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Computer Science Books/Sem7/Wt Stalling

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Computer Science Books/Sem7/Wt Stalling

WIRELESS COMMUNICATIONS AND NETWORKS SeCOND EDITION Williant Stallings • Upper Saddle River, NJ 07458 WIRELESS COMMUNICATIONS AND NETWORKS SeCOND EDITION Williant Stallings • Upper Saddle River, NJ 07458

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Library of Congress Cataloging-in-Publication Data on file Vice President and Editorial Director, ECS: Marcia J Horton Publisher: Alan Apt Associate Editor: TOni D. Holm Editorial Assistant: Patrick Lindner Vice President and Director of Production and Manufacturing, ESM: David W Riccardi Executive Managing Editor: Vince OJBrien Managing Editor: Camille Trentacoste Production Editor: Rose Kernan Cover Photo: Brand X Pictures Director of Creative Services: Paul Belfanti Art Director: Heather Scott Cover Designer: Tamara Newnam Managing Editor,AV Management and Production: Patricia Burns Art Editor: Gregory Dulles Manufacturing Manager: Trudy Pisdotti Manufacturing Buyer: Lynda Castillo Marketing Manager: Pamela Hersperger Marketing Assistant: Barrie Reinhold • •. . © 2005,2002 Pearson Education, Inc. Pearson Prentice Hall Pearson Education, Inc. Upper Saddle River, NJ 07458 All rights reserved. No part of this book may be reproduced, in any form or by any means, without permission in writing from the publisher. Pearson Prentice Hall® is a trademark of Pearson Education, Inc. The author and publisher of this book have used their best efforts in preparing this book. These efforts include the development, research, and testing of the theories and programs to determine their effectiveness. The author and publisher make no warranty of any kind, expressed or implied, with regard to these programs or the documentation contained in this book. The author and publisher shall not be liable in any event for incidental or consequential damages in connection with, or arising out of, the furnishing, performance, or use of these programs. Printed in the United States of America 10987654321 ISBN: 0-13-191835-4 Pearson Education Ltd., London Pearson Education Australia Pty. Ltd., Sydney Pearson Education Singapore, Pte. Ltd. Pearson Education North Asia Ltd., Hong Kong Pearson Education Canada, Inc., TOronto Pearson EducaCion de Mexico, S.A. de C. V. Pearson Education~apan, TOkyo Pearson Education Malaysia, Pte. Ltd. Pearson Education Inc., Upper Saddle River, New Jersey

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As always) for my loving wife and her constant companions Geoffroi and Helma

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CONTENTS Preface xi Chapter 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 Introduction 1 Wireless Comes of Age 2 The Cellular Revolution 3 The Global Cellular Network 4 Broadband 5 Future Trends 5 The Trouble with Wireless 7 Outline of the Book 7 Internet and Web Resources 10 PART ONE TECHNICAL BACKGROUND 13 Chapter 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Chapter 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 Chapter 4 4.1 4.2 4.3 4.4 4.5 4.6 Transmission Fundamentals 14 Signals for Conveying Information 15 Analog and Digital Data Transmission 22 Channel Capacity 27 Transmission Media 31 Multiplexing 36 Recommended Readings and Web Sites 40 Key Terms, Review Questions, and Problems 41 Appendix 2A Decibels and Signal Strength 43 Communication Networks 46 LANs,MANs,andWANs 47 Switching Techniques 49 Circuit Switching 50 Packet Switching 54 Asynchronous Transfer Mode 60 Recommended Readings and Web Sites 65 key Terms, Review Questions, and Problems 66 Protocols and the TCPlIP Suite 69 The Need for a Protocol Architecture 70 The TCP/IP Protocol Architecture 71 The OSI Model 75 Internetworking 77 Recommended Readings and Web Sites 83 Key Terms, Review Questions, and Problems 83 Appendix 4A Internet Protocol 85 Appendix 4B Transmission Control Protocol 88 Appendix 4C User Datagram Protocol 92 – VII

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10.6 10.7 Chapter 11 11.1 11.2 11.3 11.4 11.5 Chapter 12 12.1 12.2 12.3 12.4 .CONTENTS IX Recommended Readings and Web Sites 311 Key Terms, Review Questions, and Problems 314 Cordless Systems and Wireless Local Loop 317 Cordless Systems 318 Wireless Local Loop 329 WiMAX and IEEE 802.16 Broadband Wireless Access Standards 342 Recommended Readings and Web Sites 354 Key Terms, Review Questions, and Problems 355 Appendix llA Linear Predictive Filters 356 Mobile IP and Wireless Access Protocol 358 Mobile IP 359 Wireless Application. Protocol 373 Recommended Readings and Web Sites 394 Key Terms, Review Questions, and Problems 396 Appendix 12A Internet Control Message Protocol 397 Appendix 12B Message Authentication 401 Appendix 12C Service Primitives and Parameters 402 PART FOUR WIRELESS LANS 404 Chapter 13 13.1 13.2 13.3 13.4 13.5 13.6 Chapter 14 14.1 14.2 14.3 14.4 14.5 14.6 14.7 Chapter 15 15.1 15.2 15.3 15.4 15.5 15.6 15.7 15.8 Wireless Lan Technology 405 Overview 406 Infrared LANs 413 Spread Spectrum LANs 417 Narrowband Microwave LANs 418 Recommended Readings and Web Sites 419 Key Terms, Review Questions, and Problems 419 Wi-Fi and the IEEE 802.11 Wireless Lan Standard 421 IEEE 802 Protocol Architecture 422 IEEE 802.11 Architecture and Services 428 IEEE 802.11 Medium Access Control 433 IEEE 802.11 Physical Layer 442 Other IEEE 802.11 Standards 451 Recommended Readings and Web Sites 458 Key Terms, Review Questions, and Problems 459 Appendix 14A Scrambling 460 Bluetooth and IEEE 802.15 463 Overview 464 Radio Specification 471 Baseband Specification 472 Link Manager Protocol 491 Logical Link Control and Adaptation Protocol 495 IEEE 802.15 501 Recommended Readings and Web Sites 508 Key Terms, Review Questions, and Problems 509

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X CONTENTS APPENDICES 511 Appendix A Traffic Analysis 511 A.l Basic Traffic Concepts 512 A.2 Multiserver Models 513 A.3 Recommended Reading 519 Appendix B Fourier Analysis 520 B.l Fourier Series Representation of Periodic Signals 521 B.2 Fourier Transform Representation of Aperiodic Signals 522 B.3 Recommended Readings 525 Appendix C Data Link Control Protocols 526 C.l High-Level Data Link Control 527 Glossary 533 References 540 Index 547

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‘¥ “’;\ ••- ‘;-j-,+<-(’ “ . ~ , ; ~ PREFACE Wireless technology has become the most exciting area in telecommunications and network- ing. The rapid growth of mobile telephone use, various satellite services, and now the wireless Internet and wireless LANs are generating tremendous changes in telecommunications and networking. This book explores the key topics in the field in the following general categories: • Technology and architecture: There is a small collection of ingredients that serves to char- acterize and differentiate wireless communication and networking, including frequency band, signal encoding technique, error correction technique, and network architecture. • Network type: This book covers the important types of wireless networks, including satellite, cellular, fixed wireless access, and wireless LANs. • Design approaches: The book examines alternative design choices and assesses their relative merits. • Applications: A number of key technologies and applications have been developed on top of wireless infrastructures, especially mobile IP and wireless Web access. Throughout, there is an emphasis on both technology and on standards. The book pro- vides a comprehensive guide to understanding specific wireless standards, such as those pro- mulgated by lTD and IEEE 802, as well as standards developed by other organizations. This emphasis reflects the importance of such standards in defining the available products and future research directions in this field. This book is intended for a broad range of readers who will benefit from an understanding of wireless communications and networks, and the associated technologies. This includes stu- dents and professionals in the fields of data processing and data communications, designers and implementers, and data communication and networking customers and managers. For the professional interested in this field, the book serves as a basic reference volume and is suitable for self-study. As a textbook, it is suitable for an advanced undergraduate or graduate course. It cov- ers the material in the CS332 Wireless and Mobile Computing advanced course of the joint ACM/IEEE Computing Curricula 2001. The chapters and parts of the book are sufficiently modular to provide a great deal of flexibility in the design of courses. The book treats a number of advanced topics and provides a brief survey of the required ele- mentary topics. For the reader with little or no background in data communications, Part One and the appendices cover a number of basic topics. The book is divided into four parts: • Technical Background • Wireless Communication Technology Xl

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xii PREfACE • Wireless Networking • Wireless LAN s In addition, the book includes an extensive glossary, a list of frequently used acronyms, and a bibliography. Each chapter includes problems, suggestions for further reading, and a list of relevant Web sites. Each chapter also includes, for review, a list of key words and a number of review questions. There is a Web site for this book that provides support for students and instructors. The site includes links to other relevant sites, transparency masters of figures and tables from the book in PDF (Adobe Acrobat) format, PowerPoint slides, and sign-up information for the book’s Internet mailing list. The Web page is at WilliamStallings.comlWireless/Wireless2e.html; see Section 1.8 for more information. An Internet mailing list has been set up so that instructors using this book can exchange information, suggestions, and questions with each other and with the author. As soon as typos or other errors are discovered, an errata list for this book will be available at WilliamStallings.com. I also maintain the Computer Science Student Resource Site at WilliamStallings.com/StudentSupport.html. In the three years since the first edition of this book was published, the field has seen con- tinued innovations and improvements. In this new edition, I try to capture these changes while maintaining a broad and comprehensive coverage of the entire field. To begin the process of revision, the first edition of this book was extensively reviewed by a number of professors who teach the subject. The result is that, in many places, the narrative has been clarified and tightened, and illustrations have been improved. Also, a number of new “field- tested” problems have been added. Beyond these refinements to improve pedagogy and user friendliness, the technical content of the book has been updated throughout, to reflect the ongoing changes in this exciting field. Every chapter has been revised. Highlights include the following: • Minimum shift keying: MSK is a form of modulation that is found in some mobile communications systems. This material is now covered. • CDMA2000: The first 3G (third generation) wireless system to be deployed commercially is known as CDMA2000 1xEV-Do. A discussion of this important standard is included. • WiMAX and IEEE 802.16a: Work on wireless local loop has evolved, including the intro- duction of the WiMAX specification to provide interoperability specifications for 802.16. Chapter 11 includes new material on 802.16, including the recent 802.16a standard. • Orthogonal frequency division multiplexing: The popularity of OFDM is increasing and is used in a variety of local and wide area wireless standards. The material on OFDM has been updated and expanded. • Wi-Fi and IEEE 802.11: The coverage of 802.11a and 802.11b has been expanded significantly, and treatment of 802.11g had been added. • Data scrambling: Scrambling is a technique often used to improve signal quality. An overview of data scrambling is provided in Chapter 14.

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… PR.EFACE XIll • Wi-Fi protected access: WPA has replaced Wireless Equivalent Privacy (WEP) as the specification for providing security in wireless LANs. Chapter 14 provides coverage of WPA. • IEEE 802.15 and personal area networks: The initial 802.15.1 standard provides an official specification for Bluetooth, which was covered in the first edition as well as this edition. This edition also covers two new standards: the 802.15.3 high-speed wireless PAN standard and the 802.15.4 low-speed wireless PAN standard. • Trellis-coded modulation: TCM is a technique that provides for efficient use of bandlimited channels; it is described in Chapter 15. In addition, throughout the book, virtually every topic has been updated to reflect the developments in standards and technology that have occurred since the publication of the first edition. This new edition has benefited from review by a number of people, who gave generously of their time and expertise. The following people reviewed all or a large part of the manuscript: Dr. Albert Cheng (University of Houston-University Park), Dale W. Callahan (University of Alabama, Birmingham), Ravi Sankar (University of South Florida, Tampa), Pei Zheng (Arcadia University, Pennsylvania), and Anne Cox (Austin Community College, Texas). Thanks also to the many people who provided detailed technical reviews of a single chapter: Lars Poulsen, Howard Eisenhauer, D. E. Jennings, Paul Robichaux, John Adams, Jerry Huang, Andreas Kasenides, Munira Ahmed, Hossein Izadpanah, Aaron King, Benoit d’Udekem, Marco Casole, Kevin Peterson, Dinesh Lal Pradhan, and Cathal Mc Daid. Finally, I would like to thank the many people responsible for the publication of the book, all of whom did their usual excellent job. This includes the staff at Prentice Hall, par- ticularly my editors Alan Apt and Toni Holm; their assistant Patrick Lindner: production manager Rose Kernan; and supplements manager Sarah Parker. Also, Jake Warde of Warde Publishers managed the reviews; and Patricia M. Daly did the copy editing.

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I DUCTIO Wireless Comes of Age 1.2 The Cellular Revolution The Global Cellular Network Broadband Future Trends The Trouble With Wireless Outline of the Book Part One: Background Part Two: Wireless Communication Technology Part Three: Wireless Networking Part Four: Wireless Local Area Networks Internet and Web Resources Web Sites for This Book Other Web Sites USENET Newsgroups 1

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2 CHAPTER 1 / INTRODUCTION This book is a survey of the technology of wireless communications and networks. Many factors, including increased competition and the introduction of digital tech- nology, have led to unprecedented growth in the wireless market. In this chapter, we discuss some of the key factors driving this new telecommunications revolution. This book, and the accompanying Web site, covers a lot of material. Following the general discussion, this chapter gives the reader an overview of the book. Guglielmo Marconi invented the wireless telegraph in 1896. 1 In 1901, he sent tele- graphic signals across the Atlantic Ocean from Cornwall to St. John’s Newfoundland; a distance of about 3200 km. His invention allowed two parties to communicate by sending each other alphanumeric characters encoded in an analog signal. Over the last century, advances in wireless technologies have led to the radio, the television, the mobile telephone, and communications satellites. All types of information can now be sent to almost every corner of the world. Recently, a great deal of attention has been focused on satellite communications, wireless networking, and cellular technology. Communications satellites were first launched in the 1960s. Those first satel- lites could only handle 240 voice circuits. Today, satellites carry about one-third of the voice traffic and all of the television signals between countries [EVAN98]. Modern satellites typically introduce a quarter-second propagation delay to the signals they handle. Newer satellites in lower orbits, with less inherent signal delay, have been deployed to provide data services such as Internet access. Wireless networking is allowing businesses to develop WAN s, MANs, and LANs without a cable plant. The IEEE has developed 802.11 as a standard for wire- less LANs. The Bluetooth industry consortium is also working to provide a seamless wireless networking technology. The cellular or mobile telephone is the modern equivalent of Marconi’s wire- less telegraph, offering two-party, two-way communication. The first-generation wireless phones used analog technology. These devices were heavy and coverage was patchy, but they successfully demonstrated the inherent convenience of mobile communications. The current generation of wireless devices is built using digital technology. Digital networks carry much more traffic and provide better reception and security than analog networks. In addition, digital technology has made possible value-added services such as caller identification. Newer wireless devices connect to the Internet using frequency ranges that support higher infor- mation rates. The impact of wireless communications has been and will continue to be pro- found. Very few inventions have been able to “shrink” the world in such a manner. The standards that define how wireless communication devices interact are quickly lThe actual invention of radio communications more properly should be attributed to Nikola Tesla, who gave a public demonstration in 1893. Marconi’s patents were overturned in favor of Tesla in 1943 [ENGEOO].

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1.2 / THE CELLULAR REVOLUTION 3 3THz Infrared wireless LAN Optical communications satellite 300 GHz 30GHz 3GHz 300 MHz Terrestrial microwave Experimental communications satellite Color TV FM radio Communications satellite Cordless phone Wi-Fi Cellular phone WiMAX Ultra- wideband ZigBee 30 MHz Shortwave radio Black-and- white TV Mobile two-way radio 3 MHz 1930 1940 1950 1960 1970 1980 1990 2000 2010 Figure 1.1 Some Milestones in Wireless Communications converging and soon will allow the creation of a global wireless network that will deliver a wide variety of services. Figure 1.1 highlights some of the key milestones in the development of wire- less communications. 2 Wireless technologies have gradually migrated to higher fre- quencies. As will be seen in later chapters, higher frequencies enable the support of greater data rates and throughput. The cellular revolution is apparent in the growth of the mobile phone market alone. In 1990, the number of users was approximately 11 million [ECON99]. Today, that number is in the billions. According to the lTD (International Telecommunications Dnion),3 the number of mobile phones worldwide outnumbered fixed-line phones for the first time in 2002. The newer generation devices, with access to the Internet and built-in digital cameras, add to this momentum. There are a number of reasons 2Note the use of a log scale for the y-axis. A basic review of log scales is in the math refresher document at the Computer Science Student Resource Site at WilIiamStallings.comlStudentSupport.html. 3 A description of lTD and other standards-making bodies is contained in a supporting document at this book’s Web site.

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4 CHAPTER 1 ; INTRODUCTION for the increasing dominance of mobile phones. Mobile phones are convenient; they move with people. In addition, by their nature, they are location aware. A mobile phone communicates with regional base stations that are at fixed locations. Technical innovations have contributed to the success of mobile phones. The handsets have become smaller and lighter, battery life has increased, and digital technology has improved reception and allowed better use of a finite spectrum. As with many types of digital equipment, the costs associated with mobile telephones have been decreasing. In areas where competition flourishes, prices have dropped dramatically since 1996. In many geographic areas, mobile telephones are the only economical way to provide phone service to the population. Operators can erect base stations quickly and inexpensively when compared with digging up ground to lay copper in harsh terrain. Mobile telephones are only the tip of the cellular revolution. Increasingly, new types of wireless devices are being introduced. These new devices have access to the Internet. They include personal organizers and telephones, but now they have Web access, instant messaging, e-mail, and other services available on the Internet. Wire- less devices in automobiles allow users to download maps and directions on demand. Soon, the devices may be able to call for help when an accident has occurred or perhaps notify the user of the lowest-priced fuel in the immediate area. Other conveniences will be available as well. For example, refrigerators may one day be able to order groceries over the Internet to replace consumed items. The first rush to wireless was for voice. Now, the attention is on data. A big part of this market is the “wireless” Internet. Wireless users use the Internet differently than fixed users. Wireless devices have limited displays and input capabilities com- pared with typical fixed devices such as the Pc. Transactions and messaging will be the rule instead of lengthy browsing sessions. Because wireless devices are location aware, information can be tailored to the geographic location of the user. Informa- tion will be able to find users, instead of users searching for information. Today there is no single cellular network. Devices support one or two of a myriad of technologies and generally work only within the confines of a single operator’s network. To move beyond this model, more work must be done to define and implement standards. The lTD is working to develop a family of standards for the next-generation wireless devices. The new standards will use higher frequencies to increase capacity. The new standards will also help overcome the incompatibilities introduced as the different first- and second-generation networks were developed and deployed over the last decade. The dominant first-generation digital wireless network in North America was the Advanced Mobile Phone System (AMPS). This network offers a data service using the Cellular Digital Packet Data (CDPD) overlay network, which provides a 19.2-kbps data rate. The CPDP uses idle periods on regular voice channels to provide the data service. The key second-generation wireless systems are the Global System for Mobile Communications (GSM), Personal Communications Service (PCS) IS-136, and PCS

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1.5 / FUTURE TRENDS 5 IS-95. The PCS standard IS-136 uses time division multiple access (TDMA) while IS-95 uses code division multiple access (CDMA). The GSM and PCS IS-136 use dedicated channels at 9.6 kbps to deliver the data service. The ITU is developing International Mobile Telecommunications-2000 (IMT- 2000). This family of standards is intended to provide a seamless global network. The standards are being developed around the 2-GHz frequency band. The new standards and frequency band will provide data rates up to 2 Mbps. In addition to defining frequency usage, encoding techniques, and transmis- sion, standards also need to define how mobile devices will interact with the Inter- net. Several standards bodies and industry consortiums are working to that end. The Wireless Application Protocol (WAP) Forum is developing a common protocol that allows devices with limited display and input capabilities to access the Internet. The Internet Engineering Task Force (IETF) is developing a mobile IP standard that adapts the ubiquitous IP protocol to work within a mobile environment. The Internet is increasingly a multimedia experience. Graphics, video, and audio abound on the pages of the World Wide Web. Business communications are following the same trend. For example, e-mail frequently includes large multimedia attachments. In order to participate fully, wireless networks require the same high data rates as their fixed coun- terparts. The higher data rates are obtainable with broadband wireless technology. Broadband wireless service shares the same advantages of all wireless ser- vices: convenience and reduced cost. Operators can deploy the service faster than a fixed service and without the cost of a cable plant. The service is also mobile and can be deployed almost anywhere. There are many initiatives developing broadband wireless standards around many different applications. The standards cover everything from the wireless LAN to the small wireless home network. Data rates vary from 2 Mbps to well over 100 Mbps. Many of these technologies are available now and many more will become available in the next several years. Wireless LANs (WLANs) provide network services where it is difficult or too expensive to deploy a fixed infrastructure. The primary WLAN standard is IEEE 802.11, which provides for data rates as high as 54 Mbps. A potential problem with 802.11 is compatibility with Bluetooth. Bluetooth is a wireless networking specification that defines wireless communications between devices such as laptops, PDAs, and mobile phones. Bluetooth and some versions of 802.11 use the same frequency band. The technologies would most likely interfere with each other if deployed in the same device. Much of the development effort in new wireless technology makes use of portions of the frequency spectrum that do not, in many countries, require licensing. In the United States, two such frequency bands are Industrial, Scientific, and Medical

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6 CHAPTER 1/ INTRODUCTION (ISM) band near 2.4 GHz and the newly allocated unlicensed radio band, the Unli- censed National Information Infrastructure (UNII) band. UNII was created by the FCC (Federal Communications Commission) to allow manufacturers to develop high-speed wireless networks. In order to find enough bandwidth to satisfy needs, the band was established at 5 GHz, making it incompatible with 2.4-GHz equip- ment. The free, unlicensed portions of the radio spectrum enable manufacturers to avoid billions of dollars in licensing fees. For years, these radio frequencies were neglected, the lonely domain of cord- less phones and microwave ovens. In recent years however, spurred by consumer demand and active standards bodies, considerable research and development is underway. The first significant fruit of this activity is Wi-Fi (Wireless Pidelity), the very popular wireless LAN technology based on the IEEE 802.11 standards. In essence, Wi-Fi refers to 802.11-compatible products that have been certified as interoperable by the Wi-Fi Alliance, a body specifically set up for this certification process. Wi-Pi covers not only office-based LANs, but also home-based LANs and publicly available hot spots, which are areas around a central antenna in which peo- ple can wirelessly share information or connect to the Internet with a properly equipped laptop. Wi-Pi is examined in some detail in Chapter 14. Wi-Fi is just the first major step in utilizing these bands. Four other innovative technologies are working their way through the research, development, and stan- dardization efforts: WiMAX, Mobile-Fi, ZigBee, and Ultrawideband. We survey these technologies briefly in this section. WiMAX is similar to Wi-Pi. Both create hot spots, but while Wi-Pi can cover several hundred meters, WiMAX has a range of 40 to 50 km. Thus, WiMAX provides a wireless alternative to cable, DSL, and TlIEl for last-mile broadband access. It will also be used as complimentary technology to connect 802.11 hot spots to the Inter- net. Initial deployments of WiMAX are in fixed locations, but a mobile version is under development. WiMAX is an interoperability specification based on IEEE 802.16 and is discussed in more detail in Chapter II. Mobile-Fi is similar to the mobile version of WiMAX in terms of technology. The objective with Mobile-Pi is to provide Internet access to mobile users at data rates even higher than those available in today’s home broadband links. In this con- text, mobile truly means mobile, not just movable. Thus, a Mobile-Pi user could enjoy broadband Internet access while traveling in a moving car or train. Mobile-Pi is based on the IEEE 802.20 specifications. ZigBee functions at a relatively low data rate over relatively short distances, compared to Wi-Fi. The objective is to develop products that are very low cost, with low power consumption and low data rate. ZigBee technology enables the coordina- tion of communication among thousands of tiny sensors, which can be scattered throughout offices, farms, or factories, picking up bits of information about tempera- ture, chemicals, water, or motion. They’re designed to use little energy because they’ll be left in place for 5 or 10 years and their batteries need to last. ZigBee devices communicate efficiently, passing data over radio waves from one to the other like a bucket brigade. At the end of the line the data can be dropped into a computer for analysis or picked up by another wireless technology like Wi-Pi or WiMAX. Ultrawideband serves a very different purpose than the other technologies mentioned in this section. Ultrawideband enables the movement of massive files at

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1.7 / OUTLINE OF THE BOOK 7 high data rates over short distances. For example, in the home, Ultrawideband would allow the user to transfer hours of video from a PC to a TV without any messy cords. On the road, a passenger who has a laptop in the trunk receiving data over Mobile-Fi could use Ultrawideband to pull that information up to a handheld computer in the front seat. Wireless is convenient and often less expensive to deploy than fixed services, but wireless is not perfect. There are limitations, political and technical difficulties that may ultimately prevent wireless technologies from reaching their full potential. Two issues are incompatible standards and device limitations. As mentioned previously, in North America there are two standards for digital cellular service. Internationally, there is at least one more. A device using PCS IS-136 will not work in an area where the deployed technology is PCS IS-95. Also mentioned previously is the inability to use Bluetooth and 802.11 in the same device. These are just two examples of problems that arise when industrywide stan- dards do not exist. The lack of an industrywide standard holds the technologies back from delivering one of the true ideals of wireless: ubiquitous access to data. Device limitations also restrict the free flow of data. The small display on a mobile telephone is inadequate for displaying more than a few lines of text. In addi- tion, most mobile wireless devices cannot access the vast majority ofWWW sites on the Internet. The browsers use a special language, wireless markup language (WML) , instead of the de facto standard HTML. Most likely, no one wireless device will be able to meet every need. The poten- tial of wireless can be met but not with a single product. Wireless will succeed because it will be integrated into a variety of devices that can meet a variety of needs. The objective of this book is to provide a comprehensive technical survey of wireless communications fundamentals, wireless networks, and wireless applications. The book is organized into four parts (Figure 1.2). The reader who is already familiar with data communications and networking technology can safely skip or just skim Part One. Part Two discusses underlying principles common to all of the material covered in the remainder of the book and should be read next. Parts Three and Four are inde- pendent and may be covered in either order. Within Part Three, all of the chapters are more or less independent and can be read in any order depending on your level of interest. The same is true of Chapters 14 and 15 in Part Five. Part One: Background Part One provides a preview and context for the remainder of the book, covering basic topics in data communications as well as TCP/IP. Part One, together with the appen- dices at the end of the book, is intended to make the book as self-contained as possible.

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8 CH/\PTER 1 I INTRODUCTION Part One Tec~calBackground • Transmission fundamentals • Communication networks • Protocols and TCP/IP Part Two Wireless Communication Technology • Antennas and propagation • Signal encoding techniques • Spread spectrum • Coding and error control /Part Three Wireless Networking • Satellite communications • Cellular wireless networks • Cordless systems and wireless local loop • Mobile IP and WAP Figure 1.2 Wireless Topics Part Four Wireless LANs • Wireless LAN Technology • IEEE 802.1lIWi-Fi • Bluetooth Chapter 2: Transnlission Fundaluentals Chapter 2 provides a basic overview of transmission topics. The chapter begins with a look at some data com- munications concepts, including signaling techniques and analog and digital data transmission. The chapter then covers channel capacity, transmission media, and the concept of multiplexing. Chapter 3: COUlnlunication Networks This chapter provides an overview and comparison of basic communication network technologies, including circuit switching, packet switching, and ATM. Chapter 4: Protocols and the TCPlIP Protocol Suite Data network communication and distributed applications rely on underlying communications software that is independent of application and relieves the application of much of the burden of reliably exchanging data. This communications software is organized into a protocol architecture, the most important incarnation of which is the TCP/IP protocol suite. Chapter 4 introduces the concept of a protocol architecture and

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  1. 7 / OUTLINE OF THE BOOK 9 provides an overview of TCP/IP. Another architecture, the Open Systems Intercon- nection (OSI) reference model, is briefly described. Finally, the concept of internet- working and the use of TCP/IP to achieve internetworking are discussed. Part Two: Wireless Communication Technology This part is concerned with the underlying technology of wireless transmission and the encoding of analog and digital data for wireless transmission. Chapter 5: Antennas and Propagation Chapter 5 examines the fundamen- tal principles of radio and microwave. The chapter discusses relevant aspects of antenna performance, then looks at wireless transmission modes, and finally exam- ines the key issue of fading. Chapter 6: Signal Encoding Techniques Data come in both analog (contin- uous) and digital (discrete) form. For transmission, input data must be encoded as an electrical signal that is tailored to the characteristics of the transmission medium. Both analog and digital data can be represented by either analog or digital signals; the relevant cases for wireless transmission are discussed in Chapter 6. Chapter 7: Spread Spectruln An increasingly popular form of wireless com- munications is known as spread spectrum. Two general approaches are used: fre- quency hopping and direct sequence spread spectrum. Chapter 7 provides an overview of both techniques. The chapter also looks at the concept of code division multiple access (CDMA), which is an application of spread spectrum to provide multiple access. Chapter 8: Coding and Error Control Wireless communications systems are highly prone to error, and virtually all wireless transmission schemes include techniques for forward error correction (FEC) by adding redundancy to the trans- mitted data so that bit errors can be corrected at the receiver. Chapter 8 examines FEC in detail. In addition, Chapter 8 looks at the use of redundancy for error detec- tion, which is also found in many wireless schemes. Finally, error detection is often combined with automatic repeat request (ARQ) techniques that enable a transmit- ter to retransmit blocks of data in which the receiver has detected an error. Part Three: Wireless Networking This part examines the major types of wireless networks. These include satellite- based networks, cellular networks, cordless systems, fixed wireless access schemes, and the use of mobile IP and the Wireless Application Protocol (WAP) to provide Internet and Web access. Chapter 9: Satellite COlnmunications This chapter covers the basic principles of satellite communications. It looks at geostationary satellites (GEOS), low-earth orbiting satellites (LEOS), and medium-earth orbiting satellites (MEOS). The key design issue of capacity allocation is examined in detail. Chapter 10: Cellular Wireless Networks Chapter 10 begins with a discussion of the important design issues related to cellular wireless networks. Next, the chapter covers the traditional mobile telephony service, now known as first-generation

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10 CHAPTER 1 / INTRODUCTION analog. Chapter 10 then examines second-generation digital cellular networks, look- ing at the two principal approaches: time division multiple access (TDMA) and code division multiple access (CDMA). Finally, an overview of third-generation networks is provided. Chapter 11: Cordless Systems and Wireless Local Loop Chapter 11 looks at two technologies that bring wireless access into the residence and office: cordless systems and wireless local loop (WLL). Cordless systems have evolved from the simple single-user cordless telephones used within the home to accommo- date multiple users over much larger ranges. Sometimes called radio in the loop (RITL) or fixed wireless access (FWA), WLL is a system that connects subscribers to the public switched telephone network (PSTN) using radio signals as a substitute for copper for all or part of the connection between the subscriber and the switch. Chapter 11 looks at the design issues related to WLL and then examines the IEEE 802.16 standard. Chapter 12: Mobile IP and Wireless Access Protocol Chapter 12 exam- ines the modifications to IP to accommodate wireless access to the Internet. The chapter then examines the Wireless Application Protocol (WAP). WAP provides mobile users of wireless phones and other wireless terminals, such as pagers and personal digital assistants (PDAs), access to telephony and information services, including the Internet and the Web. Part Four: Wireless Local Area Networks In recent years, a whole new class of local area networks have arrived to provide an alternative to LANs based on twisted pair, coaxial cable, and optical fiber-wireless LANs. The key advantages of the wireless LAN are that it eliminates the wiring cost, which is often the most costly component of a LAN, and that it accommodates mobile workstations. This part examines underlying wireless LAN technology and then examines two standardized approaches to local wireless networking. Chapter 13: Wireless LAN Technology Wireless LANs use one of three trans- mission techniques: spread spectrum, narrowband microwave, and infrared. Chapter 13 provides an overview of LANs and wireless LAN technology and applications. Chapter 14: IEEE 802.11 Wireless LAN Standard The most significant set of standards defining wireless LANs are those defined by the IEEE 802.11 commit- tee. Chapter 14 examines this set of standards in depth. Chapter 15: Bluetooth Bluetooth is an open specification for wireless commu- nication and networking among pes, mobile phones, and other wireless devices. Bluetooth is one of the fastest growing technology standards ever. It is intended for use within a local area. Chapter 15 examines this specification in depth. There are a number of resources available on the Internet and the Web to support this book and to help one keep up with developments in this field.

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1.8 / INTERNET AND WEB RESOURCES 11 Web Sites for This Book A special Web page has been set up for this book at WilliamStallings.comlWirelessl Wrreless2e.html. The site includes the following: • Useful Web sites: There are links to other relevant Web sites, including the sites listed in this section and throughout this book. • Errata sheet: An errata list for this book will be maintained and updated as needed. Please e-mail any errors that you spot to me. Errata sheets for my other books are at WilliamStallings.com. • Documents: Includes a number of documents that expand on the treat- ment in the book. Topics include standards organizations and the TCP/IP checksum. • Figures: All of the figures in this book in PDF (Adobe Acrobat) format. • Tables: All of the tables in this book in PDF format. • Slides: A set of PowerPoint slides, organized by chapter. • Internet mailing list: The site includes sign-up information for the book’s Internet mailing list. • Wireless courses: There are links to home pages for courses based on this book; these pages may be useful to other instructors in providing ideas about how to structure their course. I also maintain the Computer Science Student Resource Site, at WilliamStallings.comlStudentSupport.html; the purpose of this site is to provide documents, information, and useful links for computer science students and profes- sionals. Links are organized into four categories: • Math: Includes a basic math refresher, a queuing analysis primer, a number system primer, and links to numerous math sites • How-to: Advice and guidance for solving homework problems, writing techni- cal reports, and preparing technical presentations • Research resources: Links to important collections of papers, technical reports, and bibliographies • Miscellaneous: A variety of useful documents and links Other Web Sites There are numerous Web sites that provide information related to the topics of this book. In subsequent chapters, pointers to specific Web sites can be found in the “Recommended Reading and Web Sites” section. Because the addresses for Web sites tend to change frequently, I have not included these in the book. For all of the Web sites listed in the book, the appropriate link can be found at this book’s Web site.

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12 CHAPTER 1 / INTRODUCTION The following Web sites are of general interest related to wireless commu- nications: • Vendors: Links to thousands of hardware and software vendors who currently have WWW sites, as well as a list of thousands of computer and networking companies in a Phone Directory • Wireless Developer Network: News, tutorials, and discussions on wireless topics • Wireless.com: An amazing list of links to all aspects or wireless communica- tions, networking, and standards USENET Newsgroups A number of USENET newsgroups are devoted to some aspect of data communi- cations or networking. As with virtually all USENET groups, there is a high noise- to-signal ratio, but it is worth experimenting to see if any meet your needs. The most relevant are • comp.std.wireless: General discussion of wireless standards for wide area and local area networks. This is a moderated group, which keeps the discus- sion focused. • comp.dcom.*: There are a number of data communications related news- groups that begin with “comp.dcom.”

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PART ONE 13

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14

SMISSION FUND 2.1 Signals for Conveying Information Time Domain Concepts Frequency Domain Concepts Relationship between Data Rate and Bandwidth 2.2 Analog and Digital Data Transmission Analog and Digital Data Analog and Digital Signaling Analog and Digital Transmission 2.3 Channel Capacity Nyquist Bandwidth Shannon Capacity Formula 2.4 Transmission Media Terrestrial Microwave Satellite Microwave Broadcast Radio Infrared 2.5 Multiplexing 2.6 Recommended Reading and Web Sites 2.7 Key Terms, Review Questions, and Problems Key Terms Review Questions Problems Appendix 2A Decibels and Signal Strength i\LS

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2.1 / SIGNALS FOR CONVEYING INFORJVIATION 15 The purpose of this chapter is to make this book self-contained for the reader with little or no background in data communications. For the reader with greater interest, references for further study are supplied at the end of the chapter. In this book, we are concerned with electromagnetic signals used as a means to transmit information. An electromagnetic signal is a function of time, but it can also be expressed as a function of frequency; that is, the signal consists of components of different frequencies. It turns out that the frequency domain view of a signal is far more important to an understanding of data transmission than a time domain view. Both views are introduced here. Titne Domain Concepts Viewed as a function of time, an electromagnetic signal can be either analog or digital. An analog signal is one in which the signal intensity varies in a smooth fash- ion over time. In other words, there are no breaks or discontinuities in the signal. A digital signal is one in which the signal intensity maintains a constant level for some period of time and then changes to another constant level.! Figure 2.1 shows Amplitude (volts) L..- ~ Time (a) Analog Amplitude (volts) L..––l_––L ….I- .l…-__—J_—L_–I.-_–L- ..l– ~ Time (b) Digital Figure 2.1 Analog and Digital Waveforms lThis is an idealized definition. In fact, the transition from one voltage level to another will not be instan- taneous, but there will be a small transition period. Nevertheless, an actual digital signal approximates closely the ideal model of constant voltage levels with instantaneous transitions.

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Period =T = 1/f (a) Sine wave e

Tim

A ,-, ’“- ’0… ‘-’ ~ ’t:l 0 :l

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2.1 / SIGNALS FOR CONVEYING INFORMATION 17 period (1) of a signal, which is the amount of time it takes for one repetition; there- fore, T = 1/f. Phase is a measure of the relative position in time within a single period of a signal, as illustrated later. The general sine wave can be written s( t) = A sin(21Tft + cP) (2.1) A function with the form of Equation (2.1) is known as a sinusoid. Figure 2.3 shows the effect of varying each of the three parameters. In part (a) of the figure, the fre- quency is 1 Hz; thus the period is T = 1 second. Part (b) has the same frequency and phase but a peak amplitude of 0.5. In part (c) we have f = 2, which is equivalent to T = 1/2. Finally, part (d) shows the effect of a phase shift of 1T14 radians, which is 45 degrees (21T radians = 360 0 = 1 period). In Figure 2.3 the horizontal axis is time; the graphs display the value of a signal at a given point in space as a function of time. These same graphs, with a change of scale, can apply with horizontal axes in space. In that case, the graphs display the value of a signal at a given point in time as a function of distance. For example, for a sinusoidal transmission (say, an electromagnetic radio wave some distance from a radio antenna or sound some distance from loudspeaker) at a particular instant of time, the intensity of the signal varies in a sinusoidal way as a function of distance from the source. There is a simple relationship between the two sine waves, one in time and one in space. The wavelength (A) of a signal is the distance occupied by a single cycle, or, put another way, the distance between two points of corresponding phase of two consecutive cycles. Assume that the signal is traveling with a velocity v. Then the wavelength is related to the period as follows: A = vT. Equivalently, Af = v.Ofpar- ticular relevance to this discussion is the case where v = c, the speed of light in free space, which is approximately 3 X 108 m/s. Frequency DOluain Concepts In practice, an electromagnetic signal will be made up of many frequencies. For example, the signal s(t) = (4/1T) x (sin(21Tft) + (1/3)sin(21T(3f)t)) is shown in Figure 2Ac. The components of this signal are just sine waves of fre- quencies f and 3f; parts (a) and (b) of the figure show these individual components. There are two interesting points that can be made about this figure: ’“ The second frequency is an integer multiple of the first frequency. When all of the frequency components of a signal are integer multiples of one frequency, the latter frequency is referred to as the fundamental frequency. • The period of the total signal is equal to the period of the fundamental fre- quency. The period of the component sin(21Tft) is T = 1/f, and the period of s(t) is also T, as can be seen from Figure 2Ac. It can be shown, using a discipline known as Fourier analysis, that any signal is made up of components at various frequencies, in which each component is a sinusoid. By adding together enough sinusoidal signals, each with the appropriate

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————————r————————.————————, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , - - —— - -r—— - - - - ––––– ––1- - - - - - - — - —– – –., , , , , , , , , , ,,,,,, s(t) 0.5 1.0 ———–r————————r––––– , , , , , , , , , , , , , , , , , , , , , , - - -r––––– - – ———–1- – , , , , , , , , , , , , , , , , , , , , s(t) 0.5 1.0 0.0 -f–––––‘——+_——‘I t 1.5 s 0.5 1.0 (b) A = 0.5, f = 1, <p = 0 0.0 -0.5 0.0 -1’———–‘1.———1-,’———‘1 ,,,,,,,,,, –– - - - - - -1–– —–– — — – — –, , , , , , , , , , , , , , , , , , , , , -1.0 -+–––––;———–;-’–––––;’ t 1.5 s 1.0 = 1, f = 1, <p = 0 0.5 (a) A 0.0 -0.5 , , , , , , , , , , , , , , , , , , , ,—– ———–– – ––r- – – -r––––– –– –– — –., , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , -1.0 -+———+’—-’“‘––_+’————1’ -1.0 t -1.0 0.0 0.5 1.0 1.5 s 0.0 (c) A = 1, f= 2, <p = 0 ]1’igmc 2.3 set) A sin(27Tft + r:/» t 1.5 s

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2.1 / SIGNALS FOR CONVEYING INFOR..J’vlATION 19 1.0 -+––:;;<“-…..::—+––––1—..,.,.-……,…–+––––1 0.5 -++––~-+––––I-+––+––––I 0.0 –r—––+––––f––––-–––––) -0.5 -t–—–+-r––+-I—––+-r—.f—1 -1.0 -t––––+—..,,£..–+—––+—––=-.£….—I O.OT 0.5T LOT (a) sin(2’iTft) 1.5T 2.0T 1.0 -t———j—––+—––+–––––i 0.5 -t———j———I—––+–––––i 0.0 –r-.-+–+–+—–”_–f–_-1’—–--+--—1 -0.5 -+—––+———1—––+––––/ -1.0 -+—––+––––1—––+———1 O.OT 0.5T l.OT (b) (1/3)sin(2’iT(3j)t) l.5T 2.0T 1.0 -t–+—T–T–--+—––+–f-~r+-T+–––––j 0.5 —nr—––lk-t—––++—––t+–––––j 0.0 -+—––-——+—––+———1 -0.5 -+––––+--——f–l—––++—––+-1 -1.0 -+––––+—-–,~~-+–+—––+-__/‘’–;___,f––1 O.OT 0.5T 1.0T l.5T (c) (4/’iT) [sin(2’iTft) + (1I3)sin(2’iT(3j)t)] Hgme 2.4 Addition of Frequency Components (T = lit) 2.0T amplitude, frequency, and phase, any electromagnetic signal can be constructed. Put another way, any electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases. The importance of being able to look at a signal from the frequency perspective (frequency domain) rather than a time perspective (time domain) should become clear as the discussion proceeds. For the interested reader, the subject of Fourier analysis is introduced in Appendix B.

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20 CHl\PTER 2 / TRANSMISSION FUNDAMENTALS The spectrum of a signal is the range of frequencies that it contains. For the signal of Figure 2.4c, the spectrum extends fromfto 3f. The absolute bandwidth of a signal is the width of the spectrum. In the case of Figure 2.4c, the bandwidth is 3f - f = 2f. Many signals have an infinite bandwidth, but with most of the energy contained in a relatively narrow band of frequencies. This band is referred to as the effective bandwidth, or just bandwidth. Relationship between Data Rate and Bandwidth There is a direct relationship between the information-carrying capacity of a signal and its bandwidth: The greater the bandwidth, the higher the information-carrying capacity. As a very simple example, consider the square wave of Figure 2.2b. Suppose that we let a positive pulse represent binary 0 and a negative pulse represent binary 1. Then the waveform represents the binary stream 0101. … The duration of each pulse is 1I(2f); thus the data rate is 2fbits per second (bps). What are the frequency compo- nents of this signal? To answer this question, consider again Figure 2.4. By adding together sine waves at frequenciesfand 3f, we get a waveform that begins to resemble the square wave. Let us continue this process by adding a sine wave of frequency 5f, as shown in Figure 2.5a, and then adding a sine wave of frequency 7f, as shown in Figure 2.5b. As we add additional odd multiples of f, suitably scaled, the resulting waveform approaches that of a square wave more and more closely. Indeed, it can be shown that the frequency components of the square wave with amplitudes A and -A can be expressed as follows: 4 .; sin(21rkft) set) = A X - L.J 1r k odd k=l k This waveform has an infinite number of frequency components and hence an infi- nite bandwidth. However, the peak amplitude of the kth frequency component, kf, is only 11k, so most of the energy in this waveform is in the first few frequency compo- nents. What happens if we limit the bandwidth to just the first three frequency com- ponents? We have already seen the answer, in Figure 2.5a.As we can see, the shape of the resulting waveform is reasonably close to that of the original square wave. We can use Figures 2.4 and 2.5 to illustrate the relationship between data rate and bandwidth. Suppose that we are using a digital transmission system that is capa- ble of transmitting signals with a bandwidth of 4 MHz. Let us attempt to transmit a sequence of alternating Os and Is as the square wave of Figure 2.5c. What data rate can be achieved? We look at three cases. Case I. Let us approximate our square wave with the waveform of Figure 2.5a. Although this waveform is a “distorted” square wave, it is sufficiently close to the square wave that a receiver should be able to discriminate between a binary 0 and a binary 1. If we let f = 10 6 cycles/second = 1 MHz, then the bandwidth of the signal s( t) = i X [sine (21r X 10 6 )t) + !sin( (21r X 3 X 10 6 )t) + !sin( (21r X 5 X 10 6 )t)] 1r 3 5

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2.1 I SIGNALS FOR CONVEYING INFORMATION 21 1.0 -f-+--+_+–--+––––+-/–~~~~-++––––j 0,5 –+I—––t+———++———’I+———j 0.0 -f––––+—––.+—––4———J 2.0T 0.5T LOT 1.5T (a) (4I1r) [sin(217ft) + (l/3)sin(217(3f)t) + (1/5)sin(217(5f)t)] O,OT -1.0 -+––––+--–+-+4__I-+––––+-__~~~4__—+–I -0.5 -+–––––+-——I+––––H—––+I 0.0 -+—––+––––+———-–––––1 0,5 –*—––+––––+1––––+———–1 -1.0 -+———j-1l—J~~~~~-+———l-_+…p.!lor__,~,….+.j -0.5 -f–––––ft.—––I+———H–––––Jj O,OT 0.5T LOT l.5T 2.0T (b) (4/17) [sin(217ft) + (l/3)sin(217(3f)t) + (l/5)sin(217(5f)t) + (l/7)sin(217(7f)t)] 1.0 —+—––4———l—––4–––––J 0,5 -+––––+—––+—––+–––––1 0,0 -+––––+—––+—––+–––––1 -0.5 -f———+––––I––––+—–– -1.0 -+—––+––––+––––l—–– O,OT O,5T LOT 1.5T (c) (4/17) 2, (l/k)sin(217(kf)t), for k odd Figure 2.5 Frequency Components of Square Wave (T = lIf) 2.0T is (5 X 106 ) - 10 6 = 4 MHz. Note that for f = 1 MHz, the period of the fundamental frequency is T = 1/10 6 = 10-6 = 1 f..Ls. If we treat this waveform as a bit string of Is and Os, one bit occurs every 0.5 f..LS, for a data rate of 2 X 10 6 = 2 Mbps. Thus, for a bandwidth of 4 MHz, a data rate of 2 Mbps is achieved. / ..

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22 CHAPTER:2 I TRANSMISSION FUNDAMENTALS Case II. Now suppose that we have a bandwidth of 8 MHz. Let us look again at Figure 2.5a, but now with f = 2 MHz. Using the same line of reasoning as before, the bandwidth of the signal is (5 x 2 X 10 6) - (2 X 10 6) = 8 MHz. But in this case T = lIf = 0.5 j.Ls. As a result, one bit occurs every 0.25 j.LS for a data rate of 4 Mbps. Thus, other things being equal, by doubling the bandwidth, we double the potential data rate. Case III. Now suppose that the waveform of Figure 2Ac is considered ade- quate for approximating a square wave. That is, the difference between a positive and negative pulse in Figure 2Ac is sufficiently distinct that the waveform can be used successfully to represent a sequence of Is and Os. Assume as in Case II that f = 2 MHz and T = lIf = 0.5 j.LS, so that one bit occurs every 0.25 j.LS for a data rate of 4 Mbps. Using the waveform of Figure 2Ac, the bandwidth of the signal is (3 x 2 X 10 6 )- (2 X 10 6) = 4 MHz. Thus, a given bandwidth can support vari- ous data rates depending on the ability of the receiver to discern the difference between 0 and 1 in the presence of noise and other impairments. To summarize, • Case I: Bandwidth = 4 MHz; data rate = 2 Mbps • Case II: Bandwidth = 8 MHz; data rate = 4 Mbps • Case III: Bandwidth = 4 MHz; data rate = 4 Mbps We can draw the following conclusions from the preceding discussion. In general, any digital waveform will have infinite bandwidth. If we attempt to transmit this wave- form as a signal over any medium, the transmission system will limit the bandwidth that can be transmitted. Furthermore, for any given medium, the greater the bandwidth transmitted, the greater the cost. Thus, on the one hand, economic and practical reasons dictate that digital information be approximated by a signal of limited bandwidth. On the other hand, limiting the bandwidth creates distortions, which makes the task of interpreting the received signal more difficult. The more limited the bandwidth, the greater the distortion and the greater the potential for error by the receiver. The terms analog and digital correspond, roughly, to continuous and discrete, respec- tively. These two terms are used frequently in data communications in at least three contexts: data, signals, and transmission. Briefly, we define data as entities that convey meaning, or information. Signals are electric or electromagnetic representations of data.Transmission is the commu- nication of data by the propagation and processing of signals. In what follows, we try to make these abstract concepts clear by discussing the terms analog and digital as applied to data, signals, and transmission. Analog and Digital Data The concepts of analog and digital data are simple enough. Analog data take on continuous values in some interval. For example, voice and video are continuously

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2.2 / ANALOG AND DIGITAL DATA TRANSMISSION 23 Music ……… … ~ ~ ,,,,, Noise Approximate dynamic range of music

Upper limit of FM radio “” r Upper limit “I of AM radio “” , : ~ 1 Telephone channel I : 1 1 I I 1 I I I … I I “‘”,’\ 1 ’\ I , 1 , I , 1 \ 1 \ 1 \ 1 \1t1
1








-30 dB Speech Approximate dynamic range of voice 0 ~ ..c ’0 -20 Q,l “’Cl .5 .~ ….ell l-o l-o -40 Q,l ~ 0 ~ -60 10Hz 100 Hz 1 kHz Frequency 10kHz 100 kHz Figure 2.6 Acoustic Spectrum of Speech and Music [CARN99] varying patterns of intensity. Most data collected by sensors, such as temperature and pressure, are continuous valued. Digital data take on discrete values; examples are text and integers. The most familiar example of analog data is audio, which, in the form of acoustic sound waves, can be perceived directly by human beings. Figure 2.6 shows the acoustic spectrum for human speech and for music. Frequency components of typical speech may be found between approximately 100 Hz and 7 kHz. Although much of the energy in speech is concentrated at the lower frequencies, tests have shown that frequencies below 600 or 700 Hz add very little to the intelligibility of speech to the human ear. Typical speech has a dynamic range of about 25 dB;2 that is, the power produced by the loudest shout may be as much as 300 times greater than that of the least whisper. Analog and Digital Signaling In a communications system, data are propagated from one point to another by means of electromagnetic signals. An analog signal is a continuously varying elec- tromagnetic wave that may be propagated over a variety of media, depending on frequency; examples are copper wire media, such as twisted pair and coaxial cable; 2The concept of decibels is explained in Appendix 2A.

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24 CHAPTER 2 / TRANSMISSION FUNDAMENTALS Voltage at transmitting end Voltage at receiving end Figure 2.7 Attenuation of Digital Signals fiber optic cable; and atmosphere or space propagation (wireless). A digital signal is a sequence of voltage pulses that may be transmitted over a copper wire medium; for example, a constant positive voltage level may represent binary 0 and a constant negative voltage level may represent binary l. The principal advantages of digital signaling are that it is generally cheaper than analog signaling and is less susceptible to noise interference. The principal dis- advantage is that digital signals suffer more from attenuation than do analog signals. Figure 2.7 shows a sequence of voltage pulses, generated by a source using two volt- age levels, and the received voltage some distance down a conducting medium. Because of the attenuation, or reduction, of signal strength at higher frequencies, the pulses become rounded and smaller. It should be clear that this attenuation can lead rather quickly to the loss of the information contained in the propagated signal. Both analog and digital data can be represented, and hence propagated, by either analog or digital signals. This is illustrated in Figure 2.8. Generally, analog data are a function of time and occupy a limited frequency spectrum. Such data can be directly represented by an electromagnetic signal occupying the same spectrum. The best example of this is voice data. As sound waves, voice data have frequency compo- nents in the range 20 Hz to 20 kHz. As was mentioned, most of the speech energy is in a much narrower range, with the typical speech range of between 100 Hz and 7 kHz. The standard spectrum of voice signals is even narrower, at 300 to 3400 Hz, and this is quite adequate to propagate speech intelligibly and clearly. The telephone instrument does just that. For all sound input in the range of 300 to 3400 Hz, an electromagnetic signal with the same frequency-amplitude pattern is produced. The process is per- formed in reverse to convert the electromagnetic energy back into sound. Digital data can also be represented by analog signals by use of a modem (modulator-demodulator). The modem converts a series of binary (two-valued) voltage pulses into an analog signal by modulating a carrier frequency. The resulting signal occupies a certain spectrum of frequency centered about the carrier and may be propagated across a medium suitable for that carrier. The most common modems represent digital data in the voice spectrum and hence allow those data to be prop- agated over ordinary voice-grade telephone lines. At the other end of the line, a modem demodulates the signal to recover the original data. In an operation very similar to that performed by a modem, analog data can be represented by digital signals. The device that performs this function for voice data is a codec (coder-decoder). In essence, the codec takes an analog signal that directly represents the voice data and approximates that signal by a bit stream. At the other end of the line, a codec uses the bit stream to reconstruct the analog data. This topic is explored subsequently.

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2.2 I ANALOG AND I)IGITr-\L DXL~ TRANSMISSION 25 Analog signals: Represent data with continuously varying electromagnetic wave Analog signal Analog data (voice sound waves) Telephone n.rLrl Digital data ……f——~~ (binary voltage pulses) ~Analog signal (modulated on carrier frequency) Modem Digital signals: Represent data with sequence of voltage pulses ~ JlJ1Jl Analog ~gnal • • ~. • Digital ,ignal Codec Digital data Digital transceiver Digital signal Figure 2.8 Analog and Digital Signaling of Analog and Digital Data Finally, digital data can be represented directly, in binary form, by two voltage levels. To improve propagation characteristics, however, the binary data are often encoded into a more complex form of digital signal, as explained subsequently. Each of the four combinations (Table 2.1a) just described is in widespread use. The reasons for choosing a particular combination for any given communications task vary. We list here some representative reasons: .. Digital data, digital signal: In general, the equipment for encoding digital data into a digital signal is less complex and less expensive than digital-to- analog equipment. .. Analog data, digital signal: Conversion of analog data to digital form permits the use of modern digital transmission and switching equipment for analog data.

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26 CHAPTER:2 / TR.i\NS“vUSSIONFUNDA.JVIENTl\LS Table 2.1 Analog and Digital Transmission (a) Data and Signals Analog Data Digital Data Analog Signal Two alternatives: (l)signal occupies the same spectrum as the analog data; (2) analog data are encoded to occupy a different portion of spectrum. Digital data are encoded using a modem to produce analog signal. Digital Signal Two alternatives: (1) signal consists of two voltage levels to represent the two binary values; (2) digital data are encoded to produce a digital signal with desired properties. (b) Treatment of Signals Analog Transmission Digital Transmission Analog Signal Digital Signal Is propagated through amplifiers; same treatment whether signal is used to represent analog data or digital data. Not used Assumes that the analog signal represents digital data. Signal is propagatedthrough repeaters; ateach. repeater, digital data are recovered from inbound signal and used to generate a new analog outbound signal. Digital signal represents a stream of Is and Os, which may represent digital data or may be an encoding of analog data. Signal is propagated through repeaters; at each repeater, stream of Is and Os is recovered from inbound signal and used to generate a new digital outbound signal. III Digital data, analog signal: Some transmission media, such as optical fiber and satellite, will only propagate analog signals. 0) Analog data, analog signal: Analog data are easily converted to an analog signal. Analog and Digital Both analog and digital signals may be transmitted on suitable transmission media. The way these signals are treated is a function of the transmission system. Table 2.1b summarizes the methods of data transmission. Analog transmission is a means of transmitting analog signals without regard to their content; the signals may represent analog data (e.g., voice) or digital data (e.g., data that pass through a modem). In either case, the analog signal will suffer attenuation that limits the length of the transmission link. To achieve longer distances, the analog transmission system includes amplifiers that boost the energy in the signal. Unfortunately, the amplifier also boosts the noise components. With amplifiers cascaded to achieve long distance, the signal becomes more and more distorted. For analog data, such as voice, quite a bit of distortion can be tolerated and the data remain intelligible. However, for digi- tal data transmitted as analog signals, cascaded amplifiers will introduce errors.

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2.3 / C:HANNEL CAPACITY 27 Digital transmission, in contrast, is concerned with the content of the signal. We have mentioned that a digital signal can be propagated only a limited distance before attenuation endangers the integrity of the data. To achieve greater distances, repeaters are used. A repeater receives the digital signal, recovers the pattern of ones and zeros, and retransmits a new signal. Thus, the attenuation is overcome. The same technique may be used with an analog signal if the signal carries digital data. At appropriately spaced points, the transmission system has retransmission devices rather than amplifiers. The retransmission device recovers the digital data from the analog signal and generates a new, clean analog signal. Thus, noise is not cumulative. A variety of impairments can distort or corrupt a signal. A common impairment is noise, which is any unwanted signal that combines with and hence distorts the signal intended for transmission and reception. Noise and other impairments are discussed in Chapter 5. For the purposes of this section, we simply need to know that noise is something that degrades signal quality. For digital data, the question that then arises is to what extent these impairments limit the data rate that can be achieved. The maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions is referred to as the channel capacity. There are four concepts here that we are trying to relate to one another: .. Data rate: This is the rate, in bits per second (bps), at which data can be communicated. ’“ Bandwidth: This is the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium, expressed in cycles per second, or Hertz. 1& Noise: For this discussion, we are concerned with the average level of noise over the communications path. .. Error rate: This is the rate at which errors occur, where an error is the reception of a 1 when a 0 was transmitted or the reception of a 0 when a 1 was transmitted. The problem we are addressing is this: Communications facilities are expensive and, in general, the greater the bandwidth of a facility, the greater the cost. Further- more, all transmission channels of any practical interest are of limited bandwidth. The limitations arise from the physical properties of the transmission medium or from deliberate limitations at the transmitter on the bandwidth to prevent interference from other sources. Accordingly, we would like to make as efficient use as possible of a given bandwidth. For digital data, this means that we would like to get as high a data rate as possible at a particular limit of error rate for a given bandwidth. The main constraint on achieving this efficiency is noise. Nyquist Bandwidth To begin, let us consider the case of a channel that is noise free. In this environment, the limitation on data rate is simply the bandwidth of the signal. A formulation of

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28 CIIAPTFR 2 / TRANSMISSION FUNDAMFN’I.i\LS this limitation, due to Nyquist, states that if the rate of signal transmission is 2B, then a signal with frequencies no greater than B is sufficient to carry the signal rate. The converse is also true: Given a bandwidth of B, the highest signal rate that can be car- ried is 2B. This limitation is due to the effect of intersymbol interference, such as is produced by delay distortion. 3 The result is useful in the development of digital-to- analog encoding schemes. Note that in the preceding paragraph, we referred to signal rate. If the signals to be transmitted are binary (take on only two values), then the data rate that can be supported by B Hz is 2B bps. As an example, consider a voice channel being used, via modem, to transmit digital data. Assume a bandwidth of 3100 Hz. Then the capacity, C, of the channel is 2B = 6200 bps. However, as we shall see in Chapter 6, signals with more than two levels can be used; that is, each signal element can repre- sent more than one bit. For example, if four possible voltage levels are used as signals, then each signal element can represent two bits. With multilevel signaling, the Nyquist formulation becomes C = 2B logz M where M is the number of discrete signal elements or voltage levels. Thus, for M = 8, a value used with some modems, a bandwidth of B = 3100 Hz yields a capacity C = 18,600 bps. So, for a given bandwidth, the data rate can be increased by increasing the number of different signal elements. However, this places an increased burden on the receiver: Instead of distinguishing one of two possible signal elements during each signal time, it must distinguish one of M possible signals. Noise and other impairments on the transmission line will limit the practical value of M. Shannon Capacity Forulula Nyquist’s formula indicates that, all other things being equal, doubling the band- width doubles the data rate. Now consider the relationship among data rate, noise, and error rate. The presence of noise can corrupt one or more bits. If the data rate is increased, then the bits become “shorter” in time, so that more bits are affected by a given pattern of noise. Thus, at a given noise level, the higher the data rate, the higher the error rate. Figure 2.9 is an example of the effect of noise on a digital signal. Here the noise consists of a relatively modest level of background noise plus occasional larger spikes of noise. The digital data can be recovered from the signal by sampling the received waveform once per bit time. As can be seen, the noise is occasionally suffi- cient to change a 1 to a 0 or a 0 to a 1. All of these concepts can be tied together neatly in a formula developed by the mathematician Claude Shannon. As we have just illustrated, the higher the data rate, the more damage that unwanted noise can do. For a given level of noise, we would expect that a greater signal strength would improve the ability to receive data correctly in the presence of noise. The key parameter involved in this reasoning is 3Delay distortion of a signal occurs when the propagation delay for the transmission medium is not con- stant over the frequency range of the signal.

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2.3 / CHANNEL CAPACITY 29 Data transmittted: a a a a a a a Signal: Noise: Signal plus noise: Sampling times: I I Data received: a a a a a a a Original data: a a a 100 I 10 I a “—— Bits in error ~ Figure 2.9 Effect of Noise on a Digital Signal the signal-to-noise ratio (SNR, or S/N),4 which is the ratio of the power in a signal to the power contained in the noise that is present at aparticular point in the transmis- sion. Typically, this ratio is measured at a receiver, because it is at this point that an attempt is made to process the signal and eliminate the unwanted noise. For conve- nience, this ratio is often reported in decibels: signal power SNR dB = 10 10glO–.— nOIse power This expresses the amount, in decibels, that the intended signal exceeds the noise level. A high SNR will mean a high-quality signal. 4S ome of the literature uses SNR; others use SIN. Also, in some cases the dimensionless quantity is referred to as SNR or SIN and the quantity in decibels is referred to as SNRdb or (SIN)db’ Others use just SNR or SIN to mean the dB quantity. This text uses SNR and SNRdb .

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30 CHiWTER 2 / TRANSMISSION FUNDAMENTALS The signal-to-noise ratio is important in the transmISSIOn of digital data because it sets the upper bound on the achievable data rate. Shannon’s result is that the maximum channel capacity, in bits per second, obeys the equation C = B log2(1 + SNR) where C is the capacity of the channel in bits per second and B is the bandwidth of the channel in Hertz. The Shannon formula represents the theoretical maximum that can be achieved. In practice, however, only much lower rates are achieved. One reason for this is that the formula assumes white noise (thermal noise). Impulse noise is not accounted for, nor are attenuation distortion or delay distortion. Various types of noise and distortion are discussed in Chapter 5. The capacity indicated in the preceding equation is referred to as the error-free capacity. Shannon proved that if the actual information rate on a channel is less than the error-free capacity, then it is theoretically possible to use a suitable signal code to achieve error-free transmission through the channeL Shannon’s theorem unfortu- nately does not suggest a means for finding such codes, but it does provide a yardstick by which the performance of practical communication schemes may be measured. Several other observations concerning the preceding equation may be instruc- tive. For a given level of noise, it would appear that the data rate could be increased by increasing either signal strength or bandwidth. However, as the signal strength increases, so do the effects of nonlinearities in the system, leading to an increase in intermodulation noise. Note also that, because noise is assumed to be white, the wider the bandwidth, the more noise is admitted to the system. Thus, as B increases, SNR

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