INTRODUCTION TODWDM TECHNOLOGY

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Books of Related Interest from IEEE Press

UNDERSTANDING SONETISDH AND ATM: Communications Networks for theNext MillenniumStamatios V. KartalopoulosA volume in the IEEE Press Understanding Science & Technology Series1999 Softcover 288 pp IEEE Order no. PP5399 ISBN 0-7803-4745-5

PHOTONIC SWITCHING TECHNOLOGY: Systems and NetworksHussein T. Mouftah and Jaafar M. H. Elmirghani1998 Hardcover 612 pp IEEE Order No. PC5761 ISBN 0-7803-4707-2

UNDERSTANDING LASERS: An Entry-Level Guide, Second EditionJeff HechtA volume in the IEEE Press Understanding Science & Technology Series1994 Softcover 448 pp IEEE Order No. PP3541 ISBN 0-7803-1005-5

INTRODUCTION TODWDM TECHNOLOGY

Data in a Rainbow

Stamatios v. KartalopoulosLucent Technologies, Inc.

IEEE Communications Society, Sponsor

+IEEEIEEE PRESS

The Institute of Electrical and Electronics Engineers, Inc., New York

~WILEY­~INTERSCIENCE

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Library of Congress Cataloging-in-Publication Data

Kartalopoulos, Stamatios V.Introduction to DWDM technology: data in a rainbow / Stamatios V. Kartalopoulos

p. ern."IEEE Communications Society, sponsor."Includes bibliographical references and index.ISBN 0-7803-5399-4

I. Optical communications. 2. Multiplexing. 3. Fiber optics. I. IEEE CommunicationsSociety. II. Title.TK5103.59 K36 2000621.382'7--dc21 99-049676

CIP

10 9 8 7 6 5 4 3

Dedicated to my wonderful family:to my wife Anita with deep love and appreciation

for her continuous encouragement in endeavors far beyond academic,and with the greatest love to our children, William and Stephanie,

who have brought both happiness and pride to our lives.

CONTENTS

Preface xv

Acknowledgments xvii

Introduction xix

PART I Fundamentals of Light 1

Chapter 1 The Nature of Light 31.1 Introduction 31.2 Increasing the Transportable Bandwidth of a Fiber 41.3 What Is DWDM? 41.4 What Is OFDM? 51.5 Opaque versus Transparent WDM Systems 61.6 DWDM Devices 61.7 Fundamentals of Light 7

1.7.1 The Wave Nature of Light 71.7.2 The Particle Nature of Light 7

1.8 Photometric Terms: Flux, Illuminance,and Luminance 9

Exercises 10

Chapter 2 Interaction of Light with Matter 112.1 Introduction 112.2 Transparent versus Opaque Matter 112.3 Properties of Optically Transparent Matter 12

2.3.1 Reflection and Refraction: Index of Refraction 122.3.2 Snell's Law 13

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viii

2.3.32.3.42.3.52.3.62.3.72.3.82.3.92.3.102.3.112.3.122.3.132.3.142.3.152.3.162.3.172.3.182.3.192.3.202.3.212.3.222.3.232.3.242.3.252.3.262.3.272.3.282.3.29

ExercisesReferencesStandards

Critical AngleOptical PrismsDiffractionDiffraction at InfinityGaussian BeamsDiffraction GratingsThe Huygens-Fresnel PrincipleInterference of LightAntireflection CoatingsHolographyPolarizationPolarization ExamplesPolarization by Reflection and RefractionExtinction RatioPolarization Mode Shift: The Faraday EffectPhase ShiftIsotropy and AnisotropyBirefringenceThe Quarter-Wavelength PlateThe Half-Wavelength PlateMaterial DispersionNonlinear PhenomenaHomogeneity and HeterogeneityEffects of Impurities in MatterEffects of MicrocracksEffects of Mechanical PressureEffects of Temperature Variation

Contents

131516161717181819202222222424252526272828282929303031313233

PART II Optical Components

Chapter 3 The Optical Waveguide: The Fiber3.1 Introduction3.2 Anatomy of a Fiber Cable

3.2.1 How Is Fiber Made?3.2.2 How Is the Preform Made?

3.3 Index of Refraction Profiles3.4 Fiber Modes

3.4.1 Multimode Graded Index3.4.2 Single Mode

3.5 Propagation of Light3.6 Critical Cone or Acceptance Cone3.7 Exit Cone3.8 Phase Velocity

35

37373738384041424243434444

Contents ix

3.9 Group Velocity 443.10 Modal Dispersion 45

3.10.1 Intermodal Delay Difference 453.10.2 Maximum Bit Rate 463.10.3 Mode Mixing 47

3.11 Reduction of Modal Dispersion 473.12 Chromatic Dispersion 48

3.12.1 Material Dispersion 483.12.2 Wavelength Dispersion 483.12.3 Chromatic Dispersion: Travel Time Variation 493.12.4 Chromatic Dispersion: Pulse Spread 49

3.13 Dispersion-Shifted and Dispersion-Flattened Fibers 503.14 Chromatic Dispersion Limits: ITU-T 513.15 Single-Mode Chromatic Dispersion Calculations 523.16 Chromatic Dispersion Compensation 533.17 Polarization Mode Dispersion 543.18 Fiber Attenuation or Loss 54

3.18.1 The Decibel 563.18.2 Examples 56

3.19 Fiber Spectrum Utilization 573.20 Single-Mode Fiber Cut-Off Wavelength 583.21 Fiber Birefringence and Polarization 593.22 Nonlinear Phenomena 60

3.22.1 Stimulated Raman Scattering 613.22.2 Stimulated Brillouin Scattering 623.22.3 Four-Wave Mixing 623.22.4 Temporal FWM, Near End and Far End 63

3.23 Spectral Broadening 643.24 Self Phase Modulation 653.25 Self-Modulation or Modulation Instability 653.26 Impact of FWM on DWDM Transmission Systems 663.27 Countermeasures to Reduce FWM 663.28 Solitons 67

3.28.1 A Qualitative Interpretation of Solitons 673.29 Fiber Connectors 683.30 Conclusion 69Exercises 70

Chapter 4 Optical Spectral Filters and Gratings 714.1 Introduction 714.2 Fabry-Perot Interferometer 71

4.2.1 Fabry-Perot Resonator 724.2.2 Finesse 744.2.3 Spectral Width, Line Width, and Line Spacing 754.2.4 The Fabry-Perot Filter 75

x

4.3 Bragg Grating4.3.1 Bragg Reflector4.3.2 The Bragg Condition

4.4 Fiber Bragg Gratings4.5 Tunable Bragg Gratings4.6 Dielectric Thin Film4.7 Polarizing Beam-Splitters4.8 Tunable Optical Filters4.9 Acousto-Optic Tunable Filters4.10 The Mach-Zehnder Filter

4.10.1 Tunability of the Mach-Zehnder Filter4.11 Absorption Filters4.12 Birefringence Filters4.13 Hybrid Filters4.14 Tunable Filters: Comparison4.15 Diffraction GratingsExercises

Contents

7677777780808181828385868687878889

Chapter 5 Optical Demultiplexers 915.1 Introduction 915.2 Prisms 915.3 Superprisms 925.4 Diffraction Gratings 925.5 Arrayed Waveguide Grating 935.6 Mach-Zehnder Interferometer 945.7 Spectral Filters 945.8 Acousto-Optic Filter plus Polarizing-Beam Splitter 955.9 Optical Multiplexers 95Exercises 96

Chapter 6 Light Sources6.1 Introduction6.2 Light-Emitting Diodes

6.2.1 Switching Speed and Output Power6.2.2 Output Optical Spectrum6.2.3 Input-Output Response6.2.4 Modulation Response6.2.5 Conclusions

6.3 Lasers6.3.1 The Ruby Laser6.3.2 Semiconductor Lasers

6.4 Monolithic Fabry-Perot Lasers6.5 Monolithic Bragg Lasers6.6 Distributed-Feedback Lasers

979798989899

100100100101102103104105

Contents xi

6.7 Semiconductor Quantum Well Lasers 1056.8 VCSEL Lasers 1066.9 Monolithic Tunable Lasers 107

6.9.1 Single-Frequency Lasers 1076.9.2 Multifrequency Lasers 107

6.10 Optical Comb Generators 1086.11 Chirped-Pulse Laser Sources 1096.12 Multifrequency Cavity Lasers 109

6.12.1 Advantages of WGRs 1106.12.2 Disadvantages of WGRs 110

6.13 Monolithic DFB Arrays 1106.13.1 Advantages of DFBs 1116.13.2 Disadvantages of DFBs 111

6.14 Modulators 1116.15 Laser Modules 1136.16 Supercontinuum Sources-Spectrum Slicing 114Exercises 114

Chapter 7 Photodetectors 1157.1 Introduction 1157.2 Photodetector Characteristics 1157.3 The PIN Photodiode 1167.4 The APD Photodiode 117Exercises 117

Chapter 8 Light Amplifiers 1198.1 Introduction 1198.2 Regenerators 1208.3 Optical Amplifiers 1208.4 Semiconductor Optical Amplifiers 1228.5 Erbium-Doped Fiber Amplifiers 122

8.5.1 Advantages of EDFAs 1248.5.2 Disadvantages of EDFAs 124

8.6 Praseodymium-Doped Fiber Amplifiers 1258.7 Stimulated Raman and Stimulated Brillouin

Scattering Amplifiers 1268.8 Classification of Optical Fiber Amplifiers 126

8.8.1 Power Amplifiers 1268.8.2 Preamplifiers 1278.8.3 Line Amplifiers 1278.8.4 Amplifier Standards 127

8.9 Wavelength Converters 1278.9.1 Cross-Gain Modulation 1288.9.2 Four-Wave Mixing 1288.9.3 Optical Frequency Shifter 129

Exercises 130

xii Contents

Chapter 9 Other Optical Components 1319.1 Introduction 1319.2 Optical Phase-Locked Loops 1319.3 Optical Directional Couplers 1329.4 Ring Resonators 1359.5 Optical Equalizers 1369.6 Optical Isolators 1379.7 Polarizers, Rotators, and Circulators 138Exercises 139

Chapter 10 Optical Cross-Connects 14110.1 Introduction 14110.2 Optical Cross-Connect Model 14210.3 Free Space Optical Switching 14210.4 Solid-State Cross-Connects 14310.5 Microelectromechanical Switches:

Reflector Type 14510.6 Electromechanical Switches-Mirror Array 14610.7 Switching Speeds 14810.8 Polymers 14910.9 Photochromic Materials 149Exercises 149

Chapter 11 Optical Add-Drop Multiplexers 15111.1 Introduction 15111.2 The OADM Function 15111.3 Optical Add-Drop Multiplexers 152Exercises 153References 154Standards 163Other Sources 163

PART III Coding Optical Information 165

Chapter 12 Digital Transmission and Coding Techniques 16712.1 Introduction 16712.2 Return to Zero and Non-Return to Zero 16712.3 Unipolar and Bipolar Signals 16812.4 4B/5B, 8B/10B Coding 16912.5 ASK Format 16912.6 PSK Format 17012.7 FSKFormat 171Exercises 172

Contents

Chapter 13 Decoding Optical Information13.1 Introduction13.2 ASK Demodulators13.3 PSK and FSK DemodulatorsExercisesReferencesStandards

xiii

173173173174175175176

PART IV Dense Wavelength Division Multiplexing 177

Chapter 14 DWDM Systems 17914.1 Introduction 17914.2 DWDM Network Topologies 18114.3 DWDM Applicability 182

Chapter 15 Engineering DWDM Systems 18715.1 Introduction 18715.2 ITU-T Nominal Center Frequencies 18815.3 Channel Capacity, Width, and Spacing 18915.4 Channel Bit Rate and Modulation 18915.5 Wavelength Management 18915.6 Multichannel Frequency Stabilization 19015.7 Channel Performance 19015.8 Channel Dispersion 19015.9 Power Launched 19115.10 Optical Amplification 19115.11 Fiber Type as the Transmission Medium 19115.12 Optical Power Budget 19115.13 Types of Service Supported 19215.14 Aggregate Bandwidth Management 19215.15 Protocol Used to Transport Supported Services 19315.16 Protocol for Network Management 19315.17 Network Reliability 19315.18 Network and Service Protection

and Survivability Strategies 19315.19 Network Scalability and Flexibility 19415.20 Wavelength Management 19415.21 Interoperability and Inter domain

Compatibility 19415.22 Single-Mode Power Loss Calculations:

An Example 19515.23 Channel Calculations in a Network:

Three Examples 196

xiv Contents

Chapter 16 DWDM Topologies 19716.1 Introduction 19716.2 Point-to-Point Topology 19716.3 Ring-Configured Mesh and Star Networks 19816.4 A DWDM Hub 201

16.4.1 Transmit Direction 20116.4.2 Receive Direction 201

16.5 Faults 202References 203Standards 206

PART V DWDM Current Issues and Research 209

Chapter 17 State of the Art 21117.1 Introduction 21117.2 Current Issues 211

17.2.1 Optical Materials 21117.2.2 Integration 21217.2.3 Lasers and Receivers 21217.2.4 Line Coding Techniques 21217.2.5 Optical Cross-Connect 21217.2.6 Optical Add-Drop Multiplexers 21317.2.7 Optical Memories and Variable Delay Lines 21317.2.8 Nonintrusive Optical Monitoring 21317.2.9 DWDM System Dynamic Reconfigurability 21317.2.10 Optical Backplanes 21317.2.11 Standards 21417.2.12 Network Issues 21417.2.13 Ultrahigh Speeds at Longer Spans 21417.2.14 Opaque Systems 215

17.3 Ultrafast Pattern Recognition 21517.3.1 Example: SONET/SDH 21617.3.2 Example: ATM 21717.3.3 Example: Internet Protocol 217

17.4 Current Research: Wavelength Bus 218References 219Standards 222

Acronyms and Abbreviations

Answers

Index

About the Author

223

233

241

257

PREFACE

Thousands of years ago someone tried to answer the question: Does light travel al-ways in a straight line, even if in a transparent medium, or can it follow its curva-ture? Using a bucket of water with a hole at the bottom, he discovered the latter-how simple!

Sunlight rays crossing the morning dew droplets formed a rainbow of colors.Thus the sun rays, composed of many colors, were demystified-what a simple ob-servation! Sun rays, when reflected with shining bronze shields, were redirected toselected points called estiai or foci. Furthermore, concentrated rays had so much en-ergy that they could warm up things or bum them. Soon thereafter, the glassy opti-callens was produced.

It was found that rays passing through a spherical lens did not create the best fo-cal point; today, this imperfection is known as lens sphericity. It was also discoveredthat shapes based on hyperbolas or parabolas were better suited to optical applica-tions than those based on circles or spheres.

Simple experiments and observations of the past have helped our understandingabout the nature of things. Yesterday's science fiction is today's reality. The elec-tronic properties of conductors and semiconductors help to create or detect light.When three crystals, all with different impurities, were fused together, they becamea transistor, which within a few years revolutionized the way we live. The wrist-sizecommunicator is no longer just a fantasy in comic books. Pocket-size powerful com-puters and credit-card-size communication devices are a reality. Low-earth-orbitsatellite (LEOS) communication networks are not "pie in the sky," but they are roam-ing the silent skies. At the click of a button, one can access virtually any part of theglobe and hear and see events as they happen. Optical fiber has been wrapped aroundthe globe like a ball of yam connecting all continents and transporting data at thespeed of light. Direct-to-satellite communication enables wireless connectivity atany time between any two places in the world, as well as providing global position-ing services within accurate to a few feet or inches! A single optical fiber can trans-port the information of hundreds of thousands of volumes within a second.

xv

xvi

ABOUT THIS BOOK

Preface

My interest in this field began early in my life when I pursued an undergraduate de-gree in physics. My interest in optics continued during my graduate work when Icombined electronics, materials, and interferometric techniques. During the last fewyears, I have been working on the subject of optical communications, bothSONET/SDH and ATM over SONET/SDH. My work on this subject culminated intoa book titled Understanding SONETISDH and ATM (IEEE Press, 1999).Subsequently, I had the opportunity to expand my understanding of multiwavelengthtransmission in optical media, now known as wavelength division multiplexing(WDM), mainly through my active interest in this subject during my undergraduateand graduate studies. Recently, I discovered that the notes I had been compiling overthe years had current educational value, particularly in the area of DWDM.

The intention of this book is to explain in simple language the properties oflight, its interaction with matter, and how it is used to develop optical componentssuch as filters, multiplexers, and others that are used in optical communications. Inaddition, the book provides an introduction to DWDM technology and to DWDMcommunications systems. This book is not meant to replace related standards or toprovide a complete mathematical analysis of each optical device, although mathe-matical relationships support some device functionality. DWDM is still evolving,and it is strongly recommended that the reader interested in the details of DWDMconsult the most current updated standards. I wish you happy and easy reading.

Stamatios V. KartalopoulosLucent Technologies, Inc.

ACKNOWLEDGMENTS

Throughout time, few noted achievements have been the product of an individual'seffort. Instead, they have been accomplished through the efforts of many. Similarly,the fruition of this book would have been impossible without the cooperation, dili-gence, understanding, and encouragement of a number of people.

First, I would like to extend my thanks and appreciation to my wife Anita forher patience and encouragement. Second, I express my appreciation to my col-leagues for creating an environment that fosters learning, collaboration, and encour-agement-in particular, Wayne H. Knox, Martin C. Nuss, Joseph E. Ford, MartinZimgibl, Rod Alfemess, Tom J. Ciaccia, and David J. Smith. Finally, to the anony-mous reviewers for their comments and constructive criticism; to the IEEE Pressstaff for their enthusiasm, suggestions, creativity, and project management; and to allthose who diligently worked during all phases of this production, I offer my hum-blest gratitude.

Stamatios V. KartalopoulosLucent Technologies, Inc.

xvii

INTRODUCTION

Light has fascinated mankind since the very beginning of time. Light enables us tosee things-uplifting rainbows, dramatic colors at dawn or sunset, vibrant colors offlowers and birds. Thus, it is no accident that light held a prominent position in mostphilosophies and religions.

The fascination with light has also sparked the curiosity of many scientists.Since ancient times, they have tried to decipher the nature of light and over the cen-turies, like masons laying one brick at a time to complete a wall, they have added tothis body of knowledge. Today, we know that light is an electromagnetic wavewhich, like radio waves, is subject to all laws of physics on propagation and inter-action.

Electromagnetic waves extend over a wide spectrum of frequencies (or wave-lengths). However, this spectrum is not entirely visible to the human eye. The part ofthe spectrum that we call visible light is in a narrow range (of wavelengths), from0.7 J.1m (700 nm) to 0.4 J.1m (400 nm) and from deep red to dark violet-blue. For ex-ample, the yellow light of a sodium lamp has a wavelength of 589 nm. It just hap-pens that this part of the spectrum is in the response range of our eye receptors (theretinal cones and rods). The cones enable us to perceive color, and the rods enableus to detect such minuscule quantities of light that we can see a lit candle in the darkmany miles away. Our eye receptors do not respond to frequencies below red (knownas infrared or IR) or to frequencies above violet-blue (known as ultraviolet or UV),although the eye receptors of certain animals do. If we were to ask a cat or an owlwhat its visible range is or what it sees through its eyes, we would certainly hear adifferent answer.

Since ancient times, a great deal of research has revealed that light propagatesin a straight line. However, when light is in an optically transparent pipe, then it isguided by the pipe and follows its curvature. This observation was demonstratedwith a very simple and convincing experiment. Heron of Alexandria took a bucketwith a hole in the bottom and filled it with water. As the water was gushing out ofthe hole, a curved stream of water was formed, As sunlight entered at an angle from

xix

xx Introduction

the top of the bucket, it propagated through the hole following the curvature of thestream. This experiment and others have been repeated throughout the centuries, andnew observations on the reflection and refraction of light led scientists to concentrateon light rays, which could be focused on a desired spot. (It is said that Archimedeswas able to burn the enemy's wooden fleet by this method.)

The quest for unveiling the secrets of light did not stop. Huygens studied thewave nature of light, and Fabry and Perot studied interactions of light and explainedits interferometric properties. In addition to the wave nature of light, it has beenfound that light exhibits particle properties. Initially, this raised many eyebrows as itwas met with skepticism. However, Compton's demonstration of a small lightweightpropeller in a vacuum, one side of which was black (for high absorption) and theother shiny (for high reflectance), was very convincing when light caused the pro-peller to rotate-a mechanical reaction that could be explained only by means ofwave theory.

Furthermore, many scientists studied the composition of light, and it was sepa-rated into its component wavelengths. Similarly, Zeeman studied the interaction oflight with other fields, and he split the chlorine yellow line with a strong magneticfield. The propagation properties of light in transparent materials and in opticalwavelengths were also studied. Today, many interesting materials have been devel-oped, and glass fiber is the chosen transmission medium for high-speed, high-reliability, and long-distance terrestrial and submarine communications. Currently,bit rates of up to 40 gigabits per second (Gb/s) are used in a single fiber. With wave-length multiplexing, dubbed dense wavelength division multiplexing (DWDM), theaggregate bandwidth has exceeded a terabit per second. DWDM systems with up to128 wavelengths have been announced, and DWDM with 206 wavelengths hasalready been experimentally demonstrated. A 40-wavelength DWDM system, at 10Gb/s per wavelength, has an aggregate bandwidth of 400 Gb/s-a bandwidth thatcan transport in a single fiber the contents of more than 11,000 volumes of an ency-clopedia in a second. DWDM systems with 40 Gb/s per wavelength have alreadybeen announced, and the trend continues to increase both the wavelength density andthe bit rate, as illustrated in Figure 1.1.

IN THIS BOOK

This book is organized into five parts and each part into chapters. Part I reviews thephysics of light; interferometry, diffraction, refraction, and so on, are important tounderstand before the Mach-Zehnder or Fabry-Perot interferometers are described.The optical properties of matter including nonlinear effects also are important to un-derstand before we study the transmission properties of light in fiber. Part II de-scribes the fundamentals of the optical glass fiber as a transmission medium and thetransmission properties of light through it. It also describes the fundamentals ofmany optical devices such as filters, multiplexers and demultiplexers, switches, po-larizers, light sources, and optical receivers. Part III reviews various coding tech-

Introduction

Per-fiber capacity trends

xxi

(/)

Qiccttl.co:2o3::

10 Mb/s 100 Mb/s 1 Gb/s 10 Gb/s 100 Gb/s 1 Ibis

Channelbit rate

.& = Year ofcommercialdeployment

(Copyright c 1998. Lucent Technologies. All rights reserved. Reprinted with permission)

Figure 1.1 Per-fiber capacity trends. (From Lucent Technologies, Bell Labs Technology, vol. 2 no. 2, Fall1998, p. 3.)

niques that are used in both digital and optical transmission . Part IV provides a com-prehensive, yet simplified, description of wavelength digital multiplexing (WDM)and DWDM. It also discusses DWDM system design issues, network topologies,and fault avoidance. Finally, Part V provides a discussion on the current research inthis area. At the end of Chapters 1-13 are problems and review questions. Relatedreferences are provided at the end of each part, and answers can be found before theindex.

STANDARDS

Optical transmission is specified in detail in several documents published by var-ious international standards bodies . These documents are official and voluminous,and this relatively brief book can be considered only a high-level tutorial that isdesigned to help readers understand the workings of DWDM technology.

xxii Introduction

Consequently, we strongly recommend that system designers consult the standardsfor details. The key international standards bodies in DWDM optical communica-tions are as follows:

• ITU-T and ITU-R are the International Telecommunications Union-Tele-communications Standardization Sector and the International Telecom-munications Union-Radio-Communications Sector, respectively. ITU haspublished several documents identified by "lTV-T Recommendation G.nnn"where nnn is a number that refers to a specific aspect of the system. For ex-ample, ITU-T Recommendation 0.774.01 describes the synchronous digitalhierarchy (SDH) performance monitoring for the network element.

• Bellcore was a U.S.-based organization that contributed to standards and alsopublished recommendations.

• Some other standards bodies are as follows: American National StandardsInstitute (ANSI), Association Francaise de Normalisation (AFNOR), ATM-Forum, British Standards Institution (BSI), Consultative CommitteeInternational Telegraph and Telephone (CCITT, a former name of lTV),Deutsches Institut fur Normung EV (DIN), European Association forStandardizing Information and Communication Systems (ECMA), Electron-ics Industry AssociationlTelecommunications Industry Association (EIAITIA), European Telecommunications Standardization Institute (ETSI),Frame-Relay Forum (FRF), Institute of Electrical and Electronics Engineers(IEEE), Internet Engineering Task Force (IETF), Motion Picture ExpertsGroup (MPEG), International Standards Organization (ISO), Telecommu-nications Information Networking Architecture (TINA) consortium, ComiteEuropeen de Normalisation Electrotechnique (CENELEC), Personal Com-puter Memory Card International Association (PCMCIA), and World WideWeb Consortium (W3C).