foundation of learning for anyone hoping to contribute to ... · foundation of learning for anyone hoping to contribute to this technology’s rapid evolution. —Scott Clavenna,

The horizons of optical networks are much more than high speed physical layer transport. An intelligent optical network design

must include higher network layer considerations. This is the only book currently on the market that addresses optical networks

from the physical layer to the network layer and should be valuable for those who try to understand the intricacies of what

optical networks can be.

—Vincent Chan, Professor, MIT Department of Electrical Engineering and Computer Science

This book is not only essential reading for anyone in the optical networks industry, it is important. It provides the necessary

foundation of learning for anyone hoping to contribute to this technology’s rapid evolution.

—Scott Clavenna, President, PointEast Research

The authors’ grasp of what is truly workable and worthwhile in optical networks is fundamental, and they have effectively

packaged this knowledge in an easy-to-comprehend text that will be valued to both veterans and those new to optical

networking.

—Scott Grout, President and CEO, Chorum Technologies

This is a comprehensive and authoritative work on optical networks, ranging in scope from components and systems to overall

design principles. I find the book well organized and easy to use, and I particularly like the treatment of network design and

operation. An essential book for anyone seriously interested in optical networks.

—Goff Hill, Chief Network Architect, Altamar Networks, UK

I really enjoy the bottoms-up approach taken by the authors to address fundamentals of optical components as the enablers,

optical transmission system design and engineering as the building blocks, and network architecture and its management

features that deliver applications to the network operators and services providers at the top of the food chain.

—Shoa-Kai Liu, Director of Advanced Technology, Worldcom

This book not only provides the fundamentals and details of photonics, but the pragmatic perspective presented enables

the service provider, the equipment manufacturer, and the academician to view light from a real-life standpoint.

—Mathew Oommen, Vice President, Network Architecture, Williams Communications Group

This book functions as both an introduction to optical networking and as a text to reference again and again. Great for system

designers as well as those marketing and selling those systems. Optical Networks provides theory and applications. While no

text can be truly state-of-the-art in the fast moving area of optical networking, this one comes as close as possible.

—Alan Repech, System Architect, Cisco Systems Optical Transport

This book provides the most comprehensive coverage of both the theory and practice of optical networking. Its up-

to-date coverage makes it an invaluable reference for both practitioners and researchers.

—Suresh Subramaniam, Assistant Professor, Department of Electrical and Computer Engineering, George Washington

University

This book provides an excellent overview of the complex field of optical networking. I especially like how it ties the optical

hardware functionality into the overall networking picture. Everybody who wants to be a player in the optical networking space

should have this book within easy reach.

—Martin Zirngibl, Director, Photonics Network Research, Lucent Technologies, Bell Laboratories

The Morgan Kaufmann Series in Networking Series Editor, David Clark, M.I.T. P2P Networking and Applications John Buford, Heather Yu, and Eng Lua The Illustrated Network Walter Goralski Broadband Cable Access Networks: The HFC Plant David Large and James Farmer Technical, Commercial and Regulatory Challenges of QoS: An Internet Service Model Perspective XiPeng Xiao MPLS: Next Steps Bruce S. Davie and Adrian Farrel Wireless Networking Anurag Kumar, D. Manjunath, and Joy Kuri Internet Multimedia Communications Using SIP Rogelio Martinez Perea Information Assurance: Dependability and Security in Networked Systems Yi Qian, James Joshi, David Tipper, and Prashant Krishnamurthy Network Analysis, Architecture, and Design, 3e James D. McCabe Wireless Communications & Networking: An Introduction Vijay K. Garg IPv6 Advanced Protocols Implementation Qing Li, Tatuya Jinmei, and Keiichi Shima Computer Networks: A Systems Approach, 4e Larry L. Peterson and Bruce S. Davie Network Routing: Algorithms, Protocols, and Architectures Deepankar Medhi and Karthikeyan Ramaswami Deploying IP and MPLS QoS for Multiservice Networks: Theory and Practice John Evans and Clarence Filsfils

Traffic Engineering and QoS Optimization of Integrated Voice & Data Networks Gerald R. Ash IPv6 Core Protocols Implementation Qing Li, Tatuya Jinmei, and Keiichi Shima Smart Phone and Next-Generation Mobile Computing Pei Zheng and Lionel Ni GMPLS: Architecture and Applications Adrian Farrel and Igor Bryskin Content Networking: Architecture, Protocols, and Practice Markus Hofmann and Leland R. Beaumont Network Algorithmics: An Interdisciplinary Approach to Designing Fast Networked Devices George Varghese Network Recovery: Protection and Restoration of Optical, SONET-SDH, IP, and MPLS Jean Philippe Vasseur, Mario Pickavet, and Piet Demeester Routing, Flow, and Capacity Design in Communication and Computer Networks Michał Pióro and Deepankar Medhi Wireless Sensor Networks: An Information Processing Approach Feng Zhao and Leonidas Guibas Communication Networking: An Analytical Approach Anurag Kumar, D. Manjunath, and Joy Kuri The Internet and Its Protocols: A Comparative Approach Adrian Farrel Modern Cable Television Technology: Video, Voice, and Data Communications, 2e Walter Ciciora, James Farmer, David Large, and Michael Adams Policy-Based Network Management: Solutions for the Next Generation John Strassner MPLS Network Management: MIBs, Tools, and Techniques Thomas D. Nadeau Developing IP-Based Services: Solutions for Service Providers and Vendors Monique Morrow and Kateel Vijayananda

Telecommunications Law in the Internet Age Sharon K. Black Optical Networks: A Practical Perspective, 3e Rajiv Ramaswami, Kumar N. Sivarajan, and Galen Sasaki Internet QoS: Architectures and Mechanisms Zheng Wang TCP/IP Sockets in Java: Practical Guide for Programmers Michael J. Donahoo and Kenneth L. Calvert TCP/IP Sockets in C: Practical Guide for Programmers Kenneth L. Calvert and Michael J. Donahoo Multicast Communication: Protocols, Programming, and Applications Ralph Wittmann and Martina Zitterbart High-Performance Communication Networks, 2e Jean Walrand and Pravin Varaiya Internetworking Multimedia Jon Crowcroft, Mark Handley, and Ian Wakeman Understanding Networked Applications: A First Course David G. Messerschmitt Integrated Management of Networked Systems: Concepts, Architectures, and their Operational Application Heinz-Gerd Hegering, Sebastian Abeck, and Bernhard Neumair Virtual Private Networks: Making the Right Connection Dennis Fowler Networked Applications: A Guide to the New Computing Infrastructure David G. Messerschmitt Wide Area Network Design: Concepts and Tools for Optimization Robert S. Cahn For further information on these books and for a list of forthcoming titles, please visit our Web site at http://www.mkp.com.

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Printed in the United States of America

09 10 11 12 13 5 4 3 2 1

To Our Parents

Optical Networks

A Practical Perspective

Third Edition

Rajiv Ramaswami

Kumar N. Sivarajan Galen H. Sasaki

AMSTERDAM • BOSTON • HEIDELBERG • LONDON

NEW YORK • OXFORD • PARIS • SAN DIEGO

SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO

Morgan Kaufmann Publishers is an imprint of Elsevier

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Contents

Foreword xxi

Preface to the First Edition xxv

Preface to the Second Edition xxix

Preface to the Current Edition xxxiii

1 Introduction to Optical Networks 11.1 Telecommunications Network Architecture . . . . . . . . . . . . . . . . . . . . . 21.2 Services, Circuit Switching, and Packet Switching . . . . . . . . . . . . . . . . . 5

1.2.1 The Changing Services Landscape . . . . . . . . . . . . . . . . . . . . . 81.3 Optical Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.3.1 Multiplexing Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 111.3.2 Second-Generation Optical Networks . . . . . . . . . . . . . . . . . . . 13

1.4 The Optical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151.5 Transparency and All-Optical Networks . . . . . . . . . . . . . . . . . . . . . . 221.6 Optical Packet Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241.7 Transmission Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

1.7.1 Wavelengths, Frequencies, and Channel Spacing . . . . . . . . . . . . . 261.7.2 Wavelength Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . 281.7.3 Optical Power and Loss . . . . . . . . . . . . . . . . . . . . . . . . . . 29

1.8 Network Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

ix

x Contents

1.8.1 Early Days—Multimode Fiber . . . . . . . . . . . . . . . . . . . . . . . 301.8.2 Single-Mode Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 331.8.3 Optical Amplifiers and WDM . . . . . . . . . . . . . . . . . . . . . . . 341.8.4 Beyond Transmission Links to Networks . . . . . . . . . . . . . . . . . 37

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

I Technology 45

2 Propagation of Signals in Optical Fiber 472.1 Loss and Bandwidth Windows . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

2.1.1 Bending Loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512.2 Intermodal Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

2.2.1 Geometrical Optics Approach . . . . . . . . . . . . . . . . . . . . . . . 522.2.2 Bit Rate–Distance Limitation . . . . . . . . . . . . . . . . . . . . . . . . 542.2.3 Controlling Intermodal Dispersion: Graded-Index Multimode Fiber . . 552.2.4 Multimode Fiber in Practice . . . . . . . . . . . . . . . . . . . . . . . . 57

2.3 Optical Fiber as a Waveguide . . . . . . . . . . . . . . . . . . . . . . . . . . . . 582.3.1 Wave Theory Approach . . . . . . . . . . . . . . . . . . . . . . . . . . 592.3.2 Fiber Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 632.3.3 Polarization Modes and Polarization-Mode Dispersion . . . . . . . . . 652.3.4 Other Waveguides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

2.4 Chromatic Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 702.4.1 Chirped Gaussian Pulses . . . . . . . . . . . . . . . . . . . . . . . . . . 712.4.2 Controlling the Dispersion: Dispersion-Shifted Fibers . . . . . . . . . . 75

2.5 Nonlinear Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782.5.1 Effective Length and Area . . . . . . . . . . . . . . . . . . . . . . . . . 792.5.2 Stimulated Brillouin Scattering . . . . . . . . . . . . . . . . . . . . . . . 812.5.3 Stimulated Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . 822.5.4 Propagation in a Nonlinear Medium . . . . . . . . . . . . . . . . . . . 832.5.5 Self-Phase Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 852.5.6 SPM-Induced Chirp for Gaussian Pulses . . . . . . . . . . . . . . . . . . 882.5.7 Cross-Phase Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . 902.5.8 Four-Wave Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 922.5.9 Fiber Types to Mitigate Nonlinear Effects . . . . . . . . . . . . . . . . . 95

2.6 Solitons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 992.6.1 Dispersion-Managed Solitons . . . . . . . . . . . . . . . . . . . . . . . 102

2.7 Other Fiber Technologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Contents xi

2.7.1 Photonic Crystal Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . 1032.7.2 Plastic Optical Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

3 Components 1133.1 Couplers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

3.1.1 Principle of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 1163.1.2 Conservation of Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

3.2 Isolators and Circulators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1183.2.1 Principle of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

3.3 Multiplexers and Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1213.3.1 Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1243.3.2 Diffraction Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1283.3.3 Bragg Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1293.3.4 Fiber Gratings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1323.3.5 Fabry-Perot Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1363.3.6 Multilayer Dielectric Thin-Film Filters . . . . . . . . . . . . . . . . . . . 1393.3.7 Mach-Zehnder Interferometers . . . . . . . . . . . . . . . . . . . . . . . 1413.3.8 Arrayed Waveguide Grating . . . . . . . . . . . . . . . . . . . . . . . . 1453.3.9 Acousto-Optic Tunable Filter . . . . . . . . . . . . . . . . . . . . . . . 1493.3.10 High Channel Count Multiplexer Architectures . . . . . . . . . . . . . 154

3.4 Optical Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1573.4.1 Stimulated Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1583.4.2 Spontaneous Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . 1593.4.3 Erbium-Doped Fiber Amplifiers . . . . . . . . . . . . . . . . . . . . . . 1603.4.4 Raman Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1653.4.5 Semiconductor Optical Amplifiers . . . . . . . . . . . . . . . . . . . . . 1673.4.6 Crosstalk in SOAs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

3.5 Transmitters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1723.5.1 Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1723.5.2 Light-Emitting Diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . 1823.5.3 Tunable Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1843.5.4 Direct and External Modulation . . . . . . . . . . . . . . . . . . . . . . 1923.5.5 Pump Sources for Raman Amplifiers . . . . . . . . . . . . . . . . . . . . 196

3.6 Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1983.6.1 Photodetectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1983.6.2 Front-End Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203

xii Contents

3.7 Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2053.7.1 Large Optical Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . 2073.7.2 Optical Switch Technologies . . . . . . . . . . . . . . . . . . . . . . . . 2133.7.3 Large Electronic Switches . . . . . . . . . . . . . . . . . . . . . . . . . . 220

3.8 Wavelength Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2213.8.1 Optoelectronic Approach . . . . . . . . . . . . . . . . . . . . . . . . . . 2223.8.2 Optical Gating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2243.8.3 Interferometric Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 2253.8.4 Wave Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228


4 Modulation and Demodulation 2454.1 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245

4.1.1 Signal Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2464.2 Subcarrier Modulation and Multiplexing . . . . . . . . . . . . . . . . . . . . . . 248

4.2.1 Clipping and Intermodulation Products . . . . . . . . . . . . . . . . . . 2494.2.2 Applications of SCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

4.3 Spectral Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2514.3.1 Optical Duobinary Modulation . . . . . . . . . . . . . . . . . . . . . . 2524.3.2 Optical Single Sideband Modulation . . . . . . . . . . . . . . . . . . . . 2544.3.3 Multilevel Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2554.3.4 Capacity Limits of Optical Fiber . . . . . . . . . . . . . . . . . . . . . . 255

4.4 Demodulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2564.4.1 An Ideal Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2584.4.2 A Practical Direct Detection Receiver . . . . . . . . . . . . . . . . . . . 2594.4.3 Front-End Amplifier Noise . . . . . . . . . . . . . . . . . . . . . . . . . 2604.4.4 APD Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2614.4.5 Optical Preamplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2614.4.6 Bit Error Rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2644.4.7 Coherent Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2694.4.8 Timing Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2714.4.9 Equalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272

4.5 Error Detection and Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . 2734.5.1 Reed-Solomon Codes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2764.5.2 Interleaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279

Contents xiii

Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285

5 Transmission System Engineering 2895.1 System Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2895.2 Power Penalty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2905.3 Transmitter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2925.4 Receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2945.5 Optical Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

5.5.1 Gain Saturation in EDFAs . . . . . . . . . . . . . . . . . . . . . . . . . 2965.5.2 Gain Equalization in EDFAs . . . . . . . . . . . . . . . . . . . . . . . . 2975.5.3 Amplifier Cascades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2995.5.4 Amplifier Spacing Penalty . . . . . . . . . . . . . . . . . . . . . . . . . 3005.5.5 Power Transients and Automatic Gain Control . . . . . . . . . . . . . . 3025.5.6 Lasing Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303

5.6 Crosstalk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3045.6.1 Intrachannel Crosstalk . . . . . . . . . . . . . . . . . . . . . . . . . . . 3055.6.2 Interchannel Crosstalk . . . . . . . . . . . . . . . . . . . . . . . . . . . 3075.6.3 Crosstalk in Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . 3095.6.4 Bidirectional Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3095.6.5 Crosstalk Reduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3115.6.6 Cascaded Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313

5.7 Dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3145.7.1 Chromatic Dispersion Limits: NRZ Modulation . . . . . . . . . . . . . 3155.7.2 Chromatic Dispersion Limits: RZ Modulation . . . . . . . . . . . . . . 3175.7.3 Dispersion Compensation . . . . . . . . . . . . . . . . . . . . . . . . . 3205.7.4 Polarization-Mode Dispersion (PMD) . . . . . . . . . . . . . . . . . . . 325

5.8 Fiber Nonlinearities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3285.8.1 Effective Length in Amplified Systems . . . . . . . . . . . . . . . . . . . 3295.8.2 Stimulated Brillouin Scattering . . . . . . . . . . . . . . . . . . . . . . . 3315.8.3 Stimulated Raman Scattering . . . . . . . . . . . . . . . . . . . . . . . . 3325.8.4 Four-Wave Mixing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3345.8.5 Self-/Cross-Phase Modulation . . . . . . . . . . . . . . . . . . . . . . . 3385.8.6 Role of Chromatic Dispersion Management . . . . . . . . . . . . . . . 340

5.9 Wavelength Stabilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3415.10 Design of Soliton Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3425.11 Design of Dispersion-Managed Soliton Systems . . . . . . . . . . . . . . . . . . 3435.12 Overall Design Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

5.12.1 Fiber Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3475.12.2 Transmit Power and Amplifier Spacing . . . . . . . . . . . . . . . . . . 348

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5.12.3 Chromatic Dispersion Compensation . . . . . . . . . . . . . . . . . . . 3485.12.4 Modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3495.12.5 Nonlinearities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3495.12.6 Interchannel Spacing and Number of Wavelengths . . . . . . . . . . . . 3495.12.7 All-Optical Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3505.12.8 Wavelength Planning . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3515.12.9 Transparency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 353


II Networks 367

6 Client Layers of the Optical Layer 3696.1 SONET/SDH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 371

6.1.1 Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3736.1.2 VCAT and LCAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3776.1.3 SONET/SDH Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3786.1.4 SONET Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . 3796.1.5 SONET/SDH Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . 3846.1.6 Elements of a SONET/SDH Infrastructure . . . . . . . . . . . . . . . . 386

6.2 Optical Transport Network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3896.2.1 Hierarchy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3916.2.2 Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3926.2.3 Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395

6.3 Generic Framing Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3966.4 Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399

6.4.1 Frame Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4026.4.2 Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4036.4.3 Ethernet Physical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . 4066.4.4 Carrier Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407

6.5 IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4116.5.1 Routing and Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . . 4136.5.2 Quality of Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414

6.6 Multiprotocol Label Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4156.6.1 Labels and Forwarding . . . . . . . . . . . . . . . . . . . . . . . . . . . 4176.6.2 Quality of Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4196.6.3 Signaling and Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . 420

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6.6.4 Carrier Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4206.7 Resilient Packet Ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 421

6.7.1 Quality of Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4226.7.2 Node Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4236.7.3 Fairness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424

6.8 Storage-Area Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4256.8.1 Fibre Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426


7 WDM Network Elements 4337.1 Optical Line Terminals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4367.2 Optical Line Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4387.3 Optical Add/Drop Multiplexers . . . . . . . . . . . . . . . . . . . . . . . . . . . 438

7.3.1 OADM Architectures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4417.3.2 Reconfigurable OADMs . . . . . . . . . . . . . . . . . . . . . . . . . . 447

7.4 Optical Crossconnects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4527.4.1 All-Optical OXC Configurations . . . . . . . . . . . . . . . . . . . . . . 458


8 Control and Management 4698.1 Network Management Functions . . . . . . . . . . . . . . . . . . . . . . . . . . 469

8.1.1 Management Framework . . . . . . . . . . . . . . . . . . . . . . . . . . 4718.1.2 Information Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4738.1.3 Management Protocols . . . . . . . . . . . . . . . . . . . . . . . . . . . 474

8.2 Optical Layer Services and Interfacing . . . . . . . . . . . . . . . . . . . . . . . 4768.3 Layers within the Optical Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . 4788.4 Multivendor Interoperability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4798.5 Performance and Fault Management . . . . . . . . . . . . . . . . . . . . . . . . 481

8.5.1 The Impact of Transparency . . . . . . . . . . . . . . . . . . . . . . . . 4818.5.2 BER Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4828.5.3 Optical Trace . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4838.5.4 Alarm Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4838.5.5 Data Communication Network (DCN) and Signaling . . . . . . . . . . 4858.5.6 Policing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 487

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8.5.7 Optical Layer Overhead . . . . . . . . . . . . . . . . . . . . . . . . . . 4878.5.8 Client Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 492

8.6 Configuration Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4938.6.1 Equipment Management . . . . . . . . . . . . . . . . . . . . . . . . . . 4938.6.2 Connection Management . . . . . . . . . . . . . . . . . . . . . . . . . . 4948.6.3 Adaptation Management . . . . . . . . . . . . . . . . . . . . . . . . . . 499

8.7 Optical Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5018.7.1 Open Fiber Control Protocol . . . . . . . . . . . . . . . . . . . . . . . . 503


9 Network Survivability 5119.1 Basic Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5139.2 Protection in SONET/SDH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 518

9.2.1 Point-to-Point Links . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5189.2.2 Self-Healing Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5219.2.3 Unidirectional Path-Switched Rings . . . . . . . . . . . . . . . . . . . . 5239.2.4 Bidirectional Line-Switched Rings . . . . . . . . . . . . . . . . . . . . . 5259.2.5 Ring Interconnection and Dual Homing . . . . . . . . . . . . . . . . . . 530

9.3 Protection in the Client Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5329.3.1 Protection in Resilient Packet Rings . . . . . . . . . . . . . . . . . . . . 5339.3.2 Protection in Ethernet . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5349.3.3 Protection in IP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5369.3.4 Protection in MPLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 538

9.4 Why Optical Layer Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5419.4.1 Service Classes Based on Protection . . . . . . . . . . . . . . . . . . . . 548

9.5 Optical Layer Protection Schemes . . . . . . . . . . . . . . . . . . . . . . . . . . 5499.5.1 1 + 1 OMS Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . 5529.5.2 1:1 OMS Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5529.5.3 OMS-DPRing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5529.5.4 OMS-SPRing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5539.5.5 1:N Transponder Protection . . . . . . . . . . . . . . . . . . . . . . . . 5539.5.6 1 + 1 OCh Dedicated Protection . . . . . . . . . . . . . . . . . . . . . . 5539.5.7 OCh-SPRing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5579.5.8 OCh-Mesh Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . 5579.5.9 GMPLS Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563

9.6 Interworking between Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 564Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 565

Contents xvii

Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 566Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 567References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 569

10 WDM Network Design 57310.1 Cost Trade-Offs: A Detailed Ring Network Example . . . . . . . . . . . . . . . 57710.2 LTD and RWA Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 584

10.2.1 Lightpath Topology Design . . . . . . . . . . . . . . . . . . . . . . . . . 58510.2.2 Routing and Wavelength Assignment . . . . . . . . . . . . . . . . . . . 59010.2.3 Wavelength Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . 593

10.3 Dimensioning Wavelength-Routing Networks . . . . . . . . . . . . . . . . . . . 59610.4 Statistical Dimensioning Models . . . . . . . . . . . . . . . . . . . . . . . . . . . 599

10.4.1 First-Passage Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60010.4.2 Blocking Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 601

10.5 Maximum Load Dimensioning Models . . . . . . . . . . . . . . . . . . . . . . . 60910.5.1 Offline Lightpath Requests . . . . . . . . . . . . . . . . . . . . . . . . . 61010.5.2 Online RWA in Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . 615


11 Access Networks 62911.1 Network Architecture Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 63111.2 Enhanced HFC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63611.3 Fiber to the Curb (FTTC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638

11.3.1 PON Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 648Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 649Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 651

12 Photonic Packet Switching 65312.1 Optical Time Division Multiplexing . . . . . . . . . . . . . . . . . . . . . . . . . 658

12.1.1 Bit Interleaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66012.1.2 Packet Interleaving . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66112.1.3 Optical AND Gates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 665

12.2 Synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66812.2.1 Tunable Delays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67012.2.2 Optical Phase Lock Loop . . . . . . . . . . . . . . . . . . . . . . . . . . 671

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12.3 Header Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67312.4 Buffering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 674

12.4.1 Output Buffering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67612.4.2 Input Buffering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67712.4.3 Recirculation Buffering . . . . . . . . . . . . . . . . . . . . . . . . . . . 67812.4.4 Using Wavelengths for Contention Resolution . . . . . . . . . . . . . . 68012.4.5 Deflection Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683

12.5 Burst Switching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68812.6 Testbeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 689

12.6.1 KEOPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69012.6.2 NTT’s Optical Packet Switches . . . . . . . . . . . . . . . . . . . . . . . 69112.6.3 BT Labs Testbeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69312.6.4 Princeton University Testbed . . . . . . . . . . . . . . . . . . . . . . . . 69312.6.5 AON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69412.6.6 CORD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 694


13 Deployment Considerations 70713.1 The Evolving Telecommunications Network . . . . . . . . . . . . . . . . . . . . 707

13.1.1 The SONET/SDH Core Network . . . . . . . . . . . . . . . . . . . . . 70913.1.2 Architectural Choices for Next-Generation Transport Networks . . . . 712

13.2 Designing the Transmission Layer . . . . . . . . . . . . . . . . . . . . . . . . . . 71813.2.1 Using SDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71913.2.2 Using TDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72013.2.3 Using WDM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72113.2.4 Unidirectional versus Bidirectional WDM Systems . . . . . . . . . . . . 72213.2.5 Long-Haul Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72413.2.6 Long-Haul Network Case Study . . . . . . . . . . . . . . . . . . . . . . 72513.2.7 Long-Haul Undersea Networks . . . . . . . . . . . . . . . . . . . . . . 73213.2.8 Metro Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73413.2.9 Metro Ring Case Study . . . . . . . . . . . . . . . . . . . . . . . . . . . 73613.2.10 From Opaque Links to Agile All-Optical Networks . . . . . . . . . . . 738


Contents xix

A Acronyms 747

B Symbols and Parameters 757

C Standards 761C.1 International Telecommunications Union (ITU-T) . . . . . . . . . . . . . . . . . 761

C.1.1 Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 761C.1.2 SDH (Synchronous Digital Hierarchy) . . . . . . . . . . . . . . . . . . . 761C.1.3 Optical Networking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 762C.1.4 Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 762

C.2 Telcordia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763C.2.1 Physical and Environmental . . . . . . . . . . . . . . . . . . . . . . . . 763C.2.2 SONET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763C.2.3 Optical Networking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764

C.3 American National Standards Institute (ANSI) . . . . . . . . . . . . . . . . . . . 764C.3.1 SONET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764C.3.2 Fibre Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 764

D Wave Equations 765

E Pulse Propagation in Optical Fiber 769E.1 Propagation of Chirped Gaussian Pulses . . . . . . . . . . . . . . . . . . . . . . 772E.2 Nonlinear Effects on Pulse Propagation . . . . . . . . . . . . . . . . . . . . . . . 773E.3 Soliton Pulse Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777

F Nonlinear Polarization 779

G Multilayer Thin-Film Filters 781G.1 Wave Propagation at Dielectric Interfaces . . . . . . . . . . . . . . . . . . . . . . 781G.2 Filter Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 785References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 788

H Random Variables and Processes 789H.1 Random Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 789

H.1.1 Gaussian Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 790H.1.2 Maxwell Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791H.1.3 Poisson Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 791

H.2 Random Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 792H.2.1 Poisson Random Process . . . . . . . . . . . . . . . . . . . . . . . . . . 793

xx Contents

H.2.2 Gaussian Random Process . . . . . . . . . . . . . . . . . . . . . . . . . 794Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794

I Receiver Noise Statistics 795I.1 Shot Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 797I.2 Amplifier Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 798References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 800

J Asynchronous Transfer Mode 801J.1 Functions of ATM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 802

J.1.1 Connections and Cell Forwarding . . . . . . . . . . . . . . . . . . . . . 803J.1.2 Virtual Paths . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 804

J.2 Adaptation Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805J.2.1 AAL-1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 805J.2.2 AAL-5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 806

J.3 Quality of Service . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 806J.4 Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807J.5 Signaling and Routing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 807

Bibliography 809

Index 845

Forewordby Paul E. Green, Jr.

Director, Optical Network Technology

Tellabs, Inc.

Not too many years ago, whenever one wanted to send messages effectively, therewere really only two choices—send them by wire or send them by radio. This situationlasted for decades until the mid-1960s, when the fiber optics revolution began, quietlyat first, and then with increasing force as people began to appreciate that sendingpulses of light through tiny strands of glass wasn’t so crazy after all. This revolutionis now in full cry, with 4000 strand miles of fiber being installed per day, justin the United States alone. Fiber has been displacing wire in many applications,and gradually it is emerging as one of the two dominant Cinderella transmissiontechnologies of today, wireless being the other. One of these (wireless) goes anywherebut doesn’t do much when it gets there, whereas the other (fiber) will never goeverywhere but does a great deal indeed wherever it reaches. From the earliest daysof fiber communication, people realized that this simple glass medium has incredibleamounts of untapped bandwidth capacity waiting to be mined, should the day comewhen we would actually need it, and should we be able to figure out how to tap it.That day has now come. The demand is here and so are the solutions.

This book describes a revolution within a revolution, the opening up of thecapacity of the now-familiar optical fiber to carry more messages, handle a widervariety of transmission types, and provide improved reliabilities and ease of use.In many places where fiber has been installed simply as a better form of copper,even the gigabit capacities that result have not proved adequate to keep up withthe demand. The inborn human voracity for more and more bandwidth, plus thegrowing realization that there are other flexibilities to be had by imaginative use ofthe fiber, have led people to explore all-optical networks, the subject of this book.

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xxii Foreword

Such networks are those in which either wavelength division or time division is usedin new ways to form entire network structures where the messages travel in purelyoptical form all the way from one user location to another.

When I attempted the same kind of book in 1993, nobody was quite sure whetheroptical networking would be a roaring success or disappear into the annals of “what-ever happened to . . .” stories of technology that had once sounded great on paper,but that had somehow never panned out in the real world. My book (Fiber OpticNetworks, Prentice Hall) spent most of its pages talking about technology buildingblocks and lamenting their limitations since there was little to say about real net-works, the architectural considerations underlying them, and what good they hadever done anybody.

In the last four years, optical networking has indeed really happened, essentiallyall of it based on wavelength division multiplexing, and with this book Ramaswamiand Sivarajan, two of the principal architects of this success, have redressed theinsufficiencies of earlier books such as mine. Today, hundreds of millions of dol-lars of wavelength division networking systems are being sold annually, major newbusinesses have been created that produce nothing but optical networks, and band-width bottlenecks are being relieved and proliferating protocol zoos tamed by thisremarkably transparent new way of doing networking; what’s more, there is a richarchitectural understanding of where to go next. Network experts, fresh from thenovelties of such excitements as the Web, now have still another wonderful toy shopto play in. The whole optical networking idea is endlessly fascinating in itself—basedon a medium with thousands of gigabits of capacity yet so small as to be almost in-visible, transmitters no larger than a grain of salt, amplifiers that amplify vast chunksof bandwidth purely as light, transmission designs that bypass 50 years of hard-wonbut complex coding, modulation and equalization insights, network architecturesthat subsume many functions usually done more clumsily in the lower layers of clas-sical layered architectures—these are all fresh and interesting topics that await thereader of this book.

To understand this new networking revolution within a revolution, it is neces-sary to be led with a sure hand through territory that to many will be unfamiliar.The present authors, with their rare mixture of physics and network architectureexpertise, are eminently qualified to serve as guides. After spending some time withthis book, you will be more thoroughly conversant with all the important issues thattoday affect how optical networks are made, what their limitations and potentialitiesare, and how they fit in with more classical forms of communication networks basedon electronic time division. Whether you are a computer network expert wonderinghow to use fiber to break the bandwidth bottlenecks that are limiting your system ca-pabilities, a planner or implementer trying to future-proof your telephone network,

Foreword xxiii

a teacher planning a truly up-to-date communication engineering curriculum, a stu-dent looking for a fun lucrative career, or a midcareer person in need of a retread,this volume will provide the help you need.

The authors have captured what is going on and what is going to be going on inthis field in a completely up-to-date treatment unavailable elsewhere. I learned a lotfrom reading it and expect that you will too.

Preface to the FirstEdition

Fiber optics has become the core of our telecommunications and data networkinginfrastructures. Optical fiber is the preferred means of transmission for any data overa few tens of megabits per second and over anything from a kilometer and upwards.The first generation of fiber optic networks used optical fiber predominantly as a re-placement for copper cable for transmission at higher bit rates over longer distances.The second generation of fiber optic networks is just emerging. These networks re-ally exploit the capacity of fiber to achieve overall transmission capacities of severaltens of gigabits per second to terabits per second. Moreover, they exploit routingand switching of signals in the optical domain. The rapid evolution of technology,coupled with the insatiable demand for bandwidth, is resulting in a rapid transitionof these networks from research laboratories into the marketplace.

The fundamentals of optical fiber transmission are covered well in severalbooks. There is, however, a need for a book that covers the transmission aspectsof second-generation fiber optic networks, and focuses on the networking aspectssuch as architectures, and control and management issues. Such a book would notbe complete without describing the components needed to build these networks, par-ticularly since the network architectures strongly depend on these components, anda person designing optical networks will need to be familiar with their capabilities.Thus this book attempts to cover components, transmission, and networking issuesrelated to second-generation optical networks. It is targeted at professionals who arenetwork planners, designers or operators, graduate students in electrical engineeringand computer science, and engineers wanting to learn about optical networks.

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xxvi Preface to the First Edition

Teaching and Learning from This Book

This book can be used as a textbook for graduate courses in electrical engineer-ing or computer science. Much of the material in this book has been covered incourses taught by us. Part I covers components and transmission technology aspectsof optical networking, and Part II deals with the networking aspects. To understandthe networking issues in Part II, students will require a basic undergraduate-levelknowledge of communication networks and probability. We have tried to make thetransmission-related chapters in Part I of the book accessible to networking profes-sionals. For example, components are treated first in a simple qualitative mannerfrom the viewpoint of a network designer, but their principle of operation is thenexplained in detail. Some prior knowledge of electromagnetics will be useful in un-derstanding the detailed quantitative treatment in some of the sections. Advancedsections are marked by an asterisk; these sections can be omitted without loss ofcontinuity.

With this background, the book can be the basis for a graduate course in an elec-trical engineering curriculum. Alternatively, a graduate course in a computer sciencedepartment might emphasize network architectures and control and management,by focusing on Part II, and skim over the technology portions of the book in PartI. Likewise, a course on optical transmission in an electrical engineering departmentmight instead focus on Part I and omit the remaining chapters. Each chapter is ac-companied by a number of problems, and instructors may obtain a solution manualby contacting the publisher at [email protected].

Second, we have attempted to provide an overview of much recent work inthis emerging field, so as to make the book useful to researchers in the field as anup-to-date reference. Each chapter includes an extensive list of references for thosewho might wish to explore further. The problems include some research topics forfurther exploration as well. Finally, we hope that the book will also serve as anintroduction to people working in other areas who wish to become familiar withfiber optics.

Overview of the Book

Chapter 1 offers an introduction to optical networks. Part I of the book is devotedto the technology underlying optical networks. Chapter 2 describes how light prop-agates in optical fiber, and deals with the phenomena of loss, dispersion, and fibernonlinearities, which play a major role in the design of transmission systems. Chap-ter 3 provides an overview of the different components needed to build a network,such as transmitters, receivers, multiplexers, and switches. Chapter 4 describes how

Preface to the First Edition xxvii

electrical signals are converted to light signals (the modulation process) at the trans-mitter and how they are recovered at the receiver (demodulation). Chapter 5 focuseson the physical layer design of the latest generation of transmission systems andnetworks, and the factors limiting the system performance.

Part II is devoted to a variety of networking aspects of optical networks. Chap-ter 6 describes the different first-generation optical networks that are deployed widelytoday. Chapter 7 covers broadcast and select WDM networks that are suitable forLANs and MANs. Different topologies, media-access, and scheduling methods willbe described and compared in a uniform framework. Chapter 8 describes networksusing wavelength routing. These networks are emerging from the laboratories intocommercial deployment. The chapter covers the architectural aspects of these net-works and focuses on the key design issues. Chapter 9 describes how to overlay virtualnetworks, for example, IP or ATM networks over an underlying second-generationoptical network. Chapter 10 covers control and management, including connectionmanagement, fault management, and safety management. Chapter 11 describes sev-eral significant experimental wavelength routing demonstrations, field trials, and pro-totypes. Chapter 12 describes passive optical network solutions for fiber-to-the-curband fiber-to-the-home access network applications. Chapter 13 covers the issues as-sociated with deploying the new second-generation technology in different types oftelecommunications networks. Chapter 14 covers optical time division multiplexednetworks, which are today in the research labs but offer future potential for trans-mission at very high rates on each WDM channel.

The appendices cover some of the basics of stochastic processes and graph theoryfor readers as background material for the book. The large number of symbols andparameters used in Part I (Technology) is also summarized in an appendix.

Acknowledgments

First and foremost, we would like to thank Paul Green for introducing us to thisfield and being our mentor over the years, as well as for writing the foreword to thisbook. We would like to acknowledge, in particular, Rick Barry, Ori Gerstel, AshishVengsarkar, Weyl-Kuo Wang, and Chaoyu Yue for their detailed reviews and discus-sions of part or all of the material in the book. In addition, we would like to thankVenkat Anatharam, Dan Blumenthal, Kamal Goel, Karen Liu, Roger Merel, RickNeuner, and Niall Robinson for their comments. We would also like to thank RajeshM. Krishnaswamy for performing one of the simulations in Section 10.2.2, A. Sel-varajan for answering some of our technology-related questions, and ChandrikaSridhar for helping with the preparation of the solutions manual.

xxviii Preface to the First Edition

We would also like to thank the folks at Morgan Kaufmann; in particular, oureditor, Jennifer Mann, for guiding us through the entire process from start to finishand for her efforts to improve the quality of our book, and our production editor,Cheri Palmer, for orchestrating the production of the book.

Finally, we’d like to acknowledge the invaluable support given to us by our wives,Uma and Vinu, during this endeavor, and to Uma for drawing many of the figures inthe book.

Preface to the SecondEdition

Since the first edition of this book appeared in February 1998, we have witnessed adramatic explosion in optical networking. Optical networking used to be confinedto a fairly small community of researchers and engineers but is now of great interestto a broad audience including students; engineers in optical component, equipment,and service provider companies; network planners; investors; venture capitalists; andindustry and investment analysts.

With the rapid pace in technological advances and the widespread deployment ofoptical networks over the past three years, the need for a second edition of this bookbecame apparent. In this edition we have attempted to include the latest advances inoptical networks and their underlying technologies. We have also tried to make thebook more accessible to a broader community of people interested in learning aboutoptical networking. With this in mind, we have rewritten several chapters, added alarge amount of new material, and removed some material that is not as relevantto practical optical networks. We have also updated the references and added somenew problems.

The major changes we’ve made are as follows: We have mostly rewritten theintroduction to reflect the current understanding of optical networks, and we’veadded a section called “Transmission Basics” to introduce several terms commonlyused in optical networking and wavelength division multiplexing (WDM) to thelayperson.

In Chapter 2, we’ve added significant sections on dispersion management andsolitons, along with a section describing the different fiber types now available.

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In Chapter 3, we now cover electro-absorption modulated lasers, tunable lasers,Raman amplifiers, and L-band erbium-doped fiber amplifiers, and we have signifi-cantly expanded the section on optical switching to include the new types of switchesusing micro-electro-mechanical systems (MEMS) and other technologies.

In Chapter 4, we cover return-to-zero modulation and other newer modulationformats such as duobinary, as well as forward error correction, now widely used inhigh-bit-rate systems. Chapter 5 now includes expanded coverage of chromatic dis-persion and polarization effects, which are important factors influencing the designof high-bit-rate long-haul systems.

The networking chapters of the book have been completely rewritten and ex-panded to reflect the signficant progress made in this area. We have organized thesechapters as follows: Chapter 6 now includes expanded coverage of SONET/SDH,ATM, and IP networks. Chapter 7 is devoted to architectural considerations un-derlying WDM network elements. Chapter 8 attempts to provide a unified viewof the problems associated with network design and routing in optical networks.Chapter 9 provides significantly expanded coverage of network management andcontrol. We have devoted Chapter 10 to network survivability, with a detaileddiscussion on optical layer protection. Chapter 11 covers access networks witha focus on emerging passive optical networks (PONs). Chapter 12 provides up-dated coverage of optical packet-switched networks. Finally, Chapter 13 focuseson deployment considerations and is intended to provide the reader with a broadunderstanding of how telecommunications networks are evolving. It includes a cou-ple of detailed network planning case studies on a typical long-haul and metronetwork.

There is currently a great deal of standards activity in this field. We’ve added anappendix listing the relevant standards. We have also added another appendix listingthe acronyms used in the book and moved some of the more advanced material onpulse propagation into an appendix.

While we have mostly added new material, we have also removed some chapterspresent in the first edition. We have eliminated the chapter on broadcast-and-selectnetworks, as these networks are mostly of academic interest today. Likewise, wealso removed the chapter describing optical networking testbeds as they are mostlyof historical importance at this point. Interested readers can obtain a copy of thesechapters on the Internet at www.mkp.com/opticalnet2.

Teaching and Learning from This Book

This book can be used as a textbook for graduate courses in electrical engineeringor computer science. Much of the material in this book has been covered in coursestaught by us. Chapters 2–5 cover components and transmission technology aspects of

Preface to the Second Edition xxxi

optical networking, and Chapters 6–13 deal with the networking aspects. To under-stand the networking issues, students will require a basic undergraduate-level knowl-edge of communication networks. We have tried to make the transmission-relatedchapters of the book accessible to networking professionals. For example, compo-nents are treated first in a simple qualitative manner from the viewpoint of a net-work designer, but their principle of operation is then explained in detail. Some priorknowledge of semiconductors and electromagnetics will be helpful in appreciatingthe detailed treatment in some of the sections.

Readers wishing to obtain a broad understanding of the major aspects of opticalnetworking can read Chapters 1, 6, 7, and 13. Those interested in getting a basicappreciation of the underlying components and transmission technologies can readthrough Chapters 1–5, skipping the quantitative sections.

The book can be the basis for a graduate course in an electrical engineering orcomputer science curriculum. A networks-oriented course might emphasize networkarchitectures and control and management, by focusing on Chapters 6–13, and skimover the technology portions of the book. Likewise, a course on optical transmissionin an electrical engineering department might instead focus on Chapters 2–5 andomit the remaining chapters. Each chapter is accompanied by a number of prob-lems, and instructors may obtain a solution manual by contacting the publisher [email protected].

Acknowledgments

We were fortunate to have an outstanding set of reviewers who made a significant ef-fort in reading through the chapters in detail and providing us with many suggestionsto improve the coverage and presentation of material. They have been invaluable inshaping this edition. Specifically, we would like to thank Paul Green, Goff Hill, DavidHunter, Rao Lingampalli, Alan McGuire, Shawn O’Donnell, Walter Johnstone, AlanRepech, George Stewart, Suresh Subramaniam, Eric Verillow, and Martin Zirngibl.In addition, we would like to acknowledge Bijan Raahemi, Jim Refi, Krishna Thya-garajan, and Mark R. Wilson who provided inputs and comments on specific topicsand pointed out some mistakes in the first edition. Mark R. Wilson was kind enoughto provide us with several applications-oriented problems from his class, which wehave included in this edition. We would also like to thank Amit Agarwal, ShyamIyer, Ashutosh Kulshreshtha, and Sarath Kumar for the use of their mesh networkdesign tool, Ashutosh Kulshreshtha for also computing the detailed mesh networkdesign example, Tapan Kumar Nayak for computing the lightpath topology designexample, Parthasarathi Palai for simulating the EDFA gain curves, and Rajeev Royfor verifying some of our results. As always, we take responsibility for any errorsor omissions and would greatly appreciate hearing from you as you discover them.Please email your comments to [email protected].

Preface to the CurrentEdition

Optical networking has matured considerably since the publication of the last editionof this book in 2002. A host of new technologies including reconfigurable opticaladd/drop multiplexers and sophisticated modulation formats are now mainstream,and there has been a significant shift in telecommunications networks migrating toa packet-over-optical infrastructure. We have incorporated many of these into thisrevised edition.

In Chapter 2, we expanded the discussion on multimode fiber and added sectionson photonic crystal and plastic fibers. Chapter 6 has been rewritten with new sectionson Generic Framing Procedure, Optical Transport Network, and Resilient PacketRing (RPR). The coverage of Synchronous Optical Networks (SONET) now includesVirtual Concatenation (VCAT) and the Link Capacity Adjustment Scheme (LCAS).There is also expanded coverage of Ethernet and Multiprotocol Label Switching(MPLS) that includes the development of these technologies to support carrier gradeservice. Chapter 7 is devoted to architectural considerations underlying WavelengthDivision Multiplexing (WDM) network elements, and we have updated the sectionon Reconfigurable Optical Add Drop Multiplexers (ROADMs). Chapter 8 reflectsthe changes in network management and control, including more discussion onpacket transport considerations. Chapter 9 includes network survivability of clientlayer protocols such as Ethernet, MPLS, and RPR, which is important to understandthe role of optical networks in survivability.

As with the previous editions, this book is intended to for use by a broad au-dience including students, engineers in optical component, equipment, and serviceprovider companies, network planners, investors, venture capitalists, and indus-try and investment analysts. It can be used as a textbook for graduate courses in

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electrical engineering or computer science. Please see the section “Teaching andLearning from This Book" on page xxx for some guidance on this. Instructors canobtain a solutions manual by contacting the publisher through the book’s web page,www.elsevierdirect.com/9780123740922.

We would like to acknowledge the invaluable assistance provided by Karen Liuin revising Chapter 2, especially the sections on multimode, photonic crystal andplastic fibers. We would also like to thank Ori Gerstel for insightful discussions onoptical networks and Parthasarathi Palai for inputs on the DWDM network casestudies.

1c h a p t e r

Introduction to OpticalNetworks

A s we begin the new millennium, we are seeing dramatic changes in thetelecommunications industry that have far-reaching implications for ourlifestyles. There are many drivers for these changes. First and foremost is the con-tinuing, relentless need for more capacity in the network. This demand is fueled bymany factors. The tremendous growth of the Internet and the World Wide Web, bothin terms of number of users and the amount of time, and thus bandwidth taken byeach user, is a major factor. Internet traffic has been growing rapidly for many years.Estimates of growth have varied considerably over the years, with some early growthestimates showing a doubling every four to six months. Despite the variations, thesegrowth estimates are always high, with more recent estimates at about 50% annu-ally. Meanwhile, broadband access technologies such as digital subscriber line (DSL)and cable modems, which provide bandwidths per user on the order of 1 Mb/s, hasbeen deployed widely. For example, in 2008 about 55% of the adults in the UnitedStates had broadband access at home, while only 10% had access through dialuplines of 28–56 kb/s. Fiber to the home has shown steady growth with Asian marketsshowing the highest market penetration.

At the same time, businesses today rely on high-speed networks to conduct theirbusinesses. These networks are used to interconnect multiple locations within acompany as well as between companies for business-to-business transactions. Largecorporations that used to lease 155 Mb/s lines to interconnect their internal sites arecommonly leasing 1 Gb/s connections today.

There is also a strong correlation between the increase in demand and the costof bandwidth. Technological advances have succeeded in continously reducing the

1

2 Introduction to Optical Networks

cost of bandwidth. This reduced cost of bandwidth in turn spurs the development ofa new set of applications that make use of more bandwidth and affects behavioralpatterns. A simple example is that as phone calls get cheaper, people spend more timeon the phone. This development in turn drives the need for more bandwidth in thenetwork. This positive feedback cycle shows no sign of abating in the near future.

Another factor causing major changes in the industry is the deregulation of thetelephone industry. It is a well-known fact that monopolies impede rapid progress.Monopolistic companies can take their time adapting to changes and have no incen-tive to reduce costs and provide new services. Deregulation of these monopolies hasstimulated competition in the marketplace, which in turn has resulted in lower coststo end users and faster deployment of new technologies and services. Deregulationhas also resulted in creating a number of new start-up service providers as well asstart-up companies providing equipment to these service providers.

Also, traffic in a network is dominated by data as opposed to traditional voicetraffic. In the past, the reverse was true, and so legacy networks were designed toefficiently support voice rather than data. Today, data transport services are perva-sive and are capable of providing quality of service to carry performance sensitiveapplications such as real-time voice and video.

These factors have driven the development of high-capacity optical networks andtheir remarkably rapid transition from the research laboratories into commercialdeployment. This book aims to cover optical network technologies, systems, andnetworking issues, as well as economic and other deployment considerations.

1.1 Telecommunications Network ArchitectureOur focus in this book is primarily on the so-called public networks, which arenetworks operated by service providers, or carriers, as they are often called. Carriersuse their network to provide a variety of services to their customers. Carriers usedto be essentially telephone companies, but today there are many different breedsof carriers operating under different business models, many of whom do not evenprovide telephone service. In addition to the traditional carriers providing telephoneand leased line services, today there are carriers who are dedicated to interconnectingInternet service providers (ISPs), carriers that are in the business of providing bulkbandwidth to other carriers, and even virtual carriers that provide services withoutowning any infrastructure.

In many cases, the carrier owns the facilities (for example, fiber links) and equip-ment deployed inside the network. Building fiber links requires right-of-way priv-ileges. Not anybody can dig up streets! Fiber is deployed in many different ways

1.1 Telecommunications Network Architecture 3

today—buried underground, strung on overhead poles, and buried beside oil andgas pipelines and railroad tracks. In other cases, carriers may lease facilities fromother carriers and in turn offer value-added services using these facilities. For exam-ple, a long-distance phone service provider may not own a network at all but rathersimply buy bandwidth from another carrier and resell it to end users in smallerportions.

A local-exchange carrier (LEC) offers local services in metropolitan areas, and aninterexchange carrier (IXC) offers long-distance services. This distinction is blurringrapidly as LECs expand into long distance and IXCs expand into local services.In order to understand this better, we need to step back and look at the history ofderegulation in the telecommunications services industry. In the United States, before1984, there was one phone company—AT&T. AT&T, along with the local Belloperating companies, which it owned, held a monopoly for both long-distance andlocal services. In 1984, with the passing of the telecommunications deregulation act,the overall entity was split into AT&T, which could offer only long-distance services,and a number of “baby” Bells, or regional Bell operating companies (RBOCs),which offered local services and were not allowed to offer long-distance services.Long-distance services were deregulated, and many other companies, such as MCIand Sprint, successfully entered the long-distance market. The baby Bells came to beknown as the incumbent LECs (ILECs) and were still monopolies within their localregions. There has been considerable consolidation in the industry, where RBOCshave even acquired long-distance companies. For example, RBOC Southwestern BellCommunications acquired AT&T to form AT&T Inc., and Verizon Communications(formerly the RBOC Bell Atlantic) acquired MCI. Today, the RBOCs are under threecompanies: AT&T Inc., Verizon, and Qwest. In addition to the RBOCs, there areother competitive LECs (CLECs) that are less regulated and compete with the RBOCsto offer local services.

The terminology used above is prevalent mostly in North America. In Europe, wehad a similar situation where the government-owned postal, telephone, and telegraph(PTT) companies held monopolies within their respective countries. Over the pastdecade, deregulation has set in, and we now have a number of new carriers in Europeoffering both local and long-distance services.

In the rest of the book, we will take a more general approach and classify carriersas metro carriers or long-haul carriers. Although the same carrier may offer metroand long-haul services, the networks used to deliver long-haul services are somewhatdifferent from metro networks, and so it is useful to keep this distinction.

In contrast to public networks, private networks are networks owned and oper-ated by corporations for their internal use. Many of these corporations in turn relyon capacity provided by public networks to implement their private networks, par-ticularly if these networks cross public land where right-of-way permits are required


to construct networks. Networks within buildings spanning at most a few kilometersare called local-area networks (LANs); those that span a campus or metropolitanarea, typically tens to a few hundred kilometers, are called metropolitan-area net-works (MANs); and networks that span even longer distances, ranging from severalhundred to thousands of kilometers, are called wide-area networks (WANs). We willalso see a similar type of classification used in public networks, which we study next.

Figure 1.1 shows an overview of a typical public fiber network architecture. Thenetwork is vast and complex, and different parts of the network may be owned andoperated by different carriers. The nodes in the network are central offices, sometimesalso called points of presence (POPs). (In some cases, POPs refer to “small” nodesand hubs refer to “large” nodes.) The links between the nodes consist of fiber pairsand, in many cases, multiple fiber pairs. Links in the long-haul network tend to bevery expensive to construct. For this reason, the topology of many North Americanlong-haul networks is fairly sparse. In Europe, the link lengths are shorter, and thelong-haul network topologies tend to be denser. At the same time, it is imperativeto provide alternate paths for traffic in case some of the links fail. These constraintshave resulted in the widespread deployment of ring topologies, particularly in NorthAmerica. Rings are sparse (only two links per node) but still provide an alternatepath to reroute traffic. In many cases, a meshed network is actually implemented inthe form of interconnected ring networks.

At a high level, the network can be broken up into a metropolitan (or metro)network and a long-haul network. The metro network is the part of the networkthat lies within a large city or a region. The long-haul network interconnects citiesor different regions. The metro network consists of a metro access network and ametro interoffice network. The access network extends from a central office out toindividual businesses or homes (typically, groups of homes rather than individualhomes at this time). The access network’s reach is typically a few kilometers, and itmostly collects traffic from customer locations into the carrier network. Thus mostof the traffic in the access network is hubbed into the carrier’s central office. Theinteroffice network connects groups of central offices within a city or region. Thisnetwork usually spans a few kilometers to several tens of kilometers between offices.The long-haul network interconnects different cities or regions and spans hundredsto thousands of kilometers between central offices. In some cases, another part ofthe network provides the handoff between the metro network and the long-haulnetwork, particularly if these networks are operated by different carriers. In contrastto the access network, the traffic distribution in the metro interoffice and long-haulnetworks is meshed (or distributed). The distances indicated here are illustrative andvary widely based on the location of the network. For example, intercity distancesin Europe are often only a few hundred kilometers, whereas intercity distances inNorth America can be as high as a few thousand kilometers.

1.2 Services, Circuit Switching, and Packet Switching 5

Interexchange network Interoffice network Access network

Business

Home

Central office

MetropolitanMetropolitanLong haul

Figure 1.1 Different parts of a public network.

The network shown in Figure 1.1 is a terrestrial network. Optical fiber is alsoextensively used in undersea networks. Undersea networks can range from a fewhundred kilometers in distance to several thousands of kilometers for routes thatcross the Atlantic and Pacific oceans.

1.2 Services, Circuit Switching, and Packet SwitchingMany types of services are offered by carriers to their customers. In many cases,these are connection-oriented services in that there is the notion of a connectionbetween two or more parties across an underlying network. The differences lie inthe bandwidth of the connection and the type of underlying network with whichthe connection is supported, which has a significant impact on the quality-of-serviceguarantees offered by the carriers to their customers. Networks can also provideconnectionless service; we will discuss this type of service later in this section.

There are two fundamental types of underlying network infrastructures basedon how traffic is multiplexed and switched inside the network: circuit-switchedand packet-switched. Figure 1.2 illustrates some of the differences in the type ofmultiplexing used in these cases.

A circuit-switched network provides circuit-switched connections to its cus-tomers. In circuit switching, a guaranteed amount of bandwidth is allocated to eachconnection and is available to the connection all the time, once the connection is setup. The sum of the bandwidth of all the circuits, or connections, on a link must be less


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Figure 1.2 Different types of time division multiplexing: (a) fixed, (b) statistical.

than the link bandwidth. The most common example of a circuit-switched networkis the public-switched telephone network (PSTN), which provides a nailed-downconnection to end users with a fixed amount of bandwidth (typically around 4 kHz)once the connection is established. This circuit is converted to a digital 64 kb/s circuitat the carrier central office. This network was designed to support voice streams anddoes a fine job for this application.

The circuit-switched services offered by carriers today include circuits at a varietyof bit rates, ranging from 64 kb/s voice circuits all the way up to several Gb/s. Theseconnections are typically leased by a carrier to its customers and remain nailed downfor fairly long periods, ranging from several days to months to years as the bandwidthon the connection goes up. These services are also called private line services. ThePSTN fits into this category with one important difference—in the PSTN, users dialup and establish connections between themselves, whereas with private line services,the carrier usually sets up the connection using a management system. This situationis changing, and we will no doubt see users dialing for higher-speed private lines inthe future, particularly as the connection durations come down.

The problem with circuit switching is that it is not efficient at handling burstydata traffic. An example of a bursty traffic stream is traffic from a user typing ona keyboard. When the user is actively typing, bits are transmitted at more or less

1.2 Services, Circuit Switching, and Packet Switching 7

a steady rate. When the user pauses, there is no traffic. Another example is Webbrowsing. When a user is looking at a recently downloaded screen, there is almostno traffic. When she clicks on a hyperlink, a new page needs to be downloaded assoon as possible from the network. Thus a bursty stream requires a lot of bandwidthfrom the network whenever it is active and very little bandwidth when it is not active.It is usually characterized by an average bandwidth and a peak bandwidth, whichcorrespond to the long-term average and the short-term burst rates, respectively. Ina circuit-switched network, we would have to reserve sufficient bandwidth to dealwith the peak rate, and this bandwidth would be unused a lot of the time.

Packet switching was invented to deal with the problem of tranporting burstydata traffic efficiently. In packet-switched networks, the data stream is broken upinto small packets of data. These packets are multiplexed together with packetsfrom other data streams inside the network. The packets are switched inside thenetwork based on their destination. To facilitate this switching, a packet header isadded to the payload in each packet. The header carries addressing information, forexample, the destination address or the address of the next node in the path. Theintermediate nodes read the header and determine where to switch the packet basedon the information contained in the header. At the destination, packets belongingto a particular stream are received, and the data stream is put back together. Thepredominant example of a packet-switched network is the Internet, which uses theInternet Protocol (IP) to route packets from their source to their destination.

Packet switching uses a technique called statistical multiplexing when multiplex-ing multiple bursty data streams together on a link. Since each data stream is bursty,it is likely that at any given time only some streams are active and others are not. Theprobability that all streams are active simultaneously is quite small. Therefore thebandwidth required on the link can be made significantly smaller than the bandwidththat would be required if all streams were to be active simultaneously.

Statistical multiplexing improves the bandwidth utilization but leads to someother important effects. If more streams are active simultaneously than there is band-width available on the link, some packets will have to be queued or buffered untilthe link becomes free again. The delay experienced by a packet therefore depends onhow many packets are queued up ahead of it. This causes the delay to be a randomparameter. On occasion, the traffic may be so high that it causes the buffers to over-flow. When this happens, some of the packets must be dropped from the network.Usually, a higher-layer transport protocol, such as the transmission control protocol(TCP) in the Internet, detects this development and ensures that these packets areretransmitted. On top of this, a traditional packet-switched network does not evensupport the notion of a connection. Packets belonging to a connection are treatedas independent entities, and different packets may take different routes through thenetwork. This is the case with networks using IP. This type of connectionless service


is called a datagram service. This leads to even more variations in the delays expe-rienced by different packets and also forces the higher-layer transport protocol toresequence packets that arrive out of sequence at their destinations.

Thus, traditionally, such a packet-switched network provides what is called best-effort service. The network tries its best to get data from its source to its destinationas quickly as possible but offers no guarantees. This is indeed the case with much ofthe Internet today. Another example of this type of service is frame relay. Frame relayis a popular packet-switched service provided by carriers to interconnect corporatedata networks. When a user signs up for frame relay service, she is promised acertain average bandwidth over time but is allowed to have an instantaneous burstrate above this rate, though without any guarantees. In order to ensure that thenetwork is not overloaded, the user data rate may be regulated at the input to thenetwork so that the user does not exceed her committed average bandwidth overtime. In other words, a user who is provided a committed rate of 64 kb/s may senddata at 128 kb/s on occasion, and 32 kb/s at other times, but will not be allowed toexceed the ave

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