Ie I e c 0 mm u n i cati 0 ns Transmission
Engineering
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Telecommunications Transmission
Engineering
Volume 2 - Facilities
First Edition
Technical Personnel
American Telephone and Telegraph Company, Bell Telephone Companies,
and
Bell Telephone Laboratories
Bell System Center for Technical Education
Prepared for Publication by Western Electric Company, Inc.
Technical Publications Winston-Salem, North Carolina
FIRST EDITION 1977
Printed in the United States of America
Transmission Engineering
Introduction
Communication Engineering is concerned with the planning, design, implementation, and operation of the network of channels, switching machines, and user' terminals required to provide communication between distant points. Transmission Engineering is, the part of Communication Engineering which deals with the channels, the trans- mission systems which carry the channels, and the combinations of the many types of channels and systems which form the network of facilities. It is a discipline which combines many skills from science and technology with an understanding of economics, human factors, and system operations.
This three-volume book is written for the practicing Transmission Engineer and for the student of transmission engineering in an under- graduate curriculum. The material was planned and organized to make it useful to anyone concerned with the many facets of Communi- cation Engineering. Of necessity, it represents a view of the status of communications technology at a specific time. The reader should be constantly aware of the dynamic nature of the subject.
Volume 1, Principles, covers the transmission engineering prin- ciples that apply to communication systems. It defines the charac- teristics of various types of signals, describes signal impairments arising in practical channels, provides the basis for understanding the relationships between a communication network and its components, and provides an appreciation of how transmission objectives and achievable performance are interrelated.
Volume 2, Facilities, emphasizes the application of the principles of Volume 1 to the design, implementation, and operation of trans- mission systems and facilities which form the telecommunications
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network. The descriptions are illustrated by examples taken from modern types of facilities most of which represent equipment of Bell Laboratories design and Western Electric manufacture; these examples are used because they are familiar to the authors.
Volume 3, Networks and Services, shows how the principles of Volume 1 are applied to the facilities described in Volume 2 to pro- vide a variety of public and private telecommunication services. This volume reflects a strong Bell System operations· viewpoint in its con- sideration of the problems of providing suitable facilities to meet customer needs and expectations at reasonable cost.
The material has been prepared and reviewed by a large number of technical personnel of the American Telephone and Telegraph Company, Bell Telephone Companies, and Bell Telephone Labora- tories. Editorial support has been provided by the Technical Publica- tions Organization of the Western Electric Company. Thus, the book represents the cooperative efforts of many people in every major organization of the Bell System and it is difficult to recognize indi- vidual contributions. One exception must be made, howev,er. The material in Volume 1 and most of Volume 2 has been prepared by Mr. Robert H. KUe of the Bell Telephone Laboratories, who was associated in this endeavor with the Bell System Center for Technical Education. Mr. Klie also coordinated the preparation of Volume 3.
C. H. Elmendorf, III Assistant Vice President - Transmission Division.
American Telephone and Telegraph Company
Preface
The Bell System transmission facilities network is made up of a large number of transmission systems, media, terminal equipment units, and items of apparatus that have been designed and constructed to operate efficiently and economically as an integrated communica- tions system. The network has grown rapidly in recent years and has changed remarkably with the increasingly sophisticated technological designs and processes that have emerged over the same period.
The network has evolved as one capable of providing high-quality telecommunications services economically. In addition, it is composed of facilities and equipment that give it flexibility and adaptability in the face of a wide range of environmental factors that include rural and metropolitan areas, hot and cold climates, residential and business communities, and many more. Furthermore, the entire network has proven to be adaptable to transmitting signals of a constantly chang- ing character that have resulted from the provision of new and expanded services.
Volume 2 is devoted to descriptions of the major facilities, systems, circuits, equipment, and apparatus designed and used by the Bell System to provide the required wide range of communications serv- ices. The text is organized in seven sections devoted to descriptions and discussions of (1) the network and the principal transmission media, (2) local plant facilities, (3) the major analog carrier systems
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that utilize metallic media, (4) analog microwave radio systems, (5) a wide range of digital systems, (6) transmission maintenance systems and equipment, and (7) how all these elements are brought together into an integrated communications system that serves this nation and interconnects with the facilities network of the entire world.
Section 1 provides a general description of the facility network and discusses briefly the manner in which it has evolved. A summary is also given of the characteristics of transmission media. Section 2 pre- sents descriptions of loops and station sets, voice-frequency network trunk and data facilities, and wideband facilities. The section also includes discussions of the transmission aspects of central office and customer premises switching equipment generically called Business Communications Systems.
In the third section, descriptions of analog carrier systems utilizing metallic media are presented. The section b€gins with a chapter in which the frequency division multiplex equipment is described. Basic design features of analog transmission systems are then discussed after which descriptions of systems based on wire-pair cable and coaxial cable utilization are given. Section 4 covers analog microwave radio systems. Basic design features, systems engineering, and pro- tection switching systems are first described. These general discus- sions are followed by descriptions of the features of short-haul and long-haul radio systems. Domestic satellite transmission and mis- cellaneous radio systems and services, principally mobile communi- cations services and radio paging services, are also described.
Section 5 contains descriptions of digital transmission systems. The general design features of such systems are discussed and it is shown how they differ from analog carrier systems. Digital system terminal and multiplex equipment, digital transmission lines, and digital micro- wave radio systems are treated in succeeding chapters.
Section 6 is devoted to a discussion of transmission maintenance.
New maintenance systems, which are currently playing such a prom- inent role in the field of transmission maintenance, are computer- controlled and are being utilized to fufill significant functions in record keeping, operations control, and force management. Descrip- tions of more conventional types of test equipment and the important functions still fulfilled by such equipment in transmission maintenance are also presented.
An overall view of the facilities network and how the parts fit to- gether is presented in Section 7. Some of the limitations imposed upon system use because of interferences introduced into one system by another are first described. Finally, the methods of interconnecting- the parts and ensuring compatibility are covered. In addition to system interconnection, the design and operation of some common- equipment designs are also discussed.
Introduction 0 0 0 0 0 • • • 0 • 0 0 • 0 0 0 • • • 0 • 0 0 0 0 • 0 0 0 0 • • 0 0 0 0 • 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 0 0 • iii
pore/ace 0 0 . 0 v
SECTION 1 THE FACILITY NETWORK 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 Chapter 1. The Evolution of the Facility Network 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2 1-1 Voice-Frequency Transmission Facilities 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 3 1-2 Carrier Transmission Facilities 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 • 0 0 0 0 0 • 0 • 4 Analog Cable Carrier Systems 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 • • • 0 0 0 0 0 • 0 0 0 0 0 0 • 4 Radio Transmission Systems o. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 • 0 0 • 0 0 0 0 0 • 7 Pulse Transmission Systems 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 . 0 • • 0 0 0 0 • 0 0 0 0 0 0 0 0 • 0 10 1-3 Video Transmission 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 11 1-4 Maintenance and Reliability 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 0 0 • 0 0 0 • 0 0 13 1-5 Facility Selection and Application 0 0 0 0 0 0 0 0 • 0 0 0 0 • • 0 0 0 0 • 0 0 0 0 0 0 0 • • 0 0 0 0 0 0 15 Growth Factors 0 0 0 0 0 0 0 0 0 0 0 . 0 0 . 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 0 0 0 0 0 . 0 0 0 15 Facility Forecasts 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 • 0 • • 0 • 0 0 0 • • • • 0 0 0 0 0 0 0 • • o . 19 1-6 Engineering Considerations 0 0 • 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 • 0 0 0 ' • • • • 0 • 0 • 0 0 0 0 0 . 0 • • 20 Economics . 0 • • • • • • • 0 • • • • • • • • • 0 • • • 0 • 0 • • • • • 0 • 0 • • • • • • • • • • • • • • • 0 • • • • 20 Technology ... 0 • • • • • • • • • • • • • • 0 • • • • • • 0 • " 22 Chapter 2. Transmission Media . 0 0 0 • • • • • 0 • • • 0 • 0 • • • • • • • • • • • • • • • • • • • • • • • • 26 2-1 Open Wire. 0 • 0 • • • • • 0 • • • • • 0 • 0 • • • • • 0 • 0 • • • • • 0 • • • • • • 0 , • • • • • • • • • • • • • • • • 27 2-2 Loop and Local Trunk Cables ... 0 • 0 • 0 • • • • • • • • 0 • • • • • • 0 • • • • 0 0 0 0 • 0 0 • • • 27 Physical Characteristics ... .. 0 • 0 • • • • • • • • • • • • • 0 • 0 0 • • • 0 o . 28 Transmission Characteristics '" 0 • 0 • 0 0 0 • • • 0 0 • • • • • 0 • • • • • 0 • • 0 • 0 • 0 • o . 31
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2-3 Toll Trunk Cables 40 40 44 Cable Pairs ... .
Coaxial Cables ... . ... .
2-4 Microwave Radio Transmission . . . 46
Propagation Paths ... :... 46
Antennas . . . . . . 51
Waveguides ... . . . 56
2-5 Mobile Radio Transmission ... . 57
SECTION 2 LOCAL PLANT FACILITIES 61 Chapter 3. Loops and Station Sets 63 3-1 Loop Baseband Facilities . . . .. . ... . Loop Design Plans ... . ... . Voice-Frequency Electronic Equipment ... . Bridge Lifters ... . Concentrators. . . . ... . Program Facilities ... . 64 65 ()8 78 80 80 3-2 Loop Carrier Facilities ... . 81
81 85 Analog Systems ... . Digital Systems ... . 3-3 Telephone Station Equipment The 500-Type Telephone Station Set Other Station Set Types ... . Ringing Considerations ... . Chapter 4. Voice-Frequency Trunk Facilities 89 89 93 93 102 4-1 Transmission Considerations .... 103
4-2 Negative Impedance Repeaters ... 104
Repeater Design ... 105
Gain ... ... 106
Impedance Relationships . . . 108
Disablers ... . . . 109
Applications ... 109
4-3 Four-Wire V-Type Repeaters. . . 109
Equipment Arrangements ... 111
Amplifiers ... 113
Equalizers ... 115
Four-Wire Terminating Sets ... . . . 118
Four-Wire Extension Networks ... . . . 119
Low-Pass Filter. . . 120
4-4 Metallic Facility Terminals Equipment Features Transmission Features ..
4-5 Voice-Frequency Equipment Comparisons Bandwidth and Stability ... . Return Loss and Echo ... . Losses and Transmission Level Points Signalling ... .
4-6 Echo Suppressors ... . Operation ... . Types
Enablers and Disablers
Chapter 5. Voice-Frequency Data Facilities 5-1 Transmission Channels
Access Facilities ....
Interoffice Facilities 5-2 Transmission Performance
Objectives ... . ... . Impairments
5-3 Voiceband Data Stations ... . Data Sets ... .
Analog Interface ... ' ... . 5-4 The 20B-Type DATA-PHONE Data Set ... .
Description ... . Electrical Characteristics
Chapter 6. Wideband Facilities
6-1 Wideband Data Facilities ... . Wide band Data Station ... .
Access Facilities ... . ... . Central Office and Carrier System Equipment ... .
120 121 122 123 123 124 124 125 126 126 128 131 133 134 134 137 138 138 140 141 142 143 144 144 148 153 153 154 157 161
6-2 Local Plant Facilities for DDS ... 166
Customer Premises Equipment ... 167
Access Facilities ... 168
6-3 Baseband Television Facilities . . . 170
The A2-Type Video Transmission Systems. . . 170
The A4 Video Transmission System . . . .. 174
Chapter 7. Central Office Equipment .. 177 7-1 Central Office Transmission Paths
Intraoffice Wiring ... . Switching Networks
178 178 179
7-2 Terminating and Ancillary Circui.ts ... . ... . Line Circuits ... . Trunk Circuits ... .
183 184 185
7-3 Transmission-Related Switching Operations ... . 195
196 196 Call Sequencing ... . Switched Transmission Operations ... . 7-4 Digital Switching Interfaces ... , 197
Digroup Terminal ... . . . .. 198
Voiceband Interface Units ... ... 198
7-5 Television Equipment ... . 199
7 -6 Sources and Control of Impairments ... 200
Noise ... 200
Crosstalk . . . .. 201
Coupling Circuits ... 202
Loss and Return Loss ... . . . 202
Battery Voltage ... 202
Chapter 8. Business Communications Systems. . . 204
8-1 Key Telephone Systems . . . .. 206
8-2 Equipment for PBX and Centrex Services ... ... 201)
Station Lines and Tie Trunks ... 209
PBX Types and Services . . . .. 210
Transmission Considerations ... 214
8-3 Call Distributors and Telephone Answering Systems ... 216
SECTION 3 ANALOG CARRIER SYSTEMS ON METALLIC MEDIA 221 Chapter 9. Frequency Division Multiplex ... 223
9-1 The FDM Hierarchy ... 224
Channel Banks . . . .. 226
Group and Supergroup Banks ... 229
The Mastergroup Multiplex ... 236
Multimastergroup Multiplex Equipment . . . .. 241
9-2 Design Considerations ... 246
Efficiency Factors ... 247
Transmission ... 250
Reliability ... . . . .. 253
9-3 Special Multiplex F~quipment Designs ... 254
Program Terminals '. . . . .. 254
Wideband Data Terminals ... 255
Chapter 10. System Design Features ... . 257
10-1 The Design Process ... 258
Design Relationships ... 259
Design Implementation ... 263
10-2 Equalization ... 268
Functions and Objectives ... 268
Design Types .. . . .. 270
Equalization Strategy ... 272
10-3 Ancillary Designs ... 274
Power .... , ... , ... , ... , . " .. . 274
Maintenance and Reliability . . . .. 275
Terminal Arrangements ... 275
Chapter 11. Wire Pair Carrier Systems .... . . 277
11-1 O-Type Systems ... 278
O-Type Terminals ... 278
Line Repeaters ... 28& 11-2 N-Type Repeatered Lines. . . 288
Transmission Plan ... 28fr N1 Repeaters. . . 291
N1A Repeaters ... 297
N2 Repeatered Line ... 298
11-3 N -Type Terminals ... 302
The N1 Terminal ... 303
ON Junctions and Terminals ... 305
N2 Terminal . . . .. 308
N3 Terminal ... . . . .. 311
Chapter 12. Coaxial Carrier Systems ... 317
12-1 Coaxial Systems Engineering ... 318
Transmission Media .... . . 318
Survey of Systems ... 320
Route Engineering ... 324
12-2 The L5 Coaxial Carrier System ... . . . .. 326
Transmission Design and Layout ... " 327 Maintenance and Reliability ... 338
Power System ... , ... 344
12-3 Undersea Cable Systems ... 344
System Comparisons ... 346
Design Features ... 347
SECTION 4 ANALOG RADIO SYSTEMS
Chapter 13. Microwave Radio System Design Features
13-1 The Transmission Medium ... . Transmission Medium Impairments ... . Continuity of Service ... . ... . 13-2 Repeaters and Terminals ... .
Baseband System Repeaters ... . Intermedia te-Freq uency Repeaters FM Terminals ... . 13-3 Entrance Facilities
Signal Sources ... . Wire-Line Entrance Links 13-4 Signal-to-Noise Considerations
Chapter 14. Microwave System Engineering
14-1 Operating Frequencies and System Characteristics The 2-GHz Band ... .
The 4-GHz Band The 6-GHz Band The ll-GHz Band
The 18-GHz Band ... . 14-2 Choice of System ... . 14-3 Route Selection and Layout
Site Selection ... .
Path Transmission Characteristics ... . Interference Studies ... .
Governmental Jurisdictions Chapter 15. Protection Switching ....
15-1 Continuity of Service ... .
351 353 354 355 356 358 358 360 360 362 363 364 368 371 371 372 372 372 373 373 374 376 376 380 388 393 396 398
15-2 Frequency Diversity Switching in Long-Haul Systems ... 399
The 400A System ... 400
The 100A System ... 405
The TDAS System .. . . 410
The THAS System ... , ... 414
15-3 Frequency Diversity Switching of Short-Haul Systems 415 The 400B Protection Switching System ... 416
401-Type Systems .. . . . . . 417
15-4 Space Diversity and Hot Standby Switching .. 418
15-5 Entrance Link and FM Terminal Switching The 200A Protection Switching System The 200B System ...
Chapter 16. Short-Haul Systems ... . 16-1 The TJ, TL-Type, and TM-l Systems .. .
The T J System ... . The TL-l System ... . The TM-l/TL-2 System ... .
421 422 423 424 425 425 426 426
16-2 The TM-2 Systems ... 427
Frequency Plans ... 428
Repeaters ... 429
16-3 The TN-l System ... . Frequency Plans ... . ... . Equipment Layouts ... . ... . Transmission Performance ., ... . The 5A FM Terminal ... . . . ... . Chapter 17. Long-Haul Systems 17-1 The TD-Type Systems ... . Frequency Plan .... . ... . System Layout ... . Repea ters ... ,... . ... . 17-2 The TH-3 System ... . System Considerations ... . ... . Intermediate Repeater ... , ... . Main Station Repeater ... . ... . 17-3 The TH-l System ... . ... . System Considerations. . . . ... . Repeaters ... . Chapter 18. Domestic Satellite Communications 18-1 International and Domestic Regulation 435 435 439 443 444 446 447 448 450 452 460 461 464 468 4119 470 472 477 478 18-2 Space Vehicle Considerations ... . . . . .. 481
Satellite Stabilization ... 481
Station Keeping . . . . . . 482
Satellite Lifetime ... 483
Launch Vehicles ... 483
18-3 Satellite Transmission Equipment. . . 484
Major Equipment Items ... . . . 484
Comstar Satellite Design ... . . . 486
18-4 Earth Station Equipment ... . ... . 489
489 489 Frequency Coordination ... . Earth Station Transmission Equipment ... . 18-5 Transmission System Considerations ... 494
Link Transmission Characteristics ... 494
Message Circuit Noise . . . .. . .. 499
Transmission Delay . . . .. 502
Chapter 19. Supplementary Radio Communications ... 504
19-1 Land-Mobile Communications ... . System Layout ... . ... . Channel Frequencies and Uses ... . Method of Operation. . . . ... . Channel Loading ... . 19-2 Personal Paging Services ... . Mode of Operation ... . Control Terminal ... . Transmitter ... . Receiver 504 506 506 508 511 512 512 513 514 515 19-3 Marine, Aircraft, and Railroad Customer Services . . . .. 516
Marine Services ... 516
Aircraft and Railroad Services ... . . . .. 517
19-4 Supplementary Long-Haul Radio Facilities ... 518
Tropospheric Transmission Beyond Line of Sight . . . .. 518
Overseas Radio Facilities .... . . . .. 51~
SECTION 5 DIGITAL SYSTEMS 522 Chapter 20. Cable System Design Features .. 524
20-1 Digital Terminal Signal Processing ... . . . .. 525
Sampling, Quantizing, and Companding ... 526
Coding ... 530
Multiplexing . . . .. 533
20-2 Digital Transmission Line ... 535
Signal Characteristics ... 536
Transmission Line Signal Impairments ... , 537
Repeater Characteristics ... . . . .. 542
Transmission Line Layout ... 546
20-3 Maintenance 547
Chapter 21. Digital Terminal and Multiplex Equipment 549
21-1 The Digital Multiplex Hierarchy . . . .. 550
The 24-Channel DS1 Signal . . . .. 552
The DS1C Signal . . . . . . .. 553
The DS2 Signal . . . . . . .. 556
The DS3 Signal .. . . .. 558
The DS4 Signal ... . . . . . .. 561
The Digital Data System . . . .. 563
21-2 Digital Multiplex Equipment ... . . . .. 563
21-3 Signal Characteristic Specifications .. . ... " 564 Line Rates and Codes ... 564
Synchronization .... , . . . .. 565
Elastic Stores ... 565
Pulse Parameters ... 566
Multiplex and Cross-Connect Operations ... 568
PCM Channel Banks . . . .. . ... . Channel Units ... . The D1 Channel Banks ... . The D2 Channel Bank ... . The D3 Channel Bank ., ... . The D4 Channel Bank ... . Special Purpose Terminal Arrangements 569 570 571 581 583 584 586 Chapter 22. Digital Transmission Lines .... 589
22-1 The T1 Digital Transmission System . . . .. 589
Transmission Media ... ... 590
Regenerative Line Repeater ... 591
Line Layout ... 597
Maintenance Considerations .... .... . . 598
The T1 Outstate Digital Transmission System . . . .. 600
22-2 The T1C Digital Transmission System Transmission Media ... . Regenerative Repeater ... . Line Layout ... . 601 602 602 603 22-3 The T2 Digital Transmission System ... " 603 Signal Format .. . . .. 604
Transmission Media ... 604
Regenerative Repeater ... 605
Line Layout. . . . ... " 605 Maintenance ... 606
22-4 The T4M Digital Transmission System . . . .. 606
Signal Format . . . .. 607
Transmission Medium ... 608
Regenerative Repeater . . . .. 608
Line Layout . . . .. 609
Maintenance . . . .. ... 610
Chapter 23. Digital Transmission on Radio Systems 612
23-1 The 1A Radio Digital System ... 613
Terminal Equipment ... ... . . 613
System Layout ... . . . .. 615
Engineering ... . . . .. 618
23-2 The 3A Radio Digital System ... 619
Terminal Equipment ... 620
System Layout ... ... . . . .. 625
Maintenance ... 627
23-3 The DR 18A Digital Radio System ... . . . .. 627
Terminal and Repeater Arrangements . . . 628
System Engineering and Layout . . . .. 634
Maintenance ... . . . 638
SECTION 6 TRANSMISSION MAINTENANCE 639 Chapter 24. Maintenance Systems ... 640
24-1 Network Effects on Maintenance Surveillance ... . Test Procedures ... . Test Equipment ... . Preventive Maintenance ... . Trouble Identification and Location Operating Centers ... . 641 642 642 643 644 644 644 24-2 Facility Maintenance Systems ... 645
24-3 System-Integrated Maintenance Arrangements . . . .. 645
System-Independent Maintenance Arrangements .. . . .. 653
Circuit Maintenance Systems ... . Loop Maintenance ... . Trunk Maintenance ... . Special Services Circuit Maintenance 656 656 658 661 24-4 Maintenance Support ... . 662
663 663 665 Access for Maintenance ... . Communications for Maintenance ... . Service Protection . . . Chapter 25. Test Equipment ... .668
25-1 Voiceband Loss and Delay Measuring Equipment ... 670
Test Signal Generators ... 670
Test Signal Measuring Instruments . . . .. 671
Combined Signal Generator and Measuring Instruments .... 672
25-2 Noise Mea.suring Equipment .. . . . 673 The 3C Noise Measuring Set . . . 675 The 7 A Carrier-Frequency Noise Measuring Set . . . .. 676 The 6-Type Noise Measuring Sets . . . .. 677 25-3 Impedance-Related Measuring Equipment ... 678 Return Loss Measurement . . . .. 679 Balance Testing . . . 680 Level Tracers .. . . .. .. . . .. 681 Measurement of Line Impedance ... . 682 25-4 High-Frequency Measuring Equipment . . . .. 683 Test Set~ for Analog Channel Measurements .. . . .. 683 General Purpose Test Equipment . . . .. 684 System-Related Test Sets .. . . .. 688 Test Sets for TI-Carrier Systems ... 689 25-5 Data Set and Data Loop Test Equipment ... 690 The 911N A Data Test Set . . . .. 690 The 914C Data Test Set ... 691 The 921A Data Test Set . . . .. 691 Data Test Sets For the DDS ... . . . .. 692
SECTION 7 TRANSMISSION SYSTEM INTEGRATION 694
Chapter 26. System Compatibility and Coordination 26-1 Coupling Considerations
695 696 26-2 Coordination in the Loop Plant . . . .. 697
26-3
Inductive Coordination ... . . . .. 697 Transmission Level Point Coordination ... 699 Data Loops . . . .. 700 Loop Carrier Systems . . . .. . . . .. 700 Coordina tion in the Trunk Plant . . .
Short-Haul Carrier Systems .. .
Frequency Coordination ... . 701 701 703 Chapter 27. Common System Facilities ... 705 27-1 Circuit and System Interconnection ... 706 Voice-Frequency Interconnections ... 706 Analog Carrier Systems ... 707 Digital Systems ... 718 Digital-Analog Interfaces . . . . . . .. 718 27 -2 Common Systems Equipment . . . .. 719 Synchronization . . . .. 719 Pilots and Carrier SuppJi.es . . . .. 723 Restoration . . . . . . 724
Index 726
Engineering
Section
1The Facility Network
The public and private switched message networks and the many special services circuits share a nationwide network of telecommuni- cations facilities. These facilities include transmission media, voice- frequency and carrier systems and equipment, a wide variety of terminating circuits, channelizing equipment, signalling and switching equipment, power supplies, and outside plant items of many descrip- tions. In short, the facility network comprises the telecommunications plant.
High-quality transmission is provided over this network by careful design of all of these facilities and by paying particular attention to the interactions at the interfaces. Each of the categories of facilities mentioned above has some effect on transmission. However, those having the largest and most direct effects are transmission systems and the transmission media.
Chapter 1 briefly reviews the evolution of transmission systems from the single-wire, ground-return circuits that were initially leased from telegraph service suppliers through the alphabetically desig- nated analog cable carrier and microwave radio systems to the modern digital transmission systems. Video signal transmission is also covered. The rapidly changing fields of maintenance and reliability are discussed and all of these factors are related to the importance of controlling the dynamic network changes by adequate planning and by the application of engineering principles.
Every connection established for electrical communications between two points requires some form of transmission medium. Chapter 2 describes the media most commonly used. These include open-wire lines, loaded and nonloaded multi pair cables, coaxial cables, and the atmosphere, which provides the medium for a large number of different types of radi'o transmission systems. Transmission over waveguide is also briefly discussed.
Chapter 1
The Evolution of the Facility Network
Transmission facilities include transmission media, assemblies of equipment required to make up transmission systems, and the channels derived from these systems. A network of such facilities exists to provide a wide variety of telecommunications services. In- cluded are many types of transmission systems and subsystems which have evolved with advancing technology and with increasing demands for more and different types of services.
As it becomes necessary to expand facilities and to replace portions that become obsolete, various criteria must be used in order to ac- complish the expansion in an orderly and economic manner. Selection of new facilities and the manner in which they are applied are de- pendent on many factors. The rate of growth and development of a geographic area, shifts of population and business activities, and the interaction of such factors in their influence on community of in- terests must all be taken into account. Separate consideration of these factors must be given to urban, suburban, and rural environments and accurate forecasts of loop and trunk facility needs must also be separately made.
In all these aspects of the facility network evolution, engineering control has been and must be exercised so that new plant is com- patible with the existing plant. Engineering economy studies must be made to assure that growth is efficient and that short and long range objectives are satisfied wherever possible. New technology and in- novations must be carefully evaluated and, applied to assure the satisfaction of customer demands for improved performance and new services.
2
1-1 VOICE-FREQUENCY TRANSMISSION FACILITIES
The transmission media used during the years following the inven- tion of the t~lephone were single, iron-wire conductors rented from suppliers of telegraph service. These circuits operated on the principle of ground return circuit completion and, as a result, were subj ect to many types of impairments. The advantages of paired copper con- ductors, primarily lower transmission loss and lower susceptibility to noise, were recognized very early and by 1900, virtually all existing telephone communication was over paired copper conductors. The transition from iron to copper conductors was accelerated by the development during the late nineteenth century of the hard drawing process for copper wire. Also by 1900, some cables had been manu- factured and the technique of inductive loading had been invented and was being used. These advances permitted telephone communica- tions over longer and longer distances.
With the introduction of electron tube amplifiers, transmission of telephone signals from coast to coast was accomplished in 1915.
Nearly all transcontinental circuits during those early years were open-wire lines. These were used for long distance telephony because large gauge conductors were necessary to obtain the required low line loss.
As the toll plant grew in size and complexity, problems were recognized and s'Olved one by one. In terms of transmission per- formance, these problems included the need to reduce circuit loss and noise and, as transmission paths increased in length, to control or suppress echoes. Losses were first reduced by increasing the size of conductors. Later, inductive loading was applied and, with the in- vention of the electron tube, amplification was provided. Noise perfor- mance was improved by using balanced pairs, transposition (or frogging) of pairs, staggered twisting of cable conductors, and quadded cable. Four-wire transmission, impedance matching, con- trolled losses, and echo suppressors were used to improve echo per- formance of the network. In addition, economics and operational considerations led to the adoption 'Of common battery operati'On, the development of multichannel carrier systems, and the introduction of machine switching 'Of local and toll traffic.
In the local pla~t, the transition from open-wire lines to cable and the application of inductive loading techniques permitted the develop- ment of economical subscriber loops and local trunks. Central office
equipment expanded from simple manual switchboards to large cord- type switchboards with A-board and B-board arrangements, the A-board for outgoing calls and the B-board for incoming calls. This was followed by the first switching machines such as step-by-step and panel. In the manual and early machine switching systems, the status of telephone set development and signalling requirements placed limitations on wire gauge selection in the loop and trunk plant.
The use of smaller wire gauges in the local plant was initially made possible by the application of inductive loading. Later, the in- vention of electron tubes and solid-state devices and improvements in circuit components of all types provided further important advances in voice-frequency transmission performance. These new devices and improved components have made possible the development of a wide range of amplifiers, repeaters, bridge lifters, hybrid coil transformers, impedance matching devices, noise balancing circuits, and filters. They have also led to improvements in frequency response, antisidetone features, and loop current equalization of telephone station sets. These improvements together with the development of circuits to increase loop signalling ranges have made possible the use of smaller wire gauges in the loop plant.
1-2 CARRIER TRANSMISSION FACILITIES
The history of carrier systems really begins in the early years of the 20th century and progress in all areas has been rapid. Carrier modes of transmission increase the efficiency of utilization of trans- mission media by combining (multiplexing) a large number of mes- sage signals into a single composite signal. Nearly all types of carrier modulation have been used but three are now predominant. Single- and double-sideband amplitude modulation with frequency division multiplexing of signals are commonly used to form a br'oadband signal for transmission over analog cable carrier systems. Frequency modu-
lation of a microwave carrier is used to transmit this broadband signal over microwave radio systems. Various forms of pulse modula- tion, notably pulse code modulation, are used with time division multiplex techniques for transmitting signals over digital (regenera- tive repeater) facilities. Many other combinations of these techniques are possible and some are being developed.
Analog Cable Carrier Systems
The application of electron tubes and solid-state devices and con- tinuing improvements in passive components have made it possible
to develop carrier systems providing ever wider bandwidths, thus substantially reducing the per-unit line cost of circuits. However, the distance between terminals, among other factors, does affect the point at which the use of electronics becomes more attractive than voice- frequency cable circuits.
In addition to the economic advantages of carrier systems, a number of performance improvements are realized. The velocity of propaga- tion at carrier frequencies in any cable circuit is substantially higher than at voice frequencies. This is especially true when carrier circuits are compared with loaded cable pairs. The higher velocity offers ad- vantages in respect to control of echoes and favors some types of signal transmission where absolute delay is important. Some form of four-wire transmission is necessary in broadband carrier systems;
as a result, impedances are better controlled thus permitting the operation of circuits at lower losses yet with satisfactory stability and echo performance.
The development of the A-type system in 1917 was followed by a succession of carrier systems identified by alphabetical designations.
These systems were at first developed for use on open-wire trans- mission lines. Most of these early systems, designated A through J, provided four-wire or equivalent four-wire transmission in a fre- quency band above a voice-frequency channel which could simultane- ously be provided on the open-wire pair. Two exceptions should be noted. The E-type system provided transmission for one single- sideband channel on power lines. The same band was used for both directions of transmission, a mode that was made possible by voice- frequency switching so that only one direction of transmission oc- curred at anyone time. Only three such systems were placed in service, those during the middle 1920s. The other exception was the G 1 system, placed in service in 1935. It provided a single channel above a voice-frequency channel on a single pair of wires in a double-sideband transmitted carrier mode and was the only carrier system which employed true two-wire transmission.
The J-type system provided transmission for twelve single- sideband suppressed carrier channels on an open-wire pair in the equivalent four-wire mode. A voice-frequency channel and a C-type carrier system, which had a frequency allocation between the voice- band and the J-carrier band, could be operated simultaneously on the same pair. A number of different frequency allocations were used for J carrier but all involved a first step of modulation into a group band
covering the spectrum from 60 to 108 kHz. Thus, the basic group was formed to become the foundation of the entire modern frequency division multiplex (FDM) system, during the 1930s. The K-carrier systems, developed at about the same time as the J-type, provided twelve single-sideband message channels for four-wire transmission on cable pairs rather than on open-wire lines. This was the first system in which the transmission medium was not shared with a voice-frequency channel or with another type carrier system. Due to crosstalk effects, separate cables were normally used for the two directions of transmission. However, a single cable could be used when pairs were carefully shielded from one another.
Terminal equipment for both J - and K-carrier systems used the same basic group frequencies. Both J - and K-type systems were manufactured and installed in quantity before, during, and after World War II. Manufacture of these systems has now been discon- tinued although a number of both types are still in service.
While the J- and K-type systems filled long-haul needs, short-haul analog systems were also developed. In 1950, the first N -type system was placed in service to provide twelve double-sideband transmitted carrier channels over nonloaded cable pairs for distances up to about 200 miles. The initial designs utilized electron tubes throughout.
However, the system was redesigned during the 1960s to exploit solid- state technology. The terminal equipment was again redesigned to permit the transmission of 24 single-sideband channels.
The N-type systems were originally designed for four-wire trans- mission in two frequency bands which were alternated in succeeding repeater sections by a modulation process at each repeater (frequency frogging) . This technique partially equalizes the attenuation/
frequency characteristic and minimizes the crosstalk coupling between the two directions of transmission. Another version of line design also uses the alternation of frequency positions at each repeater but utilizes the equivalent four-wire mode of transmission.
The O-type systems, made available during the same era and similar in many respects to the N-type systems, provide short-haul carrier transmission on open-wire facilities. These systems, many of which are still in service, utilize electron tubes. The equivalent four-wire mode of transmission is used and frequency frogging is employed at each repeater. In addition, the open-wire pairs must be transposed in accordance with a plan developed for carrier frequencies.
The O-type system can provide a maximum of 16 4-kHz single- sideband channels which are multiplexed in groups of four channels.
Each of the two pairs of channels in a four-channel group is trans- mitted on a common carrier frequency, one channel as an upper side- band and the other as a lower sideband. This arrangement is called twin-channel operation.
Certain O-type terminal arrangements have been adapted for use with N -type lines. This combination of O-type terminals and N -type lines is called an ON system. Up to 24 channels can be provided by this method. An ON junction is available to permit convenient inter- connection of cable and open-wire facilities. In addition, ON -type system signals can be multiplexed by standard arrangements to pro- vide for the transmission of 96 channels on microwave radio systems.
In 1929, the initial patent for a coaxial cable transmission system was granted. Cable and system development work continued from that time until 1941 when the Ll coaxial system was placed in service.
Initially, the Ll system provided transmission for 480 (later ex- panded to 600) 4-kHz message channels using separate coaxial units for each direction of transmission. The continuing development of coaxial systems has produced the 1860-channel L3 system (1953), the 3600-channel L4 system (1967), and the 10,800-channel L5 system (1974). The L5 system is currently being expanded to a capacity of 22 600-channel mastergroups (13,200 channels).
Design improvements in active devices, components, and systems have been paralleled by improvements in the performance and capa- bilities of the transmission media. From open wire, progress has been made in conductor, insulation, and sheath designs of cables and numerous advances have been made in all aspects of coaxial cable design.
Radio Transmission Systems
Early theoretical studies pointed to the possibility of using the earth's atmosphere as a transmission medium. The invention and development of the electron tube opened the way not only to cable carrier system development but also to the exploitation of the atmos- phere for radio transmission methods.
In 1915, significant experimentation in communication by radio was started. One of the principal objectives, at first, was to provide a means for communications between the United States and Europe.
By 1923, the basic feasibility had been established and intensive work was underway [1]. The first transatlantic commercial telephone service was established in 1927 when a long-wave (57-kHz) system was put into service between the United States and Great Britain.
This system employed single sideband transmission with suppressed carrier [2]. In 1928, short-wave systems were installed to operate in the 3- to 30-MHz range and service was expanded to all parts of the world. In the 1950s, overseas service was largely taken over by sub- marine cable transmission systems [3, 4].
During this period, radio transmission capability was also developed for a number of mobile services. These included high-seas ship-to- shore communication, coastal-harbor service (ship-to-ship and ship- to-shore), and mobile radio telephony to moving vehicles including automobiles, trains, and aircraft. Most important, from the point of view of modern communications, was the development of microwave radio systems.
Microwave system development was stimulated significantly by World War II developments of radar and microwave components.
There was some microwave transmission system work done for military applications and in preparation for the tremendous growth in communications services anticipated for the early post-war years.
This growth quickly materialized; it was stimulated by the pent-up demand for services that could not be satisfied during the war and by the introduction and rapid growth of television. The first experi- mental microwave radio system, called TDX, was installed between New York and Boston and service was begun in May, 1948. This system was manufactured commercially as the TD-2 System [5]. It is now the most widely used long-haul transmission system in the United States.
Many microwave systems have been developed to fill service needs in long-haul and short-haul applications. These have been designed to operate in a number of frequency bands specified for common carrier use. Since the medium must be shared by many users of communica- tion services, the allocation of frequency bands and the design and use of radio transmission equipment in the United States are subject to licensing and control by the Federal Communications Commission.
The frequency bands allocated for various types of services must con-
