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

General

Optical Connectivity

Static

Controllable

Optical Node Functionality

Static Coupler

Station and Overlay Functionality

Network Architecture Broadcast/ Select

Wavelength Router

Space Switch

w-s, w-s-t Switch

Permutation

Generalized

LDC

Purely Optical Wavelength Routed

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