over which the optical signal power is distributed and yet keep the desired wave-guiding properties of the fiber.9 The advantage of these fibers is that spreading the total optical power over a larger area reduces the optical field intensity in the core at any one point. This, in turn, reduces the various nonlinear effects discussed previously. Various designs involving special (e.g., fluoride) doping, as well as special index profiles, produce effective core areas exceeding 100 µm2 , which is roughly twice the area of standard fibers, while maintaining the desired fiber properties [Kato+98].

4.3.3

Solitons Well before the advent of fiber optic communications, it was known that a special type of solitary wave or soliton could exist in certain types of media that are both dispersive and nonlinear. Although each of these effects by itself tends to distort and broaden a propagating pulse, the right combination of dispersion and nonlinearity produces a narrow, stable pulse that propagates over long distances without any distortion whatsoever, with one effect compensating for the other. This, of course, is the ideal situation for long-distance communication. The form of a soliton can be deduced by modifying the wave equation, Equation (4.10), to include a nonlinear and time-dispersive refractive index. For a wave propagating in the z direction, we can write 1 ∂ 2 (n 2 E) ∂2 E − 2 = 0, 2 ∂z c ∂ 2t

(4.25)

where n in Equation (4.25) is of the form of Equation (4.23). By assuming a quadratic dependency of β on ω (corresponding to a linear dependence of group velocity on ω), assuming that the nonlinearities and dispersive effects are weak, and dropping “small” terms, it can be shown (e.g., see [Kazovsky+96]) that the complex envelope u(z, t) of a soliton satisfies the nonlinear Schrodinger equation: ∂ 2u j ∂u = sgn(β2 ) 2 − j|u|2 u. ∂z 2 ∂t 9

(4.26)

A challenge in designing large effective core area fibers is to keep the bending losses low. Larger core diameters tend to increase losses due to the bending of the fiber.

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t − β1z T0

Figure 4.12 Soliton.

A solution of Equation (4.26), called the fundamental soliton solution, is10 

t − β1 z jaz , u(z, t) = U0 e sech T0

(4.27)

where a = |β2 |/T02 , T0 is the pulse width, and β1 and β2 are, respectively, the first and second derivatives of β with respect to ω. Equation (4.27) represents a soliton with a shape shown in Figure 4.12. The fundamental soliton is stable in the sense that if a pulse approximating a soliton in shape and amplitude is launched on a fiber it tends toward a soliton as it propagates and thereafter retains the soliton form. Furthermore, solitons propagating in opposite directions pass through each other “transparently.” To maintain solitons over long distances, fiber amplification is required. Many soliton experiments using EDFAs and picosecond pulses were carried out in the 1990s. Typical experiments used a recirculating fiber loop containing several EDFAs to simulate long transmission links. An example is a demonstration of soliton transmission over a 15,000km distance at 5 Gbps. The distance was simulated by a loop with 27-km EDFA spacing [Mollenauer+92].

4.3.4

Photonic Crystal Fibers To this day, virtually all optical fibers used in telecommunications are variations on the theme of the core/cladding structure of Figure 4.2. However, in recent years, the feasibility of a radically new fiber structure, the photonic crystal fiber (PCF), has been demonstrated. Also known as a holey or microstructured fiber, it guides light using an arrangement of air holes running along its z axis. Figure 4.13 shows three configurations, where the white areas are air holes in the silica fiber. The holes are inserted in the fibers by stacking capillary tubes in the fiber preform. Three different methods of guiding light are used in these fibers. The one most resembling ordinary fiber propagation is the effective-index PCF, shown in Figure 4.13(a). Here the array of holes surrounding the solid core (made by omitting the center hole in the regular array) play the role of the low index cladding in an ordinary fiber. However, by properly dimensioning the holes and their spacing, the effective index of the array can be made considerably lower 10

An infinite number of higher order solutions also exist.

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(b)

189

(c)

c 2001 Figure 4.13 Three types of microstructured fibers. (From [West+01, Figure 1]. Copyright
IEEE. Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

than the cladding index of an ordinary fiber and strongly wavelength dependent: low at long wavelengths and approaching the core index at short wavelengths. As a result the condition for single mode operation (Equation [4.13]) is satisfied at all wavelengths, producing an “endlessly single-mode” fiber. A second version of the PCF is the air-clad core fiber of Figure 4.13(b), where a solid center is surrounded by a single ring of large diameter holes, which produce the effect of a solid rod hanging in air, meaning that the core-cladding index ratio is extremely large. In this case, propagation effects are quite different: the effective area seen by the propagating mode is exceedingly small, meaning that the fiber becomes highly nonlinear because of the increased power density seen by the material. Finally, the third version is the photonic band gap (PBG) fiber. An example is shown in Figure 4.13(c), where an air core is surrounded by a regular array of holes. In this case, the holes act much like a crystal lattice, interacting with the lightwave to produce photonic band gaps: wavelength bands where no propagation takes place down the fiber. One application of this type of PCF is that the guided wave can be essentially confined to the hollow core, resulting in extremely low attenuation. As with many new technologies, applications are often serendipitous: the predicted ones (e.g., broadband low loss transmission) have not materialized, but other applications, especially at the component level, are being proposed at a rapid rate. For example, by properly tailoring the hole configuration, fiber designs can be optimized for dispersion flattening, dispersion shifting and dispersion compensation [Shen+03]. Polarization maintaining PCFs have been proposed using hole arrays that produce highly birefringent fibers [Libori+01]. A number of optical signal processing applications make use of fiber nonlinearities, which can be controlled and enhanced using PCFs (see Section 4.11). An example is pulse compression using a highly nonlinear microstructured fiber [Petropoulos+03]. The array of holes forming the photonic crystal can also be used on a slab to produce waveguides in a planar configuration. In [Sugimoto+03] a directional coupler is built using this technique, where the guides are fabricated by omitting rows of holes from

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the array where the guides are located. Another application of a planar geometry is a PCF-based purely optical switch [Stapleton+07]. A directional coupler composed of photonic crystal waveguides on a silicon-on-insulator wafer is operated as a switch under optical control. Light traversing the coupler is directed to one or the other of the output ports by illuminating the coupler with an optical control signal from an 850-nm laser. The same basic structure can also be configured as a tunable filter. A particularly intriguing potential application involves the combination of PCF and semiconductor technology. The goal is to fabricate semiconductor devices within the holes of microstructured fibers so as to produce all-fiber optoelectronic devices [Badding+06]. The vapor deposition technology used in semiconductor fabrication has been adapted for this task by using pressures that are high enough to make the gases act like fluids. In this way, the chemical vapors flow into the holes of a PCF producing coatings on the walls of the holes that have the desired electronic properties. A field effect transistor has been fabricated within a fiber as a proof of the utility of these techniques. These methods open the door to many applications of microstructured fibers that extend their use beyond purely optical waveguiding devices to the optoelectronic domain; for example, embedding semiconductor pumping within a fiber laser or enhancing and tailoring nonlinearities within a fiber by inserting appropriate nonlinear semiconducting material – possible uses would be in wavelength conversion, regeneration, and other optical signal processing operations. At this writing, PCFs for practical applications are still on the beginning of the learning curve, and their success or failure will depend on how practical issues such as manufacturability, reliability, and cost are resolved. However, the prognosis is good, considering the rate at which demonstrated applications are appearing on the scene.

4.4

Amplifiers Because fiber attenuation limits the reach of a nonamplified fiber span to approximately 200 km for bit rates in the gigabit-per-second range, wide area purely optical networks cannot exist without optical amplifiers. Amplifiers are typically used in three different places in a fiber transmission link. As power amplifiers, they serve to boost the power of the signal before it is launched on the line, extending the transmission distance before additional amplification is required. Amplifiers operating as line amplifiers are located at strategic points along a long transmission link to restore a signal to its initial power level, thereby compensating for fiber attenuation. Finally, as a preamplifier, the device raises the signal level at the input of an optical receiver, which serves to improve signal detection performance (i.e., the receiver sensitivity). In each case, the desired properties are different. For power amplifiers, the important feature is high gain; preamplifiers require a low noise figure, and line amplifiers require both. Optical amplifiers are also employed at various other points in a network (for example, within an optical switching node to compensate for losses in the switch fabric). Semiconductor optical amplifiers (SOAs) were developed in the 1980s but they never had a serious impact on long-distance transmission because of a number of negative features discussed in Section 4.4.3. In

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the case of fiber amplifiers, especially the EDFA and the Raman amplifier (RA), however, the situation was quite different. The first articles on EDFAs appeared in 1987 [Desurvire+87, Mears+87]. Within a few years of that time, 9000-km unrepeatered transmission was demonstrated. Shortly thereafter, soliton experiments showed that transmission distances could be extended almost indefinitely. All of these experiments used EDFAs. It is not an exaggeration to say that these devices have revolutionized optical communications. Although the EDFA played a fundamental role in extending the reach of optical transmission systems it still had some drawbacks, including operation confined to a limited band of the optical spectrum and a nonflat gain profile. In contrast, RAs, which were first demonstrated well before the EDFA [Stolen+73] and then virtually ignored for three decades, have more recently attracted renewed interest. This stems mainly from their ability to increase both the reach and the aggregate bit rate carried on a fiber; that is, the usable fiber bandwidth. As cost has replaced performance as the driving force in technology development, other more cost-effective amplifers have come to the fore, collectively known as amplets. We begin this section with a discussion of EDFAs, followed by descriptions of RAs and SOAs and concluding with a brief look at amplets.

4.4.1

Erbium-Doped Fiber Amplifiers The EDFA belongs to a family of rare-earth-doped fiber amplifiers, the class of other possible dopants, including praseodymium (used for amplification in the 1300-nm range), neodymium (originally used for very high-power lasers), ytterbium (which has been used as a codopant with erbium), and thulium (amplifying in the S band). The important place of the EDFA in optical communications is due primarily to the fact that the properties of erbium produce amplification in a fairly wide band (approximately 35 nm) within the 1550-nm low-attenuation window in fibers. Furthermore, the EDFA has many other desirable features that will become apparent as we proceed. Three different EDFA structures are shown in Figure 4.14. In each case, the amplifier is of the traveling wave type, consisting of a strand of single-mode fiber, typically on the order of tens of meters long, doped with erbium. (The points S in the figure represent fiber splices.) The EDFA is an optically pumped device, so energy is supplied by an optical source (an LD), which injects power into the doped fiber at a wavelength matched to the characteristics of erbium (980 or 1480 nm). Pumping can be forward, backward, or bidirectional (see Figures 4.14[a] through 4.14[c], respectively). The pump is typically coupled into the transmission fiber via a wavelength-selective coupler (WSC). (EDFA modules used in the field typically include other components, such as optical isolators to eliminate reflected power, and various devices for signal power monitoring, stabilization, and control.) Amplification occurs by transfer of power from the pump wave to the signal wave as it propagates down the doped fiber. Like many other forms of amplifiers of electromagnetic radiation, the EDFA operates via a three-energy level system. The model representing this process is shown in Figure 4.15. Levels E 1 , E 2 , and E 3 are the ground, metastable, and pump levels,

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Pump Signal In

Signal Out

WSC

S

S

S

EDF

S Pump LD (a) Forward Pumping

Pump

Signal In S

EDF

Signal Out

WSC

S

S S

Pump LD (b) Backward Pumping

EDF Pump

Signal In

Pump

WSC

S

S

S

WSC

Signal Out S

S

S Pump LD

Pump LD

(c) Bidirectional Pumping

Figure 4.14 Basic erbium-doped fiber amplifier structures. (From [Desurvire94, Figure 5.4].

c 1994. Reprinted by permission of John Wiley & Sons, Inc.) Copyright


E3

N3

980 nm

N2

E2

1530 nm

N1 Figure 4.15 Energy levels in EDFA.

E1

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respectively. The populations (fractional densities) of erbium ions in the three energy levels are denoted N1 , N2 , and N3 , where N1 > N2 > N3 when the system is in thermal equilibrium (no pump or signal present). When pump and signal are present, these populations change as ions move back and forth between levels, accompanied by the emission or absorption of photons at frequencies determined by the energy-level difference. The wavelengths associated with the dominant transitions are indicated in Figure 4.15. The wavelength λ for each transition is given by the quantum relation λ = hc/E, where h is Planck’s constant and E is the difference in energy levels. In actuality, the three levels in the simplified diagram of Figure 4.15 are narrow bands, so each transition is actually associated with a band of wavelengths rather than a single line. Two pump wavelengths are typically used for EDFAs: 980 and 1480 nm. As shown in Figure 4.15, by absorbing energy from a 980-nm pump, Er3+ ions in the ground state are raised to state E 3 . The rate at which these transitions occur is proportional to N1 Pp , where Pp is the pump power. These excited ions decay spontaneously to the metastable state E 2 , and this transition occurs at a rate much faster than the rate from level E 1 to level E 3 .11 This means that in equilibrium under the action of the pump, the ion population in the ground state is reduced and accumulates largely in state E 2 . This process is referred to as population inversion because we now have N2 > N1 , the reverse of the situation in thermal equilibrium. The transition rate from level E 2 to level E 1 is very slow compared with the other transitions, so that the lifetime τ , in the state E 2 (the reciprocal of its transition rate to E 1 ) is very long (approximately 10 ms). Similar pumping action can occur at 1480 nm, in which case the ions are raised directly to the upper edge of the E 2 band. Reliable semiconductor laser pump sources have been developed for EDFAs at both the 980- and 1480-nm pump wavelengths. The wavelength band for transitions from state E 2 to the ground state is in the 1530nm range, making it ideal for amplification in the lowest attenuation window of fibers. The dominant transitions from E 2 to E 1 are radiative, which means that they are of two types: spontaneous emission and stimulated emission. In the former case, an ion drops spontaneously to the ground state, resulting in the emission of a photon in the 1530-nm band, and this appears as additive noise. Spontaneous emission noise is an unavoidable by-product of the amplification process, predicted by quantum theory. Its phase, direction, and polarization are independent of the signal. In the case of stimulated emission, an incident photon in the 1530-nm range stimulates the emission of another photon at the same wavelength in a coherent fashion (with the same direction, phase, and polarization). If the incident photon is from a signal, this produces the desired amplification of the optical field. However, the incident photon could also have originated as a spontaneous emission “upstream” on the fiber, in which case this is called amplified spontaneous emission (ASE), which represents the major source of noise in amplified fiber transmission systems.

11

The transition is dominantly nonradiative, so energy is lost to the fiber medium rather than emitted in the form of radiation.

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Gain

194

1500

1550

1600

λ (nm)

Figure 4.16 EDFA gain profile.

4.4.1.1

Gain Profile The fairly large amplification bandwidth of the EDFA is due to the finite width of the energy bands. The width of the energy bands is caused by a number of physical phenomena, including the Stark effect [Desurvire94, p. 8], which splits the main energy levels into many sublevels. Because the population is not distributed uniformly within the E 2 band, the gain is not flat. A typical plot of gain as a function of wavelength is shown in Figure 4.16. The uneven gain profile, with a peak at approximately 1530 nm, produces significant problems in a multiwavelength system when many amplifiers are cascaded over a long transmission span. Not only does uneven gain amplify different wavelengths unequally, but it also causes a large accumulation of ASE at the peak of the gain profile, which can eventually saturate the amplifier. Because amplifier cascading on long links accentuates these effects seriously, gain flattening is an important consideration in EDFAs. Several solutions to this problem are currently in use. One approach is to modify the design of the amplifier itself by using different materials such as fluoride glass [Clesca+94]. Other approaches use gain equalization via controllable attenuators (see [Ford+98] for a MEMS-based example) or inverse filtering. (See [Vengsarkar+96] for an example using fiber gratings.)

4.4.1.2

Gain Saturation The gain of an EDFA is approximately independent of the signal power as long as the pump power is made high enough so that the pumping rate is much larger than the stimulated emission rate. This is called the unsaturated gain or small-signal regime. The small-signal gain under these conditions is an increasing function of pump power. For a given fiber structure and doping, and a given pump power, there is an optimal fiber length that maximizes gain. For lengths smaller than the optimum, the pump power is not maximally utilized, and for larger lengths, pump power is exhausted somewhere along the fiber, and attenuation takes over. Typical optimal lengths are in the range of tens of meters. Maximum small-signal gains for EDFAs are typically 30 to 40 dB. All amplifiers eventually exhibit gain saturation as the signal power is increased. In the saturated case, the signal extracts so much power from the pump as it propagates down the fiber that the stimulated emission rate becomes comparable with the pumping rate.

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The larger the input signal, and the higher the unsaturated gain, the sooner saturation is out reached. As saturation increases, the gain decreases. The saturation output power Psat is defined as the output power at which the gain is compressed by 3 dB. The values of out for typical EDFAs are in the hundreds of milliwatts. It should be noted that ASE Psat also contributes to saturation in an EDFA. When input signals are very small, it is the ASE that saturates the amplifier first. This is known as amplifier self-saturation. Because saturation is a nonlinear effect, it produces a number of complications when multiple signals are being amplified. One problem is that the saturated gain for any one signal depends on the aggregate power of the other signals as well as its own power. Thus signals (as well as accumulated ASE) tend to “steal” power from each other. Useful analytical models for these effects appear in [Jopson+92, Saleh+90]. An advantageous effect of saturation is that a small amount of it in each amplifier in a cascade of several amplifiers tends to produce a self-regulating effect. Several other nonlinear effects are a consequence of this power-stealing phenomenon but on a shorter time scale. The amplifier gain at any instant in time is a function of the excited state population N2 , which is depleted momentarily by stimulated emission when a signal is present. One manifestation of this occurs when an intensity-modulated digital signal changes from a 0 to a 1. The resultant fluctuation in N2 causes corresponding gain fluctuations, which are most pronounced in the saturated regime and in the presence of large signals. Another manifestation occurs when beats from two signals spaced closely in optical frequency cause gain fluctuations at the beat (difference) frequency. The gain fluctuations affect all signals being amplified and thus can potentially produce undesirable cross-talk, with one signal’s intensity fluctuations changing the gain for the others. These effects are significant only when the gain dynamics are such that gain can vary on a time scale as fast as that of the signal fluctuations. A simplified interpretation of gain dynamics in an EDFA is based on the assumption that the maximum speed for gain fluctuations is on the order of the reciprocal of the lifetime in the excited state, which is approximately 10 ms. However, actual gain transients in EDFAs can occur on time scales of hundreds of microseconds, which cannot be predicted using the lifetime alone. (For a more detailed explanation see [Desurvire94, p. 417].) In any case, these numbers indicate that signals fluctuating on time scales more rapid than, say, 100 µs will cause no significant cross-talk in EDFAs. This corresponds to a minimum bit rate of approximately 10 Kbps to avoid cross-talk (or a WDM signal separation of approximately 10 KHz to avoid beat frequency effects). The lack of this cross-talk effect for bit rates higher than 10 Kbps is one of the important advantages of the EDFA over the SOA (see Section 4.4.3).

4.4.1.3

Noise and Noise Figure The ASE noise generated in an EDFA can be the limiting performance factor in an optical transmission link. It is therefore important to quantify this effect. For an amplifier with gain G, the ASE noise power spectral density at the output at optical frequency ν (in each polarization state) is [Agrawal97] Ssp = n sp (G − 1)hν,

(4.28)

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∆f

Filter/ Detector

Pin PD (a)

∆f

Filter/ Detector (b)

Figure 4.17 Illustration of noise figure.

where n sp , the spontaneous emission factor, is a function of the state population and approaches its minimum value of 1 with full population inversion. The ASE noise spectrum for an EDFA is roughly the same shape as the gain profile. The significance of the ASE noise is most clearly expressed in terms of SNRs and the amplifier noise figure Fn . These quantities are defined in terms of electrically detected signals in an ideal system, as illustrated in Figure 4.17. The noise figure is defined as Fn =

SNRin , SNRout

(4.29)

where SNRin is the electrical SNR seen when a signal of power Pin is converted to a photocurrent at the output of an ideal photodetector (PD) – see Figure 4.17(a). The noise in this case is shot noise due to the fact that the ideal detector is counting photons, which arrive randomly at the detector. (The detection process must be an integral part of any noise calculation, reflecting the quantum limits of lightwave transmission.) The numerator in Equation (4.29) is given by SNRin =

(R Pin )2 (R Pin )2 Pin = = , 2 σ 2q R Pin  f 2hν f

(4.30)

where (R Pin )2 is the square of the average photocurrent, σ 2 = 2q R Pin  f is the shot noise power (the variance of the photocurrent), R = q/ hν is the responsivity of an ideal detector, q is the electron charge, and  f is the bandwidth of the electrical detector. The quantity SNRout is the electrical SNR seen with the amplifier inserted before the photodetector – see Figure 4.17(b). To find SNRout , we compute the variance of the photocurrent after amplification with gain G. Because the detector acts as a square-law device, the photocurrent variance contains terms due to shot noise and ASE noise by themselves, as well as signal-spontaneous emission beat noise because of the mixing between the signal and the ASE in the photodetector. It turns out that the latter is the dominant term, provided that G 1, and most of the ASE noise is filtered out at the input of the detector. This can be done by making  f small enough to exclude extraneous noise but include the desired signal. Then we have σ 2 ≈ 4(RG Pin )(RSsp ) f

(4.31)

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so SNRout ≈

G Pin . 4Ssp  f

(4.32)

Using Equations (4.28), (4.29), (4.30), and (4.32), the noise figure is given by Fn ≈ 2n sp (G − 1)/G,

(4.33)

which corresponds to at least a 3-dB SNR degradation in the high-gain case. (In real systems, Fn is typically at least 4 dB.)

4.4.1.4

Amplifier Chains Over a long transmission link, it is necessary to use several EDFAs interconnected by fiber sections to compensate for fiber attenuation.12 The gain of each amplifier is normally adjusted so that it compensates for the attenuation on one section of fiber. The question of optimal amplifier spacing then arises. It turns out that this is a fairly complex issue that depends, among other things, on the way in which the amplifiers are pumped, effects of fiber nonlinearities, and practical issues such as amplifier accessibility, cost, and so forth. We examine a fairly simple model here, in which a fiber of length L is divided into N sections of spacing s = L/N . An amplifier is placed after each section, with a saturated gain that just compensates for the fiber attenuation on one section: G = eαs.13 The total accumulated noise power spectral density at the end of this chain (taking into account both polarization states) is then Sn = 2N Ssp = 2n sp (G − 1)N hν = 2n sp (eαs − 1)N hν.

(4.34)

Note from Equation (4.34) that for a fixed amplifier spacing the effect of accumulated noise in the cascade grows linearly with the length of the link but decreases as the amplifier spacing decreases (i.e., as the number of amplifiers increases). Thus, the optimal strategy in this case is to place a very large number of low-gain amplifiers very close together, with the limiting case being one long, distributed amplifier. Cost, however, dictates the opposite strategy! In current practice, a compromise is reached, with spacings ranging from 20 to 100 km, typically giving an SNR at the receiver of at least 15 dB. The spacings are based on constraints such as maximum permissible power on a fiber,14 effects of fiber nonlinearities, and receiver sensitivity. 12 13

14

It is also possible to design the EDFAs as distributed amplifiers in which the doped fiber is long (on the order of kilometers) and the gain is distributed throughout its length [Desurvire94, p. 122]. Exact compensation is not necessary. In a long chain, if the amplifiers are adjusted to have a small-signal gain slightly larger than the section attenuation, saturation tends to stabilize gains at a cascaded value that just compensates for the end-to-end fiber loss. This happens because if the signal amplification overcompensates for loss on the upstream stages, the resultant increased signal power drives the downstream amplifiers into saturation, thereby tending to stabilize the overall saturated gain. The maximum power is determined not only by the technological limitations – laser and amplifier output powers and fiber nonlinearities – but also by eye safety as well. The latter consideration limits power to well under 100 mW to limit risks to service personnel (see [ANSI88]).

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Normalized gain coefficient

1.2 copolarized

1.0

orthogonal 0.8

0.6 0.4 0.2

0.0 0

5

10 15 20 25 30 Pump-signal frequency difference [THz]

35

Figure 4.18 Raman gain coefficient in bulk silica as a function of frequency shift. (From

c 2004 IEEE. Used by permission of The Institute of Elec[Bromage04, Figure 15]. Copyright
trical and Electronics Engineers, Inc.)

4.4.2

Raman Amplifiers The discussion of the EDFA provides a useful framework for describing the Raman amplifier: They are both fiber amplifiers, with important similarities as well as differences, so they can often complement each other in applications. As mentioned in Section 4.3.2.4, stimulated Raman scattering (SRS) can cause transmission impairments in fibers, but it can also be used for amplification. When SRS is used for amplification, pump power is introduced into a fiber carrying an optical signal, with the pump operating at a frequency higher than the signal frequency, just as in the EDFA (and other rare-earth-doped fiber amplifiers). The pump photons interact with the material in the fiber through inelastic collisions, producing scattered photons at lower energy (and frequency) than the pump photons, with the remaining energy imparted to the fiber medium in the form of vibrational waves, called optical phonons. If the frequency of the scattered photon is the same as that of a signal photon propagating in the fiber, it can stimulate the emission of a second signal photon, thereby amplifying the signal, a process identical to that which occurs in the EDFA. The performance of the RA can be expressed in terms of a Raman gain coefficient (RGC). An illustration of the form of a normalized RGC as a function of frequency shift between the pump and signal appears in Figure 4.18. As the figure shows, RA gain is polarization dependent. The gain coefficient for copolarized pump and signal waves is an order of magnitude higher than in the orthogonally polarized case. Polarization dependence is mitigated by the averaging effect of the polarization mode dispersion in the fiber medium and can be circumvented by using either polarization diversity pumping or a single depolarized pump [Bromage04]. An important difference between the RA and the EDFA is that the energy levels of Er3+, which determine the gain profile of the EDFA (Figure 4.16) are fixed, thereby

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fixing the position of the amplification band of the device, as well as the possible pump frequencies. The amplification band for the EDFA is fixed in the vicinity of 1530 nm – the middle of C-band – which is a primary reason for its importance in optical communication but which limits its flexibility in exploiting other transmission bands in optical fibers. In contrast, for the RA it is only the pump/signal frequency difference (a band centered around 13 THz) that is fixed by the physics of the process, and any pump frequency can be used. Changing the pump frequency automatically shifts the waveband where amplification occurs. Thus the amplification band of an RA can be centered at any desired frequency in the optical fiber transmission window by adjusting the pump frequency appropriately. Furthermore, for a single pump the amplification bandwidth is large (about 6 THz in Figure 4.18), and this band can be extended by superimposing several pumps at different frequencies. This makes the RA an excellent tool for widening the usable bandwidth of long-haul WDM transmission systems beyond C- and L-bands into the S- and U-bands and beyond. The RA can be configured either as a distributed or discrete (lumped) amplifier. A typical distributed RA (DRA) consists of a long transmission fiber into which a counterpropagating (backward) Raman pump is injected. (Backward pumping reduces the effect of pump noise, as explained below.) The distributed amplification results in reducing the perceived loss along the span, which effectively improves the reach of the span and/or increases its capacity. In a discrete RA, the amplifier consists of a coil of fiber together with pump(s) and ancillary equipment for monitoring, control, and perhaps other purposes such as dispersion compensation, gain flattening, or adding and dropping channels. Isolators are used to keep the pump power from escaping into the line. The fiber medium used in the discrete case is shorter than in the DRA, but it still is typically of the order of kilometers – two orders of magnitude longer than the EDFA. A significant advantage of the discrete RA is that the amplifying fiber can be chosen at will to suit a number of criteria. For example, a dispersion compensating fiber can be used to provide dispersion compensation for the transmission fiber, with the additional benefit of improving the Raman gain coefficient [Gruner-Nielson+01]. The primary purpose of discrete RAs is generally to expand the usable bandwidth of a transmission link, whereas the primary purpose of a DRA is to improve the reach of a fiber span. When several amplified spans are placed in tandem, with lumped line amplifiers placed at the junction points, the result is a hybrid arrangement as shown in Figure 4.19. The advantage of this arrangement is illustrated by a comparison of signal powers along the line with and without the DRAs (see Figure 4.20). Without the DRAs the signal level drops linearly along each span. Due to the overall span loss a high signal power must be launched into each span, which tends to produce nonlinear impairments at the beginning of the span. But at the far end of the span, the attenuation drops the signal into the noise level. Clearly the span is too long for discrete amplification alone. However, by adding distributed amplification throughout the span, the signal initially attenuates at almost the same rate as without RA but then increases in power toward the far end of the span as it encounters stronger pump power. The DRA pump signal power is also shown, decreasing from right to left in the figure, which is why distributed gain is highest toward the far end of the span. The net effect of the distributed amplification in the spans is to improve the overall system performance by reducing noise as well as nonlinear effects.

Multiwavelength Optical Networks

Transmission Line Fiber (Raman Gain Fiber) Pump Signal(s) Coupler

Coupler

Raman Pump Unit

Optical Line Amplifier Coupler

Amplified Transmission Signal

c Figure 4.19 Hybrid distributed-discrete amplification. (From [Islam02, Figure 4]. Copyright
2002 IEEE. Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

Distributed amplification keeps the signal above the noise level at the far end of each span, so the optical SNR at the input of each line amplifier is improved. Furthermore, distributed amplification makes it possible to launch the signal into each span at a lower power level, thereby reducing nonlinear impairments due to high signal levels. There are a number of additional considerations that work for and against the RA. On the positive side, it operates in ordinary silica fibers, requiring no special materials or dopants. This makes it ideal as a means of adding distributed amplification to existing long transmission links. Furthermore, it has better ASE noise properties than the EDFA. The latter has an ASE noise power spectral density given by Equation (4.28), where n sp > 1 due to incomplete population inversion, whereas the RA noise spectrum has the same form with n sp = 1; i.e., it acts like an EDFA with full population inversion. However, there are additional sources of noise in RAs that can be more serious than ASE: in particular, multipath effects caused by reflections and double Rayleigh scattering. Rayleigh scattering causes forward propagating signals (or noise) to be scattered Original Transmission Signal

DRA Pump Signal

Nonlinear Effects

Signal Power (dB)

200

High Noise

DRA Amplified Transmission Signal

Distance

c Figure 4.20 Signal and pump power in hybrid system. (From [Islam02, Figure 5]. Copyright
2002 IEEE. Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

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backward, but when a signal encounters this phenomenon twice, the doubly scattered signal propagates in the forward direction, recombining with the original signal after a multipath delay. Discrete double reflections due to imperfections, splices, and connectors in the fibers cause similar multipath effects. Because the Raman effect is weak, long fibers are required in RAs, which tend to increase the multipath effects. Unintended reflections and Rayleigh scattering are present in all fiber systems, but they are attenuated in a passive fiber. However, when the fiber is pumped the Raman gain magnifies these effects to the point where the multipath interference places a limit on the usable gain in an RA [Bromage04]. RAs are normally less efficient than EDFAs in converting pump power to output signal power. However, their efficiency improves, exceeding that of the EDFA, at the large aggregate signal powers that occur in long-haul WDM systems with high channel counts [Islam02]. Furthermore the gain in the fiber medium depends strongly on the type of fiber being used. Because gain is proportional to pump intensity, it increases when a given amount of pump power is confined to a small fiber core. Thus, fibers with smaller cores such as DCFs produce significantly higher Raman gain. This is a particular advantage in discrete RAs, where there is some choice in the type of fiber being used. Another drawback of the RA is that it has a very fast (fs) response to pump fluctuations. This can lead to coupling of pump noise into the amplified signals. These effects can be mitigated by using a counter-propagating pump, in which case the effects of the pump fluctuations are averaged out over the length of the pumped fiber. Otherwise, they require the use of “quiet” pumps; i.e., pumps with very low relative intensity noise. In deploying RAs as distributed amplifiers, there are some other practical concerns due to the high pump powers employed. Connectors should be minimized in favor of splices to reduce reflections and attenuation, and when connectors are required they must be designed to survive the high pump powers. Also, to protect personnel, automatic laser shutdown systems must be employed.

4.4.3

Semiconductor Optical Amplifiers The structure of an SOA is similar to that of a semiconductor laser (see Section 4.5.1). It consists of an active medium (a p–n junction) in the form of a waveguide, with a structure much like the stripe geometry laser described in Section 4.5.1. The mobile carriers (holes and electrons) now play the role of the Er3+ ions in the EDFA. The energy levels of the electrons in a semiconductor are confined to two bands: the conduction band, containing those electrons acting as mobile carriers, and the valence band, containing the nonmobile electrons. A hole, representing the absence of an electron in the valence band, also acts as a mobile carrier. Mobile electrons and holes are abundant (i.e., are majority carriers) in n-type and p-type material, respectively. The two energy bands in semiconductors play a role analogous to band E 2 and E 1 in the EDFA, but they are much broader than the EDFA bands. A band gap, E g , separates the lower edge of the conduction band from the upper edge of the valence band so that the energy change involved in moving from one band to the other is at least E g . Transfer of an electron from the valence band to the conduction band (with

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the absorption of energy) results in the creation of an electron-hole pair. One way in which this occurs is through the absorption of a photon, as in a photodetector (see Section 4.6.1). The reverse phenomenon, electron-hole recombination (with release of energy), occurs either nonradiatively (by transferring energy to the crystal lattice) or radiatively, with the emission of a photon. The radiative case is of interest to us here for applications in light sources as well as amplifiers. Radiative electron-hole recombination occurs either spontaneously or through stimulated emission involving interaction with an identical photon. These two processes are analogous, respectively, to the spontaneous and stimulated emission processes in an EDFA. By proper choice of the semiconductor materials (e.g., InGaAs or InGaAsP), bandgaps that yield emission and/or absorption wavelengths in the ranges desired for optical communications (e.g., 1300 or 1550 nm) can be produced. For photon emission to occur by electron-hole recombination at an optical frequency, ν, an electron-hole pair must be present with energy levels separated by an amount E = hν. Furthermore, if the recombination is by stimulated emission, a photon of the same frequency must be present to interact with the electron-hole pair. The conditions for these effects to occur depend on the various carrier concentrations and the photon flux in the active region (the layer around the p–n junction). In an unbiased p–n junction, a “depletion layer” exists around the junction caused by diffusion of majority carriers across the junction and subsequent recombination on the other side. This creates a net charge on each side of the junction and hence a retarding electric field, preventing further diffusion and draining carriers from the layer around the junction. The depletion layer can be broadened by reverse-biasing the junction, thereby augmenting the retarding field. This is the condition for operation of the p–n junction as a photodetector. On the other hand, by forward-biasing, the retarding field is reduced, allowing more majority carriers to cross the junction, becoming minority carriers on the other side. This creates a condition favorable to recombination in the active region because once the mobile electrons from the n side cross over to the p side (at which point they become minority carriers), they encounter a large concentration of holes with which to recombine. A similar situation occurs for the mobile holes moving in the opposite direction. This effect, which increases the population of minority carriers in the active region on each side of the junction, is called minority carrier injection. The current flow through the forward-biased junction acts as an electrical pump, supplying the energy necessary to produce an inversion of the carrier population in the active region. This is analogous to the Er3+ ion population inversion in the EDFA produced by optical pumping. The light-emitting diode (LED) is a simple application of radiative recombination. It is a forward-biased p–n junction producing its radiation by spontaneous emission. This effect is called injection electroluminescence. Now suppose an optical signal is introduced into a waveguide embedded in a forwardbiased p–n junction, which we now want to use as an amplifier. By applying sufficient injection current, conditions can be established in which stimulated emission dominates spontaneous emission and absorption in the guide. At this point, optical gain is produced, and the device becomes a semiconductor amplifier. Because the energy bands are broad in a semiconductor, the SOA amplifies over a much wider band than an EDFA.

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Although its broadband gain characteristic is a positive feature, the SOA has a number of negative features. First, the carrier lifetime in the high-energy state is very short (on the order of nanoseconds). As indicated earlier, this means that signal fluctuations at gigabit-per-second rates cause gain fluctuations at those rates, producing cross-talk effects between simultaneously amplified signals. These effects do not occur in EDFAs until the bit rate drops into the 10-Kbps range. Second, because of its asymmetrical geometry, the SOA is polarization dependent. The EDFA, with its cylindrical geometry, is not. Third, the coupling losses between the fibers and the semiconductor chip reduce substantially the usable gain and output power. Fourth, the noise figure of a typical SOA is slightly higher than that of a typical EDFA due to fiber-chip coupling losses, although advances in packaging technology have improved that. Because of recent improvements in broadband SOAs [Park+05], polarizationdependent gain (PDG) and noise figure (rather than gain flatness and saturation-induced cross-talk) are becoming the predominant limiting performance factors. The best commercial SOAs can be specified having PDG as low as 0.5 dB over the C-band (30-nm bandwidth). However, CWDM-capable SOAs typically exhibit PDGs of 1 dB or more over a 70-nm band (four CWDM channels).

Quantum Dot SOAs Another improvement on the basic SOA, which shows promise in a broad range of applications is the quantum dot SOA [Akiyama+01]. Quantum dots (QDs) are extremely small semiconductor particles (of the order of 10 nm). SOAs using arrays of QDs in their active region exhibit important changes in their gain characteristics because of the way in which the QDs concentrate the injected electron-hole pairs, modifying their energy states. Some properties that have been demonstrated are extremely fast (picosecond) nonlinear gain response, very large bandwidth (a few hundreds of nanometers) high output power, and low noise figure. A typical QD SOA described in [Akiyama+06] uses InAs QDs on an InP substrate in a self-assembled array. (Electrolytic and pyrolytic synthesis techniques have been developed in which the QDs assemble themselves into highly ordered arrays.) The ultrafast gain response produces very low distortion in amplifying pulses at very high output powers (23 dBm). (Conventional and quantum well SOAs do not have the high-speed nonlinear gain response necessary to produce clean pulses at very high powers.) These QD SOAs have application as amplifiers in CWDM systems, which can benefit from their broadband characteristics, and as optical signal regenerators, which can use their high-speed response to advantage in reshaping rectangular pulses. Other potential applications are in tunable and mode-locked lasers, in wavelength conversion using FWM, and in optical gates. SOAs lend themselves to applications in which they can be combined with other semiconductor components. This includes monolithic integration with switch fabrics or WMUX/DMUXs fabricated as semiconductor planar lightwave circuits, use as on-off switches in an integrated switch fabric, and as components of optoelectronic integrated circuits (OEICs); for example, as a preamplifier in an optical receiver or a power amplifier in a laser transmitter. In addition, SOAs have found application in WDM optical networking at the metro level as described in Section 4.4.4.

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4.4.4

Amplification Trends in Metro Optical Networks: Amplets Deployment of metropolitan and regional WDM networks is accelerating, and amplets based on either fiber or waveguide technologies are potentially key components in these cost-critical applications. The term amplet is used to describe low-cost, modest performance amplifiers, which are usually not used as line amplifiers but rather as devices that compensate for loss due to other network elements. As a result amplets become enablers for new network architectures. Their characteristics include low-cost, small size, low power consumption, and high reliability. Some of their potential metro network applications include 1. Range extension on the access network side, which is usually unamplified 2. Subband aggregation where they provide valuable gain as band amplifiers to minimize signal degradation, allow incremental growth [Antoniades+04b], and compensate for dispersion compensation device losses 3. Flexible add/drop loss compensation where they can be used as wavelength agile single-channel amplifiers to overcome network element losses and provide independent control of channel launch power 4. Transmitter booster and receiver preamplifier functions 5. Compensation for excess loss in optical switch fabrics Typical EDFA amplets are intended for single-channel or band operation with the following performance considerations: output power, power stability, gain and gain flatness, polarization-dependent gain, and multipath interference. Because these amplifiers provide gain among a small set of channels they can significantly improve system performance on those channels. The challenges center around reducing cost (by reducing pump power, fiber length, and component count and using uncooled pumps). These devices are fabricated with reduced size, reasonable output powers (+15 dBm total output power is typical), and relatively wide bandwidths, so they can be used in various C-, L- and C+L-band configurations. In addition, low gain ripple and low noise figures are desirable. Available 80-µm-diameter fibers, coolerless 980 pumps, and miniature components are some of the enabling technologies making low-cost EDFA amplets possible. Nextgeneration amplet array devices may further reduce costs by sharing uncooled pumps. The erbium doped waveguide amplifier (EDWA) is another proposed amplet device, which offers the additional potential of integrability with other devices such as optical attenuators and dispersion compensators. The size of EDWAs is similar to that of SOAs, and advances in wafer processing enable lower costs at high volumes. An example of an EDWA is a 9.8-cm-long buried channel waveguide fabricated with a two-step ion exchange process in Er-/Yb-doped phosphate glass [Philipsen+00]. Commercial devices have been demonstrated involving full integration of EDWAs with pump and associated passive devices. They are small and have output powers of the order of 10–15 dBm, noise figures of the order of 6–7 dB, and gains of 13–20 dB. Cascades of such devices have been demonstrated in transparent WDM metro rings in [Reichmann+01]. SOAs incorporating improvements to solve some of the problems mentioned in Section 4.4.3 are also candidates for applications in metro and regional networks. The

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main reason for this is their compact size and competitive pricing. In addition, their broad bandwidth serves the requirements of CWDM, which has established itself as a cost-effective alternative to DWDM in business access and metro networks. A 240-km CWDM transmission system using SOAs and Raman amplification in a hybrid configuration was demonstrated in [Reichmann+06], and a discrete hybrid SOA-Raman amplifier was reported in [Iannone+06], using a highly nonlinear fiber as the gain medium. Recent articles have reported the impact of PDG on cascades of SOAs in CWDM systems and presented methodologies for deriving PDG specifications for SOAs in such systems [Antoniades+06, Roudas+07]. Several methods have also been proposed for combating the patterning effects (cross-talk) mentioned in Section 4.4.3 and these are described in detail in [Spiekman03]. Two of the main ones are (1) operating in the linear regime of the SOA and (2) gain clamping. The latter was initially demonstrated in [Francis+01] in what is called a linear optical amplifier (LOA). An LOA is essentially an SOA integrated with a vertical-cavity surface-emitting laser (VCSEL) (see Section 4.5.2). The VCSEL shares the same active region with the amplifier and is used to maintain a constant gain in the amplifier. In [Zimmerman+04] and references therein, a number of recent demonstrations of the use of amplets in metro applications are presented.

4.5

Optical Transmitters The light sources for optical transmission systems have evolved from LEDs operating in the 850-nm range in the first-generation systems (in the 1970s) to semiconductor lasers, which are still evolving today. With some exceptions for short-distance and low-bit-rate applications such as fiber to the home,15 virtually all optical transmission systems use semiconductor lasers as their light sources. This section is confined to that topic.

4.5.1

Lasers The laser (for “light amplification by stimulated emission of radiation”) was invented by Schawlow and Townes in 1958, shortly after the maser (for “microwave amplification by stimulated emission of radiation”), which performed the same operation in the microwave domain. The first solid-state (ruby) laser was demonstrated by Maiman in 1960. Lasers exist in myriad forms, including gas lasers, dye (liquid) lasers, and fiber lasers. (One type of fiber laser consists of a ring of erbium-doped fiber [i.e., an EDFA closed on itself].16) A semiconductor laser, also called a laser diode or injection laser, is basically a device for converting electrical energy to monochromatic light. The conversion process requires two things: (1) an “active” medium with properties that facilitate conversion from electrical to optical energy and (2) a geometry suitable for generating and emitting a monochromatic light beam. 15 16

See, for example, [Iannone+98]. Recall that this type of configuration can arise in optical networks through the inadvertent creation of closed optical paths. See Figure 2.13.

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Cleaved Surface

+

− p

n

Cleaved Surface

c 1991. Reprinted by Figure 4.21 Fabry–Perot laser. (From [Saleh+91, Figure 16.3-1]. Copyright
permission of John Wiley & Sons, Inc.)

The principle of operation of a laser is the same as that for any other oscillator: gain plus feedback. In the case of a laser diode, the active medium is a forward-biased p–n junction. It has optical gain as a consequence of current injection, producing electricalto-optical energy conversion. Furthermore, the active medium is enclosed in a geometry that provides optical feedback; in other words, a portion of the light that it amplifies is fed back to be reamplified, producing sustained oscillation when the net loop gain is greater than unity. A basic laser structure, known as a Fabry–Perot (FP) laser, is shown in Figure 4.21. The active region is contained in an optical cavity of rectangular geometry, with partially reflecting mirrors on two sides. The cleaved surfaces (facets) in the figure act as mirrors, providing partial internal reflection because of the difference between the refractive index of the cavity material and air. With the exception of the partially reflecting mirrors this is very similar to the semiconductor optical amplifier described in Section 4.4.3. In an FP cavity, light is reflected back and forth between the mirrors, with certain wavelengths being reinforced by constructive interference when the successive reflections of the wave are in phase with each other. These resonances occur whenever the round-trip distance between the mirrors is a multiple of the wavelength in the material. As in the case of SOAs (and LEDs), energy is converted from electrical to optical form through radiative recombination of holes and electrons in the active region. The

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necessary population inversion is achieved by current injection in the forward-biased junction. The longest wavelength of the photon-emission process is determined by the bandgap of the material, but the possible wavelengths of oscillation in the laser are limited to those reinforced by the geometry of the cavity. Lasing is achieved by increasing the optical gain sufficiently to overcome losses (including the transmitted light) from the cavity. The minimum current required for lasing is called the threshold current. It should be noted that if the cleaved surfaces in Figure 4.21 are given antireflection coatings, the device becomes a semiconductor amplifier. Light enters through one facet, is amplified, and leaves through the other.17 Practical laser structures use more complex geometries than that shown in Figure 4.21 and require special materials to produce light at the wavelengths needed for optical communication. To reduce the threshold current and to control the radiation pattern, either a stripe geometry or buried heterostructure (BH) is used. Each confines the light to a narrow region, which reduces the required threshold current and reduces the lightemitting area to a small spot instead of the broad beam shown in Figure 4.21. The optical properties of the material are “engineered” by choosing the materials (typically indium gallium arsenide phosphide [InGaAsP] for 1300-nm and longer wavelengths) and adjusting the thickness of the active layer. Multiple quantum well (MQW) structures are designed with several very thin active layers stacked up, achieving high gain at high powers [Tsang87].

4.5.1.1

Single-Frequency Operation The early laser diodes were multimode, multifrequency devices. In the FP laser, for example, the cavity can support many modes (and frequencies) of oscillation. To obtain the monochromatic light that is necessary for high bit-rate-distance products, the geometry must be modified to achieve single-frequency operation. By narrowing the active region, as in a BH laser, all cavity modes of an FP laser are suppressed except the longitudinal modes in the narrow, active strip. In this way, it operates much like a singlemode waveguide, supporting longitudinal modes associated with different frequencies. To reduce operation to a single frequency, all but one of the longitudinal modes must be suppressed, resulting in single longitudinal mode operation. A number of approaches are used for this, including cleaved-coupled cavity (C3 ) lasers, external cavity lasers, distributed Bragg reflector (DBR) lasers, and distributed-feedback (DFB) lasers. In each, a resonant element is introduced to single out the desired mode. The most commonly used light sources for high bit-rate, long-distance transmission are the DBR and DFB lasers. Figure 4.22 shows their structure. Based on the Bragg effect, a grating such as a corrugated reflecting surface or a periodic variation of refractive index in an optical medium acts as a wavelength-selective mirror. A distributed Bragg reflector (DBR) reflects light with a wavelength corresponding to twice the grating period. In effect, the grating operates as a narrow-band reflecting filter. The distributed Bragg gratings at the two ends of the DBR laser in Figure 4.22(a) replace the mirrors in an 17

In cases where the injection current is high enough to make stimulated emission dominate spontaneous emission, and yet the light loss through the facets is so high that the lasing threshold is not attained, the device emits light as a superluminescent LED.

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p

p

n

p

Active Layer

p

p

n

Active Layer

Λ

Diffraction Gratings Guiding Layer

(a)

(b)

c 1991. Figure 4.22 Single-frequency lasers. (From [Saleh+91, Figure 16.3-10]. Copyright
Reprinted by permission of John Wiley & Sons, Inc.)

FP laser, reflecting light only at wavelengths related to the Bragg grating period. In this way, the DBR laser operates in a single longitudinal mode (and frequency) defined by the Bragg grating. Similarly, in Figure 4.22(b), a periodic frequency-selective structure is introduced as a corrugation along the active layer in the DFB laser. The periodic structure produces distributed reflections (feedback) that reinforce each other only at wavelengths related to the periodicity of the corrugations. Although these devices are costly compared with multimode lasers, they are currently the workhorses of longdistance optical communications.

4.5.1.2

Mode-Locked Lasers In a multimode laser, the light that is produced has a wide spectrum, which typically manifests itself in the form of a wave with slowly and randomly varying amplitude and phase. However, light with a wide spectrum can also manifest itself in the time domain as a sequence of very narrow pulses. This happens if the relative amplitudes and phases associated with the different modes of a multimode laser are “locked” to values representing the Fourier coefficients for a periodic pulse train. This is the principle of the mode-locked laser. These devices are often used as sources of periodic picosecond pulses for soliton transmission. Pulse widths as short as 30 fs have been generated in this manner (for example, see [Saleh+91, p. 535]). Mode locking is usually implemented by modulating the laser gain periodically by driving it with a microwave oscillator and using a saturable absorber or a modulator inside the resonator.

4.5.1.3

Supercontinuum Generation Supercontinuum light generation is an interesting application of mode-locked lasers. A supercontinuum is a very broad continuous spectrum formed by propagation of

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high-power pulses through nonlinear media. First observed in the 1970s, it occurs in various nonlinear materials such as gases and semiconductors. The term supercontinuum does not cover a specific phenomenon but rather a plethora of nonlinear effects, which, in combination, lead to extreme pulse broadening. Such effects can be self-phase modulation, cross-phase modulation, and parametric four-wave mixing (FWM) as described in detail in Section 4.3.2.4. The widest spectra are obtained when pump pulses, typically generated by mode-locked lasers, are launched close to the zero-dispersion wavelength of a highly nonlinear medium. Supercontinuum light has been generated in the visible wavelength range using Ti:Sapphire femtosecond mode-locked lasers to drive nonlinear photonic crystal fibers with zero-dispersion wavelengths in the desired wavelength range. In the infrared range of interest in optical communications, supercontinua have been generated using mode-locked EDFA fiber lasers [Westbrook+05], as well as injection-locking of a mode-locked laser diode [Mori+05]. Mode-locked laser diodes are used to pump the nonlinear material because of their low-noise characteristics and because they can easily be used to generate optical pulses with high repetition frequency (50 GHz) and short pulse width (≤ 5 ps). Polarization maintaining, dispersion flattened fiber is one of the media used to reduce polarization fluctuations and control dispersion, which is extremely important for the generation of the supercontinuum [Mori+01]. One of main applications of the supercontinuum has been as a white-light source used in many characterization schemes such as interferometer-based dispersion measurements, broadband attenuation characterization, and numerous spectroscopy and microscopy procedures. Broadband sources are also needed for the low-coherence-based imaging-technique optical coherence tomography (OCT). OCT has been used extensively in a large wavelength range for both biomedical and nonbiomedical applications. However, for biomedical applications, wavelengths in the 1300- to 1500-nm wavelength range are highly attractive as they provide high penetration depth in tissue. One of the key WDM applications for supercontinuum light is the generation of a multiwavelength comb of frequencies anchored to the center frequency of the ITU grid, with well-controlled spacings. In conventional WDM sources, all wavelengths must be controlled independently, a sizable task as the number of WDM channels increases. However, a single multiwavelength source combined with a WDM demultiplexer, which can generate multiple precisely controlled optical carriers simultaneously, is an effective way to solve the issue of channel management. A supercontinuum generated by a modelocked laser is ideal for this purpose, because the spectrum of the mode-locked pulse train inherits a periodicity at the frequency of the oscillator used for mode locking. As an example, the system described in [Mori+05] generates a full grid of carriers at 25-GHz spacing locked to the 193.1-THz center frequency and seamlessly covering the S-, C-, and L-bands from 1460 to 1625 nm. It uses a mode-locked laser diode (MLLD) driven by an oscillator to produce a 25-GHz pulse train. To enhance the signal quality by reducing its relative intensity noise, a synchronizing signal from a precisely stabilized master laser diode is injected into the ML-LD. This markedly improves the noise qualities of the generated carriers. At this writing, work is continuing in the area of using supercontinuum light in WDM applications.

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Multiwavelength Optical Networks

Tunability Laser tunability is important in many multiwavelength network applications. Slow transmitter tunability (on a millisecond time scale) is required for setting up connections in wavelength-/waveband-routed networks, and rapid tunability (on a micro- or nanosecond time scale) is required for time-wavelength division multiple-access applications. Furthermore, the local oscillator in a coherent receiver must also be tunable to track the transmitter frequency. Slow tunability over a range on the order of 1 nm is effected easily via temperature control, which is used mainly for frequency stabilization (e.g., for locking a laser frequency to a frequency standard). Rapid tunability is achieved in DBR and DFB lasers by changing the refractive index, which can be done by changing the injected current in a local region around the grating. The design of the laser allows this to be done without disturbing the active region affecting the gain. This technique is used in multiple-electrode lasers to achieve tuning ranges as high as 51 nm [Amann94]. In these lasers, tuning may be either continuous or discrete. An example of the discrete case is a sampled-grating DBR laser, which can be tuned to one of 101 wavelengths spread over a 44-nm band [Delorme+98]. Another approach to rapid tunability is to use multiwavelength laser arrays with wavelengths that span the desired tuning range. One or more lasers in an array can be activated to produce simultaneous transmissions at selected wavelengths. An integrated array of 21 lasers with 3.72-nm wavelength spacing, combined and coupled into two optical amplifiers, is shown in Figure 4.23. The chip is 1 × 4 mm with the DFB laser array split into two groups of 10 and 11 lasers, visible as horizontal stripes in the figure.

c 1994 Telcordia Technologies, Inc. Figure 4.23 Laser array. (From [Soole+94, p. 59]. Copyright
Reproduced with permission.)

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The light from the lasers is fed to a star coupler (the dark area on the right), with two output waveguides fed to optical amplifiers on the far right of the chip. Photonically integrated laser arrays are cost-effective in the sense that the perwavelength cost of packaging, fiber pigtailing, and control circuitry is reduced. However, the manufacturing cost of an array is higher than that of an equivalent number of single lasers because of reduced yield with arrays.

4.5.2

Vertical Cavity Surface Emitting Lasers Vertical-cavity surface-emitting lasers (VCSELs) are a rapidly emerging transmitter technology offering a number of potential advantages over the edge-emitting lasers described in Section 4.5.1. Their designs allow the chips to be manufactured and tested on a single wafer. Large arrays of devices can be created on a single chip operating at many different wavelengths. Using this multiwavelength array, VCSELs can easily and inexpensively provide wavelength tuning over a relatively large range. The VCSEL’s uniform, single mode beam profile is desirable for coupling into optical fibers. Furthermore, they are compact, high speed, have low thresholds, and require low drive current. During the 1990s, the feasibility of long-wavelength (1.3 and 1.5 µm) VCSELs suitable for WDM applications has been demonstrated. A review of VCSEL development can be found in [Hasnain00]. Although there are a variety of VCSEL designs, they all have certain aspects in common. A generic layered semiconductor structure is shown in Figure 4.24. Two distributed Bragg reflectors of opposite doping are stacked with a cavity layer between them. The active region resides within this cavity and is composed of a number of quantum wells. Multiple quantum wells are grown within the cavity to increase the attainable output power, which is emitted vertically (up or down). The position of the quantum wells is crucial for maximizing the gain of the device. Current is injected into the active region through a current guiding structure. To achieve an array of VCSELs with different wavelengths on the same chip, a thickness gradient at the bottom or top of the substrate creates a cavity thickness variation that translates into wavelength variation. Light out Contacts p-DBR

Cavity layer Active layer

n-DBR

Figure 4.24 Typical VCSEL structure.

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Tunability of a VCSEL device can be achieved by mechanically varying the length of the VCSEL cavity, something that has been demonstrated using a micromechanical structure that essentially varies the position of a vertically suspended DBR [Hasnain00]. VCSELS have considerable potential in the cost-sensitive metropolitan networking environment. Their relatively low power can be offset by the use of low-cost SOAs. In [Filios+03], transmission experiments using 2.5-Gbps VCSELs have demonstrated the effective use of such devices in WDM metro applications.

4.5.3

Modulation Technology Lasers are modulated either directly, by varying the injected current, or externally, by passing the emitted light beam through a controllable device that changes the amplitude and/or the phase of the light. We consider directly modulated lasers (DMLs) and externally modulated lasers (EMLs) separately.

4.5.3.1

Direct Modulation The behavior of the laser under small-signal and large-signal modulation is of interest in different contexts. When modulated with an analog signal, including subcarrier signals, the modulation index is likely to be small to reduce nonlinear distortion, among other reasons. In that case, a small-signal model that assumes sinusoidal modulation is appropriate. The key performance parameter in this case is the small-signal modulation bandwidth, which is the modulation frequency at which the response drops by 3 dB. It can be shown that the modulation bandwidth is proportional to the frequency of relaxation oscillations in the laser, which increases with the square root of the output power under biased and unmodulated conditions [Agrawal02, p. 112]. Lasers designed specifically for high-speed modulation have reached bandwidths of 24 GHz. This wideband response is particularly useful for multichannel subcarrier applications, in which the aggregate modulation bandwidths of all subcarrier channels is limited by the laser modulation bandwidth when direct modulation is used (see Section 5.2.2). In digitally modulated systems, it is generally the large-signal modulation behavior that is of interest. For example, in an intensity-modulated system, the current may vary from near threshold for a 0 to twice the threshold for a 1. But because changes in current also produce changes in refractive index, they also create phase modulation (or, equivalently, frequency modulation) along with the intended intensity modulation. This effect has been termed chirp, and it produces a significant broadening of the pulse spectrum. (The parameter involved in intensity-phase modulation coupling is called the linewidth enhancement factor because the phase modulation increases the effective spectral width of the emitted light.) It has been shown [Koch+86] that the instantaneous angular frequency deviation . (i.e., chirp) φ(t) of a directly modulated DFB laser is approximately related to its output optical power P(t) by: . α φ(t) ∼ = 2




 1 d P(t) + κ P(t) . P(t) dt

(4.35)

4

40

3

30

2

20

1

10

0

0

0

0.5 Time (ns)

1

213

Frequency Chirp (GHz)

Power (mW)

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−10 1.5

c 1997. Figure 4.25 Pulse and accompanying chirp. (From [Agrawal97, Figure 3.23]. Copyright
Reprinted by permission of John Wiley & Sons, Inc.)

In Equation (4.35), α is the linewidth enhancement factor and κ the adiabatic chirp coefficient. The first term is called transient chirp, whereas the second term is called adiabatic chirp. A transient chirp-dominated DML will exhibit ringing at the bit transition points and usually the frequency of the 1s will be similar to that of the 0s elsewhere. An adiabatic chirp-dominated DML will exhibit in general two separate levels for the frequency of the 1s and the 0s with ringing during the duration of the bits. An example is shown in Figure 4.25, in which a laser is modulated with a rectangular pulse of width 500 ps (the solid line), corresponding to a 2-Gbps bit rate. The optical frequency (the dashed line) increases on the leading edge of the pulse and decreases on the trailing edge due to the transient chirp, with a total excursion of approximately 20 GHz. (The limited modulation bandwidth produces the pulse distortion.) This spectral broadening can lead to serious dispersion problems on long fiber spans. Thus, DMLs are generally avoided in favor of the more expensive externally modulated transmitters on long- and/or high-bit-rate transmission links. The situation is quite different in metropolitan and regional-area WDM networks, where transmitter costs are a paramount concern. DMLs have attracted considerable attention for 2.5-Gbps operation in both 1300- and 1550-nm bands in these applications. They have the advantages of small size, low-cost, low-driving voltage, and high available output power. Although their chirp characteristics can significantly limit the maximum achievable transmission distance over standard single-mode fibers, the interplay of

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transmitter chirp and negative fiber dispersion can be used to offset the deleterious effects of chirp (see Section 4.9.3). Negative dispersion fiber in conjunction with DMLs can expand the dispersion-limited transmission distances significantly. A figure of merit used commonly in the industry to characterize the performance of a transmitter in the presence of chirp/dispersion is laser rating. It is basically the total dispersion that can be tolerated on a link using that transmitter before the chirp/dispersion Q penalty reaches 2 dB. The laser rating is determined experimentally using back-to-back and transmitter plus fiber BER vs. received power measurements, which are then converted to Q penalties. (See Equation [4.55] for the relation between Q and BER.) Q penalties determined by simulation can also be used to approximate this parameter. At 2.5 Gbps typical commercially available 1550-nm DMLs are rated at 1800 ps/nm. This translates into a maximum transmission distance of about 90 km over SMF. In [Tomkos+01b], it is shown that this distance can be expanded to over 600 km using negative dispersion fiber. At 10 Gbps, DMLs are rated very poorly, tolerating only about 170 ps/nm (a transmission distance of only 10 km over SMF) and thus making them unlikely candidates for Metro network applications. However, the recent emergence of 10-Gbps DMLs and applications such as 10-Gb Ethernet has prompted the investigation of other technologies to extend reach. For example, electronic dispersion compensation (EDC) (discussed in Section 4.7) can double the reach of these devices where there is a large installed base of standard SMF, ruling out the use of negative dispersion fiber [Feuer+03]. The effects of chirp are also important in intensity-modulated analog systems when operated at sufficiently high modulation indices. A thorough analysis of the limits on analog transmission due to dispersion, using directly modulated lasers, appears in [Meslener84]. The effect of dispersion on a sinusoidally modulated optical carrier is examined, showing that frequency modulation effects due to direct modulation, combined with fiber dispersion, can produce serious harmonic distortion in systems that would be essentially distortion free if frequency modulation was absent.

4.5.3.2

External Modulation External modulation, although requiring extra (and costly) components, avoids the chirp problems discussed earlier. It is, therefore, the technique of choice in high-performance long-distance systems. In external modulation, the laser output is constant, and it is modulated by inserting a controllable attenuator (or phase shifter) between the laser and the fiber link. A typical external modulator uses a Mach–Zehnder (MZ) interferometer structure implemented on a LiNbO3 substrate, as shown in Figure 4.26. In this device, the light is split equally between two parallel waveguides on the surface of the substrate and is then recombined at the output. A control voltage, V, applied to the electrodes shown in the figure creates a small change in the refractive index through an electrooptic effect. This causes a change in the propagation constant and a relative phase shift at the point where the two waves combine. When no voltage is present, the relative phase shift is zero and the recombined signal exits the device without attenuation (except for the losses in the waveguides). When a voltage is applied that produces a phase shift of π between the two arms, the signal is extinguished. Thus, the device acts as

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Input Light Ii

V

0

Modulated Light Io

c 1991. Figure 4.26 Mach–Zehnder interferometer. (From [Saleh+91, Figure 18.1-5]. Copyright
Reprinted by permission of John Wiley & Sons, Inc.)

a voltage-controlled switch. These devices operate at high speeds (e.g., 40 GHz) and can be fabricated using various materials. A modification of the MZ interferometer of Figure 4.26 allows the application of control voltages on both waveguides, resulting in a device called a dual-drive MZ external modulator. This enables the generation of arbitrary quadrature signals [Ho05] that find application in transmission procedures such as optical suppressed carrier modulation. External modulators can also be used for phase modulation in coherent systems using a single waveguide with an electro-optically modulated refractive index.

4.5.3.3

Electro-absorption Modulated Lasers Transmitters that incorporate an electro-absorption modulator (typically based on InP) and a DFB laser in one package (EA-DFBs) have become attractive technologies in WDM systems and networks. The most common of these are those that involve a monolithic device, incorporating both laser and modulator on the same indium phosphide chip. Another alternative is a hybrid device, with distinct laser and modulator parts integrated in one package. Compared to transmitters using separate external modulators, typical EA-DFBs are smaller in size and potentially low cost but have relatively lower output powers, although higher output power devices have recently been reported [Moodie+03]. Demand is emerging for very compact EA-DFBs with low-drive-voltage requirements for 80- to 100-km, 10-Gbps metro applications and new short-reach (2-4 km) 40-Gbps systems. Figure 4.27 shows the operating principle of an EA-DFB transmitter. On the left is a CW DFB laser followed by the EA modulator. The active part of the modulator consists of InGaAsP epitaxially grown on top of the InP wafer. The operation of the EA

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Modulation Signal

DCbias Transmitted Light P type CW DFB laser

E field N type Active region

Figure 4.27 Typical structure of an EA-DFB transmitter.

20

2

15

1

10

0

5

−1

0

−2

−5

0

0.5

1

1.5

2

Alpha

modulator is essentially based on the fact that the reverse bias voltage creates an electric field in the active region (the device operates like a reverse-biased p–n junction). When no electric field is present the InGaAsP material is transparent, whereas when an electric field is present the material absorption coefficient increases dramatically turning the device opaque to incoming light, effectively acting as a modulator. Passive waveguides connect the DFB laser region to the modulator and then to the output fiber. We have seen that chirp is an issue in modulator performance. EA modulators have an interesting feature allowing adjustment of chirp to zero or even negative values through changes of the reverse bias voltage. The linewidth enhancement factor α, defined in Section 4.5.3.1, for DMLs, can now vary as a function of the reverse bias voltage. EA-DFBs exhibit only transient chirp as defined in Equation (4.35). Figure 4.28 shows measured absorption and linewidth

Absorption (dB)

216

−3

Reverse bias voltage (V) Figure 4.28 Absorption and chirp (linewidth enhancement factor) parameters versus reverse bias

c 2001 voltage for a typical EA-DFB transmitter. (From [Roudas+01, Figure 1a]. Copyright
IEEE. Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

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enhancement factor characteristics as a function of the reverse applied voltage for a typical EA-DFB laser. A negative linewidth enhancement factor will provide better performance characteristics over positive dispersion fiber, whereas an EA-DFB with positive α will provide better performance characteristics over negative dispersion fiber, as explained in Section 4.9. The use of EA-DFB lasers in conjunction with negative residual dispersion is preferable to their use with positive residual dispersion because higher output powers (typically by 8 dB) are achieved. This is seen in Figure 4.28 where the laser would be biased for a positive average α parameter; i.e., lower values of reverse bias voltage V. This produces lower EA absorption values resulting in higher transmitter output powers. However, the extinction ratio in this case will be worse because the absorption curve in that region is rather flat. In [Roudas+01], a model is presented that allows the calculation of system reach using an EA-DFB.

4.6

Optical Receivers in Intensity-Modulated Direct-Detection Systems The basic transmission link considered in this book involves an intensity-modulated optical transmitter and a direct detection receiver as shown in Figure 4.1. In this section, we focus on the receiver structure, consisting of the photodetector, a “front-end” amplifier (possibly incorporating equalization), and a detection circuit (in the case of digital transmission). The photodetector converts the optical signal to a photocurrent, using direct detection, and the amplifier raises the power of the photocurrent to a level sufficient for further electronic processing. In the digital case, this processing is primarily clock recovery, sampling, and threshold detection to extract the digital bit stream from the received signal. We consider each of these components next.

4.6.1

Photodetectors The photodetectors used in optical transmission systems are semiconductor photodiodes. In the simplest device, the p−n junction, the operation is essentially the reverse of a semiconductor optical amplifier. As shown in Figure 4.29, the junction is reverse biased, enlarging its depletion layer, and in the absence of an optical signal only a small, minority carrier current, called the dark current, flows. A photon impinging on the −

+

i(t ) Light

Figure 4.29 Photodiode.

p

n

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surface of the device and entering the semiconductor can be absorbed by an electron in the valence band, transferring the electron to the conduction band. Each electron/hole pair produced this way in the depletion region contributes to the photocurrent i(t). For this absorption to take place, the photon energy hν must be at least as great as the bandgap energy of the material, E g . Thus, photon absorption, and hence optoelectronic conversion, occurs only at wavelengths less than a cutoff wavelength, λc = hc/E g . Typical materials with sufficiently large λc for optical fiber transmission systems are various forms of InGaAs and InGaAsP. These are wideband devices, in which the absorption and conversion process takes place over the entire spectrum of interest in optical fiber communications. In a photodiode, the relation between the incident optical power Pin and the photocurrent I p is given by I p = R Pin , where the responsivity, R (in amperes per watt), is in turn given by R = ηq/ hν.

(4.36)

In Equation (4.36), η is the quantum efficiency of the device, which depends on the fraction of incident photons absorbed. The absorption coefficient of the semiconductor materials currently in use is high enough so that a very thin slab of material (on the order of 10 µm) is sufficient to make η ≈ 1.

4.6.1.1

PIN Photodiodes Electrons (holes) are also produced by photon absorption in the p (n) regions, but the component of photocurrent due to these effects tends to distort the photodetector response. This is because the minority carriers in the p and n regions diffuse slowly into the depletion region, resulting in a delayed response compared with the dominant, depletion region drift response. (They may also be lost by recombination before reaching the depletion region.) The PIN photodiode (for p-type, intrinsic, n-type) solves these problems by increasing the width of the depletion region, thereby causing a larger fraction of the optical power to fall on the depletion region. With the PIN photodiode, an extra layer of intrinsic material (either undoped or lightly doped) is sandwiched between the p and n regions. Its effect is to improve the responsivity of the device by capturing most of the light in the depletion region. The PIN photodiodes in a more complex doubleheterostructure geometry, which involves two extra layers of doped semiconductor, can be designed with close-to-ideal properties: near 100% quantum efficiency, absence of diffusion current, wide optical bandwidth, and large electrical bandwidths (in the tens of gigahertz).

4.6.1.2

Avalanche Photodiodes The photocurrent generated by the photodiodes discussed thus far is limited by the fact that, at best, each photon produces one electron of photocurrent. This may not be sufficient when the incident optical power is very low. The circuitry following the photodiode, including the front-end amplifier, contains unavoidable sources of thermal noise. If the level of the photocurrent is too low, the photocurrent signal is lost in the thermal noise. This is where the avalanche photodiode (APD) comes in. The APD

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resembles a PIN photodiode with an extra gain layer inserted between the i and n layers. Its purpose is to provide a current gain. If the photoelectrons produced in a photodiode are accelerated to sufficient speeds, they can produce additional electrons by colliding with the semiconductor material in a chain reaction, multiplying the number of electrons produced by each photon. This impact-ionization effect produces a current gain similar to that in a photomultiplier vacuum tube. The APD produces this result by using relatively large applied voltages to provide the necessary accelerating electric field (on the order of 105 V/cm) in the gain region of the APD. The current gain in the APD produces a much larger responsivity than in conventional photodiodes and thus is an effective means of overcoming thermal noise limitations in lower optical power systems. It does, however, introduce its own noise due to the unavoidable random nature of the electron multiplication process.

4.6.2

Front-End Amplifier: Signal-to-Noise Ratio Two types of amplifiers are typically used following the photodetector: a high-input impedance voltage amplifier and a transimpedance amplifier, which is actually a currentto-voltage converter. (The transimpedance amplifier is most frequently used in current systems.) A circuit using the latter is shown in Figure 4.30(a). The photodiode is coupled to the input of a high-gain inverting operational amplifier with gain A as shown. A smallsignal equivalent circuit model for this arrangement is shown in Figure 4.30(b). The current source Isig represents the signal current, and R and C represent, respectively, the resistance and capacitance seen looking into the amplifier. The resistance is the parallel combination of Rbias and R a /(A + 1), where R a /(A + 1) is the equivalent input impedance of the amplifier. The bandwidth of the circuit is B = 1/2π RC. Because A 1, the resistance R can be made small, so that the bandwidth B can be made large if desired. + Ra

−A Rbias

(a)

ish

+

id

Isig

R

C

υ −

(b) Figure 4.30 Transimpedance amplifier.

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Multiwavelength Optical Networks

The additional current sources, i sh and i d in Figure 4.30(b), represent the shot noise produced by the signal and the dark current, respectively. Using these sources, we find that the SNR at the output of this circuit (assuming that amplifier noise is dominated by the thermal noise in the equivalent resistance R) is given by SNR =

2 Isig 2 i sh + i d2 + i th2

.

(4.37)

In Equation (4.37), we have 2 Ish = 2q Isig B

Id2 = 2q Id B Ith2

(4.38)

= 4KTB/R,

where K and T are Boltzmann’s constant and absolute temperature, respectively. The shot noise expression is given for a PIN photodiode. A modification, including an “excess noise factor,” is necessary for APDs (for example, see [Agrawal97, p. 167]). Because all of the noise terms here are proportional to the receiver bandwidth B, this is normally chosen as small as possible, to exclude noise, but large enough to pass the signal spectrum. Clearly, to improve the SNR it is desirable to make Isig as large as possible, which can be done by using either an APD rather than a PIN photodiode or an optical preamplifier before the photodetector. In both cases the SNR will be improved because of the gain of the APD or the amplifier, until the noise generated in the amplifying device (APD or optical amplifier), together with other noise and interference accumulated on the transmission link, dominate the receiver noise. In the case of the APD, there is an optimal value of the APD gain beyond which the SNR starts to deteriorate due to the excess noise factor (see Problem 7 at the end of this chapter). It is important to recall that the SNR discussed here is defined on the electrical side of the e/o interface as opposed to the OSNR defined on the optical side, and the assumption is that the signal current is due to an uncorrupted optical signal impressed on the photodetector. The other noise and interference effects accumulated on a fiber link have not been included in Equation (4.37). They include accumulated ASE noise from amplifiers along the transmission link; waveform distortion due to fiber dispersion; distortion and cross-talk effects due to fiber and amplifier nonlinearities; and various fiber, filter, and switch cross-talk effects. These issues appear in subsequent sections, leading up to a complete treatment of end-to-end optical connections in a network setting in Section 4.13. The shot noise terms in the previous SNR expressions are based on a Gaussian approximation of the shot noise probability density. This is an approximation that is valid (in digital systems) for optical power levels corresponding to a large number of photons transmitted in each bit interval. As the bit rate is increased and/or the optical power level is decreased, we eventually arrive at a point, called the quantum limit, where only a few photons are transmitted in each bit interval. In this case, the Gaussian approximation is no longer valid, and the signal detection process must be viewed as a photon-counting process.

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Suppose a digital signal is intensity modulated by on-off keying (OOK), at a bit rate R b , with a 1 represented by an optical power 2P, for an average power of P, assuming equiprobable 1s and 0s. This gives an average of N p = 2P/ hν R b photons for each 1 transmitted, and no photons when a 0 is transmitted, assuming no other sources of photons (noise). The optimal detector in this case would declare that a 0 was transmitted if no photons are counted in the bit interval and declare that a 1 was transmitted otherwise. In this idealized model, there will never be a case when a 0 is mistaken for a 1. The opposite can happen, however. Because the counting statistics are Poisson, there is a finite probability, P(0|1) = e−N p , that a 0 is declared when a 1 was transmitted. The BER for the system in the quantum limit is, therefore, BER = 12 e−N p . For example, for a BER of 10−9 we need approximately 20 photons for each symbol 1 or an average of 10 photons per bit. This results in approximately −60 dBm for a 1-Gbps signal in the 1550-nm range. Powers in practical systems are typically well above −40 dBm, two orders of magnitude higher than the quantum limit. The approach to the quantum limit can also be interpreted in terms of its effect on information rates and channel capacity rather than BER. For an information-theoretic treatment of the quantum limit, see [Stern60].

4.6.3

Digital Signal Detection: Noise, Interference, and Bit Error Rate In a digital transmission link, the final step in the receiver is signal detection. We restrict ourselves here to the simplest (and most common) type of system: an IM/DD binary communication system using an OOK modulation format. As shown in Figure 4.31, the photocurrent is amplified and filtered, and the filter transfer function is chosen to preserve or enhance the signal waveform while suppressing noise. It typically has a bandwidth, B (in hertz), somewhat less than the signal bit rate, R b (in bits per second). A common choice is B = 0.65R b . The amplified and filtered signal is then sampled and threshold detected. Typical waveforms illustrating this process are shown in Figure 4.32. The data signal that intensity modulates the laser transmitter is assumed to be an ideal train of rectangular pulses: s(t) =

∞ 

al u(t − lT ),

(4.39)

l=−∞

Preamplifier

PD Optical Bandpass Filter

Amplifier/ Filter

y(t)

Threshold Detector

Sample Clock Recovery

Figure 4.31 Binary receiver.

a^l

Detected Data

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s(t) 1

0

1

1 t

Bit period (a)

y(t)

y1

y3

D y4

y2

t (b)

a^l 1

0

1

0

Error t

(c) Figure 4.32 Typical waveforms in an IM/DD system.

where

 u(t) =

1 −T /2 < t ≤ T /2 0 otherwise

(4.40)

and {al } represents a sequence of data symbols (1s and 0s) with bit period T . After traversing the transmission link, the receiver front end, and the filter, the transmitted data signal s(t) [see Figure 4.32(a)] appears at the input of the sampling/threshold detection unit as a distorted and noisy waveform, y(t) – see Figure 4.32(b). The clock recovery unit, shown in Figure 4.31, processes y(t) to determine at what points in time the signal should be sampled.18 Using this information, the received signal y(t) is sampled periodically at a point t0 within the bit period, chosen to provide a high probability of correct detection. The signal samples yl are compared with a decision threshold D, shown as a dashed line in Figure 4.32(b), and a decision is made on the detected data symbol: aˆ l = 1 if yl > D 18

The clock recovery unit is essentially a variable frequency oscillator, phase locked to the data signal (i.e., a phase-locked loop). For it to work properly, there must be frequent transitions between 1s and 0s in the data signal, so that the circuit is able to discern the bit interval boundaries. This is generally ensured by “scrambling” the transmitted data stream to convert long strings of repeated symbols into interspersed 1s and 0s.

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y1 D y0 t 0

Bit Period

t0

T

2T

(a)

D

t 0

T

2T (b)

Figure 4.33 Eye diagram.

and 0 otherwise. Figure 4.32(c) shows the detected data symbols aˆ l corresponding to the transmitted data stream in Figure 4.32(a). Note the presence of a bit error in the fourth symbol. The received waveform in Figure 4.32(b) is often represented in terms of an eye diagram, as shown in Figure 4.33. The eye diagram superimposes time traces corresponding to successive bit periods. This is what is seen on an oscilloscope synchronized to a submultiple of the bit rate of the transmitted signal. Figure 4.33(a) shows what a typical eye diagram would look like if all signal impairments were deterministic in nature. Each trace represents a distorted bit waveform (representing a 1 or a 0) over two bit periods, where the distortion is a result of dispersion, limited bandwidth, and deterministic nonlinearities, as well as the effects of square-law detection. All of these effects produce ISI, which is deterministically (but nonlinearly) related to the bit pattern. The ISI comes from the interfering effects of neighboring pulses that overlap in time because of dispersion. Eye diagrams are used frequently for test purposes, during which the bit stream is generated as a periodic pseudorandom test pattern with a long but finite period. In this case, individual traces contributing to the eye diagram repeat during each period of the test pattern. The cross within the “eye opening” in Figure 4.33(a) indicates a good point for sampling and setting the threshold. The traces below the cross are all due to 0s, and those above are due to 1s, which corresponds to an “open eye.” The quantity y1 − y0

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indicates the size of the eye opening. The vertex of the cross is positioned at the chosen sampling time, t0 , and threshold setting, D. If sampling and thresholding are executed as shown, the BER will be zero. Notice, however, that as the eye opening (y1 − y0 ) decreases, the allowable range of sampling and threshold settings diminishes. For small eye openings, a small amount of timing jitter in the transmitter or the receiver or a slight misplacement of the threshold can produce a catastrophic BER even in the absence of any random noise. Figure 4.33(b) shows the open eye with random noise added. Now, instead of a well-defined deterministic pattern, we have a “fuzzy” picture because the noise is not repetitive. Furthermore, the fuzziness produces occasional excursions of the waveform across the threshold, with corresponding bit errors. The more the eye closure, the more frequently an error occurs. Also, the wider the band of “fuzziness” (corresponding roughly to the random noise power), the higher the BER. An exact determination of BER requires a detailed model of the processes along the transmission path. The relation between the waveform y(t) and the original waveform s(t) is extremely complex due to a succession of linear and nonlinear operations on the signal. The original data signal intensity modulates a laser that, if modulated directly, produces an optical field containing frequency modulation (chirp) as well as the desired intensity modulation. The optical field also includes intensity and phase noise, which are responsible for the finite linewidth of the unmodulated laser. The signal then experiences linear dispersion combined with the various nonlinear distortion and cross-talk effects, discussed in Section 4.3.2.4, and these effects accumulate with the length of the fiber. The signal also accumulates ASE noise from line amplifiers and cross-talk from any optical cross-connects it traverses before reaching its destination. The resultant optical field presented to the photodetector undergoes another nonlinear operation (square-law detection) and is further contaminated by shot noise and amplifier noise before reaching the sampler and detector. Imperfections in transmitter clocking and receiver clock recovery also introduce timing jitter, which degrades the detection process, as indicated earlier. It is clearly impossible to do an exact analysis of these effects. Fortunately, in any properly designed system all of these transmission and reception impairments must be small. For if they are not, the system will not be operating at the low BER required for digital optical transmission systems. This allows us to make many simplifying approximations and still come up with some useful results on BER. We first reduce the mathematical model for the detection process to a pair of functions, p(y|0) and p(y|1), representing the probability density functions (PDFs) for the sampled amplitude at the input of the decision unit, conditioned on the transmission of a 0 or a 1 in the corresponding bit interval.19 An illustration of typical PDFs appears in Figure 4.34, which shows an ideal detection process. The basic assumption in this model is that the PDFs are conditioned only on one transmitted symbol, so the deterministic ISI effects represented by the eye diagram of 19

The reception bit interval is delayed in time with respect to the transmission bit interval due to propagation delays.

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p(y | 0) p(y | 1)

y D Figure 4.34 Ideal detection.

Figure 4.33(a) can be ignored, as well as any statistical dependencies of the random noise effects on other symbols (or other signals).20 Assuming that 1s and 0s are equiprobable, it turns out that the setting of the decision threshold D that minimizes the probability of error (BER) is at the crossover of the two PDFs, as shown in Figure 4.34. This corresponds to maximum likelihood (ML) detection (see, for example, [Lee+94]). The ML detector declares that a 0 was transmitted when p(y|0) > p(y|1) and that a 1 was transmitted when p(y|1) > p(y|0). The total probability of error in this case corresponds to the dark area in Figure 4.34, where the probability of declaring a 1 when a 0 was sent (or the reverse) is shown as the dark area to the right (or the left) of D, respectively. The picture becomes still simpler if we approximate the PDFs as Gaussian, so 
(y − yi )2 1 i = 0, 1, (4.41) exp − p(y|i) = √ 2σi2 2π σi where yi is the mean value of y when symbol i is transmitted and σi2 is the noise variance. It can be shown that the probability of error Pe using the optimal threshold setting is then given by 

2  ∞ 1 1 1 Q Q 2 √ exp − =√ , (4.42) Pe = erfc √ √ exp(−α ) dα ≈ 2 π Q/ 2 2 Q 2π 2 where Q=

y1 − y0 σ0 + σ1

(4.43)

and erfc is the complementary error function. Plots of the exact expression for Pe as a function of Q, and its exponential approximation, are shown in Figure 4.35. Note that the exponential approximation is excellent for small values of Pe , which is the range of 20

Theoretically, it is possible to include ISI effects in the detection model by extending the conditioning over the set of all neighboring symbols in the bit sequence that contribute to ISI. However, this requires a detailed model for all of the transmission impairments producing the ISI, which is, in most cases, out of the question.

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Exponential Approximation

1 .1 Pe .01 10−3 10−4 10−5 10−6

1

2

3

4

5

Q

Figure 4.35 BER as a function of Q.

interest to us here. The square of the argument Q in Equation (4.43) can be interpreted as an electrical SNR, as seen by the decision unit. Figure 4.35 illustrates the fact that the BER is a very sensitive function of Q so that very small improvements in SNR have very substantial effects on BER, with all of this being due to the fact that the noise is (approximately) Gaussian. For example, defining Q 2dB = 20 log10 Q

(4.44)

we find that Q 2dB = 14.3 dB for a BER of 10−7 , and Q 2dB = 15.4 dB for a BER of 10−9 . For the remainder of this chapter (unless otherwise noted), we will use Q 2dB as defined in Equation (4.44) whenever we refer to Q performance in a system or Q penalty for an impairment. It should be noted, however, that in the literature, including articles referred to in this chapter, the quantity Q dB = 10 log10 Q is often used when referring to Q penalty. The Gaussian approximation is usually very good when there are many independent sources of random noise (although this is not always the case). Deterministic effects contributing to ISI can be lumped into the values of yi . For example, a worst-case BER can be obtained by basing the argument y1 − y0 in Equation (4.43) on an eye diagram, obtained either by simulation or measurement. The eye opening y1 − y0 in Figure 4.33(a) corresponds to the worst case of ISI and thus provides an upper bound on BER for any assumed noise variances. The term receiver sensitivity is often used as a measure of receiver quality. We define it here to mean the average optical power required at the input to the receiver to achieve a given BER at a given bit rate R b . Typical receiver sensitivities for a BER of 10−9 and R b = 1 Gbps are −26, −36, and −45 dBm for PIN, APD, and optical preamplifier receivers, respectively. The best of these (−45 dBm) corresponds to more than 1000 photons per bit – two orders of magnitude higher than the quantum limit.

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4.6.4

227

Analog Systems: Carrier-to-Noise Ratio There are several applications in which the laser transmitter is intensity modulated by an analog signal, which may be carrying any kind of information in either analog or digital form. The following are some examples: r Cable television (CATV) distribution networks r Distribution networks for microwave wireless systems r Optical networks using microwave subcarriers (see Section 5.2.2) In analog systems, there is no decision mechanism, and the receiver performance is measured in terms of carrier-to-noise ratio (CNR). The CNR is defined as CNR =

2 Isig

σn2

,

(4.45)

where Isig represents the signal component of the photocurrent and σn2 is the total noise at the receiver. The latter is given by 2 2 2 σn2 = σRIN + σsh2 + σs−ASE + σASE−ASE + σth2 ,

(4.46)

where the terms on the right side of Equation (4.46) are the laser relative intensity noise (RIN),21 the shot noise, the two noise components caused by signal–ASE and ASE–ASE beating in the photodetector, and the receiver’s thermal noise, respectively. A quantitative treatment of these effects can be found in [Desurvire94]. The CNR for any particular system depends on a host of system parameters, which is beyond the scope of the current discussion. However, one consideration worthy of note is that when the information-bearing signal is in analog form (e.g., AM TV), it is often much more vulnerable to noise than when it is in digital form. For example, AM CATV systems require a 50- to 55-dB CNR, whereas the corresponding quantity, Q 2dB , for an acceptable digital system is approximately 25 dB. An example of the use of dispersion compensation in a high performance application of analog DWDM appears in [Lu+04]. The objective was to demonstrate bidirectional CATV transmission over 50 km of standard SMF in the face of combined dispersion and nonlinear fiber impairments. At DWDM channel spacings, the main nonlinear effect influencing transmission fidelity is XPM, producing composite second order (CSO) and composite triple beat (CTB) interference. Chirped fiber-grating dispersion compensators were used to reduce dispersion while also mitigating the nonlinear effects. In the demonstration, four λ-channels were transmitted (two in each direction) at 200GHz spacings, with each wavelength carrying 77 channels on RF subcarriers. A CNR better than 50 dB was obtained with CSO and CTB performance more than 72 and 69 dB, respectively.

21

The RIN is caused by random fluctuations in the optical field generated by the laser transmitter.

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4.7

The End-to-End Transmission Channel Referring to the layered view of the network architecture in Figure 2.1, we have so far in this chapter confined ourselves to the optical connection layer, which terminates at the e/o interface; i.e., at the transmitter and receiver. However, there is additional processing that usually occurs on the electrical side to turn an optical connection into a transmission channel. Figure 4.36, which expands schematically on Figure 4.1, shows some basic operations that occur in the electrical domain in transforming a bit stream on a transmission channel to an optical signal and then operating on the received optical signal to transform it back to a bit stream at the other end of the transmission channel. These operations complete the picture of the physical layer in Figure 2.1. Data are presented to the input of the transmission channel in the form of a stream of bits di as shown in Figure 4.36. For purposes of FEC, redundant bits may be added in an encoder, which outputs code bits c j as shown in Figure 4.38. These are then processed electrically to produce a modulating signal adapted to a given modulation format (e.g., OOK), and this signal drives the optical modulator and laser transmitter. On the receiving side, an optical receiver, exemplified by the block diagram of Figure 4.31, converts an optical signal received from a λ-channel to an electrical waveform, represented by y(t) in Figure 4.36. Again, depending on the modulation format, further processing will take place before y(t) is converted into an output bit stream. In the simplest case of OOK,

TP TP

Input data di

Encoder

OT OT

eo Modulating Opti Optical signal Code bits Electrical modulator/ Modulator/ modulator transmitter transmitter

Optical Op channel Optically detected waveform

Estimated data

Corrected data

Electrical processor

Decoder

RP RP Figure 4.36 Transmission channel processing operations.

y (t)

ti Optical detector/ tector/ receiver

OR OR

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0

1

1

0

0

229

1

(a) NRZ

t

(b) RZ t (c) DPSK 0 0

?π

000

0

π ??

π

0 t

(d) Duobinary

t Figure 4.37 Modulation formats.

clock recovery and thresholding is all that is necessary in the electrical processor to produce a detected data stream, e j , which is the estimated version of the stream c j , and may contain bit errors. However, if more elaborate modulation formats are used and/or if equalization is applied to the waveform y(t), the electrical processor is more complex. Finally, the decoder processes the detected data stream to convert it to the final output stream f i , where some or all errors in the estimated data have been corrected. We now describe some representative modulation formats and coding schemes, concluding with a brief treatment of electronic equalization.

4.7.1

Modulation Formats Various modulation formats have been used in WDM communication systems. They have two key features: the way the data are represented in the optical signal, and the keying technique. For example, in intensity modulated systems the data are represented by the intensity of the optical signal and in PSK by the phase. The keying alternatives are known as non-return-to-zero (NRZ) or return-to-zero (RZ). In the former case, the information-bearing signal occupies a full bit period, whereas in the latter case the information is contained in a pulse occupying less than the full bit period. Each can incorporate various data representations. For example, OOK is an intensity modulation representation that involves turning the transmitter power on for a 1 and off for a 0.22 It can be implemented either as NRZ (Figure 4.37[a]) or RZ (Figure 4.37[b]). Comparing the two, RZ occupies more bandwidth than NRZ because the pulses are narrower, and 22

Amplitude-shift keying (ASK) generalizes OOK to multiple levels of intensity.

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RZ has a higher peak power than an NRZ signal running at the same average power level. In DWDM systems, the interplay of dispersion and nonlinear effects may or may not favor RZ over NRZ, depending on bit rates [Hayee+99]. At moderate speeds (10–20 Gbps) in dispersion-managed systems with alternating positive and negative dispersion fibers, RZ tends to perform better than NRZ, because the latter is more affected by nonlinearities. At higher speeds (40 Gbps) NRZ tends to perform better, because the larger bandwidth occupied by the RZ pulse makes it more vulnerable to dispersion. However, the situation is more complicated than this. If the modulation depth is high, then the spectrum is narrowed, and under those conditions RZ outperforms NRZ at high speeds at transoceanic distances. Also, as we have seen, solitons (Section 4.3.3) are RZ pulses (typically picoseconds in width) that preserve their shape due to the positive effects of a combination of dispersion and nonlinearity. So far in discussing optical transmission we have focused on intensity modulated systems because of their simplicity and because they dominate in currently deployed networks. However, NRZ-differential phase-shift keying (NRZ-DPSK) and RZ-DPSK are alternative modulation formats that offer certain advantages compared to the traditional OOK formats. In conventional PSK, the phase of the carrier is modulated at the transmitter and at the receiver phase-to-intensity conversion is needed before the signal reaches the photodetector, because the photodetection process is phase insensitive. This can be done in a coherent optical system, as explained in Section 4.8. For differential PSK (DPSK) the data is represented by a phase change between successive bit periods. For example, a π-phase change might represent a 0, with no change for a 1, thus providing the differential encoding feature (see Figure 4.37[c]). At the receiver, the phase change between successive bits is measured using a delay interferometer (DI) which converts phase changes into intensity levels. In a Mach–Zehnder DI, the lengths of its arms differ by one bit period so that successive encoded bits are superimposed at its output, with the earlier bit serving as a phase reference for the later one. There will be either constructive or destructive interference at the output depending on whether a phase change has occurred between adjacent bits. In this way the phase changes are converted into intensity changes. Because the DI is actually a linear filter it can be considered to be lumped together with the optical bandpass filter in the receiver block diagram of Figure 4.31.23 To achieve maximum receiver sensitivity, a balanced receiver consisting of a two-output MZ feeding two photodiodes is required. With this configuration, the NRZ-DPSK has a 3-dB receiver sensitivity advantage over NRZ-OOK. DPSK has also been shown to be more tolerant of fiber nonlinearity penalties due to its more evenly distributed power. RZ-DPSK can offer an additional advantage compared to NRZ-DPSK because of the inherent receiver sensitivity improvement of RZ formats, as mentioned above. A comprehensive review of DPSK theory, implementation, and experimental results can be found in [Gnauk+02]. 23

Frequency-shift keying (FSK), where the bits are represented by optical frequency deviations, is another modulation format wherein the receiver requires optical filtering in front of the photodetector. See [Kaminow+87] for an early use of FSK in optical transmission.

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A number of other RZ modulation formats have been proposed such as chirped RZ (CRZ) and carrier suppressed RZ (CSRZ). The former is generated by using bitsynchronous phase modulation of the RZ signal, where the phase deviation is varied over the bit period to provide optimum performance for the particular nonlinearities and dispersion map of the system. The latter format (CSRZ) is generated from RZ by suppressing the carrier and has a narrower spectrum, which makes it more tolerant to dispersion effects. Other popular modulation formats are duobinary RZ (D-RZ) and modified D-RZ (MD-RZ) [Cheng+02]. Duobinary is a form of intensity modulation where the transmitted pulse intensity is based on the sum of two adjacent bits (see Figure 4.37[d]). Therefore, the output can take one of three values. In duobinary the intensity of each pulse is correlated with its neighbors (because sums of adjacent bits are used), and this narrows its spectral width. For this reason, these formats were shown to exhibit dispersion tolerance as described in more detail in [KaminowChap16+02]. An interesting approach to duobinary transmission [Chowdhury+06] combines optical carrier suppression and separation (OCSS) with optical duobinary modulation to obtain high channel bit rates (40 Gbps), high-density DWDM transmission (50-GHz channel spacing), and therefore very high spectral efficiency (0.8 bps/Hz), over a distance of 200 km. The objective was to demonstrate high-speed transmission using lower speed, low-cost electronic and optical technology for applications from the core network down to metro and access networks. Carrier suppression is accomplished using a dualarm lithium niobate Mach-Zehnder modulator biased at its minimum intensity output point and driven in its two arms by complementary 20-GHz CW signals. The duobinary data then intensity modulate the resultant suppressed carrier optical signal.24 This is a good example of how optical technology has matured. Although suppressed carrier techniques were commonly used for more efficient transmission since the early days of radio, it is only recently that the approach has been used in the optical domain. Furthermore, at the high bit rates used in this example, optical processing has been found to be a more cost-effective approach than other techniques for achieving high-spectralefficiency DWDM transmission over long distances.25 It should be clear from the above that each modulation format has its advantages and disadvantages. There are always trade-offs: using a complex modulation format to adapt to particular conditions on an optical path can increase maximum achievable transmission distance and bit rates but will most likely raise the cost of the transmitter/receiver equipment.

4.7.2

Forward Error Correction We now focus on the error control part of the system of Figure 4.36, condensing the other processing operations into the modulator and demodulator shown in Figure 4.38. In a communications system that employs block error-correction coding, a digital 24 25

The dual-arm LiNbO3 modulator configured for OCSS in an application to optical label switching is shown in Figure 10.47. The spectral efficiency achieved in this demonstration is comparable to that which is common for electronic transmission over copper but rare for optical fiber transmission.

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Digital Information Source

d1, d2, d3, ... , dk Data bits

(n,k) Encoder

c1, c2, ... , cn Code bits

Corruption

Digital Information Sink

f1, f2, f3, ... , fk

(n,k) Decoder

Data bits Estimates

e1, e2, ... , en

Modulator

Channel

Demodulator

Code bits Estimates

Figure 4.38 FEC encoding/decoding functions.

information source sends a data sequence comprising k bits of data to an encoder. The encoder inserts redundant (or parity check) bits, thereby outputting a longer sequence of n code bits called a codeword. On the receiving end, codewords are used by a suitable decoder to extract the original data. A code of the type just described is called an (n, k) binary block code, and its coding efficiency is k/n. A systematic binary linear block code is one whose parity check bits are calculated as a linear function (modulo 2) of its data bits. Error correcting codes have a long history and have been refined to adapt to a wide variety of channels. A commonly used family of codes are the Reed–Solomon (RS) codes. An RS code is specified as RS(n, k) with s-bit symbols. This means that the encoder takes k data symbols of s bits each and adds parity symbols to make an n symbol codeword. There are n − k parity symbols of s bits each. An RS(n, k) code can correct any pattern of up to t symbols containing any number of errors where 2t = n − k. By correcting the most probable patterns of random errors, FEC provides a coding gain (or system Q margin). A coding gain of QdB at a given BER means that the Q factor required to achieve the specified BER with FEC is QdB less than that required without FEC. A downside is that the channel bit rate must be increased to accommodate the redundant bits, and another is that the codec is complex and costly at high bit rates. However, there are many applications where FEC is worthwhile. As used in WDM longhaul transmission and undersea transmission, FEC offers the system designer significant performance margins. For example, a commonly used code is the RS(255, 239), which provides a coding gain of about 6 dB at BER 10−15 at the expense of increasing the channel bit rate by 7%. For 40-Gbps transmissions (OC-768), FEC is especially important because of the high OSNR required at the receiver and the increased impairment penalties at higher line rates. However, the only way to reach these speeds with current technology is to demultiplex to several lower speed bit streams and implement FEC with parallel processing; a costly solution. On the other hand, advances in FEC technology have made it a cost-effective possibility in metropolitan WDM networks at lower speeds. A more detailed analysis of error control techniques and their applications in optical transmission is presented in [KaminowChap17+02].

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4.7.3

233

Equalization The compensation at the receiver, of channel-induced distortion of the optical signal is called equalization. Electronic equalizers are designed to process the optically detected waveform y(t) in Figure 4.36 to cancel or mitigate intersymbol interference at the sampling instant. Because the primary cause of ISI is dispersion, these devices are also called electronic dispersion compensators (EDCs). The most common type of electronic equalizer forms a weighted sum of the present and past sampled values (taps) of the waveform y(t) taken at a sampling instant determined by a locally generated clock, and this weighted sum is applied to a threshold device to make each bit decision (see Figure 4.39). The weights wi are calculated and updated based on previous bit decisions, making this an adaptive decision-directed equalizer. The number of taps used in the equalizer should be enough to cover the time span over which interfering pulses overlap. This is the well-known transversal filter equalizer, which has a long history, having been used in telephone modems in the 1960s. However, the analog telephone channel for which this was originally designed was, to a good approximation, a linear additive noise channel, whereas the optical channel is nonlinear due to the squarelaw action of the photodetector.26 Nevertheless, experimental results show that electronic equalization still works in direct detection optical channels. One possible reason is the robust nature of the weight-setting algorithm. Basically, it just tries to do the best it can to open the eye of the receiver no matter what impairments are encountered. A number of studies have focused on equalization technology and its applications in the cost-sensitive metro WDM networking environment. In [Feuer+03], it was experimentally demonstrated that a four tap prototype EDC module roughly doubles the dispersion-limited transmission reach for a channel with a DML source running at yk

y(t)

yk−1 T

Sampler Sampler

w1

yk−2 T

yk−3 T

w22

w33

w4

+ Threshold Th detector

Estimated data

Weight We update Figure 4.39 Transversal decision-directed equalizer. 26

We assume a direct detection receiver here. If coherent detection were used and fiber nonlinearities were negligible, the channel would be linear. Also, in a direct detection receiver, if equalization was done in the optical domain, as a filtering operation in front of the detector, the channel seen by the equalizer would be linear, but this would be much more difficult to implement.

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10 Gbps over standard single-mode fiber. (As noted in Section 4.5.3.1, DMLs operating at OC-192 are an emerging low-cost technology but have severe limits in reach due to high chirp.) Transmission improvement was also demonstrated in [Woodward+03], where EDCs were used in a WDM ring with EA-modulated transmitters. System margins were significantly improved and error floors eliminated. Finally, electronic equalization can be used in conjunction with many other techniques, such as advanced modulation formats, forward error correction, and adaptive optical PMD compensators, to increase the reach of a system or network. An example of the power of combined equalization and FEC appears in [Winzer+05]. The objective here was to increase data rates in a bidirectional CWDM system from 2.5 to 10 Gbps. Successful operation was achieved on up to 80 km of standard SMF using DMLs rated for 2.5 Gbps. Electronic equalizers can be produced in integrated form so they are potentially low cost. Also, because of their robust performance characteristics, they can be used for the simultaneous compensation of a variety of impairments: chromatic dispersion, filter-induced distortion, polarization effects, and certain nonlinear effects. This may allow the system designer to relax requirements on other devices, employing lower cost transmitters, WADMs, and optical multiplexers/demultiplexers without compromising performance.

4.8

Coherent Optical Systems The basic structure of the coherent (heterodyne) receiver is shown in Figure 2.33, and repeated in Figure 4.40 for convenience. The signal and local oscillator optical fields are PD

ε (t) d νLO

iph(t)

Coupler

v (t) BPF

ED c

LO (a) Si (f ) HBPF

0

ν2 − νLO

fI

f

Sv (f )

f Spectra (b)

Figure 4.40 Heterodyne receiver.

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combined in the coupler, and the resultant field is converted to a photocurrent, i ph (t), in the photodetector – see Figure 4.40(a). The photocurrent is then filtered in the bandpass filter (BPF), of bandwidth Be , and is detected electrically in the electrical detector (ED). With signal and local oscillator powers of Ps and PLO , respectively, the photocurrent is given by    i ph (t) = R Ps + PLO + 2 Ps PLO cos (2π f I t + ϕs − ϕl ) ,

(4.47)

where R, f I , ϕs , and ϕl are the photodetector responsivity, the intermediate frequency, the signal phase, and the local oscillator phase, respectively. The effect of the bandpass filter and the electrical detector is to filter out any extraneous signals and noise and to perform square-law detection of the remaining signal, giving a resultant signal power 2 = 2R 2 Ps PLO . The power spectral densities of i ph and the output voltage, v, are of Isig shown in Figure 4.40(b). The unique property of the coherent receiver is that the signal power at the output of the detector is proportional to the local oscillator optical power PLO , whereas the thermal receiver noise is independent of PLO . The total noise power 2 is a sum of shot noise and thermal receiver noise, where the shot accompanying Isig 2 noise power σsh = 2R Be PLO is also proportional to PLO . This means that by making the local oscillator power sufficiently large, the signal and shot noise terms can be made to dominate the thermal noise. In this shot noise limit, the electrical SNR is given by S N Re =

R Ps . q Be

(4.48)

With this SNR at the input to a decision circuit, the coherent receiver performs considerably better than a typical direct-detection receiver. However, this is an ideal model that neglects possible polarization misalignment and laser phase noise in both the signal and the local oscillator waves.

4.9

Performance Impairments in a Network Environment In studying the origin of performance impairments in this chapter, we focus primarily on the various optical network devices and subsystems in isolation. The same can be said for the various performance impairments. However, an optical signal accumulates noise, distortion, and interference originating from many sources before it reaches its destination, and their cumulative effects cannot always be deduced from looking at them one by one. In this section, we examine the performance impact of several cumulative or combined effects in optical channels, including cross-talk, signal power divergence, interaction of chirp and dispersion, and PMD. As will be seen, the whole picture is more than the sum of its parts.

4.9.1

Cross-Talk The discussion of receiver performance thus far has ignored an important, perhaps the most important, source of performance degradation: interference from unwanted signals

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(i.e., cross-talk). The effects of cross-talk on receiver performance are considered next. We focus on the digital transmission case.  Suppose a signal s with optical field Es (t) = Re[ 2Is (t)e j2πνs t+φs ] is received at a direct-detection receiver equipped with a wideband photodetector followed by an electrical low-pass filter of bandwidth Be , chosen according to the bit rate (bandwidth)  of s. Accompanying the signal is an interferer x with an optical field Ex (t) = Re[ 2Ix (t)e j2πνx t+φx ]. The photocurrent at the output of the photodetector can then be written

   (4.49) i(t) = R Is (t) + Ix (t) + 2Re Is Ix e j2π (νs −νx )t+φs −φx . In Equation  (4.49), Is and Ix represent the signal and interferer intensities, respectively, and Is Ix represents a signal-interference “beat” term that appears because of the square-law detection process.27 There are three cases of interest, as shown in Figure 4.41(a), depending on the value of |νs − νx |: r ν = ν : interchannel cross-talk s x r ν ≈ ν : cochannel heterodyne cross-talk s x r ν = ν : cochannel homodyne (multipath) cross-talk s x In the first case, the nominal optical frequencies are different; in the second case, the nominal frequencies are the same but the absolute frequencies are slightly different; and in the third case, both frequencies are exactly the same (which only occurs if the two signals come from the same source). These are illustrated in Figures 4.41(b) and 4.41(c). In Figure 4.41(b), two optical connections (S, S∗ ) and (X, X∗ ) are set up with a small amount of signal x “leaking” over to receiver S∗ , as indicated by the dashed line. (This might be a result of switch cross-talk in optical node A.) The categorization of the interference in this case is based on the magnitude of the optical frequency difference. If the two connections are on different assigned wavelengths λs = λx , then |νs − νx | > Be ; that is, the beat interference term in Equation (4.49) lies outside the bandwidth of the receiver filter. In this case, it is only the interferer intensity Ix that affects signal detection and BER. This is interchannel cross-talk.28 If the two sources are transmitting on the same nominal wavelength (i.e., λs ≈ λx , and |νs − νx | < Be ), then the beat interference term in Equation (4.49) lies within the bandwidth of the receiver filter. Now, both the interferer intensity and the beat frequency term affect signal detection, with the beat frequency term constituting cochannel heterodyne cross-talk.29 Last, Figure 4.41(c) shows a case in which a small portion, s  , of the signal s leaks through a parallel path (dashed line), rejoining the desired signal at the receiver. As explained in Section 3.4.1, this violates the distinct source combining 27

28 29

This is the same effect that appears in a heterodyne receiver. As in that case, the magnitude of the beat term depends on the polarization alignment of the two contributing fields. We have chosen the “worst case” of perfect alignment. The interference at a receiver caused by interchannel cross-talk can be eliminated by placing a narrowband optical filter in front of the photodetector. Cross-talk terminology varies considerably. Our cochannel heterodyne cross-talk is sometimes called coherent common channel cross-talk.

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Cross-talk Interchannel νs ≠ νx

Cochannel νs ≈ νx Homodyne νs = νx (a)

Heterodyne |νs − νx | < Be

x* s s

A

s*

B x

x (b)

s s

A

s*

B s′ C (c)

Figure 4.41 Types of cross-talk.

condition. Because s  arrives with a delay and phase shift, it acts as an interferer. We call this common-source interference cochannel homodyne cross-talk, multipath cross-talk, or interferometric cross-talk [Gimlett+89].30 This case is the most difficult to evaluate because the sign and magnitude of the cross-talk term depends on the relative phases and polarizations of the two signals. Referring to Equation (4.49), the cross-product term at the output of the filter is √ Is Is  cos [φs (t) − φs  (t)], where an argument t has been included in the phases to reflect the fact that s is not a constant-frequency optical signal but contains fluctuations in phase (and amplitude) due to laser linewidth as well as the modulation impressed on the laser. If the signals are exactly in phase at some point in time, the effect of the beat term is to magnify the resultant signal (because the optical fields add, in this case), whereas the opposite is true if they are out of phase. When the time delay between s and s  is small (smaller than the signal’s coherence time), then the phase fluctuations are

30

Other sources of homodyne cross-talk are multiple reflections within a fiber, causing delayed replicas of the transmitted signal to be superimposed on it.

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correlated.31 For longer delays, they are uncorrelated. In the correlated case, the interferer acts to produce a multipath fading effect, similar to that in radio transmission systems in which the interference varies slowly from constructive to destructive, depending on the instantaneous phase relations. In the uncorrelated case, the phase fluctuations due to laser linewidth, as well as intensity fluctuations due to modulation, produce a rapidly fluctuating noiselike cross-talk term. In predicting performance when significant homodyne cross-talk is present, the best one can do in most cases is to assume a worstcase scenario, in which polarizations are aligned. A convenient measure of cross-talk, or for that matter any type of signal impairment, is the impairment-induced power penalty. It indicates how much additional signal power is required to maintain a specified BER in the presence of the particular impairment. Expressed in decibels, it is defined as
 Power required with impairment . (4.50) P = 10 log10 Power required without impairment The power penalty incurred with homodyne interference illustrates the concept. Consider the following model. A large number, N , of interferers are present with random phases, each with an intensity Is  =  Is . Assuming aligned polarizations, it has been shown (see [Gimlett+89, Goldstein+94]) that the PDF for the resultant aggregate interference is approximately Gaussian, which leads to a power penalty given by P = −5 log10 [1 − 4Q 2 N ],

(4.51)

where Q is the Q factor corresponding to the reference BER. For example, at a BER of 10−9 , Q = 5.9 (Q 2dB = 15.4). Using Equation (4.51) the effect of the homodyne cross-talk power penalty can be plotted as shown in Figure 4.42(a). The solid and dashed curves show the BER without and with homodyne cross-talk, respectively. The Pe

N
= 0 N
= 0.001

10−9

N
= 0.009

Error Floor

10−7

0.32 dB

Power Penalty

Received Power (dB)

Received Power (dB)

(a)

(b)

Figure 4.42 Power penalty with homodyne cross-talk. 31

The coherence time tc for a laser is approximately equal to the reciprocal of its linewidth: tc = 1/ν. A related quantity, coherence length, lc = ctc , defines the distance between two observation points of the same signal required to decorrelate the two observed signals.

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horizontal spacing between the two is the cross-talk power penalty. The dashed curve is plotted for N  = 0.001. Another curve for N  = 0.009 is plotted in Figure 4.42(b). Note that in this case there is an error floor, corresponding to the BER (≈10−7 ), at which 4Q 2 N  = 1, where the power penalty tends toward infinity. It is impossible to achieve BERs smaller than the error floor because of the nature of the cross-talk. Among the different forms of cross-talk described above homodyne is the most severe, because the cross-talk is produced by the signal itself. Therefore, increasing the signal power will not indefinitely reduce the BER, as reflected by the error floor. The precise relations between cross-talk and BER depend a many factors, including modulation format, phase, and polarization of optical signals. Procedures have been developed to take into account these factors to evaluate cross-talk-induced penalties semianalytically [Ho+99, Kamalakis+03].

4.9.2

Signal Power Divergence For a variety of reasons, it is desirable to run a WDM system with equal powers (or OSNRs) in each channel. However, signal power or OSNR imbalance can result from nonideal and uneven loss/gain functions of optical components, such as filters, MUXs/DMUXs, optical amplifiers, and couplers, from laser misalignment, from polarization-dependent loss (PDL), and/or from dynamic reconfigurations of the network that add/drop channels at different power levels. Signal power divergence accumulates as the channels propagate through an optical path. The impact of this effect on the network performance can be very significant, causing adverse effects in both the low-power and high-power channels. The weaker channels are affected the most by receiver electrical noise and interference within the network causing low OSNR, whereas the strongest channels can potentially suffer from fiber nonlinear effects. The dynamic range limits of the optical receivers can also affect performance. Lightwave transmission systems and networks with a large number of optical components (such as undersea systems or metro networks) are vulnerable to performance degradations due to PDL, which was described in Section 4.3.2.3. The total PDL along a particular optical path will depend on the initial signal polarization orientation and on the relative orientation of the polarization axes of each component in its path. Due to PMD the principal states of polarization vary randomly in a wavelength-dependent way and, as a result, the interaction of PMD with PDL introduces among other effects a wavelength-dependent variation of the loss that each signal will experience in the path. One of the resulting effects is signal power divergence among the channels (known as PDL-induced ripple) and OSNR deterioration at the receiver. The signal power divergence becomes worse through long amplifier chains as the strongest growing channels compete for gain with the weakest channels, and the unpolarized noise is attenuated differently than the signals. PDL is a probabilistic effect involving interactions of random polarization states with components along a signal path. This suggests a Monte Carlo simulation as one way to estimate the PDL-induced ripple magnitude in a typical system [Chowdhury+99]. (See the case study of Section 4.13.2.2.) Each component is represented as a birefringent

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element with random orientations of its principal axes. All signals propagate through these elements starting with some random orientation of their polarization axes and the PDL-induced ripple statistics are calculated at the receiver based on repeated simulation runs over a large set of signal and component axis orientations. The mean+5σ value of PDL-induced ripple can be used as a good figure of merit for system engineering. In [Mecozzi+04], an analytic model is presented that deals with the effect of PDL on OSNR in optical communications systems taking into account the effect of dynamic gain equalization.

4.9.3

Chirp-Induced Penalty As described in Section 4.5.3.1, on standard SMF, the chirp produced by direct modulation of a laser interacts with fiber dispersion to distort and spread out the data pulses, causing ISI and increased BER. The chirp-induced penalty can be estimated qualitatively by calculating the amount of eye degradation at the output of the receiver electrical low-pass filter [Elrefaie+88] or, more accurately, by evaluating the error probability for different receiver types. Commercially available DMLs exhibit significant variations in the instantaneous power and frequency of their waveforms, among different manufacturers and among different samples from the same manufacturer. Signal degradation occurs when positive chirp, resulting in a blue-shifted pulse leading edge and red-shifted trailing edge, encounters positive fiber dispersion. However, if the sign of the chirp or the dispersion (but not both) is reversed, pulse compression will occur over a limited transmission distance, where the red trailing edge tends to catch up with the blue leading edge. This effect actually produces a signal enhancement; i.e., a negative power penalty. To obtain this desirable chirp/dispersion effect, either negative chirping is generated in the transmitter (e.g., using an EA-DFB transmitter or an MZ modulator tuned to produce the desired chirp), a process called prechirping, or negative dispersion fiber is installed on the signal path. The chirp/dispersion interaction was analyzed using a small signal model [Koch+84]. However, this does not appear to be applicable to the large excursions in a data pulse. Instead, a phenomenological model was developed, based on experimental measurements of the peak-to-peak chirp and the chirp duration parameters [Antoniades+02]. This simple model describes the chirp/dispersion interactions and their effect on the level of the received 1s and 0s and provides accurate results in simulations.

4.9.4

Optical Filter Concatenation: Distortion-Induced Penalty A serious signal impairment that is unique to transparent optical networks is the penalty due to WDM filter concatenation. This effect is relatively small in a point-to-point optical system because a given signal usually passes through just two filters: a MUX and a DMUX. However, in a transparent optical network, a signal may be demultiplexed and remultiplexed at many network elements throughout its path before it is finally received. Thus the signal experiences the concatenation of the entire set of filters in its path. The effective spectral transfer function of the filter set is the product of all of the individual filters’ transfer functions and can therefore be much narrower in spectral width

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than that of a single filter. Consequently, optical MUX/DMUX concatenation causes signal attenuation and distortion leading to ISI, which eventually limits the maximum number of optical network elements that can be cascaded. The resulting signal spectral clipping due to the concatenation of MUX/DMUXs diminishes the signal power seen by the network elements, producing changes in the operating points of any variable optical attenuators (VOAs) and EDFAs along the signal path. Therefore, to deduce the effects of filter concatenation on signal quality it is necessary first to take into account the interaction among optical MUX/DMUXs, VOAs, and EDFAs. This information can then be used to determine the power levels of the optical signal and amplified spontaneous emission (ASE) noise in the network. A comprehensive model including the aforementioned effects of optical filter concatenation appears in [Roudas+02]. It is based on a semianalytic technique for the evaluation of the error probability at the receiver. The error probability evaluation takes into account arbitrary pulse shapes, arbitrary optical MUX/DMUX and electronic low-pass filter transfer functions, and non-Gaussian photocurrent statistics at the output of a preamplified direct-detection receiver.

4.9.5

Polarization Mode Dispersion Impact on System Performance As described in Section 4.3.2.3, PMD is a random effect in fibers. It can be a significant impairment in high-data-rate transmission systems and networks, especially for alreadyinstalled older fiber spans. PMD is defined as the statistical average of differential group delay (DGD) between the two principal states of polarization (PSP) of a signal on an optical fiber. It has been shown that to a first-order approximation, the PMD of a transparent link is the root-mean-square sum of the PMD contributions of individual components, leading to the square root in Equation (4.20). Essentially, this model implies quadrature addition of PMD for transmission fiber, dispersion compensation modules, and network elements (including amplifiers) that have PMD. Simulations and experiments have calculated the Q penalty versus DGD in a system. What is really important using this model is the value of the accumulated system DGD as a percentage of the bit duration. For example, in [Chowdhury+04] it was shown that a DGD accumulation of 33% of the bit period (e.g., 33 ps for a 10-Gbps system) produces a Q penalty of roughly 0.5dB.32 Because PMD is a probabilistic effect, total allowed PMD in a commercially deployed system is calculated to yield an acceptable outage probability. Following current ITU standards a safety factor of 3.7 is included in the PMD to assure a 10−7 outage probability. 33 = 8.92 ps tolerable PMD for the entire In the previous example, this translates into 3.7 system. In the case study of Section 4.13.2.2, the total PMD is calculated based on the quadrature addition model described above factoring in the 3.7 safety term.

4.10

Optical and Photonic Device Technology As discussed in Section 2.3, network nodes perform the operations of routing, switching, and wavelength conversion, and these functions are implemented in a variety of 32

The Q dB definition used in this study is 10 log10 Q.

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ways in specialized network elements; static routers, optical cross-connects, wavelength add/drop multiplexers, wavelength-selective cross-connects, and wavelengthinterchanging cross-connects. When deployed in operational networks these network elements take the form of complex subsystems incorporating a host of ancillary monitoring, control, and management functionality as well as the basic optical transport functions. Although some of them may include electronics to a greater or lesser degree, their essential building blocks are optical and photonic, including directional couplers (either static or dynamic), switches, wavelength-selective elements (filters), polarizers, and isolators. In some cases wavelength selectivity is combined with a switching or tunability function in the same device to produce a multiwavelength switching element. Each of these devices performs a basic linear operation required in a transparent optical network (see Section 1.3). Some optical elements are fabricated as all-fiber devices, making it easy to connect them to transmission fibers with very small coupling losses. Others are fabricated in the form of integrated configurations using techniques derived from electronic integrated circuit fabrication. Examples of fiber devices are fused fiber directional couplers and fiber Bragg grating filters. Examples of integrated assemblies are optical switch fabrics, wavelength multiplexers/demultiplexers, and fully integrated optical receivers. Controllability is effected in devices such as switches, controllable attenuators, or tunable filters either by using materials whose optical properties can be changed under external control or by using systems whose physical geometry can be changed. Examples of the former are liquid crystals whose refractive index can be changed under electrical or magnetic control, thermo-optic materials whose optical properties are temperature dependent (e.g., silica or polymers), and electro-optic materials such as lithium niobate and semiconductors, whose refractive index can be controlled through an applied electric field. The most interesting development in the category of systems with controllable physical geometry is the optical microelectromechanical system (MEMS). In optical applications, MEMS use free-space optics and electromechanically actuated moving parts to produce a wide variety of optical systems, including gain and dispersion equalizers and large optical switch fabrics. In this section, we examine the principles of operation, performance characteristics, and underlying technology of some representative optical and photonic devices, starting with couplers and switches and concluding with wavelength-selective devices, which include filters and multiwavelength switches.

4.10.1

Couplers and Switches Controllable coupler and switch technology has come full circle over the past decade: from free-space optomechanical technology to guided-wave devices and back again. Typical early examples of optical switches consisted of bulk assemblies using free-space optics – mirrors, prisms, collimators, lenses, and gratings (for wavelength-selective devices). In these devices, the physical geometry is controlled either purely mechanically or electromechanically. They generally have good switching performance (low crosstalk and loss). However, because of their complex and potentially unreliable mechanical

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structures as well as their relatively slow speed (switching times of the order of milliseconds) these have been largely supplanted by guided-wave devices, which are easier to build, more reliable, faster, and lend themselves well to manufacturing as optical integrated circuits (OICs) using planar lightwave circuit (PLC) technology. Although guided wave optical devices are the basic components of most state-of-the-art optical switch architectures, as well as many other optical subsystems, it is possible that they will eventually be overtaken by MEMS switches in applications requiring large-scale integration. This represents a return to free-space optomechanical devices but using much more sophisticated architectures, control, and fabrication techniques. In evaluating and comparing switching devices there are many criteria to take into account: speed, power consumption, cost, reliability, polarization and wavelength independence, insertion loss, and cross-talk. Because wavelength-selective cross-connects are composed of large numbers of switching devices, they are the most demanding applications of these devices with respect to most criteria except speed.33 In particular, overall system insertion loss and cross-talk are critical performance parameters, and scalability is an essential consideration. Insertion loss in an OXC is the total attenuation along an active path from an input port to an output port. Because most OXCs have multistage switch fabrics, their overall insertion loss is several times the insertion loss of one switching device. Similarly, the total cross-talk appearing at an output port of a large switch fabric includes combined effects of many cross-talk terms generated in each elementary switching device. These multiple effects make it very important to minimize insertion loss and cross-talk at the device level, and we emphasize that issue here. The effects of embedding many imperfect devices in switch fabrics are discussed in Section 4.12.1. It is useful to express the phenomena of cross-talk and insertion loss in terms of the conceptual models used in this book, taking the directional coupler of Figure 2.8 as our generic 2 × 2 switching element. The power transfer matrix of a symmetric lossless coupler is given by Equation (2.3). In real devices, it is impossible to make α = 0 or α = 1. Instead, we have α =  > 0 when the switch is set in the bar state and α = 1 −  < 1 in the cross state. As indicated in Equation (2.13), this means that in either state a fraction  of input signal power leaks into the wrong output port. The leakage parameter , usually expressed in decibels as dB = 10 log10 , is called the cross-talk of the switching element. Note that because the device is passive and lossless, the cross-talk is accompanied by an insertion loss of the same value from input P1 to output P1 .34 In many cases, a switching element is characterized by its extinction ratio between a specified pair of input and output ports, which indicates the ratio of power transferred through the switch when its state is changed from ON to OFF. For example, the coupler of Figure 2.8 might be used as an ON/OFF switch from input P1 to output P1 , where α =  in the bar state (the ON state) and α = 1 −  in the cross state (the 33

34

Because OXCs are used for provisioning and recovering optical paths on a circuit-switched basis, high speed is not required; switching times of milliseconds are sufficient. However, optical packet-switching systems require much faster switching times – of the order of nanoseconds. Real devices may exhibit different cross-talk values in each state; i.e, we may have α = 1 −   in the cross state, where   and  are different.

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OFF state). The extinction ratio is then defined as the ratio of P1 in the ON state to P1 in the OFF state, which in this case is (1 − )/. Thus, for switches with small leakage parameter , the extinction ratio in decibels is approximately the negative of the cross-talk. The following sections describe some representative switching devices, beginning with guided-wave devices, following with MEMS switches, and concluding with liquid crystal architectures.

4.10.1.1

Guided-Wave Switches Guided-wave photonic devices are frequently fabricated using PLC technology. Components such as waveguides, couplers, filters, and switches are constructed on the surface of a substrate, with the most common materials being silica on a silicon substrate, silica on silica, ion-implanted silica, titanium diffused into lithium niobate, III-V semiconductors, or polymers. PLC technology offers the possibility of integrating large numbers of similar devices on a single chip, which is useful in applications where components are replicated many times; for example, switch fabrics and DWDM multiplexers. In this way, a single module fabricated as an OIC replaces a set of bulk devices interconnected by fibers. A major advantage of integration is the cost saving involved in eliminating the interconnections, which are labor intensive, unreliable, and introduce significant coupling losses. The downside of integration is the lower yields associated with manufacturing large wafers. Silica-on-silicon-based technology has also been used with advantage in fabricating and packaging hybrid OICs composed of disparate components such as waveguides, lenses, photodetectors, and lasers. The waveguides and other PLC devices such as couplers, multiplexers, and switches are fabricated in silica and are interconnected with other nonsilica devices, constructed as surface-mounted modules, to form a complete subsystem such as an optical transceiver. This technique, known as the silicon optical bench, has advantages in versatility, cost, and performance [Henry+89, Li+97]. The most common structures for guided-wave switching devices are the 2 × 2 directional coupler, the MZ or interferometric switch, and the Y-branch switch. Each can be built using a variety of materials and fabrication technologies. Each material offers a choice of control mechanisms. Possibilities in LiNbO3 include electro-optic and acousto-optic control. Thermo-optic control is used in silica guides, either electro-optic or thermo-optic control in polymers and electro-optic or carrier injection in semiconductors. Given this wide range of choices, switching devices can usually be chosen with cost and performance properties tailored to the needs of each specific application.

Controllable Directional Coupler The operation of the controllable directional coupler is based on the fact that when two waveguides are placed close together, coupling is possible between the two guides because of the overlap in their fields. (The light is not confined completely to the guides, with an evanescent field spreading out to the surrounding medium.) Provided that the propagation constants of the waves in the two guides are the same (the phase-matching condition), light entering one guide couples into the other and vice versa. Depending on the length of the interaction region, the coupling may be partial or complete. In the

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Fibers V

Figure 4.43 Controllable directional coupler. (From [Saleh+91, Figure 21.1-7b]. Copyright

c 1991. Reprinted by permission of John Wiley & Sons, Inc.)


configuration shown in Figure 4.43, a voltage applied to the electrodes increases the propagation constant in one of the guides and decreases it in the other. In this way, the phase matching is controlled electrically, thereby controlling the coupling ratio between the ports. Referring to the conceptual model of Equation (2.3), varying the voltage varies the control parameter α and places the coupler in the cross state, bar state, or any partial coupling state between the two. The directional coupler can therefore be operated in an analog mode, sweeping it through a continuous range of states by varying the control voltage. These devices are typically fabricated in LiNbO3 using PLC technology. The propagation constant is controlled via an electro-optic effect wherein an applied electric field produces a change in the refractive index of the material. Because LiNbO3 has a large electro-optic coefficient and subnanosecond switching time, it has been a preferred material for controllable directional couplers as well as many other photonic switching devices. LiNbO3 directional couplers can achieve cross-talk down to −30 dB and fiberto-fiber insertion losses of 2–3 dB. Controllable directional couplers are also realized in other technologies. For example, PLCs using polymer waveguides on silica have been fabricated using thermo-optic control, where refractive index control is applied through a heating electrode placed over one of the guides [Keil+94]. Polymer devices have been integrated into small switch fabrics (five couplers) with a switching time of 1 ms, overall extinction ratios of 17–19 dB, and a fiber-to-fiber insertion loss of 10 dB, about half of which is due to the fiber-to-chip interfaces. Semiconductor devices are another alternative as discussed in [Renaud+96]. In one approach, integrated switch fabrics are fabricated using InGaAsP waveguides, which offer a number of control possibilities, either using an electro-optic effect or carrier injection to control refractive index. A typical example is a 4 × 4 switch made up of six

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Phase Modulator Guide 1

3-dB Coupler

+

3-dB Coupler

− Guide 2

+

c 1988 AcaFigure 4.44 Mach–Zehnder switch. (From [Korotky+88, Figure 11.4]. Copyright
demic Press. Used with permission of Elsevier.)

couplers using carrier injection, with a 15-dB overall insertion loss and −15-dB crosstalk. Although insertion loss is generally high in semiconductor-based switches, this can be offset by the use of SOAs integrated on the same wafer as the switching devices. However, attenuation followed by amplification necessarily augments the noise level.

Mach–Zehnder Interferometric Switch The MZ interferometer, configured as a single input/output device, was described in Section 4.5.3.2. An electro-optically controlled version of this device, used as a 2 × 2 switch, is shown in Figure 4.44, where 3-dB couplers terminate the two arms of the device. In a symmetric device (both arms the same length) when no voltage is applied to the phase modulator, signals entering guide 1 or 2 exit on guide 2 or 1, respectively. If we designate this as the cross state, then it is switched to the bar state when a control voltage shifts the relative phases of the waves traversing the two arms by any odd multiple of π . One drawback of the MZ switch is that the couplers must have an exact 3-dB ratio to achieve a perfect cross state. Imperfect fabrication leads to undesirable cross-talk in the cross state. A typical switch of this type fabricated in LiNbO3 has fiber-to-chip-to-fiber insertion loss of 4 dB and cross-talk of −25 dB [Pohlmann+91]. Its switching time is less than 1 nanosecond. Other versions have been built using other technologies. For example, typical parameters using the electro-optic effect in InGaAsP semiconductors are 3 dB of insertion loss and −16 dB of cross-talk. Mach–Zehnder switches have also been built in silica on a silica substrate using thermo-optic control. Because heating is involved, switching times are in the millisecond rather than subnanosecond range. In a typical device [Kasahara+02], a thin film electrical heater is deposited on one of the arms of the MZ structure to control the switch state. Applying heat to the arm induces a change in refractive index and a phase change of π in the signal on that arm, thus changing the state of the switch. In thermally actuated devices, especially those used in large arrays, it is important to reduce the required switching power while maintaining good switching speed and low cross-talk. The switch in this example achieves 5.2 ms switching time using 90 mW of power. It has over 30 dB of extinction ratio (−30 dB cross-talk) and about 1 dB of insertion loss, which is almost all due to fiber-to-chip coupling. It is essentially polarization independent. The thermo-optic versions of the MZ switch have also been fabricated using polymer waveguides on a silicon substrate. Polymers offer easy and low-cost fabrication and low power consumption with relatively fast switching times. An example is a device using

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Thermo-optic heater

Intersection

247

 Output port 1 OFF Output port 2

Input port 1 Optical Signal



ON 1st MZ I 2

nd

MZ I

3-dB coupler

Figure 4.45 Two-stage Mach–Zehnder switch. (Adapted from [Goh+99, Figure 1]. Copyright c 1999 IEEE. Used by permission of The Institute of Electrical and Electronics Engineers,
Inc.)

asymmetric X-junctions to replace the 3-dB couplers in Figure 4.44 and employing heaters on the two guides to change their index of refraction [Oh+98]. It has −20 dB of cross-talk, 4.5 dB of insertion loss, and a 2-ms switching time while consuming only 10 mW of power.

Cascaded Mach–Zehnder Switches Although −30 dB of cross-talk is very good performance for a single device, when many such devices are combined into a large switch fabric the accumulated cross-talk may be too high to keep the total interference to an acceptable level. Thus, it is important to make the cross-talk produced by each device as low as possible. One approach is space dilation, but it requires replacing each switch by a set of four switches (see Section 4.12.1.4). A more economical approach that works in many applications (see Section 4.12.1.1), but yields an incomplete switch, is the two-stage MZ switch shown in Figure 4.45 [Goh+99]. It consists of two thermo-optic MZ switches fabricated using silica-based PLC technology as in [Kasahara+02]. When both elementary switches are in the bar state, the paths from 1 to 1 and 2 to 2 are active (called the OFF state in this case). Assuming that the control parameter α has the same value for both MZs, the overall power transfer matrix for the switch is:


(1 − α) α2

0 (1 − α)

 .

(4.52)

To evaluate cross-talk, suppose α =  when an MZ is in its bar state and α = 1 −  in the cross state. When the cascaded switch is in the ON state (both MZs in the cross state) the path from 1 to 2 is active, but this is an incomplete 2 × 2 switch, because there is no path from 2 to 1 in either switch state. Referring to Equation (4.52), when the switch is in the OFF state the leakage onto the path from 1 to 2 is  2 ; i.e., the cross-talk in decibels is reduced by a factor of 2 compared to that of a single MZ. This is known as second-order cross-talk (see Section 4.12.1.4). In the ON state, cross-talk from 1-1 is , that is, first order. Thus it is advisable to use this configuration in applications where it is only the OFF state cross-talk that is of concern (see Section 4.12.1.1, for an application).

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e1

od

tr lec

e

few mrad ele ctr

od

e2

Adiabatic region

c 1996 IEEE. Figure 4.46 Y-branch switch. (Adapted from [Renaud+96, Figure 8]. Copyright
Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

Y-branch Switches The Y-branch switch, shown in Figure 4.46, can be used as a 1 × 2 switch, in the reverse direction as a 2 × 1 switch, or in a combination of four as a 2 × 2 switch. The principle of operation of the 1 × 2 device is that the input wave propagates to the output arm with the higher refractive index, which can be controlled by various methods, depending on the materials used. The Y-branch has numerous advantages over switches like the directional coupler and the MZ, which both involve controlled phase relations. The advantages include polarization independence, relative insensitivity to manufacturing tolerances, wide optical bandwidth, and a step-like switching curve under electro-optic control. It has been called a “digital switch” because of this control behavior. A typical electro-optically controlled device fabricated in LiNbO3 has less than −16 dB of crosstalk with a control voltage of 60 V over the entire optical fiber transmission window. Various versions of the Y-branch have also been fabricated using semiconductors with optical bandwidths of more than 50 nm. For example, a carrier-injection controlled InGaAsP device has −18 dB of cross-talk, requires 30 mA of switching current, and has a 10-dB fiber-to-chip-to-fiber insertion loss when four devices are integrated into a 2 × 2 switch [Renaud+96]. Finally, thermo-optically controlled polymer devices have also been used in Y-branch configurations, with −20 dB of cross-talk and a 2.5-dB insertion loss. The Y-branch is a very versatile device as a building block for large switch fabrics. For example, switches containing a total of 88 Y-branch elements have been fabricated in LiNbO3 on a single 8 × 80-mm wafer [Murphy+97].

Gate Arrays All switching devices discussed thus far have been passive lossy devices: no power is supplied to optical signals as they traverse the switch, and insertion losses can be significant. A different approach is the active gate array switch, fabricated as a PLC with active elements, supplying gain as well as control. Semiconductor gate array switches have been demonstrated, with a typical architecture shown in Figure 4.47. In the figure, which shows the actual layout on a chip, a passive splitting network is followed by an array of amplifier gates (SOAs), which are in turn followed by a passive combining network. In this realization, additional SOAs are used at the input and output ports to

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c 1995 IEEE. Used by Figure 4.47 Gate array switch. (From [van Berlo+95, Figure 1]. Copyright
permission of The Institute of Electrical and Electronics Engineers, Inc.)

help compensate for splitting and combining losses as well as other excess losses in the PLC. Note that this is essentially the same arrangement with the same functionality as the generalized switch architecture of Figure 2.18. The monolithic structure uses a combination of passive and active waveguides for the splitter/combiners and the SOAs, respectively. The I/O amplifiers are run at a constant injection current, and each gate is controlled by varying its injection current: using zero current to set it to the OFF (absorbing) state and sufficient injection current in the ON (gain) state to yield an overall fiber-to-chip-to-fiber insertion loss of less than 2 dB. The 4 × 4 configuration demonstrated in [van Berlo+95] was designed to be largely polarization insensitive (less than 1 dB polarization dependence on most paths), which is one of the challenges of semiconductor gate array switches. It has a 40-dB extinction ratio on most paths. Crosstalk in PLC gate arrays is generated by various sources, including waveguide crossings. In this example, cross-talk varied from −45 dB to −17 dB, depending on the active connections, with the higher value probably due to fabrication imperfections.

Bubble Switch The bubble switch, a product of Agilent Technologies, is a novel variant of the PLC switching fabric. Here, a crossbar grid like that of Figure 2.14 is fabricated in the form of intersecting waveguides on a silica chip. In the realization shown in Figure 4.48, the waveguide chip is sealed to a silicon controller chip, which contains an array of controllable electric heating elements at the cross-points. As shown in the figure, each cross-point has a trench carved out of it, which is filled with a fluid whose refractive index matches that of the waveguides. As long as fluid is present, the cross-point is “open” and the light signal in each guide takes a straight path through it. To close the cross-point, its heater is activated causing the fluid to vaporize, producing a bubble, which lowers the refractive index in the trench. As a result, the trench wall encountered by light entering on the IN guide in the figure acts as a mirror, deflecting the IN signal to the OUT guide through total internal reflection. For operation in the crossbar configuration of Figure 2.14, the IN/DROP guide would correspond to a horizontal input bus and the ADD/OUT guide would correspond to a vertical output bus. These switches have been demonstrated in sizes up to 32 × 32 with overall insertion loss less than 9 dB, cross-talk from −50 to −55 dB, and switching times of approximately 8 ms.

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Multiwavelength Optical Networks

ADD Signal Fluid

IN Signal

Trench

Center Heater Optical Waveguide Sidewall Metalization

Bubble DROP Signal

OUT Signal

Figure 4.48 Laser-activated bubble switch element. (From [Hengstler+03, Figure 1]. Copyright

c 2003 IEEE. Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)


4.10.1.2

Microelectromechanical Systems: MEMS These systems, fabricated using ingenious micromachining techniques, had their first applications in the early 1990s as microscopic actuators, motors, sensors, and similar devices. In the optical communication field, they have been applied to dispersion compensation (Section 4.3.2.3), dynamic gain equalization (Section 4.4.1.1), and reconfigurable wavelength add/drop multiplexing (Section 4.10.5.3). However, their most ambitious application is in large-scale free-space integrated optical switch fabrics, the focus of this section. The first generation of MEMS-based switch fabrics are known as 2D switches. They have a planar architecture, with an N × N switch consisting of an integrated array of N 2 moving mirrors, used to make free-space optical beam connections between sets of N input and N output fibers in crossbar fashion. Typically fabricated in polysilicon on a silica substrate, they use either free rotating or torsion hinged mirrors actuated electrostatically. An example of the free rotating case, used to realize an optical crossbar switch of the form of Figure 2.14, is shown in Figure 4.49(a), where a mirror in a position (i, j) is set in the vertical position to deflect a light beam from input fiber i to output fiber j. A translation plate, shown in Figure 4.49(b), is used to “flip” each mirror up or down. Because of the small mirror size, switching times of submilliseconds are achieved. Cross-talk in these systems is better than −60 dB for an 8 × 8 switch. Although 2D MEMS switches have many good features, including low cross-talk, wavelength, polarization and bit-rate independence, compactness, relative ease of fabrication, and low power consumption, there is a scalability problem due to insertion loss. Because of the divergence of the optical beams in free space, the coupling loss between

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251

Switch Mirror

Microlens

Silica Substrate (a) Schematic Drawing of the Micromachined Free-Space Matrix Switch

Switch Mirror

Hinge Joint Actuated Translation Plate Silica Substrate (b) Working Principle of the Switch Mirrors

Figure 4.49 2D mechanical switch using micromachined mirrors. (From [Lin98, Figure 1].

c 1998 IEEE. Used by permission of The Institute of Electrical and Electronics Copyright
Engineers, Inc.)

input and output fibers grows significantly with the size of the switch. For example, fiber-to-fiber insertion loss on the longest path through a switch of this type increases from 2.9 to 6.9 dB for 16 × 16 and 32 × 32 switches, respectively. Note also that the path lengths through the switch are not uniform, so insertion loss varies from one connection to another. Because of this, the 2D switch has a practical size limitation of about 32 × 32 to keep insertion loss below 10 dB. Although this may seem like a large switch, it is not large at all in DWDM switch configurations. For example, consider a typical DWDM switch using the opaque O-O-O architecture of Figure 4.88. It carries one wavelength on each port of the core switch fabric, giving the overall switch complete nonblocking WIXC functionality. With as few as 10 wavelengths on each link, a 32 × 32 switch fabric in this configuration can serve no more than three input and output fibers.

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Multiwavelength Optical Networks

optical signal Lens array

Fi Fiber array

ME MEMS mirror array Op Optical path MEMS array

Fi Fiber array Le Lens array

(a)

(b)

c 2002 IEEE. Used by Figure 4.50 3D MEMS switch. (From [Chu+02, Figure 3]. Copyright
permission of The Institute of Electrical and Electronics Engineers, Inc.)

The second generation, called a 3D MEMS-based switch, largely circumvents the scalability problem, but the 3D system requires twice as many mirrors and a very challenging multistage optical structure and control system. Figure 4.50 shows a “Z-structure” for an N × N 3D switch. Light beams from an array of N input fibers are focused on an array of tilting mirrors, with one mirror dedicated to each input fiber, with a similar arrangement used for the output fibers. As shown in Figure 4.51, these mirrors are supported by gimbals so they can tilt (under electrostatic or electromagnetic control) in two directions. To set up a connection between input fiber i and output fiber j, the input mirror i is tilted to focus its beam on output mirror j, which also has to be tilted to direct the incoming beam to fiber j. Note that to accommodate all possible connections, the angular position of each mirror must move through a wide range of twodimensional states. An advantage of this geometry is that all paths are approximately the √ same length and the path length grows as N instead of N , reducing the insertion loss problem – insertion losses of less than 10 dB are possible with ports numbering in the hundreds. There are many design challenges. Some basic requirements include: high spring stiffness and low mirror mass for fast mirror response time, large (of the order of 1 mm) mirror diameter, high reflectivity, large radius of curvature, and large tilt angle (up to 10◦ ) together with stability in the face of temperature changes and vibrations. Mirror arrays are typically fabricated using surface micromachined polysilicon or single-crystal silicon (SCS) starting with a silicon-on-insulator (SOI) wafer. Fabrication techniques well established for integrated electronics are used in each case. Whereas a metal coating is required to achieve good reflectivity in polysilicon, the necessary mirror properties,

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253

c 2003 IEEE. Used Figure 4.51 3D gimbaled mirror. (From [Greywall+03, Figure 1]. Copyright
by permission of The Institute of Electrical and Electronics Engineers, Inc.)

flatness, temperature stability, and high reflectivity, are achievable in SCS, together with good spring properties, without metallization. Although magnetic actuators have been used in MEMS switches [Bernstein+04], the electrostatic approach is preferred because it is relatively simple to integrate electrostatic actuators into a silicon-based structure. Whether electrostatic or magnetic, a sophisticated control system is required to achieve reasonably fast switching times (several milliseconds) while controlling power consumption and maintaining accurate beam pointing in the face of external perturbations (temperature change, shock, and vibration), as well as parameter drift and manufacturing imperfections. Feedback control is the best way to achieve these ends, as well as to combat the natural instability of electrostatic actuators.35 However, feedback systems require mirror position sensors to generate the feedback signals, as well as sophisticated control algorithms and considerable real-time computing power capable of implementing these algorithms in a system that may have thousands of mirror controllers operating simultaneously. Because of their significantly increased complexity, 3D MEMS-based switches are currently economically competitive with their 2D counterparts only in very large port count switches.

35

A spring-restrained electrostatically controlled mirror tends to be unstable beyond a certain angular displacement limit, because under a constant voltage (or charge) the electrostatic force increases as its tilt angle increases, producing a “snap down” effect until it reaches a mechanical stop.

254

4.10.1.3

Multiwavelength Optical Networks

Liquid Crystal Switch Architectures Liquid crystals are materials that have anisotropic refractive properties that can be changed under the influence of an applied electric field. This gives them special polarizing properties in response to an applied control voltage. In conjunction with other optical elements, including mirrors, polarizers, lenses, and other birefringent (polarizationsensitive) crystals, LCs can be used to spatially deflect light signals and modulate their phase and intensity. The molecules in LC material tend to line up with each other, producing their anisotropic optical properties, and the application of an electric field changes the orientation of the molecules. Two types of LC materials that are commonly used in display and communications applications are twisted nematic crystals and smectic crystals. The former act as controllable polarization rotators that can be deactivated by an applied electric field; response times are slow (of the order of milliseconds). The latter exhibit ferroelectric polarization behavior, acting as polarizers whose axis of polarization can be switched between two states by the application of an electric field; they are relatively fast, with switching times of the order of 10 microseconds. Liquid crystals are most commonly used in spatial light modulators (SLMs) where a slab of LC material sandwiched between other elements is arranged to produce an array of pixels that are controlled through a grid of electrodes to produce prescribed light patterns. This is a mature technology, having been widely used for many years in consumer applications such as computer and television displays. Applications in optical switching require a higher degree of manufacturing precision, but the material offers many advantages: low insertion loss across the full optical transmission window, scalability because of the inherent parallelism of the SLM, low power consumption, and low cost. An example of a recent application is the LC-based holographic optical switch. A holographic phase-modulation pattern created on an SLM can be used to steer a light beam to a desired target by diffraction. This opens up many possibilities for free-space switch architectures operating much like MEMS switches but without moving parts. One possible configuration is shown in Figure 4.52, where a 1 × N switch is built around a reflective SLM consisting of an LC slab backed by a mirror [Fracasso+03]. Light from a single input fiber is spread by the lens system onto the surface of the SLM, which consists of individually controllable pixels; the mirror

positive lens (achromat doublet)

programmable beam steerer

single-mode fiber array

f

f

c 2003 Figure 4.52 Liquid crystal holographic switch. (From [Fracasso+03, Figure 3]. Copyright
IEEE. Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

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255

Figure 4.53 Two hologram N × N liquid crystal holographic switch. (From [Manolis+02, Figure 1]. c 2002 IEEE. Used by permission of The Institute of Electrical and Electronics Copyright
Engineers, Inc.)

causes the light to make two passes through the slab. Each pixel is controlled to produce a specified phase shift in the reflected light, which is refocused by the optical system into a beam impinging on one of the output fibers. Deflection of the beam to a given output fiber is achieved by electronically writing a precalculated hologram into the pixels of the SLM. Thus, the SLM acts much like an electronically controllable mirror. A 1 × 8 switch has been demonstrated using smectic LCs [Crossland+00]. It is polarization insensitive with 17 dB of insertion loss, cross-talk ranging from −19 to −40 dB (depending on the switch setting), and a switching time of less than 20 µs. The high insertion loss in this case, is in large part because of properties of the LC material used here, which constrain the hologram to binary phase modulation. The architecture of a more elaborate twohologram N × N switch is shown in Figure 4.53 [Manolis+02]. In this case, each input fiber and each output fiber has a dedicated hologram, and the SLMs are transmissive rather than reflective. To set up a connection between input fiber i and output fiber j, the input hologram is programmed to deflect its beam onto output hologram j, which also must be programmed to direct the incoming beam to fiber j. This structure is similar to a 3D MEMS switch architecture. A 3 × 3 switch was demonstrated with a 19-dB insertion loss, and −35 to −40 dB of cross-talk. Although LC holographic switches have stringent optic alignment requirements, errors in alignment can be corrected by building them into a modified hologram. Comparing the cross-talk and insertion loss figures to MEMS switches, the latter have the advantage.36 However, the LC switch has no moving parts and can achieve speeds in the microsecond range. Wavelength selectivity can also be built into LC switching devices (see Section 4.10.5.2).

4.10.2

Reciprocity A fundamental property of Maxwell’s equations in linear isotropic media is reciprocity [Ramo+94]. When applied to guided-wave optical systems, reciprocity has a simple informal interpretation: If a wave propagates from one point (an input) in a system to

36

The comparison is somewhat unfair at this writing because the LC switch is at an earlier stage of development than the MEMS switch. Current developments in the LC television display field may spill over to optical switching applications.

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Multiwavelength Optical Networks

a1 1 b1 a2 2 b2

.. .

an n bn

Figure 4.54 Illustration of reciprocity.

other points (outputs), then waves injected at those outputs will propagate backward to the input in exactly the same way. A more quantitative definition of reciprocity is illustrated by the n-port device shown in Figure 4.54. Suppose it contains an arbitrary interconnection of linear components all built from isotropic materials. In Figure 4.54, the quantities ak and bk represent normalized complex amplitudes of incident and reflected waves, respectively. The normalization is such that 12 Re[ak ak∗ − bk bk∗ ] is the net average power into port k. In the  case of passive devices, the net power into the device, 12 Re[ k (ak ak∗ − bk bk∗ )], must be nonnegative, and in lossless devices it must be 0. If a and b represent the vectors of incident and reflected waves, respectively, then their relationship is defined by b = Sa,

(4.53)

where S is called the scattering matrix for the device. For reciprocal systems, S is symmetric. This implies that the individual responses (reflected waves) to single excitations (incident waves) are related by bi /a j = si j = b j /ai . Because these are relations among complex amplitudes, they are defined for each optical frequency and are generally frequency dependent. A frequency-dependent S represents any frequencyselective element such as a filter, WMUX, or WDMUX. If the system is passive, we have x∗t [I − S∗t S]x ≥ 0 for all x, which means that the expression in brackets is a Hermitian matrix. If it is lossless, S∗t = S−1 ; that is, S is a unitary matrix. If it is lossless and reciprocal, S∗ = S−1 . For example, the scattering matrix S for the 2 × 2 coupler in Figure 2.8 is a 4 × 4 matrix, which can be related to the power transfer matrix in Equation (2.3) as follows. Relabeling the output ports 1 and 2 as 3 and 4, respectively, we can partition S into four 2 × 2 submatrices Si j , where S 21 is the matrix relating the two incident waves at the input ports to the two reflected (outgoing) waves at the output ports. (For an ideal

Enabling Technology

coupler, Sii = 0.) It can be shown that S 21 = S 12 =

√

1−α √ j α

√  j α . √ 1−α

257

(4.54)

The power transfer matrix of Equation (2.3) follows directly from Equation (4.54). The most important consequence of reciprocity for our purposes is that if we know the optical field (and thus power) relationships for signals traversing a system in one direction, these relationships must also hold for signals traversing the same system in the opposite direction. The “system” may be a single device, a network element, or a whole network, as long as it is linear and reciprocal. It is for this reason that the standard building blocks of our systems can be used for pairs of similar operations simply by reversing inputs and outputs: splitters become combiners,37 and multiplexers become demultiplexers.

4.10.3

Nonreciprocal Devices One reason for introducing the concept of reciprocity here was to point out that, in certain applications, nonreciprocal devices are needed. The simplest example is the fiber amplifier. If an EDFA is inserted in a fiber to amplify signals propagating from left to right, it will also amplify signals propagating in the opposite direction. Because there are often undesired signals moving in the wrong direction in a fiber (for example, due to small reflections at various points in a system), it is important to prevent this backward propagation. This requires a nonreciprocal device, called an isolator. The principle of the isolator is shown in Figure 4.55. Incident light is vertically polarized by polarizer A , experiences a 45◦ clockwise rotation as it passes through the Faraday rotator, and continues on through polarizer B, which is oriented to allow it to pass unattenuated. A wave entering in the opposite direction through B starts off at the 45◦ polarization angle, is rotated counterclockwise another 45◦ , resulting in a horizontal polarization, and is therefore blocked by polarizer A . These opposite directions of polarization rotation can only occur in nonreciprocal devices. Faraday rotators are made of special materials that have nonisotropic dielectric properties in the presence of a constant applied magnetic field. Another application of the Faraday rotator is in the circulator shown in Figure 4.59. The directions of circulation of the forward- and backward-propagating waves, as described in Section 4.10.4.3, can be obtained only in a nonreciprocal device.

4.10.4

Optical Filtering Technology One of the factors determining how efficiently the fiber bandwidth can be exploited is the maximum packing density of wavelengths and wavebands.38 This in turn is dependent 37 38

The inevitability of combining losses is demonstrated easily using reciprocity. See Problem 9 at the end of this chapter. The distinction between wavelengths and wavebands was explained in Section 2.2. There, the discussion was generic and focused on wavebands. In this section, the focus is on specific devices that may be used

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Multiwavelength Optical Networks

45° 45°

B

Transmitted Wave

45°

x Polarizer B Incident Wave

y

Faraday Rotator

Polarizer A (a) 45° Reflected Wave B

45°

x Polarizer B y

Faraday Rotator

Polarizer A (b)

c 1991. Reprinted by Figure 4.55 Optical isolator. (From [Saleh+91, Figure 6.6-5]. Copyright
permission of John Wiley & Sons, Inc.)

on wavelength filtering and switching technology. The two critical areas of application of wavelength-selective devices in multiwavelength networks are in optical receivers (either fixed or tunable) and wavelength-selective switches (WSSs). Tuned (or tunable) optical receivers must select signals on a given wavelength and reject others on different wavelengths appearing on the same fiber. Undesired signals at different wavelengths that reach the photodetector constitute interchannel cross-talk, which degrades receiver performance just as cochannel (same wavelength) cross-talk does. The ideal filtering devices for performing the selection at the receiver should have a rectangular transfer function: flat passbands with linear phase shift (constant delay) over the passband, and high attenuation in the stop band. An ideal filter transfer function is impossible to achieve, and a real filter only approaches this ideal, with nonflat passband, nonlinear phase, sloping “skirts,” limited attenuation in side lobes and other either on a waveband or wavelength level. To conform to current usage, the term wavelength is usually used inclusively here and encompasses wavebands as well as wavelengths. Cases where wavelengths and wavebands are handled differently are exceptions to this rule.

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259

irregularities in its stop band. Flat passband and linear phase is needed to faithfully reproduce the transmitted signal at the detector. Steep skirts and absence of side lobes mean less spacing (guard band) is needed between the wavelengths sharing a fiber to minimize interchannel cross-talk. The passbands and the guard bands between them necessarily must be somewhat wider than minimum values calculated under ideal conditions to allow tolerance for laser and component misalignments, drifts, and imperfections. (See Section 4.13.2.1 for an example of the laser misalignment problem.) Similar issues arise in the design of WSSs. In most cases, WSSs use the threestage architecture of Figure 2.21, requiring wavelength demultiplexing, followed by space switching, followed by remultiplexing. The criteria concerning flatness and guard bands that apply to receiver filters also apply to the demultiplexing/multiplexing transfer functions here, except that the requirements are usually more stringent. Because an optical signal most likely traverses many demultiplexers and multiplexers on an end-toend optical path, as mentioned in Section 4.9.4 the overall transfer function seen by the signal is the result of cascading many elementary filtering and switching components. Each component has variable characteristics because of manufacturing imperfections, aging, and temperature, so the bandwidth of the end-to-end transfer function seen by the signal can be much narrower than the bandwidth of an individual filtering element and far from the ideal rectangular form. The problems caused by these imperfect filter characteristics are compounded by laser misalignment, with the overall effect translating into waveform distortion, ISI, and interchannel cross-talk.39 If a system is not designed with appropriate tolerances, these effects eventually render purely optical paths traversing many WSSs unusable. To allow tolerances for imperfections, passbands and guard bands for independently routed wavelengths must be considerably larger than the spacings dictated by optical receiver requirements alone. It is for this reason that waveband routing, which wastes less bandwidth in guard bands, is generally more spectrally efficient than wavelength routing (see Section 2.2). The devices most widely used for optical filtering and WMUX/WDMUX applications are Fabry–Perot (FP) filters, multilayer interference (MI) filters, fiber and waveguide gratings, and arrayed waveguide gratings (AWGs). Although the FP filter has been known since the 19th century, the remaining devices only became viable commercial products since the mid-1990s. Important criteria for these devices are passband flatness, good side lobe suppression, high out-of-band attenuation, polarization independence, low PMD, low insertion loss, and insensitivity to environmental conditions; e.g., temperature, shock, and vibration. Tunability properties are also of interest, including a wide tuning range, simple and low power control mechanism, and high tuning speed. It goes without saying that cost is primordial, and this is related to simplicity, manufacturability, and reliability.

39

An analytical study of this problem, based on realistic filter characteristics, appears in [Roudas+97].

260

4.10.4.1

Multiwavelength Optical Networks

Fabry–Perot Filters The FP filter (resonator), also known as an etalon, consists of a rectangular cavity with flat parallel mirrors on opposite sides as shown in Figure 4.56(a). We have already seen an example of this in the FP laser of Figure 4.21. With perfect mirrors and a lossless medium in the cavity, lightwaves propagating normal to the mirrors will be reflected back and forth, with constructive interference between waves in phase with each other on successive passes through the cavity. This only occurs for certain wavelengths – the resonant wavelengths, with values, λn = 2d/n, where d is the mirror spacing and n is an integer. In a filter, the reflectivity of the mirrors is less than 100% so that some light can enter and leave the cavity. Also the medium is lossy, so the waves are attenuated by a factor r < 1 on each round-trip through the cavity. In this case, the discrete set of resonant wavelengths, λn , or equivalently resonant frequencies νn = c/λn , is replaced by a continuous periodic spectral distribution for the light intensity I in the cavity, as shown in Figure 4.56(b). The spacing, c/2d, between adjacent frequencies νn is called the free spectral range (FSR). The width of each resonance at its half power points, called the full width at half maximum (FWHM), increases with increasing r . The input/output frequency response of the filter has the same shape as the spectrum of Figure 4.56(b). Note that the frequency response in one passband is far from the ideal rectangular form, with rounded top and wide skirts. This is a disadvantage in DWDM applications, where interchannel cross-talk is an issue. However, an imperfect passband is less important in CWDM, where larger guard bands are used between channels. In most WDM applications (e.g., tuned receivers or WADMs) a filter with a single passband is desired for selecting one wavelength out of many in the WDM spectrum. In such cases, the periodic shape of the FP frequency response can be a problem unless d

In

Out

Mirrors

(a) Filter Structure I

FSR FWHM

ν νn−1

νn

(b) Intensity Spectrum

Figure 4.56 Fabry–Perot filter and its spectral response.

νn+1

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the FSR is large enough to contain the full usable WDM spectrum. The requirement of a large FSR combined with the requirement of an FWHM small enough to select a single channel out of a large number of closely spaced channels dictate a filter with a large ratio FSR/FWHM, which is called the finesse of the filter. A way of achieving effectively single-passband operation is to modify the basic FP resonator to greatly increase its finesse. One approach is to make the mirrors in Figure 4.56(a) frequency selective by replacing them with distributed Bragg reflectors, as is done in the DBR laser of Figure 4.22(a). Tunability can be imparted to the basic FP resonator by changing the mirror spacing d. This is conveniently accomplished in fiber FP filters, where an FP resonator is formed in the space between the ends of two fibers, which are coated to serve as the mirrors. The cavity itself may be filled with air or fiber, and the spacing is typically controlled through a piezoelectric actuator. A typical commercial device has 2.5-dB insertion loss, 35-GHz passband, and tunability of 60 nm. Tunable FPs can also be constructed by filling the cavity with a medium whose refractive index can be varied, thereby changing the resonant frequencies. A rapidly tunable filter acts like a wavelength selective switch, which is ON when its passband is aligned with a selected wavelength and OFF when the passband is detuned from that wavelength. Tunable/switchable liquid crystal fiber FPs have been demonstrated using an applied electric field to change the index of a smectic LC in the cavity. Tuning range of 12 nm has been achieved with switching time less than 30 µs, finesse of 166, and insertion loss of 4.1 dB [Bao+96]. A more sophisticated version of the FP filter that incorporates both single-passband operation and tunability has been built using InGaAs/InP technology and a MEMS-based actuator [Irmer+03]. An FP cavity is enclosed between two DBRs, which are fabricated using micromachining techniques to form layers of InP separated by air gaps. One DBR is integrated into a membrane, which is displaced under electrostatic control to provide tuning. The resultant device can be tuned over 140 nm with a 3- to 5-nm passband. It blocks all other wavelengths in the entire optical fiber transmission window. Tuning times are in the 20-ms range.

4.10.4.2

Multilayer Interference Filters The operation of the MI filter is illustrated in Figure 4.57(a). When used as a WDMUX (or a drop filter), several input signals at different wavelengths enter the device at point 1, with the signal to be dropped exiting at point 4. The filter consists of a glass substrate on which are deposited several layers of dielectric thin films (on the right side of the substrate in the figure). The reflections between the layers allows a selected wavelength to be transmitted to the right and reflects the remaining wavelengths back through the glass and out the left side (point 2). In this case, wavelengths λ1 and λ2 enter on the left, λ1 is transmitted and dropped on the right, and λ2 is reflected and exits on the left. Because this is a reciprocal device, it acts as a WMUX if operated in the reverse direction, as shown in Figure 4.57(b). Now, channels on λ1 and λ2 enter at points 4 and 2, respectively, and are multiplexed, exiting at point 1.

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Multiwavelength Optical Networks

Input Signals λ1, λ2

Output Signal λ2

1

3

2

4

DMUX Operation

Transmitted Signal λ1

(a) Demultiplexing Mode

Output Signals λ1, λ2

Input Signal λ2

1

3

2

4

MUX Operation

Input Signal λ1

(b) Multiplexing Mode

Figure 4.57 MI filter.

The MI filter is a cavity resonator similar to the FP filter. It is built using multiple layers of transparent dielectric materials of differing refractive index, deposited on a glass substrate. A cavity consisting of a film of lower index material is sandwiched between two dielectric reflectors consisting of several alternating layers of high and low index thin films. The result is a filter with essentially the same spectral characteristics as the FP with DBR mirrors described in Section 4.10.5. One of the attractive features of the MI filter as opposed to the FP filter is that by using a stack of several cavities a multicavity device with prescribed characteristics can be obtained. The filter center frequency and passband shape can be adjusted by the appropriate choice of layering geometry and dielectric material, producing a close-to-ideal optical frequency response: flat passband and steep side slopes. Tunable MI filters can be built by using materials whose refractive index is controllable. In [Domash+04], a class of filters is demonstrated using amorphous silicon as the high index film in single- and multicavity devices. This material is useful for a tunable filter, because its high thermo-optic index coefficient facilitates thermal control. Dual cavity filters have been demonstrated with tunability over 40 nm and less than 1 dB of insertion loss. Using the same technology, a class of low-cost switchable filters has also been demonstrated. A conventional fixed multicavity MI filter is modified by adding to it a thermo-optically tunable cavity of the type just described. By adjusting the resonances of the various cavities properly, the device is arranged so that at room temperature the cavities are mismatched, so the filter reflects all optical wavelengths. When a control is applied to raise the tunable cavity to a “resonant” temperature, the complete stack becomes transmissive in its design passband. In effect this becomes an ON/OFF switched filter, which can be used in various multiwavelength switching applications.

Enabling Technology

GRIN Lens

GRIN Lens

GRIN Lens ns

Le

RIN

GRIN Lens

263

Channel 1

Channel 3

Channel 5

G

Channel 2

ns

IN GR

Le

GRIN Lens

Channel 7

ns

Channel 4

Le

RIN

G

ns

Channel 6

IN

Fused Silica Block

Le

GR

Channel 8

c 1996 IEEE. Used by Figure 4.58 MI filter array. (From [Scobey+96, Figure 5]. Copyright
permission of The Institute of Electrical and Electronics Engineers, Inc.)

Extensions of this idea produce “hitless” tunable filters, where the MI filter is designed to be thermally switched from one center wavelength to another without passing through any undesired intermediate filter states. An integrated array of MI filters deposited on both sides of a substrate can be used as a multiwavelength WDMUX/WMUX as shown in Figure 4.58. The figure shows a schematic representation of the operation of the device as a WDMUX. Graded refractive index (GRIN) lenses are used to collimate the light before directing the signal at a slight angle to the MI filter. A first wavelength (channel 1) is dropped and collected by another collimating lens. All other wavelengths are reflected to the left. In this way, the remaining light is reflected from filter to filter with a single channel removed at each reflection. Typical parameter values are wavelength channel spacing of 100 GHz and insertion loss of 7 dB for 16 wavelengths.

4.10.4.3

Fiber and Waveguide Gratings We have already seen the Bragg grating as a component of a DBR laser in Section 4.5.1.1, where it is used as a wavelength-selective reflecting filter. Although gratings have been used as basic wavelength-selective elements in free space optical devices since the beginnings of optical communications, it was not until the advent of the fiber Bragg grating (FBG) that grating-based devices found wide application in filters, WMUX/WDMUXs, WADMs, and similar apparatus. The FBG is manufactured by “writing” the grating into a fiber containing photosensitive material. This is done by exposing the fiber to a periodic pattern of UV light, which acts on dopants in the fiber to produce a corresponding periodic variation of its refractive index. The FBG can be designed to have very good

264

Multiwavelength Optical Networks

Circulator Bragg Grating Port 1

Input

Port 2

Λ3

λ1, λ2, ..., λ8

λ1, λ2, λ4, ... λ3

Port 3

λ3 Figure 4.59 FBG used as a drop filter.

reflective filtering properties: flat response in the reflection band, 30- to 40-dB rejection of out-of-band signals, and insertion losses of the order of tenths of a dB. The use of the FBG as a drop filter is shown in Figure 4.59. Multiple wavelengths propagate from left to right in the fiber, passing from port 1 to port 2 in the circulator. Wavelength 3 is reflected from the FBG, entering port 2, after which it exits at port 3. Imperfect reflection of the dropped signal will cause leakage of a portion of its power onto the fiber outbound from port 2. This may cause interchannel cross-talk if a channel at wavelength 3 is added downstream, so it is important to keep this leakage small. One way of building a WDMUX with FBGs is by using a passive splitter, with each arm containing an FBG drop filter for one wavelength. This, however, incurs a splitting loss proportional to the number of wavelengths being demultiplexed, so it is practical for only a small number of wavelengths. Typical FBG drop filters are available for systems with wavelength channel spacings on the ITU grid (50 or 100 GHz) with insertion loss of 2 to 3 dB (including circulator loss). By adding a second circulator to the right of the grating in Figure 4.59 the drop filter is turned into a WADM, adding and dropping the wavelength selected by the grating. The FBG can be made tunable by changing the pitch of the grating through either stretching or compression. Tunable FBG filters have been built using piezoelectric actuators with tuning times of the order of milliseconds over a range of 45 nm [Iocco+99]. Bragg gratings can also be written directly into planar waveguides using the same UV fabrication principles. Thus, for example, an MZ grating-based WADM can be fabricated using PLC technology, as shown in Figure 4.60. The basic MZ switch is modified by writing identical Bragg gratings into its two parallel waveguides. The grating reflects

Input Drop

1

Channel waveguide waveguide

3 Add Output

2

Coupler

Grating

Coupler

4

c 1997 Figure 4.60 A Mach–Zehnder WADM. (From [Kaminow+97, Figure 7.14a]. Copyright
Academic Press. Used with permission of Elsevier.)

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a selected wavelength λ entering on the Input port so that it exits on the Drop port. Other wavelengths in the input signal are transmitted through the grating and exit on the Output port. If the wavelength λ is present at the Add port, it will be reflected by the grating and combined with the other input wavelengths at the Output port. These devices have been fabricated using both silica and polymer waveguides. WMUX/WDMUXs can be constructed by cascading MZ WADMs in an arrangement similar to the MI filter WDMUX/WMUX in Figure 4.58. A four-wavelength polymer WDMUX/WMUX designed for a 400-GHz grid has uniform 2.5-dB insertion loss with very good channel isolation. Another grating application is the chirped FBG, where the pitch of the grating is varied along its length, to produce wavelength dependence of the group delay of the reflected signal. With appropriately designed chirp, these gratings can be used as dispersion compensation devices (see Section 4.3.2.3). Still other applications involve long period fiber gratings, which operate on a principle different than the Bragg effect. A grating with a period of tens of microns, many times the wavelength of the transmitted light, does not reflect the light in the fiber. Instead, it acts to couple the light from the core into the cladding where it is lost. Long-period gratings designed to have specified transmission spectra have been used to advantage for EDFA gain equalization where the grating transmission spectrum approximates the inverse of the nonuniform EDFA gain profile (see Section 4.4.1.1). Using two long-period gratings written side by side in a fiber, the flattened gain profile is reduced to variations of no more than 0.45 dB over a 37-nm band [Harumoto+02].

4.10.4.4

Arrayed Waveguide Gratings The AWG or phased array (PHASAR) has the functionality of the static wavelength router, shown conceptually in Figure 2.10, but it operates using interference principles in an elegant fully intergrated PLC. A signal entering a given input port on a given wavelength is routed to an output port determined by its wavelength. If only a single input port is used with n output ports, the device acts as a WDMUX, or if operated in the reverse direction, it acts as a WMUX (reciprocity again). In an m × n system, it creates a static routing pattern determined by the geometry of the device. Typically fabricated in silica on silicon technology, the AWG is a phased array of multiple waveguides with path length differences between neighboring guides arranged to create the desired phase relations at the output coupler Co [Dragone+91] (Figure 4.61). The input signals pass through a combining coupler Cc and then through the waveguides and exit at the output coupler. The phase differences between signals routed through the different paths cause different wavelengths from the same input port to be focused on different output ports. By proper design of the waveguide geometry, an AWG can be arranged to multiplex/demultiplex a given number of λ-channels on a given grid, say 16 channels on the 100-GHz ITU grid. In idealized form, the power transfer matrix of the AWG has the properties of a Latin router as described in Section 2.3.1.2. When a multiwavelength signal is present on a given input port, each of its λ-channels is routed to a different output port. The routing rule for multiwavelength signals from each different input port produces a different permutation of the constituent λ-channels at

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Cc Co λ1 λ1, λ2, ..., λn λn

Figure 4.61 Arrayed waveguide grating.

the output ports, with the property that all channels appearing at a given output have distinct wavelengths. The channel and wavelength permutation rules are determined from the geometry of the waveguides and couplers. Of course, the filtering properties of the AWG are imperfect, so the usual effects of insertion loss and interchannel cross-talk are present. In addition the AWG has a finite FSR as in FP filters. Nevertheless, AWGs have been demonstrated with impressive characteristics, and much progress has been made since their invention in 1990. This includes solving problems of temperature and polarization sensitivity as well as achieving flat passband shapes, low cross-talk, and low insertion loss. Devices have been demonstrated for 32 channels at 100-GHz spacing with 2.1-dB insertion loss, 49-GHz channel FWHM, and −28-dB cross-talk and for 256 channels at 25-GHz spacing with 2.7-dB insertion loss, 14.4-GHz channel FWHM, and −33-dB cross-talk [Hibino00]. As with the FBG, the AWG is versatile enough to be adaptable to many different fabrication technologies and applications. In addition to silica-on-silicon fabrication, AWGs have been built as semiconductor PLCs, an approach that facilitates monolithic integration with SOAs, lasers, and photodiodes [Yoshikuni02]. Applications include integrated wavelength-selective switches (Section 4.12.2), dispersion slope equalizers (Section 4.3.2.3), integrated WDM receivers, and multiwavelength modulators.

4.10.5

Multiwavelength Switch Technology Transparent waveband-space switches can be realized in different ways depending on how and where wavelength selectivity (filtering), tuning, and switching are included in the switch fabric. The most straightforward approach is an interconnection of separate bulk wavelength-selective devices (e.g., filters and WMUX/WDMUXs) with photonic space switches (e.g., the architecture of Figure 2.21). Other approaches, which are discussed in this section, conform closely to our definition of a generic multiwavelength switch, involving combining, filtering, tuning, and switching in one package. These range from integrated monolithic devices to combinations of full integrated devices with other components.

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We have seen that devices designed as static filters, or WMUX/WDMUXs, can often be converted to tunable or switched devices by controlling their parameters externally. For example, the reflection band of an FBG can be displaced by stressing it to change its grating period. In this way, it can be switched under external control to reflect power in different bands, acting as an ON/OFF switch in those bands. Other devices are designed from the start to be MWSs, switching several wavelengths independently. We describe several of these in the next sections and conclude with a discussion of wavelength dilation, a means of improving the performance of MWS-based wavelength-selective switches.

4.10.5.1

Acousto-Optic Tunable Filters As illustrated in Figure 4.62, the acousto-optic tunable filter (AOTF) described in [Smith+90] is a two-input/two-output device constructed on a LiNbO3 substrate, which acts as a 2 × 2 controllable and wavelength-selective directional coupler. The basic AOTF implements a switching operation by “flipping” the polarization of the input wave and hence is inherently polarization dependent. The device shown in the figure is a polarization-independent version of the elementary AOTF, implemented by polarization diversity techniques using polarization beam splitters (PBSs). An input is present on the upper left fiber, and it is split by the PBS into its two polarized components as shown. (The transverse magnetic [TM] and transverse electric [TE] polarizations correspond to the vertical and horizontal components with respect to the substrate surface.) The TM component then traverses the upper waveguide and exits on the right. An electric interdigital transducer (IDT) formed by electrodes on the surface, driven by a radiofrequency source, creates a surface acoustic wave that propagates in the same direction as the lightwaves. Through an acousto-optic effect in the material, this forms the equivalent of a moving grating, which can be phase matched to an optical wave at a selected wavelength by choosing the correct frequency of the electrical excitation. A signal that is phase matched (the selected signal) is “flipped” from the TM to the TE mode, so the Acousto-optic Polarization-Flipping Filter

Output PBS

Input PBS TM

TE

TE

TM

Input

IDT

Figure 4.62 Acousto-optic tunable filter.

Acoustic Beam in Acoustic Waveguide

Unselected Output Selected Output

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Multiwavelength Optical Networks

output PBS directs it to the lower output. The unselected signal exits on the upper output. A similar polarization-flipping and -selecting process takes place for the TE component on the lower guide. A signal entering on the lower port of the input PBS on the selected wavelength undergoes the same operations, exiting on the upper output port. Thus the device is in the cross state for the selected wavelength. By superimposing excitations of several frequencies, several wavelengths can be selected simultaneously, which makes this an MWS, as shown conceptually in Figure 2.22. A normally unwanted by-product of the interaction is that the optical frequency of a selected signal is Doppler shifted by an amount equal to the acoustic excitation frequency, typically of the order of 100 MHz in LiNbO3 devices. The switch states of AOTFs can be controlled independently for each wavelength over a continuum of values so they can be configured to act as 2 × 2 wavelength-selective LDCs. They can be tuned over very wide ranges of wavelengths (up to 200 nm), and switching times are of the order of microseconds. The original versions had problems with insertion loss and side-lobe suppression. However, they were improved to have 30-dB side-lobe suppression and low loss [Jaggi+98]. Typical parameter values are: about 1 nm wavelength spacing, and 2 dB fiber-to-fiber insertion loss. AOTFs have also been built using other structures and materials. In [Sapriel+02], a TeO2 crystal is used as the acousto-optically controlled medium. A 1 × 2 AOTF switch was demonstrated using a piezoelectric transducer driven by an RF signal to launch an acoustic wave into the crystal. This generates a moving grating, propagating collinear to an input lightbeam. The device has an input fiber and two output fibers for bar state and cross state outputs. As in LiNbO3 devices, a wavelength is selected by adjusting the frequency of the excitation so that a phase-matching condition exists between the acoustic wave and the selected input wave. Unselected wavelengths exit the device on the bar state fiber, whereas the selected λ-channel is deflected by diffraction to the cross state fiber. This is a polarization-dependent effect, but the device is made polarization independent by inserting polarization splitters and half-wave plates at the input and output ports. Devices have been demonstrated with 0.75-nm passband, 53-µs switching time, and worst-case cross-talk at 4-nm channel spacing about −25 dB. Performance is poorer at smaller channel spacings, however. Another approach is the all-fiber AOTF. These operate by using the acousto-optic excitation either to switch a lightwave between modes in a two-mode fiber [Park+02] or to couple light between core and cladding modes in a single-mode fiber [Satorius+02]. In both cases an acoustic flexural wave is launched along the fiber using a piezoelectric transducer. In [Satorius+02], a core block is inserted into the fiber to prevent signals in the core from passing the blocking point. A phase-matching condition between the flexural wave in the fiber and a selected wavelength propagating in the core causes the selected signal to be converted to a cladding mode while the unselected signals remain in the core and are blocked. The selected signal propagating in the cladding bypasses the core block and is then converted back to the cladding mode through the inverse of the original mode conversion process. An interesting by-product of these two conversions is that the optical frequency shift incurred in most AOTFs is cancelled out in this device. Prototypes have been built with 3.5-nm passband, −17-dB sidelobes, 29-dB extinction ratio, and 5-dB insertion loss. By sandwiching a single AOTF between a circulator and a

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Faraday rotator/mirror a two-pass device is formed with superior characteristics: 2.5-nm passband, −29-dB sidelobes, and 38-dB extinction ratio, but the insertion loss increases to over 12 dB because of the two passes through the device and the additional losses in the accompanying elements.

4.10.5.2

Liquid Crystal Multiwavelength Switch Multiwavelength switches can also be realized using liquid crystals as described in [Patel+95]. The simplest polarization-dependent form of a 1 × 2 liquid crystal switch is shown schematically in Figure 4.63. An input signal (indicated as input 1) is dispersed spatially (demultiplexed) using a grating device. Each wavelength component is directed through one pixel of an electrically controlled liquid crystal SLM, onto a polarizationsensitive deflective element (e.g., a calcite crystal). The SLM is a twisted nematic LC slab that rotates the polarization of a beam traversing it when no electric field is applied but leaves the polarization unchanged in the presence of an applied electric field. The controlled polarization rotation in combination with the polarization-selective deflection produces a controlled beam displacement, with the displaced beam remultiplexed onto output 2. If undisplaced, it is remultiplexed onto output 1. In the figure, a multiwavelength signal is applied to input 1, and one of its wavelengths (the dark beam) is shown deflected to output 2. In this fashion, an array of beam deflectors under independent electrical control produces an MWS from input 1 to outputs 1 or 2. A similar operation is executed on a second signal entering on input 2 (not shown in the figure), so the two operations result in a 2 × 2 MWS or reconfigurable WADM. These devices can be built with flat

Grating

Grating

Output 2 Output 1

Input 1

Liquid Crystal Array Element

Beam Displacer (e.g., calcite)

c 1995 IEEE. Used by Figure 4.63 Liquid crystal MWS. (From [Patel+95, Figure 1]. Copyright
permission of The Institute of Electrical and Electronics Engineers, Inc.)

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Multiwavelength Optical Networks

passbands and can accommodate a large number of channels. [Ranalli+99] describes a 40-wavelength device with 50 GHz of channel separation, −30 dB of interchannel crosstalk, and less than 7 dB of insertion loss. It is polarization insensitive using polarization diversity techniques similar to those used for the AOTF in Figure 4.62.

4.10.5.3

MEMS-Based Wavelength-Selective Switches A different approach to wavelength-selective switching uses free-space optics to combine the wavelength selectivity of a grating with optical beam switching using micromirrors. The result is a reconfigurable WADM whose block diagram is shown in Figure 4.64(a) [Ford+99]. The device has multiwavelength inputs on its IN and ADD ports, and multiwavelength outputs on its PASS and DROP ports. The combination of WMUX, WDMUX, and single-channel reflecting/transmitting switches places the WADM in either the pass-through state (switch reflection) or the ADD/DROP state (switch transmission) for each wavelength. A λ-channel entering the IN port is passed through the upper circulator to the PASS port when a switch reflects power at that wavelength. When a switch is in the transmission state (which is bidirectional), a λ-channel on the corresponding wavelength entering the IN port is transmitted via the switch and the two circulators to the DROP port, and similarly a λ-channel entering the ADD port is transmitted to the PASS port. The optics are shown in Figure 4.64(b). Inputs from the IN and ADD ports are directed via the circulators and collimating lenses to a grating, which is used as both a WDMUX and WMUX. (The IN signal enters on the upper beam in the figure and the ADD signal on the lower beam.) The demultiplexed channels pass through

Circulator

PASS

IN

Grating ADD

WDM Mux

C2

Fold Mirror

Single-Channel Switches (reflect/ transmit)

Collimation Lenses

WDM Mux

DROP

Focus Lens IN

DROP

ADD

C1

PASS

Circulator

(b) Free-space optics

(a) Block diagram IN

PASS

IN PASS DROP

ADD

v

v (c) Mirror states

c 1999 IEEE. Figure 4.64 A MEMS-Based WADM. (From [Ford+99, Figures 1–3]. Copyright
Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

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an additional lens system to be focused on the device plane where an array of tilting mirrors is located. (The λ/4 plate is used to reduce PDL in the grating.) Figure 4.64(c) shows the operation of the tilting mirrors. For each wavelength in the PASS state, the mirror, which is hinged in the middle, is tilted to the left by an electrostatic actuator. This reflects the IN beam backward along its same path through the lens system and grating (this time acting as a WMUX) to circulator C1 . The ADD beam is deflected and dumped. In the ADD/DROP mode, the tilting mirror deflects the ADD beam to the PASS port and the IN beam to the DROP port. A WADM has been demonstrated for 16 wavelengths at 200-GHz spacing. It has a 0.7-nm passband at FWHM, a 5-dB (8 dB) insertion loss on the PASS (DROP) path, a 32 dB extinction ratio, −30 dB of cross-talk, a 0.2-dB PDL, and a 20-µs switching time, which is extremely fast for a MEMS device. MEMS-based 1 × K multiwavelength switches have been constructed using a similar approach. In this case, the optical system directs an N wavelength signal from a single input fiber via a grating demultiplexer to an array of N single-axis micromirrors with electrostatic control. Each mirror now requires analog control to steer its beam to one of K output fibers. Another pass through the same lens and grating system is used for multiplexing and focusing the beam onto the selected output fiber. A 1 × 4 prototype has been demonstrated, designed for 128 wavelengths on a 50-GHz grid [Marom+02]. It has an insertion loss of less than 5 dB and wide flat-top passbands with low interchannel cross-talk, making it suitable for operation at relatively high bit rates (10 Gbps) under realistic transmission conditions.

4.10.5.4

MI Filter-Based Switches As explained in Section 4.10.4.2, the MI filter can be designed to be switchable from a completely reflecting state (OFF) to a bandpass transmission state (ON) using thermal control. With this technique, the WMUX/WDMUX arrangement of Figure 4.58 can be employed as a building block for a WADM by replacing its static MI filters by switchable filters. Figure 4.65 illustrates the concept [Domash+04]. When all filters are in the OFF state, all wavelengths entering on the IN port are passed on to the OUT port, and no wavelengths present on the ADD fibers are transmitted. When a filter in the DMUX is switched ON, its wavelength is extracted from the multiwavelength signal on the IN port and exits on the corresponding DROP fiber. Similarly, when a filter in the MUX is switched ON, and a signal is present on its ADD port, that channel is added to the multiwavelength signal on the OUT port. In this way, switching, wavelength selectivity, and variable attenuation can all be combined in the same device. (The insertion loss through the MI filter passband can be varied by varying its temperature.)

4.10.5.5

AWG-Based Switches Three related examples of monolithic integration of the AWG with various types of switching devices have been developed at NTT laboratories. The first is a WADM realized as a PLC using silica on silicon, combining MZ switches with four AWGs on a chip [Okamoto+96]. Its logical architecture is the three-stage arrangement of Figure 2.21, with two input and two output fibers. Two of the AWGs serve as the firststage WDMUXs and two as the third-stage WMUXs. Each 2 × 2 second-stage switch is

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Multiwavelength Optical Networks

DMUX

MUX Express

IN

?λii

λi

λ?i+1

λ?i+1

λ?i+2

λi+2

λ?i+3

λi+3

OUT

Switchable/Variable Add/Drop Drop λi − λi+3

Add λi − λi+3

c 2004 IEEE. Figure 4.65 An MI filter-based WADM. (From [Domash+04, Figure 16]. Copyright
Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

realized as a double-gate switch using four thermo-optically controlled MZ switches in the space-dilated configuration of Figure 4.80. In this way fabrication imperfections can be tolerated in the MZ switches while maintaining low cross-talk. WADMs are reported for 16 channels with −28.4 dB of cross-talk, 7.8–10.3 dB of on-chip insertion loss, and a switching time of less than 1 ms. In [Suzuki+98] a 2 × 2 multiwavelength switch with the same logical architecture as in the previous example is described, but it is built around a single AWG monolithically integrated with an array of 2 × 2 switches, this time realized as single thermooptically controlled MZ switches. The AWG does fourfold duty as two WDMUXs and WMUXs, requiring a total of 2N + 2 input and output ports in an N wavelength system. Multiwavelength signals on the two input fibers are fed to two ports of the AWG, which demultiplexes them to 2N output ports. Pairs of these demultiplexed signals on the same wavelength are fed back to other ports on the AWG via 2 × 2 switches. This second pass through the AWG remultiplexes them onto two output ports connected to the output fibers. A demonstration device was reported for four wavelengths on a 100-GHz grid, fabricated using silica on silicon PLC technology. Its performance was as follows: cross-talk of less than −10 dB (largely due to manufacturing imperfections in the MZ switches), fiber-to-fiber insertion loss of 7.3–8.5 dB, and a switching time of 1 ms. A third example is a 1 × 1 (ON/OFF) multiwavelength switch called a channel selector. It uses semiconductor PLC technology to integrate AWGs with SOA gate switches [Yoshikuni02]. The logical architecture is the N wavelength 1 × 1 case of Figure 2.21, with an array of N ON/OFF SOA gates placed between a WDMUX and a WMUX, each implemented as an AWG. Reported performance of an eight-wavelength device on a 200-GHz grid is cross-talk of less than −40 dB, an extinction ratio greater than

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50 dB, and no net insertion loss because of the gain of the SOAs. However, the SOAs are polarization sensitive, introducing a PDG of 1–2 dB.

4.10.5.6

Wavelength Dilation The three-stage architecture of Figure 2.21 for a WSS requires m space switches for a system operating on m wavelengths together with a pair of WDMUX/WMUXs for each input/output port. The space switches can be of any form, either generalized or permutation. In the latter case, the WSS becomes a wavelength-selective cross-connect. As pointed out in Section 2.3.2.4, any space-switch fabric can be converted to a WSS fabric by replacing each elementary switching device with an equivalent MWS. In this approach, no WDMUX/WMUXs are required. In the case at hand, the m-fold replication of space switches is replaced by a fabric of MWSs operating on m wavelengths. Unfortunately, many types of elementary (2 × 2) MWSs are either costly or impossible to realize when m is large and/or the wavelength spacings are small. A compromise solution is a wavelength-dilated architecture, as shown in Figure 4.66 [Sharony+92]. At first glance, this appears to have the worst qualities of both the three-stage and the MWS approach. It is a three-stage architecture similar to Figure 2.21, but each spaceswitching layer is replaced by an MWS fabric. The advantage is that there are fewer MWS layers in the wavelength-dilated architecture than there are space switches in the

λ1, λ4, λ7 MWS1

WDF 2 In 1

1 8×8

1

λ1 − λ8

2

Out 1

2, 5, 8

λ1 − λ8

2, 5, 8 3 3, 6

3, 6

λ2, λ5, λ8 In 2

In 8

MWS2

λ3, λ6 MWS3

Figure 4.66 Wavelength-dilated switch.

Out 2

Out 8

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Multiwavelength Optical Networks

configuration of Figure 2.21. Furthermore, each MWS fabric operates on fewer and more widely separated wavelengths than the single MWS fabric would process. This is accomplished by “combing out” subsets of wavelengths with spacings that are multiples of those in the original channel set. In the example shown in Figure 4.66, an 8 × 8 switch operating on eight wavelengths is shown, with a factor of three wavelength dilation. It requires three 8 × 8 MWS fabrics, MWS1 , MWS2 , and MWS3 . The wavelengths on each input fiber are demultiplexed using a tree of wavelength-dropping filters (WDFs) and are remultiplexed using a similar tree operated in the reverse direction. The wavelengths dropped/added by each WDF are indicated with an arrow under the WDF. The set of wavelengths λ1 , λ4 , and λ7 is directed from each input fiber to MWS1 ; the set λ2 , λ5 , and λ8 is directed to MWS2 ; and λ3 and λ6 are directed to MWS3 . Note that each MWS must operate on two or three wavelengths with spacings that are three times as large as the original set. Thus, large guard bands are present between adjacent wavelengths, reducing interchannel cross-talk as well as the cost of the components. Because the WDFs are static devices (e.g., fiber Bragg gratings) their cost is considerably less than the switching devices, which have been reduced in number.

4.11

Wavelength Conversion and Signal Regeneration Wavelength converters can be used as components of WIXCs and (in combination with static wavelength routers) to implement WSXCs (see Section 2.3.3). The use of the WIXC as a means of removing bottlenecks in wavelength-routed networks is illustrated in Figure 2.28. This improves performance in the higher layers of the network architecture, reducing blocking in dynamic routing, increasing overall load carrying ability, and making protection and restoration simpler. Although the wavelength conversion function is not an inherently nonlinear operation, a wavelength converter may be either transparent or opaque depending on whether it is implementated using “weakly” or “strongly” nonlinear devices. Signal regeneration is an operation that cleans up digital signals that have been distorted and contaminated with noise as they traverse a path through a network. There are three levels of regeneration: 1R (reamplification), 2R (reamplification and reshaping), and 3R (reamplification, reshaping, and retiming). 1R is really not regeneration at all and will not be discussed further. The other two are highly nonlinear operations; they cannot be performed on superimposed channels. Regeneration increases the maximum bit rate that can be supported on an optical channel for a given BER or, equivalently, decreases the BER at a given bit rate, without necessitating a reduction of the signal impairments along the optical path. The result, which is felt at the physical layer, is to effectively extend the reach of the transmission path. Although linear amplification can improve transmission performance by increasing the OSNR, it is the nonlinearity of the regeneration process that enables it to improve performance in a way that no linear device can do. Thus, for example, the OSNR at the receiver, and resultant BER at the end of a long optical path can be improved by placing optical amplifiers at frequent

Enabling Technology

o e

275

e o

PD

Amplifier

Input λ1

Laser λ2

Modulator

Modulated Output λ2

Figure 4.67 Optoelectronic wavelength converter.

intervals along the path (see Section 4.4.1.4). However, the resultant improvement in BER (compared to no amplification) will not be as much as if regenerators are used at the same points. Given the current state of electronic as compared to photonic technology, the simplest and most cost-effective way of implementing wavelength conversion and regeneration is in the electronic domain, first converting the signal to electrical form, doing the signal processing electronically, and then reconverting it to optical form. This OEO approach is shown conceptually in Figure 4.67. An intensity-modulated signal at wavelength λ1 is converted to electrical form in a photodetector, amplified, and used to modulate a laser operating at a different wavelength λ2 . (This is basically a bare-bones transponder, as described in Section 2.5.) It will accept a signal at any wavelength within the range of the detector, and output a signal at wavelength λ2 , independent of the input wavelength. Because the photodetector is a square-law device this arrangement is inherently nonlinear and hence opaque: two superimposed signals at different wavelengths cannot be converted simultaneously. An advantage of this device is that it can easily be modified to perform regeneration. By inserting electronic pulse-shaping circuits after the amplifier, this becomes a 2R regenerator, and if clock recovery and gating circuits are added, 3R regeneration results. However, this makes the device signal format dependent; the range of acceptable bit rates and modulation formats is constrained by the electronic circuitry. Furthermore, the electronic approach has a speed limitation and it is power hungry, with its cost increasing rapidly as the electronic speed limits are approached. Given these cost and performance limitations, all-optical approaches, where signals remain in the optical domain, are becoming an increasingly viable alternative to the OEO method.

4.11.1

All-Optical Wavelength Conversion All-optical wavelength conversion can be performed as either an opaque or transparent (nearly linear) operation. The methods of implementing opaque conversion typically

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Multiwavelength Optical Networks

depend on electro-optic phenomena in materials such as semiconductors, involving strong nonlinearities. As in the OEO converter, the output wavelength is independent of the input wavelength. On the other hand, transparent conversion, known also as coherent, wave-mixing, or parametric conversion, depends on relatively weak nonlinearities. Viewed from the point of view of the optical channel transparent conversion behaves linearly, so several superimposed λ-channels can be converted in the same device. As opposed to the opaque converters, the wavelengths of the input and output signals of a transparent converter are related. The parametric approach is typically an order of magnitude faster than the electro-optic approach (subpicosecond rather than tens of picoseconds), but it consumes more power. Because opaque conversion depends on strong nonlinear effects, it lends itself naturally to regeneration as well. We consider the transparent case here (for conversion only), followed by opaque systems for conversion and regeneration in the next section. Transparent devices for wavelength conversion are based on FWM or difference frequency conversion, which are produced by nonlinear effects in fibers and waveguides. Although we have defined transparency as being the same as linearity, the weakly nonlinear effects considered here together with filtering produce a system that behaves linearly for all practical purposes and is thus a transparent converter. The conversion process results from the mixing of signals at different optical frequencies in a device having a power-law nonlinearity. If two signals at frequencies ω1 and ω2 are combined in a device with an nth-order nonlinearity, in the expansion of the resultant output terms there is one of frequency (n − 1)ω1 − ω2 . This phenomenon produces the cross-talk problems on fibers that were discussed in Section 4.3.2.4. In that case, it was indicated that when two signals at neighboring optical frequencies ω1 and ω2 are impressed on a fiber with a cubic nonlinearity, an undesired cross-talk signal at a third frequency 2ω1 − ω2 can appear, which is called four-wave mixing. This effect can be turned to an advantage for wavelength conversion. If we let ω1 = ω p , the pump frequency, and ω2 = ωs , the signal frequency, and call ωc = 2ω p − ωs the converted signal frequency, this becomes a frequency converter [Inoue+93]. Interchanging the pump and signal frequencies, another possible frequency for the converted wave is ωc = 2ωs − ω p . Although this is a transparent conversion process and therefore potentially very useful, it is usually a very weak (inefficient) effect in fibers. However, high-efficiency conversion has been achieved using highly nonlinear fibers (HNLFs) and high pump powers. Conversion efficiencies up to −5 dB have been achieved with 26-dBm pump power, using 0.2 m of low-dispersion slope HNLF to shift the wavelength of a 160-Gbps picosecond pulse train from 1556 to 1578 nm [Igarashi+04]. Tunability is an important property of a frequency converter. Using a bismuth-oxide PCF as the HNLF in an FWM device, a widely tunable converter has been demonstrated [Chow+07]. With 1 meter of PCF a converter tunable over 35 nm was built, operating on a 10-Gbps NRZ signal with a power penalty of 1.2 dB and conversion efficiency of −19 dB. The difference frequency converter (DFC) was discussed in Section 2.3.3 [Yoo+95, Yoo+96]. This is based on a square law rather than cubic nonlinearity. The efficiency of the effect depends on how well phase matching is realized. It can be enhanced by special design of the waveguide in which the interaction occurs. Figure 4.68 shows

Enabling Technology

λ1

λ2

2λp

λ2′

277

λ1′

Relative Power (dB)

5 −5 −15 −25 −35

1520

1530

1540

1550

1560

Wavelength (nm) (a)

Conversion Efficiency (dB)

−16

−19

−22

−25

−28 1490

1520

1550

1580

1610

Wavelength (nm) (b)

Figure 4.68 Performance of a difference frequency converter. (From [Yoo+96, Figure 2]. Copyright

c 1996 AIP. Used by permission of The American Institute of Physics.)


the performance of a DFC based on a periodically domain-reversed aluminum gallium arsenide (AlGaAs) waveguide. Figure 4.68(a) illustrates the simultaneous conversion of two signals at 1528 and 1534 nm to 1555 and 1549 nm, respectively. Note the spectrum “reflection” based on the pump wavelength (771 nm), as described in Figure 2.24. The various processes involved in fabrication render the device polarization independent, with a conversion efficiency of approximately −17 dB over a band of approximately 90 nm, as shown in Figure 4.68(b). (The solid and dashed curves in the figure represent points for two different polarization states.) More recently, using periodic poling (producing the domain reversal effect mentioned above) in a LiNbO3 waveguide, better than −10 dB conversion efficiency has been achieved for wavelength shift of 60–70 nm at data rates up to 160 Gbps [Brener+00]. Because the parametric converters reflect the signal spectrum, they reverse its phase as a function of frequency (phase conjugation). This makes them useful for dispersion

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compensation (see Section 4.3.2.3). For example, dispersion compensation using a DFC placed halfway down a link has been demonstrated successfully at 100 Gbps over 160 km of dispersive fiber. The performance of DFC and FWM wavelength converters may improve as highly nonlinear PCFs become available for this application.

4.11.2

Opaque Wavelength Conversion and Signal Regeneration The family of opaque devices and subsystems for wavelength conversion and regeneration is large and diverse, but a conceptual picture that encompasses most of them is shown in Figure 4.69. It is assumed throughout that the data format is intensity modulation. A data signal at wavelength λ1 and a probe signal at wavelength λ p are applied to the input(s) of a gating device. The data act as a control, turning the gate on or off to allow or prevent the passage of the probe signal. In the case of 2R regeneration, the tap, timing recovery, and clock are missing. The probe is then a CW signal, which is gated on when a data pulse is present and off when a pulse is absent. For 3R regeneration, a small portion of the data signal is tapped off to be used for timing recovery. Using the timing information an optical clock signal synchronized to the data is generated in the form of a pulse train at wavelength λ p . Sometimes the synchronization signal is applied as a third input to the gate. After gating by the incoming data signal this becomes the regenerated bit stream. (Note that a similar clock recovery arrangement in an optical

Op Optical Gating device

Data Signal λ1 Tap

Timing Ti recovery

Clock Clock

Data Threshold

Probe: CW Pulsed

Output

Figure 4.69 Opaque conversion and regeneration.

Output λp

Probe signal λp CW/pulsed

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HNLF

out

in SA

OBPF

c 2002 IEEE. Figure 4.70 SA-based regenerator. (From [Matsumoto+02, Figure 1]. Copyright
Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

receiver is shown in Figure 4.31. However, that is performed on the electrical side, after detection of the optical signal by the PD.) A commonly used configuration uses an SOA as the gating device, where the data signal and CW probe are superimposed as a common input. The data pulses representing 1s are inserted at high-enough intensity to drive the SOA into saturation, and thus as a consequence of cross-gain-modulation (XGM), the intensity of the probe signal appearing at the output is low. With no input pulse present, the probe is amplified, and thus a replica of the data signal appears at the output (with inverted polarity) at wavelength λ p . (The data signal at wavelength λ1 must be removed by filtering at the output of the device.) Because XPM is also present in the SOA, the output signal is chirped, and this can be used to advantage to reformat the output pulse using appropriate filters [Leuthold+04]. Furthermore, using a pulsed probe signal this configuration can be used for 3R regeneration [Chayet+04]. Similar results are obtained if the gating device in Figure 4.69 is a saturable absorber (SA). If the probe signal is eliminated and pulse-reshaping devices are included at the output of the SA, this becomes a 2R regenerator without wavelength conversion [Matsumoto+02] (see Figure 4.70). The SA has the property that it is opaque at low input intensities and transparent at high intensities. Thus, it has the effect of blocking the low-intensity noise fluctuations in the data signal when no pulse is present while passing the pulses without attenuation. Although this eliminates noise when pulses are absent, it does not reshape the data pulses. To do reshaping the pulses at the output of the SA are passed through a HNLF followed by an optical bandpass filter (OBPF). The HNLF produces SPM that, in combination with anomalous dispersion in the fiber, results in pulse compression. The narrowband filter has the effect of equalizing pulse amplitudes coming out of the HNLF, all of which results in 2R regeneration. Better performance in regeneration is obtained with interferometric gating devices such as the MZ configuration of Figure 4.71. In one arrangement, an SOA is used as the nonlinear device (NL) in one arm of the MZ, and the input data and probe signals are applied to the SOA at wavelengths λs and λ p , respectively. As a result of XPM, a Input (λs) NL Probe (λp)

OF MZ

Figure 4.71 Nonlinear Mach–Zehnder regenerator.

Output (λp)

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4.5 km DCF 3 dB coupler

input signal PC

3 dB coupler probe

modulated probe

Figure 4.72 Nonlinear optical loop mirror regenerator. (From [Yoo96, Figure 6(b)]. Copyright c 1996 IEEE. Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)


pulse present in the data will cause a phase shift of the probe signal in the SOA arm. The filter OF at the output removes the data signal and leaves the gated probe carrying a replica of the data signal. If the MZ is designed for noninverting regeneration, there is no transmission when no data pulse is present, and maximum transmission at the peak of a pulse. This is a 2R regenerator and wavelength converter, with output wavelength λ p . It can be upgraded to 3R regeneration using a pulsed probe. These systems can be designed with counter propagating data and probe signals so that no filtering is required to remove the data signal at wavelength λs from the output. Another interferometric regeneration and conversion device is the nonlinear optical loop mirror (NOLM). As shown in Figure 4.72 a probe is launched into the loop via a 3-dB coupler, splitting into equal clockwise and counterclockwise components. These normally interfere destructively at the output and the probe is extinguished there. If, however, the phase relation between the two components is altered, the interference is no longer destructive and the probe appears at the output. This differential phase shift can be induced under the control of a data signal, introduced in the coupler shown in the figure. (Polarization controllers, PC, are shown because this is a polarization-dependent device.) Fiber nonlinearity in the loop (typically several kilometers long) causes XPM between the data signal and the probe thereby changing the phase relations between the two counter-circulating components. This produces a gating effect on the probe, causing a regenerated pulse at the probe wavelength to appear at the output. There is currently renewed interest in NOLM devices for wavelength conversion and regeneration, because of the possibility of using highly nonlinear microstructured fibers in the loop, greatly reducing the necessary fiber length. Another configuration of the optical loop mirror is the SLALOM (semiconductor laser amplifier in a loop optical mirror), where the fiber nonlinearity is replaced by an SOA [Eiselt+95]. This device is capable of various nonlinear operations on optical signals, including 3R regeneration, two-pulse correlation, time division demultiplexing, and alloptical gating, where one optical pulse controls the switching of another. An application of the SLALOM to all optical header processing for optical packet switching appears in Section 10.2. An advantage of the SLALOM over the NOLM is that it does not require a long nonlinear fiber.

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Comparing the various approaches to wavelength conversion, FWM and the DFC have performance advantages in WDM networks because they are transparent and hence can handle multiple wavelengths simultaneously. They also have the best noise properties. However, fabrication is difficult. Among regenerators/converters, interferometric devices depending on XPM have a performance edge over devices depending on XGM or attenuation saturation (SOAs and SAs) due to the better extinction ratios in the former. On the other hand, SOAs using XGM have some practical advantages: conversion gain and a simple structure. Whether wavelength conversion and/or signal regeneration is appropriate in any particular application depends, of course, on various trade-offs. Does the importance of maintaining transparency outweigh the signal quality improvements obtained with regenerators? Do the performance improvements gained at the physical level by regenerators and at the network level by wavelength converters justify the cost of these devices? The answers to these questions continue to change as the technology evolves.

4.12

Optical Switch Architectures The conceptual view of switch architectures presented in Section 2.3 focused on generic functionality while intentionally avoiding details of specific physical implementations. That is, it gave us an abstract concept of a switch without being overwhelmed with the details of implementation. However, with the description of the underlying switching devices behind us we can now move on to physical implementation, which requires a more concrete picture. In the next sections, we describe some alternative physical realizations of switches that produce the desired functionality taking into account technological constraints and cost-performance trade-offs. Space switch fabrics are considered first, followed by wavelength-selective switches. Because space switches usually form the core of WSSs, the performance of the former are a key to the performance of the latter.

4.12.1

Space Switches Some representative space switch architectures were described in Section 2.3.2, characterized as permutation switches, generalized switches, and linear divider-combiners (LDCs). In this section, we revisit the space switch, focusing on physical layer issues: losses, cross-talk, and fabrication complexity. We begin with crossbar architectures, followed by multistage switches and a discussion of switch fabric cross-talk. Although some of these structures can be used as generalized switches and LDCs, we focus the discussion here on permutation switch functionality.

4.12.1.1

Crossbar Switches The crossbar switch of Figure 2.14 dates back to the electromechanical telephone switching systems of the 1950s. A connection between an input-output pair was made using electromagnetic actuators to move the corresponding horizontal and vertical metal crossbars to make contact at the cross-point. The characteristic feature of a crossbar switch

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Multiwavelength Optical Networks

Input 1

2

3

4

1’

2’

3’

4’

Output

Figure 4.73 Optical crossbar switch.

is that each connection is made by closing a single cross-point, which means that n 2 cross-points are required for an n × n (nonblocking) switch. There are many ways of implementing this concept in the optical domain, with significant differences in performance. Three possible realizations of an optical 4 × 4 crossbar are shown in Figures 4.47, 4.73, and 4.74, which we refer to as OXB1, OXB2, and OXB3, respectively. All of these lend themselves to PLC technology. Although at first glance they appear quite different, their common characteristic is that a single elementary switching device is set for each

1

1’ 2’

2 Input

Output 3

3’ 4’

4 Figure 4.74 Path-independent loss crossbar switch.

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connection through the switch. The version of OXB1 realized with SOA gate arrays (on-off switches) and two-stage splitters and combiners has already been discussed in Section 4.10.1.1. The splitting and combining losses, which are its main drawback, are compensated by amplifiers in this case, but large switches of this type are not feasible because the combining and splitting losses grow as n 2 . OXB2 and OXB3 are based on arrays of 2 × 2 switches. In OXB2, if the switch at a cross-point is OFF, light from an input guide continues on its horizontal path through the cross-point. If the switch is ON, light is deflected from the horizontal guide, downward on the vertical guide as indicated by the bent arrow symbols at the cross-points. When all couplers are in the OFF state, there are no active connections through the switch. Note that the 2D MEMS switch of Figure 4.49 does in free space what OXB2 does using waveguides, where a MEMS mirror set in the horizontal (vertical) position is equivalent to an OXB2 switch in the OFF (ON) state. Furthermore, note that what is required at each cross-point in OXB2 is an incomplete 2 × 2 switch of the type shown in Figure 4.44, where the OFF state allows the two guides to cross without interchanging signals, and the ON state diverts power from the input (horizontal) guide to the output (vertical) guide while dumping any power that might be incoming on the vertical guide. (A [one-sided] MEMS mirror has exactly the same characteristics.) No connection is ever required from the vertical guide to the horizontal guide. Insertion loss in OXB2 is better than in OXB1, but its losses are not uniform, the longest path in Figure 4.73 traversing seven couplers, and the shortest traversing only one. This inequality of path length also produces unequal levels of cross-talk on each path. In contrast to OXB2, all paths in OXB3 are the same length and traverse the same number of switches, so that it has path-independent insertion loss [Shimoe+87], a significant advantage. As laid out in Figure 4.74, OXB3 bears more resemblance to a multistage switch than a crossbar. However, it operates like a traditional crossbar in that only one cross-point is activated to establish each connection. In the 4 × 4 example shown in the figure, light propagates from left to right in input and output waveguides that are coupled to each other by an array of sixteen 2 × 2 switches, just as in OXB2. Now, each guide has a bend in it to provide the left-to-right signal flow. As in OXB2, light remains in its guide when passing through a cross-point whose switch is set to the OFF state, but light is diverted from an input guide to an output guide when the switch is in the ON state. Each path traverses four switches. Once more, the incomplete 2 × 2 switch is useful here, as suggested by symbols at the cross-points indicating the direction of signal flow in the ON state. We leave it to the reader to verify that if incomplete 2 × 2 switches are used at the cross-points of OXB2 and OXB3, there is a unique path, with one ON switch setting, for each input-output connection, and the switch is strict sense nonblocking. (The OXB3 configuration is also adaptable to 2D MEMS.) It is interesting to note that if complete 2 × 2 switches are used at the cross-points in OXB2 and OXB3, more than one possible path may exist for a given input-ouput connection, but badly chosen paths will lead to blocking. Hence, these switches are wide-sense nonblocking when built with complete 2 × 2 switches. High-performance OXB3-type switch fabrics have been constructed as monolithically integrated structures using silica PLC technology [Goh+99, Kasahara+02]. An 8 × 8

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#8

#4 #7

#2 #5

85 mm

#3

#6 #1

85 mm Figure 4.75 Circuit layout for 8 × 8 optical crossbar switch. (From [Kasahara+02, Figure 15].

c 2002 IEEE. Used by permission of The Institute of Electrical and Electronics Copyright
Engineers, Inc.)

example is shown in Figure 4.75. It is fabricated on an 85 × 85-mm silica chip, where each rectangle in the figure contains one stage of eight couplers. Each coupler is a thermo-optic MZ switch. Using two-stage switches to improve extinction ratio and reduce cross-talk, 8 × 8 units have been demonstrated with the following characteristics: switching time of 2 ms, aggregate cross-talk of −43 dB (assuming equal input powers), average insertion loss of 5.1 dB, and extinction ratio of 60.3 dB.

4.12.1.2

Routers/Selectors Another switch fabric, shown in Figures 4.76 and 2.19, is known as a router/selector.40 These figures show identical topologies, but the switching elements are grouped differently, with the former having fewer connection crossovers. Thus, in Figure 4.76, all switches can be fabricated as an integrated component on a single substrate with only 12 waveguide crossovers. These structures can be implemented with both the splitting and combining stages, made up of switching devices, or with one or the other of the stages implemented with static devices, saving half of the switches [Spanke87]. However, the latter case exhibits poorer insertion loss and cross-talk properties (see Section 4.12.1.4). Because all required switches are 1 × 2 or 2 × 1, this architecture is adapted to Y-branch implementation. A monolithic 4 × 4 semiconductor PLC device based on Y-branch switches has been demonstrated using current injection. It is polarization insensitive and has −13 dB cross-talk for each switch. In their unbiased state, the switches act as 3-dB couplers, so this fabric can be operated as an LDC, with broadcast and multiaccess capability.

40

The δ−σ architecture, shown in Figure 2.19(a) as a 4 × 4 LDC, is identical in form to the router/selector. The only difference is that the elementary switches in the LDC are allowed to run through a continuum of states, whereas those in the router/selector are used only in the cross and bar states.

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Figure 4.76 Router/selector.

4.12.1.3

Benes Switches So far we have focused on strictly nonblocking switch configurations, which typically have more controllable devices than necessary but have good cross-talk properties. Rearrangeably nonblocking switches, exemplified by the Benes family, are more economical with respect to elementary switching devices but usually at the cost of increased cross-talk. An n × n Benes switch has 2 log2 n − 1 stages with n/2 2 × 2 switches per stage. Figure 4.77 shows a 4 × 4 Benes fabric. The 8 × 8 version appears in Figure 2.17. As indicated in Section 2.3.2.1, the Benes fabrics can be converted to strictly nonblocking Cantor switches by using log2 n-fold space dilation. Space dilation is also useful for reducing cross-talk. For example, if only one of the two ports on each input element is used and only one of the two output ports on each output element is used, the 8 × 8 Benes switch of Figure 2.17 becomes a twofold dilated 4 × 4 switch with better cross-talk properties (see [Hinton+93]). Benes optical switches have been fabricated in various technologies. An 8 × 8 dilated Benes switch is described in [Watson+90]. Fabricated as a LiNbO3 PLC using directional couplers, it was designed for operation in time division-switching applications, with a switching time of 2.5 ns and cross-talk of less than −20 dB. Another more ambitious example is a 64 × 64 Benes switch implemented using liquid crystal technology [Noguchi98]. This uses the polarization-deflection approach

Figure 4.77 Benes switch.

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Multiwavelength Optical Networks

employed in the liquid crystal MWS of Figure 4.63, in an 11-stage Benes fabric. The 64 inputs are arranged in an 8 × 8 array and transmitted through a series of 11 liquid crystal SLMs interposed between calcite crystal beam routers. The SLMs implement the switching function by rotating the polarization of the beams under electrical control, and the beam routers together with arrays of lenses direct the signals between stages based on their polarization states. The complete switch fabric is polarization insensitive, with average insertion loss of 9.5 dB, average cross-talk of −25 dB, and average PDL of 0.21 dB.

4.12.1.4

Switch Cross-Talk As indicated in Section 4.9.1, cross-talk can occur in a network when interfering signals travel on unintended “leakage” paths and interfere at a receiver. But a switch fabric is itself a connection network, and because of the many paths in a typical switch, many cross-talk terms can be generated. Switch cross-talk terms can be categorized by their order. As indicated in Equation (2.13), when any real 2 × 2 switching element is in the bar or cross state, a fraction  of the input signal power leaks into the wrong output port. (This also holds for 1 × 2 elements such as the Y-branch switch.) A leakage path through a switching fabric that passes through k of these “wrong-way” hops attenuates the power of a signal traversing it by an amount kdB . This is called kth-order cross-talk. Clearly, the higher its order, the less troublesome the cross-talk; so an important figure of merit for a switch fabric is the lowest possible order of cross-talk that can exist in it. Examples of cross-talk terms of various orders in three different realizations of 4 × 4 switches are shown in Figures 4.78 and 4.79. Intended paths are shown in bold and leakage paths are shown dashed. All connections are on the same nominal wavelength. In each case, a small portion of a signal applied to an input port leaks over to an undesired output port. In the Benes switch of Figure 4.78(a), a portion of the signal on input port 3 leaks to output port 1, passing through one wrong-way hop, which makes this a first-order heterodyne cross-talk. In addition, there is a second-order homodyne

1

2 1 2

1 2

3 4

3 4 (a) Benes Architecture

Figure 4.78 Orders of cross-talk.

3

4

(b) Router/Selector Architecture

Enabling Technology

Dump Switches

Active Splitters

Passive Splitters

Cross-talk Reduction Switches

287

Active Combiners Active Combiners

Figure 4.79 Enhanced performance switch.

(multipath) cross-talk term due to the leakage path from input port 1 to output port 1. A second-order heterodyne cross-talk path is shown in the router/selector switch of Figure 4.78(b), where the three switches on the dashed path are assumed to be set in the bar state, and a third-order heterodyne path is shown in the enhanced performance switch of Figure 4.79. In each case, the switch architecture is structured so that the orders shown in these illustrations are the lowest possible for the switch. In general, increasing the order of cross-talk requires adding additional switching elements. For example, the 4 × 4 enhanced performance switch is constructed with 44 controllable, active switching elements, as well as four static splitters, to limit crosstalk to third-order and above [Murphy+97]. For comparison, the Benes switch has first-order cross-talk but requires only six elements, whereas the router/selector switch has only second-order cross-talk and higher and requires 24 elements. In the case of the router/selector, one might economize on switching elements by replacing the first two stages of switches in Figure 4.78(b) by a fixed dividing network consisting of 3-dB splitters. However, this would raise the level of cross-talk on the dashed path to first order. Even though there would be an additional attenuation of 6 dB on that path, this is significantly poorer cross-talk performance than in the version with 24 switching elements. It is worth noting that the fabrics that are most economical in element usage generally produce multipath cross-talk, which has the most damaging effect on signal quality (see Section 4.9). Conversely, switches designed with extra elements usually have no multipath cross-talk. For example, we have seen that the Benes switch, as shown in

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Multiwavelength Optical Networks

Figure 4.80 Space dilation.

Figure 4.78(a), has a multipath cross-talk path in parallel with the desired path between input 1 and output 1. No such paths exist in the router/selector and enhanced performance switches. One way of increasing systematically the order of cross-talk in any switch fabric is by space dilating each 2 × 2 switch. As shown in Figure 4.80, a set of four 2 × 2 switches can be configured to produce a single space-dilated 2 × 2 switch with second-order cross-talk. (Only one input is used on each switch on the left, and only one output is used on each switch on the right, so that Y-branch switches can be used instead of 2 × 2 switches.) A complete fabric of these space-dilated switches would have the order of its cross-talk increased by a factor of 2, but with a component count increased by a factor of 4. For example, using space dilation for each elementary switch, the 4 × 4 Benes switch requires 24 elements, which is the same number as the 4 × 4 router/selector switch. In addition to having less cross-talk, switch fabrics with extra elements are usually more versatile. Thus, for example, we have seen that the router/selector configuration can be used as an LDC with one-to-many (multicast) as well as many-to-one connections, provided that its elementary switches are allowed to assume intermediate states. (The gate array crossbar of Figure 4.47 has this property using only two-state elementary switches.) Furthermore, cross-talk levels can often be reduced by appropriately setting unused switching elements; these have “don’t care” states. For example, with the switches set as shown in Figure 4.78(b) cross-talk leaks along a path from input port 2 to output port 1. It is second order because it passes through two wrong-way hops. However, if the second (unused) switch on this path is set in the cross state, the cross-talk is reduced to third order.

4.12.2

Wavelength-Selective Switches The generic wavelength-selective switches and wavelength-interchanging switches discussed in Section 2.3 are commonly known as optical cross-connects (OXCs). We examine these in more detail now, beginning with the wavelength add/drop multiplexer, also commonly known as an optical add/drop multiplexer (OADM) (a 2 × 2 special case of the OXC) and concluding with fully configured OXCs.41 41

As networking evolves, terminology evolves with it. For consistency we generally use the term WADM in referring to a generic wavelength add/drop multiplexer. However, in recent applications exemplified by the cases that follow, usage has shifted from WADM to OADM. We use WADM and OADM interchangeably henceforth.

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289

Optical Add/Drop Multiplexers The OADM has emerged as one of the key building blocks for networks at the metro and regional level. Used for adding and dropping wavelengths from intermediate points along a transmission link or a ring, the role of OADMs in a typical network is illustrated in Figure 4.81. They offer a simple and effective way to access a general mesh network along links or within ring subnets. The OADM can be static or reconfigurable, and in the latter case it is sometimes called an ROADM. Initially developed in the early transparent optical network testbeds of the 1990s, OADMs are now deployed in the current network infrastructure. Two fundamental designs are the parallel and the serial architectures shown in Figures 4.82(a) and 4.82(b), respectively. The 2 × 2 controllable couplers in the parallel realization make this a reconfigurable OADM. The variable optical attenuators shown in the figure are used to equalize the individual λ-channel power levels, which may vary due to upstream signal power divergence or losses within the OADM. Because of the insertion loss involved in the demultiplexing process, the parallel design is more suitable for high add/drop channel counts, whereas the serial design is limited to applications with low add/drop channel counts. The add/drop modules in the serial design are normally fixed devices, but a practical advantage of this arrangement is that individual modules can be added as needed. Both designs are well known and widely utilized [Wagner+96]. An OADM design based on hierarchical, multistage optical multiplexing and demultiplexing of wavebands and wavelengths is shown in Figure 4.83. The motivation behind distinguishing between bands and wavelengths is that in many cases it is not necessary for every wavelength to be demultiplexed/multiplexed at every node if it carries no traffic for that specific node and consequently all or some of the wavelengths should have the ability to bypass any optical node. Optical node bypassing is a very cost-effective

OADM

OXC

Figure 4.81 OADMs in a network.

Multiwavelength Optical Networks

λ1 λ2

ED EDFA

EDFA MUX

D M U X

M U X

DMUX

290

λm

(a) Parallel

Wavelength add/drop

Wavelength add/drop

λ1

λm (b) Serial

Figure 4.82 Parallel (a) and serial (b) OADM architectures with capability for m wavelength

add/drops.

Wavelength access W adding/dropping/switching ….

….

wavelength DMUX

Band module n . . . Band module 2 Band module 1 Band DMUX Band access access

working fiber West Fibe Fiberraccess access

East

protection fiber Figure 4.83 Functional diagram of an OADM based on wavebands and wavelengths. (Adapted from

c 1999 IEEE. Used by permission of The Institute of Electrical [Saleh+99, Figure 12]. Copyright
and Electronics Engineers, Inc.)

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solution because (1) it avoids the use of more expensive equipment that might not be needed (i.e., individual channel MUX/DMUXs) and (2) saves on amplification costs because it results in reduced insertion losses on an optically transparent path to its destination. At the same time, the band-level demultiplexing/multiplexing capability allows for a cost-effective modular upgrade approach. In this device, a band WDMUX/MUX combination first separates and combines signals at the waveband level. In combination with a manual patch panel this allows for selective waveband add/drop or pass-through. Each dropped waveband goes through an additional level of WDMUX/MUX at the wavelength level implemented in an individual band module, and these modules can be added on an as-needed basis enabling modular node expansion. Each band module can have its own channel-conditioning capability such as amplification (dark triangles), attenuators for power equalization, and possible optical dispersion compensation modules. Switching at the wavelength level can be performed manually or controlled electronically. The number of band modules that are required can be minimized by concentrating the wavelengths being add/dropped at the node into the smallest possible number of wavebands. A grooming process for this purpose is discussed in Section 4.12.2.2 and Chapter 7. Because protection is essential when OADMs are deployed in the field, the configuration shown in Figure 4.83 is shown with two fibers: working and protection, along with line amplifiers. The box labeled fiber access allows for bypassing the OADM as well as implementing (1 + 1) protection in a ring network. The multiplexed signal outbound from the band WMUX is bridged between the eastbound working fiber and the westbound protection fiber. Also, the signals inbound on the working and protection fibers are monitored, and when a failure is detected on the working fiber the WDMUX is switched over to the protection fiber, which carries a copy of the signal on the working fiber. Another OADM architecture called Broadcast & Select (B&S) [Boskovic+02] is shown in Figure 4.84. Essentially it replaces the WDMUX/MUX and switching functions in Figures 4.82 and 4.83 by tunable receivers and transmitters. A reconfigurable wavelength-selective device can selectively pass or attenuate the power of any λ-channel on the pass-through path under external control, providing power equalizing and blocking functionalities in a single unit. A device performing this function using liquid crystal technology [Ranalli+99] has been manufactured under the name dynamic spectrum equalizer (DSE). Its dynamic capabilities make the above design a fully reconfigurable OADM. All incoming traffic is split into the drop and the pass-through directions using conventional 3-dB splitters. On the drop side power is further split using 1:N passive devices where N is the number of accessing stations (clients). Optical filters can then separate the intended traffic at each station with the possibility of accessing a number of different channels in a dynamic way if the filters are made tunable. The DSE is then used on the pass-through path to block each channel that has been dropped and selected on the first stage, whereas at the same time equalizing all the pass-through channels. On the adding side, channels can simply be inserted by combining the powers from a set of N transmitters (tunability is also possible). Signal conditioning is provided using input/output amplifiers, the dynamic spectrum equalizer, and optical dispersion compensators (not included in Figure 4.84).

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DSE DSE

A C

DSE DSE

S F RX

TX

TX

RX

F - Optical filters S - 1 to N power splitter A - Optical amplifier C - 3 dB coupler DSE - Dynamic spectrum analyzer Figure 4.84 Typical B&S OADM architecture. Assumes (1 + 1) protection. (Adapted from

c 2002 IEEE. Used by permission of The Institute of [Boskovic+02, Figure 2]. Copyright
Electrical and Electronics Engineers, Inc.)

When used in a ring, (1 + 1) protection can be provided using a bridge-and-select scheme much like that described for the OADM of Figure 4.83, assuming that one of the fibers is a working fiber and the other is a protection fiber (carrying the same information in the opposite direction). Bridging of the output signals to the two output fibers is accomplished by feeding the two transmitters with the same electrical signal, and selection is done by monitoring the outputs of the two receivers and switching from one to another in case of a failure. Another B&S architecture is shown in Figure 4.85. It is similar to the previous one except that the DSEs are replaced by 4 × 1 wavelength selective switches, which select either pass-through channels from inputs 1 and 2 or add channels from inputs 3 and 4. In 1

4 x1 1 4x

In 2

Out 1

Out 2 4x1 4x1

Out 3

RX RX

Out 4

RX RX

In 3

In 4

TX TX RX RX

RX RX

TX TX TX TX

TX TX

Figure 4.85 Typical 4 × 4 ROADM based on 4 × 1 wavelength selective switch and B&S-type of

c 2004 IEEE. Used by architecture. (Adapted from [Antoniades+04a, Figure 15]. Copyright
permission of The Institute of Electrical and Electronics Engineers, Inc.)

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In the figure the fibers are oriented to make this look more like a 4 × 4 switch than an ROADM, and indeed it is. In an ROADM working into a bidirectional fiber pair, as in Figure 4.84, there is no path from an input fiber to the other output fiber, only a pass-through path to the same output fiber. This limited connectivity characterizes a typical ROADM. In the architecture of Figure 4.85, however, all paths are possible because of the 4 × 1 switches. Signals from input ports 1 and 2 are distributed to the four output ports using 1 × 4 splitters. Outputs 3 and 4 can selectively receive λ-channels from each fiber using 1 × N splitters and tunable filters just as described in the basic B&S design of Figure 4.84. The number of passive splits/combines on the drop/add sites is dictated by how many channels need to be accessed from each fiber. The higher that number, the more insertion loss (and consequently amplification costs) will be incurred. A realization of this architecture has been proposed using a 1 × 4 MEMS switch described in [Marom+02] (see Section 4.10.5.3). The ROADM architecture of Figure 4.85 is essentially a two-stage switch with a passive (static) first stage and a controllable second stage. This arrangement can be reversed. A configuration has been proposed consisting of a controllable wavelength-dropping stage, followed by a passive combining stage for the passed-through wavelengths and the added wavelengths [Doerr+03]. The wavelength-dropping stage is realized using a silica-based monolithic PLC.

Optical Cross-Connects When examining OXC architectures it is important to distinguish between functionality and implementation. Figures 4.86, 4.87, and 4.88 show typical implementations of an OXC. The first is a transparent architecture, suitable for a node of a transparent network.

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Multiple transparent optical switch matrices

c 2004 IEEE. Used by Figure 4.86 Transparent OXC. (From [Ellinas+04, Figure 1]. Copyright
permission of The Institute of Electrical and Electronics Engineers, Inc.)

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c 2004 IEEE. Used Figure 4.87 Opaque O-E-O OXC. (From [Ellinas+04, Figure 3a]. Copyright
by permission of The Institute of Electrical and Electronics Engineers, Inc.)

It is based on multiple layers of O-O-O switch fabrics, one for each wavelength. The terminology O-O-O as used here means that the signals enter the switch fabric in optical form, remain in the optical domain in the switch, and exit in optical form. (That generally means that the signal is not modified in any significant way in passing through the switch, although there may be exceptions if optical signal processing occurs.) Three links are shown incident on the switch, each represented as a single fiber pair carrying

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c 2004 IEEE. Used Figure 4.88 Opaque O-O-O OXC. (From [Ellinas+04, Figure 3b]. Copyright
by permission of The Institute of Electrical and Electronics Engineers, Inc.)

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multiple wavelengths. They are demultiplexed/multiplexed on a wavelength basis and each layer switches signals on the same wavelengths. (We assume for simplicity that no wavelength conversion, either optical or electrical, occurs in the switch.) This is the three-stage architecture of Figure 2.21, but with additional input/output ports on the switch fabric layers for access stations (transceivers) connected to the network via this OXC. Because signals remain in the optical domain at all times, with no wavelength conversion, compatibility of all links and access stations connected to this OXC requires that all λ-channels must be on a common set of wavelengths; for example, the ITU grid. Compatibility may entail additional requirements as well, depending on the degree of transparency. There is no such thing as an absolutely transparent network! For example, the modulation format, power level, and signal bandwidth or bit rate may also be specified or constrained in some way. Power and bandwidth constraints are obviously necessary to prevent interchannel cross-talk. Signals inserted into the network on the access links must comply to these constraints, and for this reason the interface been the access station and the OXC in this case is called a compliant interface. In our generic terminology, this is a WSXC. For an OXC with a total of n input/output ports (including the access links), carrying m wavelengths, this has m switch fabric layers, each consisting of an n × n space switch. The next two implementations are opaque. Figure 4.87 shows an O-E-O switch fabric serving the same set of links and access stations, but this time there is no continuity of the optical signal. Signals entering the switch fabric all connect to it via short-reach fiber interfaces operating in a standard format at a standard wavelength (usually 1.3 µm). Signals entering on the links, which would normally be on the ITU wavelength grid, must be demultiplexed on a wavelength basis, after which each λ-channel is converted from its own wavelength to the standard format and wavelength by a transponder, which is a receiver/transmitter combination including some degree of electronic signal regeneration. The inverse operation takes place in the transponders on the outbound links. The presence of the transponders makes this an opaque node; because electronic processing is involved here, the signals on the links must be transmitted and received by the transponders at standard speeds and formats. It should be noted that the receiving side of the inbound transponder and the transmitting side of the outbound transponder are the long-reach transmitters and receivers discussed in Sections 4.5 and 4.6, respectively. If they are working into very long or poor-quality links they must be high performance (read expensive) devices. Input signals on the access links are assumed here to be exchanged with the switch fabric in the standard short-reach format. The switch fabric itself is electronic, which means that another O/E (E/O) conversion is needed at each input (output) port of the switch fabric, necessitating a transceiver at each I/O port. An opaque OXC with a total of n I/O ports carrying m wavelengths requires a single mn × mn electronic switch fabric, and each port must run at the speed of the λ-channel it is serving. This becomes difficult, power-consuming, and expensive in the electronic domain as bit rates move beyond 10 Gbps toward 40 Gbps and beyond. It should be recalled that an mn × mn switch is considerably more complex than m n × n switches. However, a non-blocking mn × mn switch fabric allows any input wavelength to be translated to any output wavelength on any output port, giving the opaque switch

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WIXC functionality as compared to the lesser WSXC functionality of the transparent switch. A different opaque approach shown in Figure 4.88 eliminates the electronic fabric. Now the switch is O-O-O, where the signals in standard short-reach optical format are interfaced directly to an mn × mn optical switch, which could be realized in any of the technologies discussed earlier in this chapter. The transponders remain, so that this is still opaque, but there is no need for any high-speed electronics in the switch or at its edge. At higher speeds and large wavelength count, transparent switches are simpler, more flexible, more reliable, and less costly than the opaque version. For example, no changes are required if channel speeds or modulation formats are changed. To compare switch complexity, suppose there are 4 I/O ports with 64 wavelengths per fiber. The opaque switch fabric is then 256 × 256. In a Benes realization (rearrangeably nonblocking) this requires 1920 2 × 2 switching elements, while the transparent version requires 384 (see Section 4.12.1.3). If a crossbar realization is used, the opaque switch requires over 65,000 switch points as compared to 1024 for the transparent switch. Furthermore, the transponders add a significant extra cost in the opaque case. The advantage of the transparent architecture is even more impressive if switching is done on a waveband rather than wavelength basis, in which case the number of layers in the switch fabric is proportionately reduced. Nevertheless, opaque switches have a number of practical advantages. They automatically regenerate all signals passing through the switch thereby preventing the accumulation of optical signal impairments, they can monitor the signal overhead bits for control and management purposes, and their wavelength conversion properties facilitate efficient use of network capacity and fault recovery. At this writing, the cost and complexity associated with large high-speed opaque switches is not an issue because demand has not yet materialized at the higher speeds and larger sizes. However, that can change in the future. One way of exploiting the advantages of both types of switches is in a translucent network, composed of transparent nodes mixed with opaque nodes at strategic locations. Properly designed this approach can achieve the best of both worlds. A way of improving the effectiveness of an OXC architecture is by doing the switching at both the waveband and wavelength levels. Figure 4.89 shows an example of a hybrid switch, sometimes called a multigranular switch. Traffic on the incident links is grouped into three wavebands, w1 , w2 , and w3 , and terminating traffic is on wavelengths λ13 and λ23 in band w3 . The switch has a hierarchical structure with a waveband-selective top layer (WSXC) switching w1 and w2 and dropping w3 down to the wavelength layer (λSXC). There, it is demultiplexed and switched at a λ-channel granularity. The wavelengths λ13 and λ23 are dropped and added at the local access stations. To put this in the context of a nonhierarchical architecture, imagine that the top layer, WSXC in Figure 4.89, represents a transparent OXC of the form of Figure 4.86 with three optical switch matrices, each operating on wavebands rather than wavelengths. (The individual links are not shown.) Now suppose the add/drop ports for band w3 are connected to the wavelength layer λSXC, which passes the demultiplexed terminating wavelengths on to the access stations. In general this switch requires one λSXC for each band being dropped

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to the wavelength layer. To make this architecture efficient, the traffic must be properly assigned to λ-channels and wavebands. Normally, most of the traffic passing through a typical OXC is transit traffic. If the traffic assignments are grouped into wavebands in such a way that the terminating traffic is contained in just a small number of bands and the transit traffic occupies a larger number of bands then only a few bands need to be dropped and wavelength demultiplexed at the node, and the complexity and cost of the OXC is diminished. This traffic assignment technique is an example of grooming at the network level. We take up the grooming problem in a network context in Chapter 7.

4.13

Performance Evaluation: Methodology and Case Studies In the discussion of transmission impairments such as dispersion, noise, and fiber nonlinearities, we began by treating each in isolation to obtain some meaningful results on performance constraints and to yield some insight on the problem. Typically we found an approximate limit on transmission link length and/or bit rate due to each type of impairment. In Section 4.9, we took the next step of examining and modeling the accumulated effects of certain impairments, but still in a relatively simple context, focusing on one impairment at a time. In a wide-area multiwavelength network, however, a purely optical connection traverses many different components (fibers, amplifiers, WMUXs, WDMUXs, and cross-connects), accumulating distortion, noise, and interference as it propagates. As noted, the problem of accumulated transmission impairments on long, nonregenerated paths is one of the negative features of optically transparent networks. Thus, the prediction of the combined effect of all these impairments is of crucial interest to the network designer. As we shall see, the complexity of the problem suggests that the best approach to performance evaluation in this case is simulation. As optical networks have come of age and new photonic and optoelectronic technologies have proved their viability in the real world, computer modeling and simulation are playing an increasingly

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important role. It is a cost-effective way of performing virtual prototyping and system engineering and testing before the actual network buildup and deployment. However, a “brute force” approach to simulation is doomed to failure because of the sheer number of components involved and their complexity. In the following sections, we describe an efficient method of physical-layer simulation and apply it to some case studies. The general objectives are r To suggest a way of improving the computational efficiency of physical-layer simulations r To illustrate how the reach of an optical path is constrained by combinations of technological limitations r To illustrate how computer simulation and modeling can be used as a tool for network design and engineering r To use the case studies as an illustration of the current trends in network architectures

4.13.1

Physical-Layer Simulation: Three-Step Approach Computer simulation can be used effectively for engineering the physical layer of WDM optical networks, producing computational accuracy while maintaining speed and topological flexibility. A number of simulation packages intended for the network optical layer exist today. The operation of the majority of these tools is based on the well-known time- and frequency-domain simulation principles [Jeruchim+92]. Unfortunately, these techniques are inefficient when dealing with very wideband systems, which is the case in WDM networks. Simulation of WDM networks requires a large simulation bandwidth, mainly because of the large aggregate bandwidth of the optical signals and the wide spectral range of the ASE noise. Moreover, such networks contain a large number of optical paths over which simulations need to be performed. In this context, complete time- and frequency-domain simulation is a very time-consuming process. As a result, a multistep computationally efficient simulation methodology was introduced in [Antoniades+02] that represents a semianalytic method of performance modeling. The strategy of the overall simulation is to begin with a wavelength-domain simulation of a network (presented in detail in [Roudas+00]) to find pertinent signal, noise, and interference power levels only. This information is then used as a basis for a more refined time-domain simulation of specific signals of interest. This two-step approach enables handling systems with sizes that would render them intractable with other methods. The above approach can then be expanded with a third step that enables efficient iterations of performance calculations that help with the engineering and design of specific topologies. The first step involves a wavelength-domain simulation of the entire network being analyzed. This is basically a low-resolution frequency-domain approach, and it is limited to linear systems. Figure 4.90 illustrates the general methodology. In a (high-resolution) frequency-domain simulation, shown in Figure 4.90(a), a multiwavelength signal is represented by sets of samples of the complex signal spectra, with a resolution bandwidth that is a small fraction of the bit rate in the case of digital signals. However, in wavelength

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simulation a much coarser resolution is used, as shown in Figure 4.90(b).42 In this case, the spectral components of the quantities of interest (WDM signal and ASE noise powers) are grouped into larger bins so that the fine structure of the frequency spectrum is lost. However, the loss of resolution is offset by a gain in computational speed. Typical effects studied in the wavelength domain are evaluations of EDFA gain profile, ASE noise accumulation, and cross-talk in various network elements. The resolution bandwidth (or bin size) appropriate to multiwavelength networks is on the order of 0.1 nm to ensure that adjacent signals fall in different bins. The rationale for using a coarse resolution is as follows. First, the optical frequency characteristics of typical optical components generally vary very slowly over the bandwidth of an individual signal, so their action results in a simple scaling of each signal by a constant gain and a change of carrier phase. Second, noise spectra are also slowly varying functions of frequency. Third, insofar as the various interactions within the network are concerned, it is usually sufficient to characterize the signals themselves as constant, unmodulated carriers.43 A result of the undersampling performed in the wavelength-domain simulation is that switching between time and frequency domains is not possible. Thus, this first simulation step is used to calculate the average signal and cross-talk powers and OSNR and not to evaluate the waveform evolution of channels through the optical components of the network. The cross-talk-induced penalties, nonlinear effects in the fiber and other optical components, and polarization effects are not captured in the wavelength-domain 42 43

Similar approaches exist under different names (for example, “power-budget simulation” [Rotolo+95]). This still allows for the study of effects such as the influence of aggregate signal power levels on EDFA gain and ASE noise because the signal variations due to modulation are much too rapid to affect these properties of the EDFA.

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simulations. These are studied only on individual paths of the network under study in the time-domain part of the simulation (step 2). As a result, the time-consuming time/frequency approach is executed only on selected worst-case paths in the network. Wavelength-domain simulations and analysis identify these worst-case paths. The second part of the simulation is a conventional time- and frequency-domain computation that is run to study the performance of the optical paths that were identified in the previous step. Having used the wavelength-domain simulation to determine the pertinent signal, noise, and interference power levels, the rest of the network is now suppressed, with each selected path viewed as an equivalent channel. Each equivalent channel is characterized by its dispersion and attenuation, together with the noise and interfering signals accumulated along the path, with fiber nonlinearities and polarization effects included. The time-/frequency-domain simulations are used to calculate the error probability (Pe ) for the selected channels as well as to obtain eye diagrams. Once Pe is obtained the Q factor can be used as a convenient parameter for system performance characterization. Based on a Gaussian approximation for noise and interference, the relation between BER (equivalent to Pe ) and Q was defined in Equation (4.42) as 1 BER = erfc 2

Q √ 2



−Q2

e 2 ≈ √ Q 2π

(4.55)

so Q is determined from BER by inverting this expression. Consecutive time-/frequencydomain simulations on a selected path can be run to determine the Q penalty for each impairment. For example, in the case of optical cross-talk, cross-talk-induced Q penalty is defined as the difference in Q 2dB at the optical receiver, observed at a given error probability Pe for the case of no cross-talk in the system, and Q 2dB observed under the effects of cross-talk. This approach helps the system/network designer quantify the impact of impairments such as fiber nonlinearities, polarization effects as well as linear optical cross-talk and distortion due to optical filter concatenation. Summing this up, the two-step approach essentially separates macroscopic first-order effects examined in the first step (e.g., power levels and noise spectra) from microscopic second-order effects examined in the second step (e.g., dispersion and signal waveform distortion). The first step uses a linear model of the network excluding any nonlinearities and operates at low resolution, whereas the second-step model may include both linear effects (e.g., cross-talk) and nonlinear effects (e.g., SPM). This essentially treats the results of nonlinearities as perturbations on the dominant linear effects. It is normally valid if nonlinearities are small compared with linear effects, which is the case in most network applications. This leads to a third simulation step that has been applied in metro network examples in [Antoniades+04a]. It is a budgeting approach based on Q penalties for the various impairments being studied, obtained either from the second step of the simulation or through stand-alone impairment models. A simplified baseline Q performance estimator is run where the Q factors of all the channels being studied on the selected worst-case paths are calculated. This is done by keeping track of the signal and ASE noise powers from the optical transmitter, through the various components to the

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optical receiver. Channel power divergence, dispersion/chirp, ASE noise and receiver noise terms are taken into account, but all other impairments whose Q penalties are to be determined are excluded at this step. Finally at the receiver the other impairments being studied (fiber nonlinearities, PMD, cross-talk, and so on) are budgeted one at a time in the form of Q penalties, as discussed in Section 4.9. It must be noted that this is not a simplistic Q budget because the margin allocated for each impairment is not predetermined but is calculated from a corresponding impairment model obtained in the second step of the simulation. The motivation behind the third step is to make the design problem tractable. Ideally, the network engineer should be able to explore all possible combinations of component parameters in the network to optimize the design. In practice this is impossible because of “parameter space explosion”: there are just too many parameters to explore. Thus we divide the problem into two parts. The first part (steps 1 and 2) uses a baseline model that can be simulated efficiently with several design iterations if necessary. The second part (step 3) refines the first step using Q penalties computed based on the results of the baseline simulation to take into account effects that are assumed not to change much from iteration to iteration. Further iterations can be run at this level to refine the design. In this way, the designer can “tweak” network performance, derive component specifications, determine the possible dependencies of specific parameters on network size, and in general better explore the performance versus cost trade-offs.

4.13.2

WDM Network Simulation Case Studies The following three case studies illustrate the implementation of the simulation approach described above. The first case study pertains to the MONET testbed ([see Wagner+96]) and focuses on issues in long-haul transmission, whereas the second and third are inspired by more recent work in the industry that focused on the metro environment, reported in more detail in [Antoniades+04a].

4.13.2.1

Study of a Wavelength Add/Drop Multiplexer Chain The effects of filter imperfections and laser misalignments were studied in a system consisting of a chain of 50 WADMs [Antoniades+97]. This might correspond, for example, to a 50-node multiwavelength national scale (long-haul) network. The objective was to determine how many WADMs can be cascaded before the OSNR drops below an acceptable level. The system that was simulated is shown in Figure 4.91. It is an eight-wavelength system, in which one channel is continued end to end, and the others are added and dropped periodically at various WADMs along the path. Each WADM is constructed as shown in Figure 4.92(a). [The same design as Figure 4.82(a).] The WDMUXs and WMUXs consist of cascades of MI filters, with each signal passing through eight filters in each WADM. The variable attenuators are servo driven to equalize the signal levels, which are amplified nonuniformly by the EDFAs. Figure 4.92(b) is a block diagram that represents the modules and their interconnections used in simulating the WADM. The filter characteristics used in the simulation were based on measured

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transmittance and reflectance of actual components. In the simulation, the EDFA gains and ASE noise contributions are calculated based on aggregate signal power, using the models described in [Roudas+99] and references therein. Some typical results are shown in Figure 4.93, illustrating combined effects of laser misalignment and filter cascading. In Figure 4.93(a), the power spectral densities (PSDs) of the signal and ASE noise are shown after 24 WADMs, with a laser misalignment of 40 GHz in channel 5, which is the end-to-end channel being studied. (The powers shown are totals in a bandwidth of 0.1 nm, which is the bin size.) Despite the misalignment, which places it at the edge of the filter passbands, the variable attenuators are able to maintain uniform signal levels but at the expense of increasing the noise level in channel 5. This reduces the OSNR in that channel to approximately 20 dB, which is more than sufficient to sustain a BER of 10−9 . After 50 WADMs, it is impossible to equalize the signal level, and it drops below the noise level. (Similar results for laser misalignments of 30 GHz or less show satisfactory OSNRs through the full chain of 50 WADMs.) It must be noted that the results presented above pertain to wavelength-domain simulations (step 1) focusing on OSNR-related effects only. Fiber nonlinearities and PMD are studied and budgeted separately in the second and third simulation steps to ensure proper system operation.

4.13.2.2

Modeling Performance Impairments in a Metro WDM Network Composed of Interconnected Rings As commercial deployment of optical networks has taken hold, the design focus has shifted from government-funded consortia (like MONET), to for-profit networking

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c 2004 IEEE. Used by permission of The cated. (From [Antoniades+04a, Figure 3]. Copyright
Institute of Electrical and Electronics Engineers, Inc.)

activities. A significant part of this work involves investigating the feasibility of transparency in metropolitan optical networks. These are typically in the form of interconnected optical rings, which have evolved historically from opaque SONET rings and their electrical ancestors. Ring topologies were preferred due to their efficient fiber sharing and resiliency. As shown in Figure 4.94, this type of network has feeder and distribution sections, referred to also as interoffice (IOF) or core, and collector or edge, respectively. The feeder sections aggregate traffic originating from the distribution sections and interconnect them while the edge rings distribute and collect traffic from individual business or campuswide networks (shown as buildings in Figure 4.94). Feeder rings can also have transfer points (points of presence) to the long-haul backbones and may also include direct customer connection sites. The network of Figure 4.94 models six edge rings attached to an IOF ring. Each edge ring, with a typical circumference of about 15–30 km, can interconnect N users (access nodes) that can be small businesses, campus networks, or Internet service providers (ISPs). Edge rings are interconnected with the IOF ring using hub nodes that provide the cross-connect capabilities. However, individual customers may directly connect to the network through the hubs. Traffic aggregation occurs at either the access or the hub node sites and can involve a wide range of service types such as Internet Protocol (IP), asynchronous transfer mode (ATM), or frame relay (FR) over SONET/SDH and

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Ethernet. For the architecture of Figure 4.94, it was assumed that client services are assigned the entire optical wavelength even if the demand does not reach OC-48 or OC-192 and that a mixture of access-to-access or access-to-hub connections exist. The physical layer model for each hub node used in this case study is the node shown in Figure 4.83, involving hierarchical optical multiplexing and demultiplexing of bands and wavelengths (see Section 4.12.2.1). Thirty-two channels are considered in the network and these are partitioned into two major bands (C- and L-bands). Each band is further partitioned into two-to-six other subbands with individual wavelengths spaced 200 GHz apart and with a guard spacing between bands. Optical dispersion compensating modules (i.e., a length of negative dispersion fiber) are assumed on each band module to compensate for the dispersion on the IOF ring. More details of the actual network design as well as typical component and system parameters used in the simulations are presented in [Antoniades+02]. Access nodes are assumed to be implemented using a serial design that provides for the add/drop of the appropriate band on the first level (band MUX/DMUX) and then selects the appropriate channel on the second level (channel MUX/DMUX). The goal here is to derive a network design that can be upgradable to higher bit rates without the need for costly equipment modifications, so both OC-48 and OC-192 bit rates were investigated. In designing and engineering the proposed network the three-step simulation process was employed. Figure 4.94 illustrates the worst-case path (between nodes A and B) for the case study. The path from A to B is shown in light gray and the return path in black. The path happens to be the longest as well as the one that passes through the most optical components. The optical signal is added on the first node of an edge ring, traverses the whole edge and IOF rings, and is dropped at the most distant node of the last edge ring. A general design assumption is that each connection is bidirectional with both directions on the same nominal frequency. This will become important during the study of cross-talk contributions presented below. In addition to the performance engineering issue, the question of how big a transparent network can be supported is of great importance. Focusing on the OC-192 (10 Gbps) bit rate, which needs to be supported as a direct upgrade for the OC-48 (2.5 Gbps) case, three important network parameters are identified r Power at the receiver (depends on channel launched power and loss on the final access ring) r OSNR at the receiver r Total uncompensated fiber length on the worst-case path The interplay of the above three parameters limits the size of the network. In the above case study, IOF and edge rings of circumference 100 and 30 km, respectively, were used; the above numbers were derived as a result of a network parameter study presented in [Antoniades+02]. Wavelength-domain simulation is first performed to calculate OSNR, channel power level, and cross-talk information for the network. Wavelengths are routed in such a way that worst-case cross-talk is produced. In particular, the focus is on a given wavelength λ1 that travels the worst-case path shown in Figure 4.94. It is assumed that λ1 is being reused

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c 2002 IEEE. Used by permission Figure 4.94. (From [Antoniades+02, Figure 11]. Copyright
of The Institute of Electrical and Electronics Engineers, Inc.)

at each access ring in the network as well as for the return path from node B to node A. The received signal at node B will contain a large number of cochannel cross-talk terms that have accumulated due to imperfections in filtering and switching at the different hub nodes as well as at access node A. (This is a cochannel heterodyne cross-talk effect as explained in Section 4.9.) Figure 4.95 presents a power level histogram of all the cochannel cross-talk terms present at node B that are obtained from wavelength-domain simulation. The strongest cross-talk contribution is generated at the access node A and is due to the return path signal. Part of this dropped signal will leak and combine with the added signal (same nominal frequency) due to imperfections in the isolation of the band MUX/DMUX used at each access node. The power level of the added signal as well as the filter isolation performance will be the determining factors for this effect. In this case study, this cross-talk term is 21 dB below the signal level and dominates all other terms, which are of the order of −40 to −50 dB. Based on the above and assuming OC-192 connections using EA-modulated transmitters, time-domain simulation is performed on the worst-case path to determine Q penalties for various impairments. For example, the power levels of the 30 dominant cross-talk terms are used in time-domain simulation on the worst-case path and the Q penalty is calculated based on simulated BER, converted to Q via Equation (4.55) as explained in Section 4.13.1. Figure 4.96 shows the Q crosstalk penalty versus the different levels of the dominant cross-talk term. It is clear that a tighter cross-talk level should be maintained at each access node to achieve a 2-dB

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Dominant cross-talk term level (dB) Figure 4.96 Cross-talk-induced Q penalty in dB versus dominant cross-talk term power level. c 2002 IEEE. Used by permission of The (Adapted from [Antoniades+02, Figure 12]. Copyright
Institute of Electrical and Electronics Engineers, Inc.)

penalty as a target cross-talk budget in the network design. This cross-talk performance can be realized by tighter cross-talk filter specifications on each band MUX/DMUX or by additional filtering at each access node, with the side effect of increasing the passthrough insertion loss of each one of these nodes. If the latter solution is chosen, only five access nodes (as shown in Figure 4.94) along with 19 km of fiber can be supported. Each node can add and drop one band and thus each access ring can potentially support up to 20 wavelengths of the total of 32, which provides adequate network connectivity. Additional penalties must be included for other effects. For filter concatenation, it has been shown that a 2.0-dB distortion-induced Q penalty is an achievable budget value in the case of EA-modulated and externally modulated transmitters at OC-192 [Tomkos+01a]. Calculations based on the model for PMD described in Section 4.9.5 yield a PMD-induced Q penalty of 0.2 dB (negligible). Finally the Q penalty because of nonlinearities obtained for the above 32-channel system is approximately 0.2 dB, which is close to negligible. This is due to the fact that the channel launched power is limited to the region where fiber nonlinearities are not dominant (about 0 dBm per channel is launched into the fiber). The last step of the simulation consists of simple budgeting calculations based on the Q penalties obtained above, focusing on the best and worst performing channels on the identified path of Figure 4.94. Power equalizers should be an integral part of the hub node design because they help mitigate the power divergence among the channels. One of the sources of power divergence is component PDL. Simulations based on the model in Section 4.9.2 have shown that in this network up to 0.5 dB PDL-induced channel

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22

Q (dB)

20

18 BER 10

12

16

14

1520 1530 1540 1550 1560 1570 1580 Wavelength (nm)

1590 1600 1610

Figure 4.97 Q-channel performance for the worst-case path of Figure 4.94 assuming OC-192 bit c rate and EA-modulated transmitters. (Adapted from [Antoniades+02, Figure 14]. Copyright
2002 IEEE. Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

power divergence will accumulate between each hub. The power equalizers are therefore designed to handle the above effect in addition to other sources of power divergence such as dynamic add/drops due to network reconfigurations. Figure 4.97 shows the Q parameter versus wavelength obtained using the simulation methodology on the metro ring composed of six interconnected access rings each having five add/drops (Figure 4.94). In the entire design, the channels will traverse a total of 38 km of dispersion uncompensated fiber (19 km in each access ring) and 85 km of dispersion compensated fiber on the IOF ring. An additional 2-dB Q margin for component aging is included in the results. Typical chirp/dispersion transmitter rating parameters are used in the simulations as described in [Antoniades+02].

4.13.2.3

Engineering the Performance of a DWDM Metro Network Deployment Scenario In the next study, the focus moves to a different scenario in which four subnets composed of interconnected rings resembling those in the previous example are joined via a large ring or mesh. The transparency of each set of interconnected rings is now discarded in favor of a more traditional SONET/SDH architecture as shown in Figure 4.98. This represents a current evolutionary scenario in a highly populated metro region [Elby+02]. All the subnet rings shown in the figure are based on (opaque) SONET/SDH equipment and the move to transparency involves only the superhubs that are to be joined in a ring or a mesh connectivity pattern. In typical densely populated metropolitan areas, superhubs number only a few and are typically spaced 30 to 50 km apart. They can be implemented using ROADMs of the type described in Section 4.12.2.1. Eliminating the

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SONET OC-12/48 (collector rings)

SONET OC-48/192 (inter-Hub rings)

Central Office (End Office) Hub Ring DWDM using ROADM (inter-Superhub wavelength paths)

Superhub

(only in some locations)

Figure 4.98 A DWDM metro network deployment scenario. All rings represent typical SONET OC-12/48/192 designs. DWDM is deployed only between the superhub nodes (dark squares) in ring (solid) or possible mesh (dotted) configurations. (From [Antoniades+04a, Figure 11]. c 2004 IEEE. Used by permission of The Institute of Electrical and Electronics Copyright
Engineers, Inc.)

details of the subnets, the remaining transparent interconnection network is shown in Figure 4.99. (This is the mesh interconnection alternative.) In this case, assuming minimum hop routing, the longest route between node pairs is 70 km. Thus paths in this study are much less affected by fiber impairments than those in the other case studies of Section 4.13.2.2. As a result, the first part of the simulation procedure is easily skipped and the problem becomes an exercise in how best to take advantage of performance versus cost trade-offs: essentially a budgeting approach A

B

25 km

5 km

5 km 45 km

C

D

45 km

50 km

20 km

20 km

E

20 km

F

Figure 4.99 DWDM metro network case study based on the network deployment scenario presented

in Figure 4.98. Nodes represent only superhub stations with typical distances (not drawn to scale). c 2004 IEEE. Used by permission of The Institute (From [Antoniades+04a, Figure 12]. Copyright
of Electrical and Electronics Engineers, Inc.)

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(simulation step 3). We start by focusing on a connection from node A to node D. Path A-B-D is a minimum hop path for this connection, with length 30 km. For the protection path, we select path A-F-D, which spans 70 km. In case this is not available, the reprovisioned protection path will be A-C-E-F-D which is 65 km long. For the purposes of this study the focus is on the worst-case path A-F-D (70 km), and OC-192 services will be engineered based on that. Superhubs are assumed to have similar ROADM equipment whose basic architecture is shown in Figure 4.85 for a typical 4 × 4 design. For this study, 40 channels were used with 100-GHz spacing. Access to 26 channels is assumed on each fiber at the drop side (and similarly for the add side). The 4 × 1 wavelength-selective switches on the output of these devices include the switching, equalization, and blocking capabilities described in Section 4.12.2.1. Insertion loss of 5 dB and in-band cross-talk of −50 dB were used in the simulation models for this ROADM based on work presented in [Marom+02]. For reasons stated above, effects like filter concatenation, cross-talk, and nonlinearities will have significantly less importance than in the previous case study. For example, the number of optical filters through which the signal passes is now only 3, which results in negligible penalty (see [Downie+03]). The cochannel cross-talk Q penalty is budgeted at only 1.0 dB based on the model presented in Section 4.9.1. Fiber nonlinearities are almost negligible for the channel spacing, bit rate, and per-channel launched power in this case (1 dBm/channel) and are budgeted at 0.2 dB. PMD is budgeted at 0.4 dB following the modeling approach presented in Section 4.9.5, and PDL-induced ripple effects and PDL/PMD interactions are ignored considering the small number of optical components that the signal traverses before being regenerated. A safety margin of 2.0 dB is allocated for component aging and factored into the budgeting model. This case study uses OC-192 DML transmitters (Section 4.5.3.1), which are a costeffective choice in metro networks, but which are limited in transmission distance due to dispersion/chirp interaction. Hence, optical dispersion compensators (Section 4.3.2.3) and electronic dispersion compensation modules (Section 4.7.3) are included on the links. Budgeting simulations are run for (1) the case where only optical dispersion compensation is used, and (2) the case where the system designer uses EDCs at the receiver of path A-F-D of Figure 4.99 and engineers all 40 possible channels through it. Figure 4.100 presents the results for the two simulation runs described above. In the case where EDCs are used at the receiver, a phenomenological model for the EDC, derived from [Feuer+03], is used in the simulation procedure. It turns out that EDCs without optical dispersion compensation cannot prolong DML reach beyond 20 km. However, EDCs were shown in this study to provide a 3.2-dB Q margin in dealing with the residual dispersion left by basic optical dispersion compensation modules (no dispersion slope compensation) (see Figure 4.100). This could be allocated to mitigate other impairments such as PMD, which can be significant in networks using older fiber. The performance margin might also be used to relax specifications on individual components. An example would be gain ripple of amplifiers and/or other optical components. The performance evaluation studies presented in this section illustrate the role that simulation can play in estimating the performance of large systems subject to complex transmission and switching impairments. Because simulation is accomplished rapidly and efficiently, the results can be used to provide considerable insight to the network

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20 with EDC 19

18

Performance margin

17

16

without EDC

15

14

Figure 4.100 Simulation results for path A-F-D in Figure 4.99 comparing Q-channel performance c 2004 IEEE. with and without EDC. (Adapted from [Antoniades+04a, Figure 14]. Copyright
Used by permission of The Institute of Electrical and Electronics Engineers, Inc.)

designer. To paraphrase Richard W. Hamming [Hamming62], the purpose of simulation is insight, not numbers.

4.14

Problems 1 What is the critical angle, the acceptance angle, and the numerical aperture of a step-index fiber with a core refractive index of 1.47 and a cladding refractive index of 1.44? 2 (Intermodal dispersion) Consider a multimode step-index fiber of length L and refractive indices of n 1 and n 2 for the core and the cladding, respectively. The broadening of a pulse injected in such a fiber can be estimated by considering the time difference in the propagation of rays following the shortest and the longest paths to the receiver. If this time difference must be limited to a fraction of the bit period, find the maximum bit rate that can be transmitted through a fiber with the following parameters: n 1 = 1.47, n 2 = 1.44, and L = 10 km. 3 (Resolution of the Helmholtz equation) Consider a step-index fiber of radius a and refractive indices of n 1 and n 2 for the core and the cladding, respectively; assume unbounded cladding. Using separation of variables to solve Equation (4.11), show that any field component can be written in the form  A Jl (κr ) exp(− jlφ) exp(− jβz) r ≤ a U (r, φ, z, ω) = B K l (γ r ) exp(− jlφ) exp(− jβz) r > a,

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where A and B are arbitrary amplitude constants, β is the propagation constant, κ 2 =

  2 kn (k 2 − β 2 ), γ 2 = n 12 − β 2 , and Jl (.) and K l (.), respectively, are the Bessel function of the first kind and the modified Bessel function of the second kind of integer order l. (Note that U is a function of ω through the dependence of the solution parameters on the wavenumber k.) 4 For a cascade of N independent sources of PMD, where the ith source contributes an average time differential of τPMD i , the overall average time differential is   N  2 τPMD =  τPMD i. i=1

Suppose an optical path passes through several WSXCs, each one containing a cascade of four lithium niobate switches (τPMD = 2 ps each). What is the maximum number of WSXCs that can be traversed before the PMD becomes significant, if the signals are OC-48? OC-192? 5 A transmission link is composed of fiber with an attenuation of 0.3 dB/km, with EDFAs spaced every 60 km. All EDFAs are identical, with amplifier gain equal to fiber loss on the preceding fiber segment. The amplifier spontaneous emission factor is 1.5. Bandwidths of any optical filters in the link are large compared to the bandwidth (5 GHz) of the low-pass electrical filter preceding the electrical detector. Assume that thermal noise in the receiver is negligible and that the signal-ASE beat noise is dominant. (a) What is the maximum distance a signal with transmitter power of −13 dBm and wavelength of 1550 nm can propagate before the electrical SNR drops below 20 dB? (b) Repeat the calculation if the EDFAs are spaced every 30 km and their gain is adjusted to compensate for the new fiber loss. Assume the same spontaneous emission factor. Comment on the results. 6 (Upper bound on EDFA gain) An optical signal, with average power Psin and wavelength λs and an optical pump with average power Ppin and wavelength λ p , enter an erbium-doped fiber. From energy conservation, show that an upper bound on the EDF gain is G ≤1+

λ p Ppin . λs Psin

7 (APD SNR) A receiver is composed of an avalanche photodiode and a highimpedance front end. The APD is illuminated with constant optical power, P. The average photocurrent is enhanced by an average multiplication factor M¯ compared to the PIN photodiode case. The shot noise generated in the photodetection process can be approximated as white Gaussian noise, with a power-spectral density of q M¯ 2+x R P, where q is the electron charge, R is the PIN photodiode responsivity given by Equation (4.36), and x is an excess-noise factor depending on the material (e.g., x = 0.3 to 0.5 for silicon APDs). Both the shot and thermal noise are filtered by the low-pass filter composed of the parasitic capacitance of the APD and the load resistance.

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Derive the expression for the signal-to-noise ratio at the input of the electronic amplifier and show that there is an optimum value of the average multiplication factor M¯ for which the SNR is maximized. Also show that at the shot-noise limit, the PIN photodiode provides better SNR performance than the APD. 8 Suppose that the n ports of the device shown in Figure 4.54 are grouped into n 1 input ports and n 2 output ports, where n 1 + n 2 = n. Assume that when optical signals are applied to the input ports only, there are no reflected waves at those ports. (a) Derive the relation between the n 2 × n 1 power matrix, A [Equation (2.4)], and the scattering matrix. (b) Now suppose that the input and output ports are interchanged and that when optical signals are applied to only the (new) input ports, there are no reflected waves at those ports. Again, derive the relation between the new power transfer matrix, A , and the scattering matrix. (c) If the device is reciprocal, show that A = At . 9 Consider a passive reciprocal element with n + 1 ports. Suppose that when the first port is used as an input and the others are used as outputs, it acts as a 1:n power splitter. Show that, operated in the opposite direction, it is an n:1 combiner with a combining loss of 1/n. 10 Redo the previous problem in the case in which the element is a 1:n demultiplexer. Show that it is a multiplexer when operated in the opposite direction. 11 (Filter cascading) In large networks, an end-to-end path often traverses many optical filters contained in WSXCs and other components along the path. In this problem, we study the bandwidth-narrowing effects of cascading for two types: FP and MI filters. The (low-pass equivalent) amplitude transfer function of an FP filter can be modeled using a first-order Butterworth function, whereas an MI filter can be modeled using a third-order Butterworth. The general nth order Butterworth function is defined as |H ( f )| =  1+

1  2n , f fc

where f c is the (3 dB) cutoff frequency. (a) Assuming cascades of 10, 20, 30, and 50 filters, plot the overall power transfer function of the cascades in dB for each filter type. (b) Compare the narrowing effects for the two filter types. Which seems more suitable for large-scale networks? (c) What is the cutoff frequency of a chain of MI filters with individual cutoff frequency of 100 GHz? (d) Comment on the effects of random filter center-frequency misalignments on the effective bandwidth of a cascade of filters. Which of the two types do you think would exhibit the best performance as a result of filter misalignments? (e) Comment on how bandwidth narrowing and filter misalignment affect tolerances to laser-tuning errors.

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