Understanding Telecommunicationsand Lightwave Systems

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IEEE Press Editorial BoardStamatios V. Kartalopoulos, Editor in Chief

M. Akay M. Eden M. S. NewmanJ. B. Anderson M. E. El-Hawary M. PadgettR. J. Baker R. J. Herrick W. D. ReeveJ. E. Brewer R. F. Hoyt G. Zobrist

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Technical ReviewersMichael Newman, CSI Telecommunications, San Francisco, CA

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Understanding Telecommunicationsand Lightwave Systems

An Entry-Level Guide

Third Edition

John G. NellistConsultant, Savita Enterprises, Ltd.

IEEEIEEE Press

WILEY-INTERSCIENCE

JOHN WILEY & SONS, INC.

This text is printed on acid-free paper. ©

Copyright © 2002 by the Institute of Electrical and Electronics Engineers, Inc. All rights reserved.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in anyform or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise,except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, withouteither the prior written permission of the Publisher, or authorization through payment of theappropriate per-copy fee to the Copyright Clearance Center, 222 Rosewood Drive, Danvers, MA01923, (978) 750-8400, fax (978) 750-4744. Requests to the Publisher for permission should beaddressed to the Permissions Department, John Wiley & Sons, Inc., 605 Third Avenue, New York,NY 10158-0012, (212) 850-6011, fax (212) 850-6008, E-Mail: PERMREQ @ WILEY.COM.

For ordering and customer service, call 1-800-CALL-WILEY.

Library of Congress Cataloging in Publication Data is available.

ISBN 0-471-15032-0

Printed in the United States of America.

1 0 9 8 7 6 5 4 3 2 1

This book is dedicated to the memory of Elliott M. Gilbert(1924 - 2000)

A good friend and a brave Marine, who, in WWII,fought in the bloodiest conflictin the South Pacific, Iwo Jima.

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Contents

Introduction 11 The Evolution of Telecommunications 3

The Telegraph 3The Telephone 3Wireless Communications 4AT&T Monopoly 4Trans-Canada System 4Semiconductors 5Digital Communications 5Satellite Communications 6Fiber Optics 6AT&T Break-Up 6Telecommunications Act of 1996 8The Meltdown 10Conclusion 10

2 Analog Transmission 12Conclusion 15Review Questions for Chapter 2 15

3 Digital Transmission 17Conclusion 19Review Questions for Chapter 3 20

4 Basic Multiplexing Techniques 21Brief History 21FDM 21TDM 22Pulse-Code Modulation 22Conclusion 27Review Questions for Chapter 4 27

5 Switching Hierarchy 28Brief History '. 28Divestiture 30Conclusion 32Review Questions for Chapter 5 32

vii

VIII Contents

6 North American Digital Hierarchies 34Conclusion 37Review Questions for Chapter 6 37

7 Transmission 38Transmission Level 39Via Net Loss (VNL) 40All-Digital Network 40Echo Suppressors and Echo Cancellers 43Split Echo Suppressor 44Full Echo Suppressor 45Digital Echo Canceller 45Conclusion 46Review Questions for Chapter 7 47

8 The Local Subscriber Loop 48Brief History 48The Local Loop 48Digital Subscriber Carrier Systems 50Asymmetric Digital Subscriber Lines 52Conclusion 54Review Questions for Chapter 8 55

9 Microwave Radio 56Brief History 56Digital Radio 56Design Considerations 58SONET Radio 60Conclusion 61Review Questions for Chapter 9 61

10 Satellite Communications 63Brief History 63Geostationary (GEO) Satellites 65Parking Slots 66Transponders 67The Footprint 68Time Delay 68Global Positioning System 70MEO and LEO Satellites 71Direct-to-Home Satellite System 75Satellite Operators 75Conclusion 76Review Questions for Chapter 10 76

11 Switching Systems 78The Strowger Switch 78Panel Switching Syste 79Crossbar Switch 79

Contents IX

The First Electronic Switch 80The Digital Switch 81The Optical Switch 84Optical Switches with Electrical Cores 85All-Optical Switches 85Conclusion 90Review Questions for Chapter 11 90

12 Private Branch Exchange 92Brief History 92Analog PBX 92Digital PBX 93Conclusion 95Review Questions for Chapter 12 96

13 Traffic Considerations 97Brief History 97CCS 97Grade of Service 98Internet Congestion 99Conclusion 101Review Questions for Chapter 13 101

14 Video Transmission 102Brief History 102Video Compression 102Switched Data Services 103Video Codecs 103Videophone 104Multimedia 105Conclusion 107Review Questions for Chapter 14 108

15 Wireless 109Brief History 109Cellular Telephone Service 109The Mobile Unit 111Personal Communications Services (PCS) 113The Standards 114The Future 115Conclusion 117Review Questions for Chapter 15 119

16 The Computer 120Brief History 120Digital Computers 120Microprocessors 123Hard Drive 123End of an Era 125

X Contents

Internet Appliances 125The Evolution of the Silicon Chip 127Bluetooth 127Conclusion 128Review Questions for Chapter 16 130

17 The Internet 131Brief History 131Internet Access 132Asymmetric Digital Subscriber Line 133Very High Speed Digital Subscriber Line 134Integrated Services Digital Network 134Cable Modems 136Satellite 137Local Multipoint Distribution System 138Circuit Switched vs. Packet Switched 139Internet Telephony 142Conclusion 143Review Questions for Chapter 17 144

18 Lightwave Systems 145Brief History 145New Transcontinental Carriers 149Lightwave Undersea Systems 150Evolution of Lightwave Systems 150Optical Fiber Cable 154Optical Fiber Transmission Parameters 157Fiber Connectors 163Optical Emitters 165Optical Detectors 167Tunable Lasers 168All-Optical Networks 169Dense Wavelength-Division Multiplexing 169Optical Amplifiers 171Optical Cross-Connects 173System Design Considerations 175Synchronous Optical Network 178Asynchronous Transfer Mode 186SONET Ring Networks 190System Availability 192Cable Placement Choices 193Cable Placement Techniques 195Placement of Underground Cable 196Placement of Aerial Cable 197Placement of Direct Buried Cable 199Field Splicing 203Mechanical Splicing 205Field Testing Using an Optical Time Domain Reflectometer 205

Contents XI

Fiber-to-the-Home 209Video-on-Demand (VOD) 209Fiber-to-the-Curb 210Local Area Network 213Ethernet 214IBM Token Ring 215LAN Topology 216Wireless LANs 218Fiber Optic LANs 218Conclusion 224Review Questions for Chapter 18 225

Telecommunications Glossary 228Bibliography 250Index 251Answers to Review Questions 259About the Author 269

Credit:

Eight figures were reprinted with permission from Under-standing Modern Telecommunications and the InformationSuperhighway by John G. Nellist and Elliott M. Gilbert. ArtechHouse, Inc., Norwood, MA, USA. www.artechhouse.com

Introduction

There is growing recognition throughout the world that futureeconomic and social progress will depend on effectivecommunications. The exiting Internet has created a newinteractive network economy. This new economy is now largerthan the old industrial economy, which included automanufacturing, construction and food services. The speed andforce with which this new network economy is moving is notyet recognized by the general public; however, the influence itwill have on how people live and work will be global andunalterable. With Internet traffic now growing tenfold yearly,there is an insatiable demand for more bandwidth.

The common goal is a global digital all-optical network thatwill have the capacity to carry voice circuits, Internet traffic,music and video. Lightwave communications systems haveclearly emerged as the only technology that can meet thebandwidth requirements of this new network economy.

All over the world, optical fibers are replacing othertransmission media in a wide variety of applications, includinglong-haul trunking systems, metropolitan telephone networks,undersea transmission links and the local loop to the home.Optical fiber provides the important requirements of widebandwidth, reliability and security. It is immune to most ofthe technical and regulatory constraints of traditionalcommunication systems.

The race to build a cheaper, more efficient all-optical networkthat will provide almost unlimited bandwidth has just begun.Optical amplifiers, optical cross-connects, dense wavelengthdivision multiplexers and optical switches are some of the

1

2 Introduction

devices that are being developed. A number of new companiesare building transcontinental backbone networks of fiber opticcable linking all major cities in the world. These networks arebased on Internet Protocol (IP) technology.

The third edition of Understanding Telecommunications andLightwave Systems has been completely updated to reflect theadvances that have been made in the telecommunicationsindustry as a result of the explosive growth of the Internet.

This new edition includes a revised Chapter 15 on the wirelessrevolution and the introduction of third-generation cell phoneswith microbrowser capability. It also includes an expandedChapter 11 with a section on optical switching and a revisedand expanded Chapter 18 on lightwave systems. Chapter 16,The Computer and Chapter 17, The Internet, have been added.The Computer covers the evolution of this machine and itsfuture potential. The Internet examines this multimediastructure which has created the new network economy that ischanging the way people communicate at the most fundamentallevels.

Although this book deals primarily with the technical andregulatory trends in North America, the basic technology iscommon to telephone networks throughout the world. Theydiffer only in the political, regulatory and societal conditionsexisting in each country.

The book has proven to be very popular in colleges anduniversities as a basic text for Man & Technology courses. Ithas also been used for a series of telecommunication seminarspresented to senior citizens attending Eldercollege.

This book aims to provide an overview of telecommunicationsas well as an introduction to the exciting technology of lightwavecommunications. With the range of applications continuing toexpand with growing speed, this technology promises to turnthe electronic age into the age of optics.

1The Evolution ofTelecommunications

The TelegraphFor centuries, long distance communications had been carriedout by means of signal fires, lamps and flashing mirrors.However, the electric telegraph developed by Samuel Morsearound 1835 actually launched long distance communications.By 1843, a telegraph line was constructed from Washington,D.C. to Baltimore. The first message sent over this line was"What hath God wrought?" which was certainly a profoundquestion in view of the developments that followed.

By 1850, Western Union was formed in Rochester, New York.Its purpose was to carry messages in a coded form (a dot and adash) from one person to another over a privately controlledbut publicly accessible network.

The TelephoneAlexander Graham Bell demonstrated his invention, thetelephone, on March 10,1876. Bell displayed the telephone atthe 1876 World's Fair in Philadelphia. The Bell TelephoneCompany was formed in 1877 to produce the telephonecommercially. In the same year, Western Union created theAmerican Speaking Telephone Company as a competitor to Bell.By 1880, the American Bell Telephone Company was organizedto serve as the parent company. Two years later, WesternElectric Company was purchased to ensure a ready supply oftelephones and related equipment. In 1885, the company wasincorporated and became the American Telephone andTelegraph Company (AT&T).

3

4 1 The Evolution of Telecommunications

Wireless Communications

Guglielmo Marconi, an Italian physicist, was the inventor ofwireless communications, radio telegraphy. At the turn of thecentury many scientists believed that the curvature of the earthwould limit practical use of the wireless to a distance of 100 -200 miles. However, in 1901, Marconi proved them wrong bytransmitting a message across the Atlantic Ocean.

In 1906, Lee DeForest announced his invention, the audiontube, the forerunner to the electron tube. The electron tubewas used in radio to pick up faint electromagnetic signals andboost them a thousand times stronger than the received signal.Although it was intended to improve the sensitivity ofradiotelegraph receivers, telephone engineers saw its potentialfor boosting long-distance telephone signals as well.

AT&T Monopoly

By 1910, AT&Thad gained control of Western Union. The U. S.Department of Justice threatened to institute an antitrust suitagainst AT&T in 1913, so it then agreed to dispose of WesternUnion. In 1934, the U. S. Congress created the FederalCommunications Commission (FCC) and defined its powers inthe Communication Act (1934). The FCC has jurisdiction overinterstate and foreign commerce in communications but nottelecommunications within a state. This is regulated by thestate public utility commissions.

Trans-Canada System

In Canada, the Trans-Canada telephone system was turnedup for service on January 25,1932, using an all-Canadian route.The system consisted of two open-wire pairs strung on poleswith voice repeaters spaced about 200 miles apart. In 1948,an additional open wire pair was strung across Canada topermit the installation of the first three-channel carrier system.This method enabled more than one telephone circuit to becarried over a pair of open wires at the same time. Ten years

1 The Evolution of Telecommunications 5

later, the first microwave radio (TD-2) was turned up for serviceacross Canada. The microwave radio provided better qualitylong-distance circuits and more reliability than the open wire.

Semiconductors

Scientists at Bell Laboratories in the United States introducedthe transistor in 1947. The transistor is a solid-state deviceand does not require a heated cathode like the vacuum tube.Within a few years, the Bell Telephone System containedmillions of transistorized elements.

By 1959, Texas Instruments and Fairchild Semiconductorssuccessfully produced integrated circuits with transistors,capacitors and resistors placed on a square of silicon. Now,with an entire set of integrated circuits mounted together on aboard, the entire board, which cost only a few dollars, could beremoved and replaced in the event of a problem.

Digital Communications

In 1962, Bell Labs designed the first commercial pulse-codemodulation (PCM) cable carrier system. The introduction ofthe Bell System's T-carriers was the beginning of a trend towarddigital communication in the telephone network.

Up until 1968, AT&T specified that only equipment furnishedby AT&T could be attached to AT&T facilities. The CarterElectronic Corp. wanted to connect their mobile radio systemto the telephone network. In the landmark 1968 CarterfoneDecision, the FCC ruled that the AT&T restriction wasunreasonable. This ruling gave birth to the interconnectindustry, thus speeding the development of private switch-boards and other devices for interconnection to the telephonenetwork.

In 1969, a second landmark decision by the FCC permittedMicrowave Communications, Inc. (MCI) to begin constructionof a microwave radio system from St. Louis to Chicago. Thisprivate line system offered direct competition with Bell Tele-

6 1 The Evolution of Telecommunications

phone's toll network. This decision opened the door to otherspecialized common carriers.

Satellite Communications

The world's first commercial communication satellite waslaunched on April 6, 1965 by the Communications SatelliteCorporation (Comsat) in the United States. This satellite,named Intelsat 1 or Early Bird, was placed in geostationaryequatorial orbit over the Atlantic Ocean. Although the UnitedStates was a pioneer in the establishment of an internationalsatellite network, Canada had the first domestic satellitesystem. The satellite, designated Anik A1, was launched inNovember 1972 and began commercial service on January 11,1973.

Fiber Optics

The development of optical fiber for use in the telecommu-nications industry did not begin until the mid 1960s. It wasinitiated in 1966 by the publication of a paper by Dr. C. K. Kao(ITT England), in which he stated that pure optical fiber wastheoretically capable of guiding a light signal with very littleloss. By 1970, Corning Glass Works in the United States wasable to produce a fiber of sufficient purity for use intelecommunications. To make the fiber, Corning used a methodof synthesizing silica glass. The raw materials were vaporizedand deposited inside a length of quartz glass tubing, which wasthen collapsed into a rod and drawn into a fiber. The technologyfor the other two key elements of a fiber optic system, the laserand photodiode, had been developed independently during theprevious ten years.

AT&T Break-Up

Up until 1982, AT&T provided 85% of all local telephone serviceand nearly 97% of all long-distance telephone service. Then in1982, AT&T and the U. S. Justice Department agreed to anantitrust settlement worked out by Judge Harold H. Greene.

1 The Evolution of Telecommunications 7

It required the divestiture of the 22 local operating companies.The United States was divided into 160 local access andtransport areas (LATAs). The Bell operating companies (BOCs)were allowed to provide only local telephone service within theLATAs. They were forbidden from providing inter-LATAservice. Each of the 22 companies was incorporated into oneof seven regional Bell operating companies (RBOCs), as shownin Figure 1-1.

On January 1, 1984 AT&T's monopoly of telephone com-munications in the United States was over. The seven RBOCswere now all independent of AT&T, but, they were restricted toproviding local telephone service within their own area.However, subject to Judge Greene's approval, they beganentering many new business areas.

The deregulation of the long-distance telephone industry in theUnited States created opportunities for other telephone carriersto establish networks of their own. AT&T's major competitors,MCI and Sprint, began building nationwide lightwave (fiberoptic) networks similar to the AT&T network.

Figure 1-1 Seven Regional Bell Operating Companies

8 1 The Evolution of Telecommunications

Telecommunications Act of 1996

In February, 1996, the U. S. Congress passed the Telecom-munications Act of 1996, which allowed long distance carriersand cable television companies to provide long distance access,local telephone services and data transmission services tosubscribers in addition to television programs. It also allowedthe RBOCs to provide long-distance service to its subscribersand to build video dial tone networks so that they can becomecommon carriers to providers of television programming.

This radical measure removes the regulatory barriers that haveseparated the telephone, cable and broadcast industries. Inparticular, it allowed long-distance operators such as AT&T,MCI and Sprint to compete in regional markets, and localoperators (called the Baby Bells) to compete in the long-distancemarket.

In 1997, SBC (the Southwest Bell Company RBOC) purchasedPacific Telesis (another RBOC) while Nynex and Bell Atlantic(two other RBOCs) merged, putting back together some of whatJudge Greene rent asunder in his Modified Final Judgment of1984. In 1994, Pacific Telesis had spun off AirTouch, theirwireless division, but in 1999, British wireless giant Vodafonebought AirTouch for $56 billion.

The fear of competition in the new liberalized markets and theexplosive growth of the Internet have caused a rash of mergersand strategic alliances in the telecommunications industry. In1999, SBC announced it would buy Ameritech Corp. for $62billion, the largest merger in telecommunications history. In2000, Bell Atlantic and GTE merged to form Verizon, the largestlocal phone company in the United States. U. S. West, whichhad split off its cable division, MediaOne, in 1995, soldMediaOne to AT&T for $54 million. Two weeks later, in June2000, Qwest bought US West for $58 billion. U. S. telecom-munications companies continue to race for partners as thevoice and data business becomes increasingly global and theInternet blurs country borders. Table 1-1 lists the top ninemergers.

1 The Evolution of Telecommunications 9

Table 1-1 Top Nine Mergers

Not all mergers have succeeded however. For example, theproposed $129 billion merger of MCI WorldCom and Sprint,the second and third largest phone companies in the UnitedStates, did not succeed. The U. S. Justice Department blockedthe merger on the grounds it would increase prices for millionsof consumers. Meanwhile, AT&T, which had spun off LucentTechnologies in 1996, has split into four separate operations:consumer long distance, business services, broadband (mostlycable) and wireless. With long-distance revenues falling, AT&Tand WorldCom are losing interest in this sector. But theremaining Baby Bells — Qwest, BellSouth, Verizon and SBC— have taken over a large share of this market in New Yorkand Texas. They are currently lobbying to enter the other 48states that bar incumbents from competing.

In Canada, B. C. Telecom in British Columbia and Telus Corp.in Alberta agreed on a $6.5 billion merger. The deal created acompany that can compete nationally with Bell Canada, thenation's largest phone company. Telus has since purchasedClearnet Communications Inc., based in Toronto, for $3.1billion. The deal made Telus the nation's largest wirelesstelephony provider with 1.8 million customers. It gave the

AOL

Ameritech

Qwest

Airtouch

MediaOne

Tele-corn Inc.

GTE

Bell Atlantic

SBC Com

$ billion

Time Warner 350.0

SBC Com 62.6

U. S. West 58.0

Vodafone 56.0

AT&T 54.0

AT&T 53.6

Bell Atlantic 53.4

Nynex 51.0

Pacific Telesis 50.0

10 1 The Evolution of Telecommunications

company a national wireless presence three years earlier thanif it had to build a network from the ground up.

The MeltdownOver the past two years, 160 million kilometers of optical fiberhave been laid around the world as companies spent $35 billionto create networks based on Internet Protocol (IP) technology.Four new transcontinental carriers, Qwest, Global Crossing,Level 3 and 360networks laid most of this fiber.

As a result, there was a glut of fiber capacity just as the NorthAmerican economy began to slowdown. Global Crossing's shareprice plunged 89%. Cash starved 360networks failed to makean interest payment of $11 million and was forced to seekbankruptcy protection.

Major telephone carriers cut their spending on telecom-munication equipment. The sales slump, immediately hit themajor suppliers, Nortel Networks, Lucent Technologies andCisco Systems. All three companies had been pursuing astrategy based on aggressive expansion and acquisition. Assales of telecommunication equipment declined, these firmshave reported record loses. Altogether, more than 100,000 jobshave been eliminated in the telecommunications industry overthe past year. The technology companies that fuelled thebooming economy of the late 1990s are now going through aperiod of adjustment.

ConclusionToday, only about 10% of American homes have high-speedaccess to the Internet. The demand for a low cost, always on,broadband connection to small offices and homes will act as acatalyst for new entrepreneurs and investors. The new networkeconomy is here to stay. The Internet will continue to growand the demand for more bandwidth will continue to increase.The glut of fiber is only temporary and the technologycompanies will once again propel the new economy

1 The Evolution of Telecommunications 11

The Internet and the new network economy is forcing themerger of telecommunications, television, computers,publishing and information services into a single interactiveinformation industry. The existing global telecommunicationnetwork, which is partly optical and partly electrical, will beunable to keep up with the insatiable demand for bandwidth.As a result, the race is on to build an all-optical network.

On one side are the huge equipment manufactures, NortelNetworks, Lucent Technologies and Cisco Systems. On theother side are hundreds of smaller companies working on asingle problem, such as how to build the first all-opticalswitching machine. For those companies that can develop amore efficient all-optical network, which will provide almostunlimited bandwidth, the pay off could be enormous.

2 Analog Transmission

When a person speaks, the vocal cords vibrate, producingsounds that are carried to the mouth. The sounds produced inspeech contain frequencies that are in the 100 to 10,000 hertz(Hz) frequency band.

The notes produced by musical instruments occupy a muchwider frequency band than that occupied by speech. Someinstruments have a fundamental frequency of 50 Hz or less,while many other instruments can produce notes in excess of15,000 Hz.

Sound waves when they enter the ear, cause the eardrum tovibrate, thus producing signals. These signals, in the form ofelectric currents, are sent to the brain where they areinterpreted as sound.

The human ear can distinguish frequencies between 30 Hz and16,500 Hz. The average human voice ranges between 200 Hzand 5,000 Hz, as shown in Figure 2-1. Telephone companycircuits operate over a range of frequencies from 300 Hz to 3,400Hz. This is sufficient to make a person's voice recognizableand understandable.

Sound moves, through the air in waves. The shorter thewavelength, the higher the frequency, or pitch, of the sound is.Most sound waves, including our voices, are made up of manydifferent frequencies and degrees of loudness.

Electricity moves through telephone wires in much the sameway as sound waves move through the air. In the case ofelectricity, electrons bump against other electrons, sending theirenergy from one end of the wire to the other. Speech is

12

2 Analog Transmission 13

Figure 2-1 The Spectrum of Human Voice

transmitted by these electrical waves with the electricity inthe wires vibrating in the same pattern as the sound waves.

Referring to Figure 2-2, when a person speaks into themouthpiece of a telephone, the sound waves made by thevibration of the vocal cords strike a thin diaphragm, causing itto vibrate. As the sound waves compress the air against thediaphragm, tiny carbon granules through which electric currentis flowing are closely packed together. This creates a goodelectrical path for the current.

When the sound waves become less dense, the diaphragmsprings back to its original position and the granules of thecarbon move farther apart. This change reduces the flow ofcurrent. The diaphragm moves back and forth many hundredsof times per second in a pattern corresponding to the soundwaves striking it. Thus, the amount of electricity flowingthrough the carbon granules varies, generating a signal. It isthis electrical signal that is sent through the wires to thereceiver at the other end.

At the receiver end, the electrical signals drive an electro-magnet, whose varying force causes a diaphragm in the receiverto vibrate. The vibrations passing through the air between

14 2 Analog Transmission

Figure 2-2 Analog Transmission

the receiver diaphragm and your ear are duplicates of the soundwaves that struck the diaphragm of the transmitter at the otherend.

The direct current (DC) for the telephone instrument is providedby a 48-volt battery at the local telephone office and fed to thetelephone over a twisted pair of copper wires designated "tip"and "ring." When the user lifts the handset from the cradle,the instrument draws the direct current from the line whenthe switch hook closes (see Figure 2-3). The carbon microphonesin the original handset have been replaced by miniature self-polarized capacitive microphones, called electrets. The originalelectromagnetic receivers have been replaced by piezoelectricreceivers.

In the late 1960s and early 1970s, a transducer for an electronicringer was introduced that eventually replaced the hammerand bell ringer that was invented in 1878. The rotary dial hasbeen largely replaced by the push-button keypad. The keypadcontrols the circuitry that generates either dial pulses or thetones for touch-tone dialing.

2 Analog Transmission 15

Figure 2-3 Telephone Power Supply

Conclusion

Information is produced and transmitted over the telephonenetwork as electrical signals. These signals have two forms:analog and digital. Analog signals are continuous and can bethought of as electrical voltages that vary continuously withtime. Digital signals are a series of on/off pulses and will beexamined in Chapter 3.

Transmission in the telephone network was completely analoguntil 1962, when digital transmission was introduced. Mostcustomer loops are still analog because of the tremendousinvestment that telephone companies have in this equipment.However, due to the new competition in the telephone indus-try and the introduction of new technologies, this situation ischanging rapidly, as we will see in the following chapters.

Review Questions for Chapter 2

1. What is the frequency range of the human ear?2. What is the frequency range of the average human voice?

16 2 Analog Transmission

3. What is the frequency range of a typical telephone companycircuit?

4. Explain how the telephone set is provided with power fromthe telephone office.

5. Identify the device that replaced the original carbonmicrophones in the telephone set.

6. Identify two types of signaling that may be accomplishedwith a telephone set.