THE IRWIN
`
`HANDBOOK OF.
`TELECOMMUNICATIONS
`
`~ARR|S883|PR|0001249
`
`Petitioner ARRIS Group, Inc.’s
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`fl
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`ARRIS883IPRI0001249
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`

`

`FOURTH EDITION
`
`
`
`THE IRWIN
`HANDBOOK OF
`TELECOMMUNICATIONS
`
`JAMES HARRY GREEN
`
`McGraw-Hill
`
`.
`
`New York San Francisco Washington, DC. Auckland Bogota
`Caracas Lisbon London Madrid Mexico City Milan
`Montreal New Delhi San Juan Singapore
`‘ Sydney Tokyo Toronto
`
`ARRIS883|PRI0001250
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`

`

`Library of Congress Cataloging-in—Publication Data
`
`Green, James H. (James Harry)
`The Irwin handbook of telecommunications / by James Harry Green.—~4th ed.
`p. cm.
`ISBN 0-07-135554-5
`
`1. Telecommunication systems—United States.
`States%ommunication systems.
`I. Title.
`
`2. Business enterprises—United
`
`TK5102.3.U6 G74
`621.382 21—dc21
`
`2000
`
`99-044782
`
`McGraw-Hill
`A Division of The McGraw—Hill Companies
`
`:2
`
`Copyright © 2000, 1997, 1992, 1989 by Pantel, Inc. All rights reserved. Printed in the United States of
`America. Except as permitted under the United States Copyright Act of 1976, no part of this publica—
`tion may be reproduced or distributed in any form or by any means, or stored in a database or re-
`trieval system, Without the prior written permission of the publisher.
`
`234567890 DOC/DOC 09876543210
`
`ISBN 0-07-135554-5
`
`This book was set in 10/ 13 Palatino by Carlisle Publishers Service.
`
`Printed and bound by R. R. Donnelley 8: Sons Company.
`
`McGraw—Hill books are available at special quantity discounts to use as premiums and sales promo—
`tions, or for use in corporate training programs. For more information, please write to the Director of
`Special Sales, Professional Publishing, McGraw-Hill, Two Penn Plaza, New York, NY 10121—2298. Or
`contact your local bookstore.
`
`This publication is designed to provide accurate and authoritative information in regard to the subject
`matter covered. It is sold with the understanding that neither the author nor the publisher is engaged
`in rendering legal, accounting, or other professional service. If legal advice or other expert assistance
`is required, the services of a competent professional person should be sought.
`
`—From a Declaration of Principles jointly adopted by a committee of the
`American Bar Association and a Committee of Publishers.
`
`This book is printed on recycled, acid-free paper containing
`a minimum of 50% recycled, de~inl<ed fiber.
`
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`

`

`CONTENTS
`
`PREFACE xv
`
`INTRODUCTION xix
`
`LIST OF FIGURES
`
`xxiii
`
`LIST OF TABLES
`
`xxxi
`
`PART ONE
`
`PRINCIPLES OF TELECOMMUNICATIONS SYSTEMS
`
`Chapter 1
`
`A Brief History of Telecommunications
`
`3
`
`A Short History of the Bell System 4
`Standards
`16
`
`Chapter 2
`
`Introduction to Voice Communications
`
`21
`
`The Public Switched Telephone Network 22
`
`The Major Telecommunications Systems
`Fundamentals of Multiplexing 26
`Analog and Digital Transmission Concepts
`Switching Systems
`29
`
`23
`
`28
`
`Numbering Systems
`
`31
`
`Interexchange Carrier Access to Local Networks
`
`32
`
`Private Telephone Systems
`
`32
`
`Summary 33
`
`Chapter 3
`
`Introduction to Data Networks
`
`35
`
`Structure of the Internet
`
`Types of Data Networks
`
`37
`
`39
`
`Quality of Service
`
`41 '
`
`Why Convergence?
`Standards I 44
`
`43
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`
`
`vi CONTENTS
`
`Chapter 4
`
`Data Communications Principles 47
`
`Data Communications Fundamentals
`
`48
`
`Data Network Facilities
`
`60
`
`Data Communications Equipment
`
`66
`
`Applications
`
`77
`
`
`Chapter 5
`
`Pulse Code Modulation 81
`
`Digital Carrier Technology 82
`
`Digital Transmission Facilities
`
`87
`
`The Digital Signal Hierarchy 92
`
`Synchronous Optical Network/Synchronous
`Digital Hierarchy (SONET/SDH)
`95
`Digital Cross-Connect Systems
`98
`
`100
`T1/E1 Multiplexers
`Voice Compression 102
`Voice/Data Multiplexers
`
`105
`
`105
`Applications
`Evaluation Considerations
`
`106
`
`
`Chapter 6
`
`Outside Plant
`
`111
`
`Outside Plant Technology 112
`Electrical Protection 119
`
`Subscriber Loop Carrier
`
`126
`
`128
`Range Extenders
`Cable Pressurization 128
`
`Applications
`
`128
`
`
`Chapter 7
`
`Structured Wiring Systems
`
`131
`
`Overview of Structured Wiring 132
`
`134 '
`EIA/TIA Standards
`National Electrical Code® (NEC)
`
`143
`
`Applications
`
`143
`
`i
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`
`
`t CONTENTS
`
`Chapter 8
`
`
`Data Communications Protocols 147
`
`Protocol Terminology and Functions 8 148
`The Open Systems Interconnect Model
`151
`
`Transport Control Protocol/Internet Protocol (TCP/IP)
`
`155
`
`Point-to-Point Protocol (PPP)
`
`164
`
`Chapter 9
`
`
`Local Area NetWork Principles 167
`
`Local Area Network Technology 168
`Local Area Network Standards
`180
`
`High-Speed Ethernet
`
`187
`
`Gigabit Ethernet
`
`190
`
`Network Operating Systems
`Other LAN Application Issues
`
`190
`195
`
`Chapter 10
`
`Common Equipment 199
`
`Relay Racks and Cabinets
`
`199
`
`Distributing Frames
`
`200
`
`202.
`Ringing and Tone Supplies
`Alarm and Control Equipment
`202
`Power Equipment
`203
`
`Applications
`
`209
`
`PART TWO
`
`CIRCUIT SWITCHING SYSTEMS
`
`Chapter 11
`
`
`Signaling Systems 215
`
`Signaling Technology 217 _
`Trunk Signaling Systems
`222
`Common Channel Signaling 224
`Private Line Signaling 226
`Applications
`228
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`
`
`viii CONTENTS
`
`
`Chapter 12
`
`Circuit Switching Technology 231
`
`Network Terminology 232
`Network Architecture
`233
`
`Early Switching Systems
`
`239
`
`Access to the Local Exchange Network 249
`
`Applications
`
`253
`
`
`Chapter 13
`
`Local Switching Systems 255
`
`Digital Central Office (DCO) Technology 256
`
`Local Central Office Equipment Features
`
`265
`
`Common Equipment
`
`266
`
`The Advanced Intelligent Network (AIN)
`Local Central Office Service Features
`269
`
`268
`
`Applications
`
`276
`
`
`Chapter 14
`
`Tandem Switching Systems 279
`
`Tandem Switch Technology 280
`Tandem Switch Features
`281
`
`Applications
`
`290
`
`PART THREE
`
`
`
`TRANSMISSION SYSTEMS
`
`
`Chapter 15
`
`Lightwave Communications 297
`
`Lightwave Technology 298
`
`Lightguide Cables
`
`300
`
`Fiber-Optic Terminal Equipment
`
`305
`
`Lightwave System Design Criteria
`
`307
`
`Fiber Optics in the Local Loop 309
`
`Undersea Fiber-Optic Cables
`
`310
`
`Applications
`
`311
`
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`
`
`ix
`
`
`Chapter 16 '
`
`Microwave Radio Systems 317
`
`Microwave Technology 317
`Microwave Characteristics
`322
`
`Microwave Systems
`
`324
`
`Microwave Antennas, Waveguides, and Towers
`
`333
`
`Applications
`
`334
`
`Chapter 17
`
`Satellite Communications 339
`
`Satellite Technology 343
`
`Earth Station Technology 346
`GEO Satellite Transmission 349
`
`Representative Satellite Services
`
`352
`
`Applications
`
`356
`
`Chapter 18
`
`
`Mobile, Cellular, and PCS Radio Systems 359
`
`Conventional Mobile Telephone Technology 360
`Private Mobile'Radio Service
`363
`
`Cellular Mobile Radio 367
`
`Personal Communications System (PCS)
`
`378
`
`Applications
`
`379
`
`'
`
`Chapter 19
`
`Wireless Communications Systems 383
`
`Unlicensed Wireless Phones
`
`384
`
`Wireless LAN5
`
`386
`
`Wireless Mobile Data
`
`389
`
`The' Wireless Local Loop 392
`
`Radio Paging 393
`
`Applications
`
`394
`
`Chapter 20
`
`Video Systems
`397-
`
`Video Technology 398
`Cable Television Systems
`
`401
`
`Video Compression 405
`Videoconferencing 406
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`
`x CONTENTS ‘
`
`
`
`Digital Television 412
`
`Video Services and Applications
`
`415
`
`Applications
`
`417
`
`PART FOUR
`
`
`
`CUSTOMER PREMISES EQUIPMENT
`
`
`Chapter 21
`
`Local Area Network Equipment 423
`
`Servers
`
`425
`
`LAN Segment Equipment
`
`430
`
`Repeaters, Bridges, Switches, Routers, and Gateways
`
`433
`
`Applications
`
`443
`
`Equipment Evaluation Criteria
`
`448
`
`Chapter 22
`
`Station Equipment 455
`
`Telephone Set Technology 455
`Station Protection 459
`
`Coin Telephones
`
`460
`
`462
`Cordless Telephones
`464
`Answering Equipment
`Conference Room Telephones
`Line Transfer Devices
`464
`
`464
`
`Applications
`
`465
`
`Chapter 23
`
`Key Telephone Systems 471
`
`Key Telephone System Technology 472
`
`Applications
`
`478
`
`Chapter 24
`
`Private Branch Exchanges 485
`
`PBX Technology 486
`
`Principal PBX Features
`
`492
`
`Call-Accounting Systems
`
`504
`
`Applications
`
`504
`
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`
`
`
`
`Chapter 25
`
`Automatic Call Distribution Equipment
`
`511
`
`Call Distribution Technology 512
`
`Call-Handling Elements
`
`514
`
`ACD Features and Operation 519
`Networked Call Centers
`521
`
`Integrated PBX versus Stand-Alone ACD 522
`Other Call Distribution Alternatives
`523
`
`Applications
`
`525
`
`Chapter 26
`
`Computer-Telephone Integration 535
`
`CTI Technology 536
`
`CTI Applications
`
`542
`
`Justifying CTI
`Standards
`547
`
`545
`
`Chapter 27
`
`Voice-Processing Systems
`
`549
`
`Voice ‘Mail
`
`551
`
`Automated Attendant
`
`557
`
`Interactive Voice Response
`
`558
`
`Digital Recording 560
`
`Speech Recognition 560
`
`Digital Announcers
`
`561
`
`Applications
`
`562
`
`Chapter 28
`Centrex Systems. 571
`
`Centrex Features
`
`572
`
`Applications
`
`575
`
`Chapter 29
`
`Electronic Messaging Systems '581
`
`Messaging Systems
`
`582
`
`The X.400 Messaging Protocol
`
`Simple Mail Transfer Protocol
`Electronic-Mail Systems
`588
`
`585
`
`587
`
`Unified Messaging 588
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`
`xii
`‘
`
`'
`
`CONTENTS
`
`Electronic Document Interchange (EDI)
`
`589
`
`Applications
`
`590
`
`
`Chapter 30
`
`Facsimile Systems
`
`593
`
`Facsimile Technology 594
`Facsimile Features
`597
`
`Applications
`
`604
`
`PART FIVE
`
`
`
`TELECOMMUNICATIONS NETWORKS
`
`
`Chapter 31
`
`Enterprise Networks
`
`609
`
`Why Private Networks?
`
`610
`
`Building Blocks of the Enterprise Network 612
`Private Voice Networks
`617
`
`Virtual Data Networks
`
`618
`
`Private Network Development Issues
`
`621
`
`Applications
`
`625
`
`
`Chapter 32
`
`The Integrated Services Digital Network 627
`
`ISDN Technology 628
`ISDN Architecture
`631
`
`Always On/Dynamic ISDN 634
`
`Digital Subscriber Line
`
`635
`
`Applications
`
`637
`
`
`Chapter 33
`
`Broadband Networks
`
`639
`
`Fibre Channel
`
`641
`
`Fiber Distributed Data Interface
`
`643
`
`Switched Multimegabit Data Service
`
`647
`
`Gigabit Ethernet
`
`653
`
`Applications
`
`654
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`
`
`Chapter 34
`
`
`xiii
`
`Wide Area Data Networks 657
`
`Data Network Facilities
`
`659
`
`Multidrop Networks
`
`665
`
`Packet Switching 670
`
`Very Small Aperture Terminal (VSAT)
`
`674
`
`Applications
`
`676
`
`‘ Chapter 35
`
`Frame Relay 681
`
`Frame Relay Technology 682
`
`Voice Over Frame Relay 688
`SNA Over Frame Relay 689
`
`Applications
`
`690
`
`Chapter 36
`
`Asynchronous Transfer Mode 695
`
`ATM Technology 696
`LAN Emulation 705
`
`Applications
`
`707
`
`Chapter 37
`
`Internetworking 711
`
`The Internet Design Problem 712
`Internet Architectures
`713
`
`Backbone Networks
`
`717
`
`Applications
`
`721
`
`Chapter 38
`
`VOice and Data Convergence 725
`
`Mixed-Media Requirements
`The H.323 Standard 729
`
`726
`
`Facilities for Mixed—Media Transmission 733 .
`
`734
`System Configurations
`Application Considerations
`
`738
`
`Applications
`
`740 .
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`CONTENTS
`xiv
`
`Chapter 39
`
`Network Management Systems
`
`743
`
`Network Management and Control
`
`744
`
`Simple Network Management Protocol
`
`747
`
`Common Management Information Protocol
`
`755
`
`Customer Network Management
`
`757
`
`Applications
`
`757
`
`Chapter 40
`
`Future Developments in Telecommunications
`
`763
`
`Internet
`
`765
`
`Convergence
`
`766
`
`The Future of Circuit Switching 767
`Multimedia over Cable
`768
`
`Mergers and Acquisitions
`
`769
`
`Speech Recognition 770
`Wireless
`771
`
`Competitive Local Service
`
`771
`
`Competition and Regulation 773
`
`APPENDIX A
`
`
`
`World Wide Web Addresses of Selected Telecommunications
`
`Manufacturers, Carriers, Vendors, and Organizations
`
`775
`
`APPENDIX B
`
`Telecommunications Acronym Dictionary 785
`
`GLOSSARY 797 '
`
`BIBLIOGRAPHY 821
`
`INDEX 823
`
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`
`
`CHAPTER 5
`
`
`Pulse Code Modulation
`
`All telephone sessions begin as an analog signal. In the majority of the cases,
`the analog voice signal is converted to digital for the bulk of its journey to the
`other end of the connection. The analog portion of the connection may be as short
`as the length of the handset cord, with the analog-to—digital conversion done in
`the telephone instrument. This is the case with most PBXs and all ISDN sessions.
`If the customer premises equipment has an all—analog connection to the central
`office, voice travels to the central office as an analog signal. If the central office is
`digital, the voice is converted in the switching system’s line circuit. If the switch—
`ing system is analog, the voice is converted in the trunk circuit and travels to a
`distant central office as a digital signal. The process for converting between dig-
`ital and analog is known as pulse code modulation (PCM), which is the fundamen—
`tal building block of today’s switching and transmission systems.
`Alec Reeves,an ITT scientist in England, patented the PCM method of con-
`verting analog voice signals to digital in 1938. Although the system was technically
`possible then, it wasn’t economically feasible. Pulse generating and amplifying cir-
`cuits required vacuum tubes and their size and power consumption consigned
`PCM to the shelf for another 20 years. Following the development of the transistor,
`PCM became commercially feasible in the 19605, and with the development of
`large-scale integration in the following decade, cost, size, and power consumption
`continued to drop, even in the face of high inflation.
`Today, digital technology has all but replaced analog technology in transmission
`systems and is rapidly replacing it in switching systems. Virtually all new central 0f-
`fices, PBXs, and even most key systems use digital technology. The advantages are sub—
`stantial. Analog circuits do not lend themselves well to integrated circuitry, whereas
`digital circuits use the same manufacturing techniques that have resulted in dramatic
`cost reductions in electronic devices. Digital switching systems can interface with dig-
`ital multiplexing systems directly without the need for an analog-to-digital conversion.
`
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`82 PART 1 Principles of Telecommunications Systems
`
`Digital circuits are also less susceptible to noise. In analog circuits, noise is additive, in—
`creasing with system length, but in a digital carrier the signal is regenerated at each re-
`peater. Over a properly engineered system, the signal arrives at the receiving terminal
`with quality unimpaired and a bit error rate several orders of magnitude less than ana-
`log. Furthermore, digital signals can be compressed into a fraction of their original
`bandwidth, while analog signals occupy a fixed amount of bandwidth.
`
`DIGITAL CARRIER TECH NOLOGY
`
`The basic digital multiplexing system is known as T carrier. The name came from
`the Bell System’s carrier designation system, which assigned letters to eaCh'suc-
`cessive model that Bell Laboratories designed. The T1 carrier system used in North
`America consists of 24.channels, each of which occupies a bandwidth of 64 kb/ 5.
`A T1 digital carrier system samples a voice signal, converts it to an 8—bit coded dig-
`ital signal, interleaves it with 23 other voice channels, and transmits it over a line
`that regenerates the signal approximately once per mile on twisted-pair copper
`. wire. The European version, known as E1, uses the same sampling method, but ap-
`plies 32 channels to the line, of which two channels are used for signaling. Al—
`though this discussion refers to voice channels, the T1 /E1 signal is channelized
`only if the terminating equipment does so. The entire bandwidth of the T1 /E1 sig—
`nal can be used between devices such as routers, which are used to interconnect
`LANs. Other devices such as T1/E1 multiplexers can divide part of the bit stream
`into voice channels and part into a wider channel for LAN interconnection.
`Tl / E1 also can be transmitted over digital radio and over fiber optics with re—
`generators spaced at Wider intervals. The digital signal is encoded and decoded in
`a digital central office or on one of several types of terminal devices. A channel bank
`combines 24 or 32 voice and data circuits into a T1 /E1 bit stream. A Tl/El multi-
`plexer, which is described later, breaks the bit stream into smaller increments than
`a channel bank and supports both voice and data signals. T1 /E1 lines can be di-
`rectly connected to digital PBXs and a variety of other devices such as key systems
`and automatic call distributors.
`A digital signal is developed by a five-step process, consisting of the following:
`
`I
`
`.
`
`9 Sampling
`
`6 Quantizing
`
`9 Encoding
`
`O Companding
`
`0 Framing
`
`Sampling
`According to Nyquist’s theorem, if an analog signal is sampled at a rate twice the
`highest frequency contained within its bandwidth, enough intelligence is retained
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`
` (:HAPTER 5 Pulse Code Modulation
`
`83
`
`
`FIGURE 5-1
`
`Voice Sampling
`
`Sampled
`voice
`signal
`
`
`
`<—-—Samples
`
`
`
`‘————-—— Envelope of voice signal
`
`PAM
`signal
`
`in the samples to reconstruct the original signal. The range of human hearing is ap—
`proximately 20 to 20,000 Hz, but the frequency range in a voice signal is much nar-
`rower. Communications channels filter the voice to a nominal bandwidth of 4000
`
`Hz (actually 300 to 3300 Hz). Therefore, a sampling rate of 8000 times per second
`is sufficient to encode a voice signal for communications purposes. A PCM system
`does exactly this. The output of the sampling process is a pulse amplitude modulated
`(PAM) signal, shown in Figure 5-1.
`
`‘ Quantizing, Encoding, and Companding
`
`The amplitude of the pulses from the sampling circuit is encoded into an 8—bit word
`by a process called quantizing, which is illustrated in Figure 5-2. The 8-bit word pro—
`vides 28 or 256 discrete steps, each step corresponding to the instantaneous ampli~
`tude of the speech sample. The output of the encoder is a stream of octets, each rep~
`resenting the magnitude of a single sample.
`The quantizing process does not exactly represent the amplitude of the PAM
`signal. Instead, the output is a series of steps, which, as shown in Figure 5-3a, does
`not precisely represent the original waveform. The error is audible in the voice
`channel as quantizing noise, which is present only when a signal is being transmit—
`ted. The effects of quantizing noise are greater with low—amplitude signals than
`with high. To overcome this effect, the encoded signal is compressed to divide low-
`level signals into more steps and high-level signals into fewer steps, as shown in
`Figure 5—319. When the signal is decoded at the receiving terminal, reversing. the
`compression process expands it. The combination of expansion and compression
`is called companding. In the United States companding follows a formula known as
`mu law coding. In Europe, the companding formula is a slightly different form
`known as A law coding. Although the two laws are incompatible, they differ only
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`
`84 PART 1 Principles of Telecommunications Systems
`
`FIGURE 5-2
`
`Ouantizing
`
`—— __ 11110011
`________
`————————————————— 111110000
`
`
`
`11111111
`
`________________ _111oo110
`_________
`11100010
`_______________ - 11010010
`
`10100010
`
`_ _ _ _ __ __ fl ——————— 10001000
`
`
`
`01001001
`— -— 01000010
`01001001
`- 00010110
`00010001
`
`- 00001001
`00000101
`00000000
`
`slightly. lTU-T recommendation (3.711 defines the standard for both algorithms
`and the process for converting between them.
`
`Framing
`
`The PCM voice signal is encoded in the terminating device and merged with other
`voice channels. Each channel generates a bit rate of 64 kb/ s (8000 samples per sec—
`ond X 8 bits, or 1 byte, per sample). The 24 channels in the North American system
`produce the frame format shown in Figure 5-4. A single framing bit is added to the
`192 bits that result from the 24 8—bit bytes. A 193—bit frame, 125 microseconds (us) A
`in duration, results. The frame repeats 8000 times per second for a total line rate of
`1.544 Mb/s. The framing bits follow a fixed pattern of zeros and ones throughout
`12 frames. This repetitive sequence of 12 frames is called a supeffmme. The 1.544—
`Mb / 5 rate results from the following:
`
`8,000
`
`samples per second
`
`X 8
`
`bits per sample
`
`64,000
`X 24
`
`bits per channel
`channels
`
`1,536,000
`
`bits per second
`
`+ 8,000
`
`framing bits per second
`
`1,544,000
`
`bits per second total
`
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`CHAPTER 5 Pulse Code Modulation 85
`
`FIGURE 5-3
`
`Companding in a PCM Channel
`
`
`
`Quantized
`I
`representation
`
`Original
`
`signal
`
`
`
`
`a. High-Level Signal
`
`Step size
`
`b. Low-Level Signal
`
`FIGURE 5-4
`
`PCM Frame
`
`lit—A78 Word
`‘
`
`1
`
`Framing bit ‘1
`
`24
`
`\——————— 24 8~bit words (192 bits)——-—/
`
`|<-———————-——-—-—-————-—-———125 us frame
`
`
`
`This system of multiplexing is known as byte interleaved multiplexing. It is also
`referred to as synchronous multiplexing. The terms synchronous and asynchronous
`are somewhat overused in telecommunications, and can result in confusion when
`
`applied to devices as diverse as an asynchronous terminal and asynchronous
`transfer mode, or ATM. Nevertheless, it is important to understand that the T car—
`rier modulation scheme results in a bit stream in which any individual channel can
`be detected simply by its position in the transmission frame, which is a multiple of
`8 bits. Each byte position in the transmission frame is called a time slot, and the
`process is known as time division multiplexing or TDM.
`European digital carrier systems, which are known as E1 systems, use the
`same 64-kb/ 5 channel bit rate but multiplex 32 rather than 24 channels for a
`2.048-Mb / 5 bit rate. Of the 32 channels, 30 are used for information channels, one
`
`'
`
`channel is used for frame alignment, and one for signaling. Companding and bit
`rate differences make North American and European digital carrier systems in-
`compatible. This incompatibility was of little consequence When the systems
`were developed in the 19605 because they would not be interconnected. With the
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`.86 PART 1 Principles of Telecommunications Systems
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`
`
`development of undersea fiber-optic cable, however, the issue of end—to-end con—
`nectivity of incompatible systems became a significant problem. This incompat-
`ibility came to be one of the driving forces behind SONET/SDl-I, which is dis—
`cussed later in this chapter.
`
`Bit-Robbed Signaling
`
`T1 carrier’s original purpose was interoffice trunking in metropolitan areas. Ana—
`log carrier systems required, with a few exceptions, outboard devices to signal over
`the channel. Since signaling is a binary function, it was feasible to use a portion of
`the T carrier signal itself to convey the ON— or OFF-hook status of the channel. See
`Chapter 11 for further discussion of how signaling systems work.
`V
`The original digital channel banks used the least significant bit in every sixth
`frame for signaling. This technique is known as bit robbing. Within a superframe,
`the bit that is robbed from the sixth frame is called the A bit, and the one that is
`robbed from the 12th frame is called the B bit. The distortion resulting from bit rob-
`bing has no effect on voice signals or data signals that are modulated with a mo—
`dem. The forced errors render a circuit unusable, however, for a 64—kb/s digital
`data signal. Therefore, devices that are designed to operate over superframe lines
`use only 7 of the 8 bits. At a sampling rate of 8000 samples per second, this leaves
`a usable signal of 7 X 8000 or 56 kb/s. Special data channel units provide direct
`digital access to the usable bandwidth.
`
`Extended Superframe
`
`Within a superframe, the framing bits synchronize the channels and signaling,
`but otherwise carry no intelligence. Also, the signaling bits reduce the data-
`carrying capacity of a T1 channel by 8000 bits per second. As the LECs and IXCs
`convert to common channel signaling between central offices, the in—band sig—
`naling capability of T1 is no longer required. Clear channel capability, which is '
`one of ISDN’s features, eliminates the bit-robbed signaling and introduces a re—
`vised Tl format known as extended superfmme (ESP). Under ESP, the 8000—b/s
`framing signal, also called the Fe channel, is multiplexed to provide 2000 b / s for
`6-bit cyclical redundancy checking (CRC) on the bit stream. A 4000-b/s facility
`data link (FDL) is used for end-to-end diagnostics, network control, and main—
`tenance functions such as forcing loopback of a failed channel. The remaining
`2000 b/s are used for framing and signaling. ESF is supported by ANSI as the
`T1403 standard.
`.
`'
`The CRC code operates in the same manner as data—link error detection,
`which is discussed in Chapter 4. It does not, however, correct errors; it only detects
`them. The CRC code is calculated at the source and then again at a terminal or in-
`termediate point. If an error is detected, the equipment can flag the fault before a
`hard failure occurs. The receiving equipment calculates the performance of the fa—
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`CI—IAPTER 5 Pulse Code Modulation
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`87
`
`cility from the CRC results and stores it or sends the information back to the orig—
`inating equipment over the FDL.
`ESF requires that terminating equipment and repeaters be compatible. Some
`ancillary equipment, such as the channel service unit (CSU), which is described
`later, can be designed to operate under either. SF or ESF rules. In contrast to SF,
`which uses a line-coding scheme known as alternate mark inversion, ESF uses bipo—
`lar with 8—zer0 substitution (BSZS) as its line—coding method, and this method is dis-
`cussed later.
`
`DIGITAL TRANSMISSION FACILITIES
`
`The basic digital transmission facility is a T1 /E1 line, which has an office repeater
`at each end feeding twisted-pair wire, with digital regenerators spaced every
`6000 feet (ft). The function of the office repeater is to match the output of the
`channel bank to the impedance of the line and to feed power over the line to the
`repeaters. The line repeaters regenerate the incoming pulses to eliminate distor—
`tion caused by the cable. The 6000—ft spacing was selected to install repeaters in
`manholes, which are placed at 6000—ft intervals to match the spacing of load coils
`in voice frequency cables.
`Digital signals in North America are applied to twisted-pair wire in groups
`of 24, 48, and 96 channels called T1, T1C, and T2. T1 signals originate in channel
`banks, digital central office switches, T1 multiplexers, PBXs, and other T1-compat—
`ible devices. The higher bit rates of TlC and T2 are developed by higher-order mul-
`tiplexing as described later. Digital signals also can be applied to fiber optics, Ini-
`crowave radio, satellite, or coaxial cable for transmission over longer distances.
`
`Digital Timing
`
`T1 / E1 signals are synchronized by loopback timing, in which synchronizing
`pulses are extracted from the incoming bit stream, as discussed in a later section.
`The PCM output of the terminating device is encoded in the bipolar format that is
`described later. The transition of each 1—second bit is detected- by the repeaters and
`the receiving terminals and is used to keep the system in synchronization. If more
`than 15 consecutive zeros are transmitted on a digital facility, the receiving end
`may lose synchronization. To prevent this, the channel bank inserts a unique bit
`pattern that is detected by the receiving end and restored to the original pattern.
`This technique, called bit stufi‘ing, is used by digital carrier systems to prevent loss
`of synchronization.
`
`The T1 IE1 Carrier System
`
`Figure 5-5 is a block diagram of a T1 /E1 carrier system. The primary elements of the
`system are the channel banks and the repeaters. The other elements, the distributing
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`88 PART 1 Principles of Telecommunications Systems ‘
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`
`
`FIGURE 5-5
`
`Block Diagram of a T1 Carrier System
`
`
`Voice and
`Voice and
`
`
`signaling
`signaling
`
`leads
`leads
`
`
`DSX
`ORB Line
`TDF
`'
`0-200 miles
`
`Digital
`channel
`bank
`
`Voice and
`signaling
`leads
`
`+__.
`Voice and
`signaling
`leads
`
`
`Digital
`
`_.
`
`ORB
`
`DSX
`
`TDF - Trunk Distributing Frame
`DSX - Digital Signal Cross-connect Frame
`ORB - Oiiice Repeater Bay
`T - Transmit
`R - Receive
`
`FIGURE 5-6'
`
`Block Diagram of a PCM Channel Bank
`
`Transmit
`
`N Low pass N
`iilier
`
`Receive
`
`N
`
`B-kHz
`sampler
`
`‘
`
`
`
`23 other
`channels
`
`Coder
`
`PCM
`
`Digital
`processor
`
`1.544 Mbls line
`
`N Demultiplexer
`.
`gate
`
`23 other
`channels
`
`
`
`1.544 Mb/s line
`Digital
`III
`
`
`processor and
`‘l h"
`
`synchronizer
`
`
`frame and digital cross-connect frame, are provided for ease of assignment and
`maintenance. In this diagram channel banks are used for the purpose of illustration,
`but most T1 /E1 circuits today are terminated in a device other than a channel bank.
`PBXs, T1 /E1 multiplexers, routers, and other devices that use the full bandwidth of
`a T1 /E1 are diminishing the use of channel banks.
`
`Channel Banks
`A T1 channel bank consists of 24 Channels called a digroup. Some manufac-
`turers package two digroups in a 48-channel framework. The 48 channels
`share a common power supply and other common equipment. Figure 5-6 is a
`block diagram of a digital channel bank. The channel bank has a metal frame—
`work with backplane wiring designed to accept plug—in common equipment
`and channel units.
`»
`
`A variety of plug-in channel units are available to provide special transmis-
`sion functions. Numerous signaling options are available. Foreign exchange (FX)
`service is a combination of special signaling and transmission service. It is used by
`LECs to connect a telephone line in one exchange to a station located in another. FX
`channel units are equipped for direct connection to metallicloops and have provi—
`sions for ringing telephones and for adding gain to long loops.
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`' CHAPTER 5 Pulse Code Modulation
`89
`
`FIGURE 5-?
`
`T1 Carrier Line Signals and Faults
`
`Unipolar signal
`
`Bipolar signal
`
`Bipolar signal with jitter
`
`Bipolar signal
`with bipolar violation
`in bit 5
`
`Program channel units replace two or more voice channel units and use the
`added bit stream to accommodate a wider Channel for use by radio and television
`stations, wired music companies, and other applications that require a wide audio
`band. Program channels with 5-kHz bandwidth replace two voice channels, and
`15-kHz units replace six voice channels.
`
`T Carrier Lines
`
`T1/E1 carrier lines can be extended on twisted—pair wire for about 200 miles,
`although most private and common carrier applications are considerably
`shorter because longer circuits are usually deployed over radio or fiber-optic
`facilities. A superframe T carrier line accepts a bipolar signal from the cormected
`device, as shown in Figure 5—7. A bipolar signal, also called alternate mark in—
`version, assigns Os to a zero voltage level. One signals are alternately : 3 volts
`(V). The bipolar signal offers two advantages. First, the line signaling rate is
`only half the data rate of 1.54.4 Mb/s because in the worst case where a signal
`composed of all ls, the signal would alternate at only 772 kb/s. The second ad—
`vantage is the ability of a bipolar signal to detect line errors. If interference or
`a failing repeater adds or subtracts 1—second bits, a bipolar violation results,
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`90 PART 1 Principles of Telecommunications Systems
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`which indicates a fault in a T carrier system. A bipolar violation occurs when
`two 1 bits of the same polarity arrive in sequence.
`Clear channel capability requires the T carrier system to support any pattern
`of bits, including long strings of Os. ESF systems replace the straight bipolar signal
`with a coding scheme known as bipolar with 8~zero substitution (13825). In this sys-
`tem, any string of eight Os is replaced with an intentional bipolar violation at the
`fourth and seventh bits. The receiving equipment, normally the channel service
`unit, detects the bipolar Violation and replaces it with a string of eight Os. The B8ZS
`_ coding scheme is not compatible with earlier T carrier lines, but most modern re—
`generators, office repeaters, channel service units, and connecting equipment are
`ESF compatible.
`
`Office Repeaters and Channel Service Units
`
`An office repeater terminates the T1 / E1 line at each central office. In the receiving
`direction, the office repeater performs normal regenerator functions, but it is pas-
`sive in the transmit direction. Its transmit function is to couple the bipolar signal
`to the line and to feed power to the line repeaters. A special type of repeater called
`a channel service unit (CSU) terminates the customer end of a T1./ E1 line that
`feeds customer premises. Unlike a DDS signal where a DSU is required to convert
`the line signal from bipolar to the unipolar signal required by the terminal equip—
`ment, the T1 / El device produces the bipolar signal. Increasingly, the CSU func-
`tion is built into the channel bank or the multiplexer. The CSU fulfills the follow-
`ing functions:
`
`9 Terminates the circuit, including lightning and surge protection.
`
`Regenerates the signal.
`Loops the digital signal back to the originating