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Copyright © 2000 John Wiley & Sons, Ltd

Understanding Data Communications (Third Edition)

Published Online: 05 Oct 2001 Author(s): Gilbert Held ISBN: 0470841486 (Electronic) 0471627453 (Print) Book Description:

Now in its third edition, Understanding Data Comunications, provides a comprehensive introduction to the field of data communications for both students and professionals. Assuming no prior knowledge of the field, it presents an overview of the role of communications, their importance, and the fundamental concepts of using the ISO's 7-layer approach to present the various aspects of networking.

• Covers the evolving high speed network access via digital subscriber line, cable modems and wireless communication.

• Examines the role of regulatory and standardization bodies, the operation of the Internet and the use of a variety of electronic applications.

Copyright © 2000 John Wiley & Sons, Ltd

• Includes a series of comprehensive questions covering the important concepts from each section.

• Describes the digital network used by communications carriers and the methods used to obtain access to the digital highway.

• Discusses frequency division multiplexing which forms the foundation for the operation of several types of high speed digital subscriber line.

Aimed at the senior level undergraduate and graduate computer science student, it is also essential reading for data processing professionals and those involved in computer science and data communications. Acknowledgements: Many thanks to iota@flyheart, who not only provided the related material but give me some advice and making-tips. Spornsored by:

iota

Made by:

NutZ9 or NutFanZ

May 14, 2003

UNDERSTANDINGDATA COMMUNICATIONS

Understanding Data Communications: From Fundamentals to Networking.Third Edition Gilbert Held

Copyright # 2000 John Wiley & Sons LtdPrint ISBN 0-471-627453 Online ISBN 0-470-84148-6

UNDERSTANDINGDATA COMMUNICATIONS

From Fundamentals to NetworkingThird Edition

Gilbert Held4-Degree Consulting

Macon, Georgia,USA

JOHN WILEY & SONS, LTDChichester . NewYork . Weinheim . Brisbane . Singapore . Toronto



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Library of Congress Cataloging-in-Publication Data

Held, Gilbert, 1943-Understanding data communications: from fundamentals to networking / Gilbert Held.p. cm.

ISBN 0-471-62745-3 (alk. paper)1. Data transmission systems. 2. Computer networks. I. Title

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4 CHAPTER TITLE

CONTENTS

Preface xix

Acknowledgements xxi

1 Communications in a Modern Society 11.1 Applications 1

1.1.1 Data collection 21.1.2 Transaction processing 31.1.3 Conversational time sharing 51.1.4 Remote job entry 71.1.5 Message switching 71.1.6 Value-added carriers and electronic mail 81.1.7 Office automation 121.1.8 Electronic commerce 141.1.9 Satellite transmission 16

1.2 Constraints 161.2.1 Throughput 171.2.2 Response time 181.2.3 Bandwidth 181.2.4 Economics 19

1.3 Emerging Trends 191.4 Review Questions 20

2 Basic Telegraph and Telephone Operations 232.1 Evolution of Communications 232.2 Telegraphy 24

2.2.1 Operation 242.2.2 Morse code 262.2.3 Morse code limitations 272.2.4 Start–stop signaling and the Baudot code 282.2.5 Bits and codes 29

2.3 Telephony 322.3.1 Principle of operation 322.3.2 Sound wave conversion 342.3.3 The basic telephone connection 362.3.4 Switchboards and central offices 372.3.5 Numbering plans 392.3.6 Geographic calling areas and network routing 402.3.7 The world numbering plan 43

2.4 Review Questions 43



3 Basic Circuit Parameters, Measurement Units andMedia Overview 47

3.1 Basic Circuit Parameters 473.1.1 Frequency and bandwidth 473.1.2 The telephone channel passband 49

3.2 Measurement Units 503.2.1 Power ratios 503.2.2 Signal-to-noise ratio 523.2.3 Reference points 54

3.3 Media Overview 563.3.1 Twisted-pair cable 563.3.2 Coaxial cable 613.3.3 Microwave 633.3.4 Fiber-optic transmission 64

3.4 Channel Capacity 673.4.1 Bit versus baud 673.4.2 Nyquist relationship 673.4.3 Shannon’s law 68

3.5 Structured Wiring 693.5.1 The wiring closet 693.5.2 The EIA/TIA-568 standard 69


4 Fundamental Data Transmission Concepts 754.1 Analog Line Connections 75

4.1.1 The analog switched line 764.1.2 Analog leased line 794.1.3 Dedicated line 824.1.4 Switched network vs leased line economics 83

4.2 Types of Service and Transmission Devices 844.2.1 Digital repeaters 854.2.2 Modems 864.2.3 Acoustic couplers 874.2.4 Analog facilities 894.2.5 Digital facilities 934.2.6 Digital signaling 934.2.7 Representative AT&T digital offerings 96

4.3 Transmission Mode 984.3.1 Simplex transmission 984.3.2 Half-duplex transmission 994.3.3 Full-duplex transmission 1004.3.4 Terminal and mainframe computer operating modes 101

4.4 Transmission Techniques 1034.4.1 Asynchronous transmission 1034.4.2 Synchronous transmission 105

4.5 Types of Transmission 1064.6 Wide Area Network Transmission Structures 107

4.6.1 Mainframe computer-based network structure 1084.6.2 LAN network structure 1094.6.3 LAN internetworking structure 110

4.7 Line Discipline 1114.8 Transmission Rate 113

4.8.1 Analog service 1134.8.2 Digital service 114

vi CONTENTS

4.9 Transmission Codes 1154.9.1 Morse code 1154.9.2 Baudot code 1164.9.3 BCD code 1164.9.4 Extended binary-coded decimal interchange code (EBCDIC) 1164.9.5 ASCII code 118


5 Terminals, Workstations and WAN and LANNetworking Overview 125

5.1 Terminals 1265.1.1 Interactive terminal classification 1265.1.2 Terminal evolution 127

5.2 Workstations and Other LAN Components 1415.2.1 Network interface card 1415.2.2 Hubs 1425.2.3 File server 1435.2.4 Print server 1455.2.5 Other types of servers 146

5.3 Wide Area Networking Overview 1465.3.1 Multiplexing and data concentration 1465.3.2 Front-end processor 1515.3.3 Network configurations 151

5.4 Local Area Networking Overview 1525.4.1 Repeaters 1535.4.2 Bridges 1535.4.3 Routers 1545.4.4 Gateways 155


6 Representative Standards Organizations:the OSI Reference Model 159

6.1 National Standards Organizations 1606.1.1 American National Standards Institute (ANSI) 1606.1.2 Electronic Industries Association (EIA) 1616.1.3 Federal Information Processing Standards (FIPS) 1636.1.4 Institute of Electrical and Electronic Engineers (IEEE) 1636.1.5 British Standards Institution (BSI) 1646.1.6 Canadian Standards Association (CSA) 164

6.2 International Standards Organizations 1646.2.1 International Telecommunications Union (ITU) 1646.2.2 International Standards Organization (ISO) 165

6.3 De facto Standards 1676.3.1 AT&T compatibility 1686.3.2 Cross-licensed technology 1696.3.3 Bellcore/Telcordia Technology 1696.3.4 Internet standards 170

6.4 The OSI Reference Model 1716.4.1 Layered architecture 1726.4.2 OSI layers 1736.4.3 Data flow 176

6.5 IEEE 802 Standards 1776.5.1 802 committees 1776.5.2 Data link subdivision 179


CONTENTS vii

7 The Physical Layer, Cables, Connectors, Plugsand Jacks 183

7.1 DTE/DCE Interfaces 1847.1.1 Connector overview 1867.1.2 RS-232-C/D 1887.1.3 Differential signaling 1987.1.4 RS-449 2007.1.5 V.35 2027.1.6 RS-366-A 2037.1.7 X.21 and X.20 2047.1.8 X.21 bis 2077.1.9 RS-530 2077.1.10 High Speed Serial Interface 2987.1.11 High Performance Parallel Interface 2147.1.12 Universal Serial Bus 2167.1.13 IEEE 1394 (FireWire) 218

7.2 Cables and Connectors 2227.2.1 Twisted-pair cable 2227.2.2 Low-capacitance shielded cable 2237.2.3 Ribbon cable 2237.2.4 The RS-232 null modem 2237.2.5 RS-232 cabling tricks 225

7.3 Plugs and Jacks 2267.3.1 Connecting arrangements 2287.3.2 Telephone options 2307.3.3 Ordering the business line 2317.3.4 LAN connectivity 232


8 Basic Transmission Devices: Line Drivers,Modems, and Service Units 235

8.1 Line Drivers 2368.1.1 Direct connection 2368.1.2 Using line drivers 239

8.2 Modem Operations 2438.2.1 The modulation process 2438.2.2 Bps vs. baud 2468.2.3 Voice circuit parameters 2468.2.4 Combined modulation techniques 2478.2.5 Mode of transmission 2538.2.6 Transmission techniques 2548.2.7 Modem classification 2558.2.8 Limited-distance modems 2568.2.9 Line-type operations 2578.2.10 Reverse and secondary channels 2578.2.11 Equalization 2588.2.12 Synchronization 2608.2.13 Multiport capability 2608.2.14 Security capability 2618.2.15 Multiple speed selection capability 2618.2.16 Voice/data capability 2628.2.17 Modem handshaking 2628.2.18 Self-testing features 2638.2.19 Modem indicators 2658.2.20 Modern operations and compatibility 265

viii CONTENTS

8.3 Intelligent Modems 2898.3.1 Hayes command set modems 2898.3.2 Key intelligent modem features 2968.3.3 Microcom Networking Protocol (MNP) 3028.3.4 Data compression 3068.3.5 MNP Class 5 compression 3068.3.6 MNP Class 7 enhanced data compression 3088.3.7 V.42bis 311

8.4 Broadband Modems 3128.4.1 Telephone and cable TV infrastructure 3138.4.2 Cable modems 3178.4.3 DSL modems 324

8.5 Service Units 3308.5.1 The DSU 3318.5.2 The CSU 331


9 Regulators and Carriers 3359.1 Regulators 336

9.1.1 US regulatory evolution 3369.1.2 International regulatory authorities 342

9.2 Carrier Offerings 3439.2.1 AT&T system evolution 3439.2.2 The Bell system 3459.2.3 The regional Bell operating companies 3469.2.4 AT&T service offerings 3499.2.5 Regional Bell operating company offerings 355

9.3 ATM Overview 3569.4 Review Questions 357

10 Transmission Errors: Causes, Measurements andCorrection Methods 359

10.1 Causes of Transmission Errors 35910.2 Performance Measurements 360

10.2.1 Bit error rate 36010.2.2 Bit error rate tester 36010.2.3 BERT time 36210.2.4 Performance classifications 36210.2.5 Block error rate testing 36410.2.6 Error-free second testing 365

10.3 Error Detection and Correction Techniques 36510.3.1 Asynchronous transmission 36510.3.2 Synchronous transmission 370


11 The WAN Data Link Layer 37711.1 Terminal and Data Link Protocols: Characteristics

and Functions 37811.1.1 Transmission sequence 37911.1.2 Error control 379

11.2 Types of Protocol 38011.2.1 Teletypewriter protocols 38011.2.2 PC file transfer protocols 385

CONTENTS ix

11.2.3 Bisynchronous protocols 39511.2.4. Digital Data Communications Message Protocol (DDCMP) 40011.2.5 Bit-oriented line control procedures 402


12 Increasing WAN Line Utilization 40912.1 Multiplexers 410

12.1.1 Evolution 41012.1.2 Device support 41012.1.3 Multiplexing techniques 411

12.2 Control Units 43912.2.1 Control unit concept 44012.2.2 Attachment methods 44012.2.3 Unit operation 44212.2.4 Breaking the closed system 443


13 Local Area Networks 44913.1 Origin 44913.2 Comparison with WANs 450

13.2.1 Geographical area 45013.2.2 Data transmission and error rates 45013.2.3 Ownership 45113.2.4 Regulation 45113.2.5 Data routing and topology 45113.2.6 Type of information carried 452

13.3 Utilization Benefits 45213.3.1 Peripheral sharing 45313.3.2 Common software access 45313.3.3 Electronic mail 45313.3.4 Gateway access to mainframes 45313.3.5 Internet access 45313.3.6 Virtual private network operations 454

13.4 Technological Characteristics 45413.4.1 Topology 45413.4.2 Comparison of topologies 45613.4.3 Signaling methods 45713.4.4 Transmission medium 46013.4.5 Access methods 460

13.5 Ethernet Networks 46513.5.1 Original network components 46513.5.2 IEEE 802.3 networks 46813.5.3 Frame composition 49013.5.4 Media access control overview 49513.5.5 Logical link control overview 49513.5.6 Other Ethernet frame types 498

13.6 Token-Ring 50413.6.1 Topology 50413.6.2 Redundant versus non-redundant main ring paths 50613.6.3 Cabling and device restrictions 50713.6.4 Constraints 51013.6.5 High speed Token-Ring 51413.6.6 Transmission formats 51513.6.7 Medium access control 52413.6.8 Logical link control 527


x CONTENTS

14 Basic LAN Internetworking 53114.1 Bridge Operations 531

14.1.1 Types of bridge 53114.1.2 Network utilization 544

14.2 The Switching Hub 54614.2.1 Basic components 54614.2.2 Delay times 54714.2.3 Key advantages of use 54914.2.4 Switching techniques 54914.2.5 Port address support 55314.2.6 Switching architecture 55614.2.7 High-speed port operations 55714.2.8 Summary 558

14.3 Router Operations 55814.3.1 Basic operation and use of routing tables 55914.3.2 Networking capability 56014.3.3 Communication, transport and routing protocols 56114.3.4 Router classifications 56314.3.5 Routing protocols 566


15 Digital Transmission Systems andEquipment 577

15.1 The T and E Carriers 57815.1.1 Channel banks 578

15.2 T1 Multiplexers 59615.2.1 Waveform-based voice digitization modules 59715.2.2 Vocoding 59815.2.3 Hybrid coding 60115.2.4 T1 multiplexer employment 602

15.3 The T3 Carrier 60515.3.1 T3 circuit types 60615.3.2 Evolution 60615.3.3 T3 framing 609

15.4 DDS, ASDS and KiloStream facilities 61515.4.1 Applications 61615.4.2 ASDS 61615.4.3 KiloStream service 617

15.5 Integrated Services Digital Network (ISDN) 61915.5.1 Concept behind ISDN 61915.5.2 ISDN architecture 62015.5.3 Network characteristics 62115.5.4 ISDN layers 625


16 Network Architecture 63116.1 SNA Overview 632

16.1.1 SNA elements 63416.1.2 System Service Control Point (SSCP) 63416.1.3 Network nodes 63416.1.4 The physical unit 63516.1.5 The logical unit 63516.1.6 SNA network structure 63516.1.7 Types of physical unit 637

CONTENTS xi

16.1.8 Multiple domains 63716.1.9 SNA layers 63916.1.10 SNA developments 64116.1.11 SNA sessions 641

16.2 Advanced Peer-to-Peer Networking (APPN) 64416.2.1 APPC concepts 64416.2.2 APPN architecture 64516.2.3 Operation 646

16.3 TCP/IP 64916.3.1 The rise of the Internet 65016.3.2 The TCP/IP protocol suite 65116.3.3 Applications 65316.3.4 TCP/IP communications 66316.3.5 The Internet Protocol (IP) 66416.3.6 Domain Name Service 679

16.4 Internetworking 68116.4.1 SNA gateway operations 68216.4.2 Supporting multiple protocols 69016.4.3 Data Link Switching 693


17 Packet Networks 69717.1 Packet Switching Overview 69817.2 X.25 Networks 700

17.2.1 Development period 70017.2.2 Need for PADs 70017.2.3 X.25 layers 70517.2.4 Methods of connection 70817.2.5 Utilization costs 70917.2.6 Tymnet 71117.2.7 Network information 71317.2.8 Features 71317.2.9 Protocol conversion 71517.2.10 LAN interconnectivity 716

17.3 Frame Relay 71717.3.1 Comparison to X.25 71717.3.2 Standards 71917.3.3 Network access 72017.3.4 Frame construction 72117.3.5 Service parameters 72917.3.6 FRAD features 73417.3.7 Voice over Frame Relay 740


18 Communications Software 74918.1 Terminal Emulation Software Features 749

18.1.1 Hardware utilization 75218.1.2 Software utilization 75318.1.3 Operational consideration 75418.1.4 Documentation 75718.1.5 Dialing 75718.1.6 Transmission 76218.1.7 Performance efficiency 76618.1.8 Performance flexibility 77018.1.9 Security performance 772

xii CONTENTS

18.2 Terminal Emulation Program Examination 77418.2.1 Procomm Plus for Windows 77518.2.2 HyperTerminal 77718.2.3 IBM PC/3270 780

18.3 Web Browsers 78318.3.1 Microsoft Internet Explorer 78418.3.2 LAN operation 788


19 Fiber-Optic, Satellite and Wireless TerrestrialCommunications 791

19.1 Fiber-Optic Transmission Systems 79219.1.1 System components 79219.1.2 Transmission advantages 79919.1.3 Limitations of use 80119.1.4 Utilization economics 80219.1.5 Carrier utilization 80519.1.6 SONET 806

19.2 Satellite Communications Systems 81019.2.1 Operation overview 81019.2.2 Satellite access 81019.2.3 Very small aperture terminal (VSAT) 81219.2.4 Low earth orbit satellites 812

19.3 Wireless Terrestrial Communications 81419.3.1 Cellular communications 81419.3.2 Wireless LANs 820


20 Evolving Technologies 82320.1 ATM 823

20.1.1 Cell size 82320.1.2 Scalability 82420.1.3 Transparency 82520.1.4 Traffic classification 825

20.2 The ATM Protocol Stack 82520.2.1 ATM Adaptation Layer 82520.2.2 The ATM Layer 82620.2.3 Physical Layer 827

20.3 ATM Operation 82720.3.1 Components 82720.3.2 Network Interfaces 82920.3.3 The ATM cell header 83020.3.4 ATM connections and cell switching 833

20.4 Virtual Private Networking 83520.4.1 Rationale for use 83620.4.2 Reliability 83720.4.3 Problem areas 837


Index 841

CONTENTS xiii

PREFACE

Man’s constant quest to communicate has resulted in a quantum leap intechnology related to data communications. For the past quarter century themaximum obtainable transmission rate on many types of communicationsfacilities has doubled every three to five years. During the past few years thisgrowth rate has accelerated, with emerging technologies providing a trans-mission capability an order of magnitude or more above what were consideredhigh operating rates just a year or two ago. Accompanying this growth and, inmany cases, providing the impetus for the technological developments thatmade such growth possible are communications-dependent applications.

Today, data communications can be considered as the fiber that binds amodern society together. The past measurement of the strength of a nation,measured in the number of tons of steel manufactured per year, has essen-tially been replaced by the installed base of personal computers, workstationsand other types of computational facilities, as well as the network structuresthat link those computers to one another. Unless stranded in a very remotelocation, you will use one or more communications facility almost every day ofyour life.

Due to the importance and, in many instances, our dependence uponcommunications, a detailed understanding of their evolution, technology andfuture directions is beneficial to most persons that work in a business, hightechnology, government or university environment, and provides a drivingforce for writing this book.

This book dates back to 1977 when the founding editor of DataCommunications magazine, the renowned Harry Karp, asked me to developa seminar to explain the characteristics, operation and utilization of datacommunications components which are the building blocks upon whichnetworks are constructed. The resulting seminar, which I have continued toteach in both the United States and Europe, provided the basis for writing DataCommunications Networking Devices, which has been blessed by readerdemand to justify four editions. From teaching several data communicationsand computer courses at the university level, I became aware of many of thelimitations of currently available books, including Data CommunicationsNetworking Devices. What my students desired was a comprehensive bookthat assumes no prior knowledge of communications and which presents



concepts and theory, and relates practical experiences in a manner useful forpersons involved or planning to work with data communications within anorganization.

This new edition of Understanding Data Communications was written forboth the student and the professional who wish to obtain a solid foundationconcerning how data communications systems operate, why, where, andwhen certain types of equipment should be networked together, and the role ofevolving communications technologies. In revising this book I continued toinclude and expand upon many basic communications concepts. History hasa way of repeating itself and knowledge of how older communications systemsoperate that may not appear to be particularly important yesterday may beextremely useful tomorrow when attempting to understand the operation andutilization as well as limitations associated with a new technology. One keyexample of this is frequency division multiplexing, a technology consideredrelatively obsolete by the 1980s but which now forms the foundation for theoperation of several types of high speed digital subscriber lines that representa new generation of modem technology. Thus, while a major emphasis of thisbook is upon modern communications equipment and transmission systems,as an educator I felt it was important to include historical information and anoverview of older technology that illustrates important concepts that areapplicable for understanding modern technologies.

In developing this book I used a layered approach, building upon theknowledge presented in each prior chapter. This layered approach facilitatesthe utilization of this book as a one-semester course at a high undergraduateor at a first-year graduate level.

Throughout this book I have included numerous illustrations, tables andschematic diagrams to illustrate concepts, theory and practice. I believe thismaterial will facilitate the use of this book long after a reader completes thecourse that it is used in, and will provide a reference for future endeavors incommunications. Finally, at the end of each section I have included acomprehensive series of questions that cover many of the important conceptscovered in the section. These questions can be used as a review mechanismprior to going forward in the book.

As both a professional author and an educator I highly value feedback. Youcan write to me through my publisher whose address is on page iv of thisbook, or you can communicate with me via email at [email protected]. Letme know if I committed the sin of omission and need to include other topics, ifyou feel I devoted too much space to a particular topic, or any other area youmay wish to comment upon.

Gilbert HeldMacon, GA

xvi PREFACE

ACKNOWLEDGEMENTS

The preparation of a manuscript that gives birth to a book requires thecooperation and assistance of many people. First and foremost, I must thankmy family for enduring those long nights and for missing weekends while Idrafted and redrafted the manuscript and reviewed proof pages.As an ‘old fashioned’ author, I prefer the pen and paper to the modern

convenience of the word processor. Although this may appear peculiar whenwriting on modern technology, my lifestyle of plane hopping, finding incom-patible electrical outlets when traveling throughout the world and the extraweight of a portable computer makes pen and paper a most convenient mech-anism of expression. Due to my method of writing I am indebted to Mrs CarolFerrell who worked on the first edition of this book, and toMrs Linda Hayes andMs Junnie Heath, who worked on the second edition. Once again, I amindebted to the fine effort of Mrs Linda Hayes who also worked on the thirdedition of this book. Linda, as well as Junnie and Carol, were responsible forturning my handwritten manuscript revisions into the word processing filesthat were used for the creation of each edition of this book. Last but not least,one’s publishing editor, editorial supervisor and copy editor are the criticallink in converting the author’smanuscript into a book. Thus, I would again liketo thank Ian McIntosh and Ann-Marie Halligan for providing me with theopportunity to author three editions of this book, and Robert Hambrook andSarah Lock and Sarah Corney for their fine efforts in moving my original andrevised manuscripts through the production process.



1COMMUNICATIONS IN A

MODERN SOCIETY

The main objective of this chapter is to obtain an appreciation of the use ofcommunications to enhance our daily work and recreation. To accomplishthis, we will look at nine typical types of communication applications.Although an in-depth examination of many application areas will be deferredto later chapters in this book, the overview of communication applicationspresented in this chapter will illustrate our society’s dependence upon theflow of timely and accurate information. Since there are many trade-offsinvolved in the design and operation of different communications systems, wewill also focus our attention upon three key constraints and their effect upondifferent types of information flow in the second part of this chapter. Eventhough this is an introductory chapter it is important to understand thedirection of technology as it relates to the field of data communications. Thus,the concluding section in this chapter will provide an overview of emergingtrends and their potential effect upon your ability to communicate.

1.1 APPLICATIONS

The evolution of data communications has been nothing short of phenom-enal. During a period slightly exceeding a century, the primitive telegraph hasbeen replaced by a wide variety of networks that are the glue which binds ourmodern society together. As we perform our daily operations, it is mostdifficult to avoid coming into contact with an application that is notdependent upon data communications. Although we may take communica-tions-related applications for granted, without the ability to communicatedata, the banking, transportation and retail industries, as well as others,could not provide customers with an acceptable level of service. For otherindustries, such as publishing and finance, as well as many governmentagencies, the ability to rapidly communicate information is indispensable totheir successful operation. Even the ability of countries to pursue policy isdirectly affected by communications. For example, in warfare the ability tosuccessfully communicate can provide the margin which differentiates victory



from defeat. This is vividly illustrated by the Gulf War, during which missileswith TV guidance, ‘smart’ bombs that could be directed down elevator shafts,and the ability to rapidly share intelligence gathered from the battlefieldresulted in one of the most decisive military campaigns conducted in thehistory of warfare.In this chapter we will examine a variety of applications that illustrate the

important role of data communications in a modern society. This examina-tion should provide readers with an insight into the ubiquitous nature ofcommunications-dependent applications, as well as knowledge of some of themany industries that benefit from the ability to rapidly and accuratelytransmit information.

1.1.1 Data collection

Although many small firms still use manual time and attendance methods,the simple mechanical ‘clock-punch’ machine used in large industrialcorporations and by companies with hundreds or thousands of employeesis essentially only seen in movies of the 1960s or earlier. Today, most largeorganizations, as well as many firms with fewer than a hundred employees,use integrated data collection systems to track employee time and attendancedata. Typically, employees insert their badges into a badge reader when theyarrive at work or on the factory floor. Similarly, at break times, lunch andwhen they leave the premises, they insert their badges into a similar reader atthe location where they ‘clocked-in’ or at another location.Each badge reader recognizes and reads a magnetic strip on the badge, a

series of vertical lines or perhaps hole punches that convey the uniqueidentity of the employee. After reading the information, the badge readertransmits it to a computer center that may be located on the factory floor, inthe same building or hundreds, or even thousands, of miles away.Once the badge reader has transmitted the information it has read from the

badge, the processing performed by the computer can range from simple timeand attendance record keeping to the sophisticated alerting of managementpersonnel to potential problems. Some problems that management might bealerted to include too few employees to perform a factory assembly function,excessive overtime or tardiness of employees. Within many organizations, thedata collection facilities are integrated into the payroll system, relegating theuse of time and attendance clerical employees to correcting such mistakes asforgetting to ‘punch-in’ or ‘punch-out’.A second pervasive example of data collection can be viewed by visiting

many fast food retail chains. As you convey your order of a hamburger, largefries and shake to the clerk, you will probably notice that they press codedkeys with symbols indicating each item on an electronic cash register type ofdevice. Although that device functions as a cash register, totaling yourpurchase, adding applicable sales tax and computing change based upon yourpayment, it is also a data collection device more commonly known as a point-of-sale terminal. As the clerk presses a coded key, the information concerningthe sale of each item is transmitted to a small computer system where it isrecorded onto a diskette, cassette or other type of storage device. At the close

2 COMMUNICATIONS IN A MODERN SOCIETY

of business or at a designated time, the computer system will print reports ofthe income received at each point of sale terminal to assist management incash reconciliation as well as a summary report of items sold in the store.Taking the automation process a few steps further, some computer systemsare programmed to automatically call a franchise distribution center orindependent vendors. The computer will then electronically order suchnecessary supplies as hamburger wrappers, straws, napkins and cups, aswell as meat patties and bags of french fries.

1.1.2 Transaction processing

Also known as inquiry-response, transaction processing is the key tocustomer support in the transportation and financial service industrieswhere instant access to database information is required. Transactionprocessing differs from data collection in the fact that data transmitted to acomputer in a transaction processing system can be used to immediatelyupdate a database. While this difference may appear trivial at first, it is thebasis for ensuring that two persons do not purchase the same airline seat onthe same flight, a bank customer does not charge an item beyond his or hercredit limit, as well as other transactions dependent upon the immediateupdating of information contained in a database.Three of the more common uses of transaction processing include stock

broker order entry systems, national credit card systems and automatic bankteller terminal operations. Although the actual execution of an order forsecurities varies based upon the market on which the security is traded andcan be affected by other factors, in many instances an order to buy a securitycalled into one stockbroker’s office will be transmitted to a centralized mar-ket, where it is matched against an order to sell a security from a customer ofa different security firm.Today investors in securities have several methods they can use in addition

to the traditional call to a registered representative. Some stock brokeragecompanies enable customers to bypass the registered representative andenter orders directly by punching keys on their telephone. Other companiesestablished online Internet sites, enabling millions of investors to conductelectronic transactions.In this book we will use the term Internet with a capital I to reference the

global network of interconnected networks. In comparison, we will use theterm internet to reference the connection of two or more public or privatenetworks.Figure 1.1 illustrates the initial or ‘home’ page of Waterhouse Securities,

one of the pioneers of online brokerage accounts. Waterhouse, like manyother brokerage and non-brokerage firms, established a presence on the Inter-net for electronic commerce. Their computer, referred to as a server, displaysan initial screen referred to as a home page when accessed. In Figure 1.1 theWaterhouse home page is shown viewed through a Netscape browser, a soft-ware program that allows you to connect to literally an unlimited numberof servers operated by an expanding universe of companies establishing apresence on the Internet. Note the $12 flat fee trading statement in the middle

1.1 APPLICATIONS 3

of the screen. A few years ago a typical purchase or sale of a few hundredshares of stock could result in a commission charge of over $100. Thus,online transaction processing is revolutionizing the manner by which con-sumers can perform a variety of tasks, ranging from purchasing stocks andairline tickets to validating bills for payment. Similar to the manner by whichthe refrigerator displaced the need for the iceman, electronic commerce canbe expected to make many business obsolete.Another popular example of transaction processing is the use of a credit

card. Most major credit card companies have national and, in many instances,international credit authorization systems. When a customer makes a pur-chase in excess of a predefined amount, the merchant inserts the credit cardinto a terminal device and enters the amount of the purchase via a keyboard.Once the transmit key is pressed, the terminal transmits the credit card num-ber and purchase amount to a computer system. First, the credit card numberis electronically checked against cards reported lost or stolen, after which theamount of the purchase is added to the outstanding balance and compared tothe maximum authorization limit for the credit card account. If the credit cardis not lost or stolen and the authorization limit has not been exceeded, thetransaction is accepted. If the transaction is rejected, the merchant may haveto place a call to the credit card processing center to obtain additionalinformation about the card.


Figure1.1 Through theuse ofabrowser you canaccessanonlinebrokerage firmand per-form different financial transactions at a fraction of the cost associated with the use of atraditionalbrokerage firm that requiresyou to use a broker

Other stores that are part of a chain may use point-of-sale terminals thatboth authorize sales as well as transmit data, which is used by corporatemarket analysts to spot purchasing trends and to examine the relationshipbetween the price of a product, its sales and geographical sales area. Somechain stores integrate their point-of-sale system with inventory control, usingmerchandise sale information transmitted with credit authorization data totrack store sales and serve as a mechanism for the distribution of newmerchandise to their stores.While many readers have first-hand knowledge of the operation of bank

teller terminals, for other readers their operation may be a slight mystery. Inessence, a bank teller terminal can be considered to be a point-of-saleterminal that either dispenses information in the form of updating a passbookor dispenses cash and electronically updates one’s account. The mostconventional type of bank teller terminal simply dispenses information inthe form of updating accounts and its operation depends upon the bank clerkwho enters deposit or withdrawal information and accepts or dispenses cash.The second type of bank teller terminal, more formally known as an Auto-matic Teller Machine or ATM, dispenses predefined packets of cash, such as$10, $20, $50 or $100.A person using an ATM first inserts his or her bank card and the machine

reads and transmits magnetic coded information on the card to the bank’scomputer system. Assuming that the card was not reported lost or stolen andgobbled up by the machine, the computer will prompt the customer to enterhis or her personal identification number, commonly referred to as a PIN. ThePIN can be viewed as a secret number known only by the customer and his orher bank and serves to verify the identity of the person using the bank card.Thus, if the correct PIN associated with the card is not entered by thecustomer at the numeric keyboard of the ATM, the request for cash will not begranted. If the request is granted, after the cash is dispensed the customer’saccount is debited by the amount dispensed, with many banks adding aservice fee which both pays for the facilities required to support the ATMsystem and contributes to their profit margin.In addition to the previously mentioned transaction processing applica-

tions, other common examples of the use of this communications basedtechnology include airline, hotel and automobile reservation systems. The keyto the successful operation of each system is the ability of a terminal operatorto query a database to determine the availability and cost of an airline trip,hotel room or a particular type of vehicle.

1.1.3 Conversational time sharing

The high cost of large scale computers resulted in the development of timesharingasamethod to enablemanyusers to share the computationalpower of acommon facility. In a time sharing environment, each user obtains the use of asmall fraction of time of the central processor knownas a time slice. If the user’sjob is not completed during the allocated time slice, the job is queued by theoperating system for service by subsequent assignments of time slices.

1.1 APPLICATIONS 5

The development of interpretive languages, such as the Beginners AllPurpose Symbolic Instruction Code (BASIC), as well as Formula Translation(FORTRAN), Common Business Oriented Language (COBOL) and Program-ming Language One (PL/I) compilers to operate under time sharing permitsapplication programmers to develop and test their programs prior to placingthem into a production environment. Since tens to hundreds, and in somecases thousands, of persons could create and execute programs concurrentlyon a time sharing system, their utilization made computing more economicalthan classical batch systems where one job must be completed prior to thestart of the next job. Although the growth in the use of personal computers hasconsiderably reduced the demand for time sharing, it is still an importantcomputational facility in some large organizations.Figure 1.2 illustrates a typical example of a modern time sharing appli-

cation. In this example the main screen display for a version of IBM’s Office-Vision calendar and electronic mail system is shown. This particular versionof OfficeVision operates on an IBM mainframe computer which can supportthousands of users.Until the advent of personal computing, only time sharing extended the

computational power of computers via communications facilities to terminalslocated on users’ desks to provide ‘desktop computational capability’. Evenwith the growth in the use of personal computers, there are many applications


Figure 1.2 One example of a modern time sharing application is IBM’s OfficeVision’scalendarandelectronicmailsystem.OfficeVision operateson several types of minicompu-ters and mainframes, with the latter supporting up to several thousand users

which, because of data storage capacity or processing power requirements,are restricted to operating in a time sharing environment. Due to this, the useof time sharing systems can be expected to coexist with personal computingfor the foreseeable future.

1.1.4 Remote job entry

There are many types of data processing jobs, such as accounting and payroll,that require execution in a continuous manner. Organizations with diverselocations may prefer to use one or a few data processing centers to processpayroll and accounting data.To facilitate the timely processing of accounting and payroll data, most

organizations that use centralized data processing centers employ remotebatch transmission facilities. Typically, accounting and payroll data collectedover a period of time at distributed locations are formed into a batch ofrecords. At a predefined time or during a predefined time interval, the batchedrecords are transmitted to the centralized data processing centers. There, thebatched records received from the remote locations are combined and used asinput to the organization’s accounting and payroll programs.During the 1960s and 1970s, physically large minicomputer based remote

batch terminals were primarily employed to transmit batch data to central-ized data processing locations. At these locations, mainframe computers wereused to process the data received from the remote locations. By the late 1980s,many minicomputer-based remote batch terminals had been replaced by theuse of personal computers to perform batch transmission applications. In the1960s and 1970s, many batch terminal configurations included such periph-eral devices as card readers, disk or magnetic tape storage units and highspeed printers, while some terminals also supported interactive cathode raytube terminals. By the use of interactive terminals, clerks could enter datathroughout an accounting or payroll period. The data were then stored on diskor tape and transmitted to the central computer facility for processing. By thelate 1980s, the tape and disks of many minicomputers had been replaced bythe use of personal computer fixed disk and diskette on-line storage.Some batch terminals include the capability to perform local data

processing, executing small data processing jobs while transmitting largerjobs to the corporate mainframe. The results of those jobs, called systemoutput (SYSOUT), as well as accounting reports, checks and other data, can bedirected from the mainframe to the batch terminal via a communicationsfacility where the data can be directly printed or stored on tape or disk for laterprinting. When first stored on tape or disk, the printing of the stored data isknown as printer spooling.

1.1.5 Message switching

Message switching represents one of the earliest merging of communicationsand computer technologies. Beginning in the early 1950s, several computer

1.1 APPLICATIONS 7

manufacturers developed software specifically designed for message switch-ing applications. Companies that purchased hardware and software obtainedthe capability to either develop an internal message switching facility for theirorganization or provide a commercial service that other organizations couldsubscribe to.Early message switching systems required terminals to be permanently

connected via a communications facility, precluding their use for otherapplications. Messages entered via a terminal were transmitted to a centralcomputer facility where their heading was first examined. The messageheading included information concerning the subscriber or subscribers thatit was to be distributed to, the originator of the message and could includesuch optional information as its subject and priority. Depending upon thestatus of the destination subscriber’s terminal, the message might beimmediately switched to an output line routed to the destination terminalor stored on disk or tape. If the destination terminal was not busy servicing apreviously transmitted message or sending data, the message might beswitched directly to its destination. lf the destination terminal was in use, themessage would be stored. Then, when the destination terminal becameavailable, the message would be retrieved from storage and forwarded to itsdestination. Due to this type of operation, message switching is commonlyreferred to as a store and forward system.The use of message switching systems initially centered upon business and

commercial activity. As the use of message switching increased, additionalapplications were developed, such as the electronic delivery of money ordersthat was well publicized by a series of television commercials. Althoughmessage switchingasa technologyhasessentially beensucceededbyelectronicmail and value-added carrier services it provideda foundation for themovementof data between terminal users, Thus, it represents an underlying technologywhich formed thebasis for theevolutionofmoremodern technologies that todaydeliver electronic mail to tens of millions of persons each day.

1.1.6 Value-added carriers and electronic mail

The proliferation of the use of personal computers in the home and officeduring the late 1970s and early 1980s served as a driving force for the growthof value-added carriers and the introduction of a new type of messageswitching known as electronic mail. Essentially, value-added carriers can beconsidered as a new type of communications utility. By leasing communica-tions lines from telephone companies and installing specialized computers,the value-added carriers developed extensive communications networks. Theuse of these networks was fostered by many companies connecting theircomputers to network nodes, permitting persons from their organization orother companies to access the carrier’s network from hundreds of locationsacross the United States and via the entry of a code to be routed to theappropriate computer facility.Although the initial use of value-added carriers was primarily by business,

during the late 1970smany individuals began to subscribe to a variety of infor-mation retrieval services that provided financial, weather and text retrieval


from selective databases. Some value-added carriers expanded into informa-tion utilities, adding their own computational facilities to their network toprovide subscribers access to a variety of information services as well as theuse of electronic mail facility. Other value-added carriers added electronicmail facilities for business users while providing a communications trans-portation facility for other users to access numerous commercial electronicmail services that were established during the 1980s.One of the first, if not the first, commercially available electronic mail

services was MCI’s MCI Mail. MCI Mail was developed in the period prior to theexpansion of the Internet for commercial use. At one time this text basedelectronic mail system was one of the most popular forms of electronic com-munications in use. Figure 1.3 illustrates the use of the HyperTerminalapplication bundled into Windows 95 and Windows 98 to access MCIMail. Although Windows represents a graphic user interface (GUI), you can-not use the point and click capability of the operating system when workingwith MCI Mail. Instead, you must enter commands in the form of text, suchas ‘scan inbox’ shown in the lower portion of Figure 1.3. In this examplethe command would result in the listing of five messages in the author’sINBOX. To read each message would then require the entry of an appropriate‘read’ command.

1.1 APPLICATIONS 9

Figure1.3 MCIMailwasone ofthe first commercialelectronic mailsystems.Althoughstillused by this author, the popularity of the Internet where subscribers can perform manyfunctions in addition to electronic mail has diminished the demand for systems strictlydevoted to email

The growth in the use of the Internet makes the use of an Internet ServiceProvider (ISP) more attractive than the use of an electronic system restrictedto mail delivery. Through an account with an ISP, both business andresidential users can perform a number of functions in addition to sendingand receiving electronic mail. MCIWorldCom, which represents the merger ofMCI Communications and WorldCom, offers Internet access as well asnumerous voice and data services, with the number of its Internet accountsnow greatly exceeding its number of MCI Mail accounts, illustrating howadvances in one area of communications can result in the rapid or gradualobsolescence of another area.The use of a more modern electronic mail system is shown in Figures 1.4

and 1.5. Figure 1.4 illustrates the initial CompuServe mail center display.CompuServe was originally one of the earliest information utilities thatprovided subscribers access to shareware programs, news and weatherinformation, and chat rooms in addition to electronic mail service. Figure 1.5illustrates the point and click ease of use of CompuServe for creating anelectronic message. After clicking on the icon labeled ‘New’ in Figure 1.4 thescreen display was changed to the ‘Create Mail’ screen shown in Figure 1.5.Clicking on the button labeled ‘Recipients’ resulted in the display of thewindow labeled ‘Message Recipients’ shown in the middle of Figure 1.5. Notethat by clicking on the rectangle labeled ‘Address Book’ a list of predefinednames and addresses is displayed. Then another few clicks enables a person


Figure 1.4 The CompuServe Mail Centre screen display provides userswith agraphic user

to select a recipient which in this example is the author’s MCI Mail address.Although you still have to enter the subject and body of the message, throughthe use of Windows’ cut and paste capability you could prepare your messageusing a word processor and either attach it as a file or copy and paste themessage into the area of the window reserved for the body of the message.The primary differences between message switching and electronic mail are

in the areas of terminal connection and message delivery. Initially, messageswitching systems required terminals to be directly connected to the messageswitching computer via dedicated communications facilities. In comparison,electronic mail systems were developed to enable terminal and personalcomputer users to use the public switched telephone network on a temporarybasis to send or receive a message, permitting the terminal or personal com-puter to be used for other applications. Concerning message delivery, initiallymessage switching systems were restricted to delivering messages to termi-nals directly connected to the message switching computer. In comparison,most electronic mail systems provide a variety of message distribution optionsto include the conversion of an electronic message to hardcopy and its deliveryby the postal service or via courier. Today, the use of electronic mail can rangein scope from a corporation distributing new product announcements, toan individual bidding on a home or sending a birthday greeting to a friendor relative.

1.1 APPLICATIONS 11

Figure1.5 Using the graphic user interface of the CompuServe Mail Centre facilitatesthecreation of electronic mailmessages

1.1.7 Office automation

Until the introduction of the microprocessor-based personal computer office,automation operations were highly centralized, with a mainframe orminicomputer typically used to provide computational resources to theemployees of an organization. Those computational resources were usuallylimited to text processing and financial applications, and required theestablishment of a communications infrastructure that could result in thetransmission of information over hundreds or thousands of miles to performrelatively simple functions by today’s computer environment, such asdeveloping a mailing list or creating a form letter.The use of a corporate mainframe for office automation functions repre-

sented perhaps the earliest example of client–server computing. Through theearly 1980s dumb terminals without microprocessor based intelligence wereused to communicate with corporate mainframe computers. The terminal,serving as a client, would send a request to themainframe which functioned asa server, servicing the processing requirements of humerous clients. This typeof client–server computing resulted in the development of hierarchical struc-tured networks in which terminals were connected to control units whichin turn were connected to the mainframe. The control unit can be viewedas a line sharing device which enabled two or more terminals to contend foraccess to relatively expensive communications lines and mainframe com-puter ports. Figure 1.6 illustrates an example of the mainframe-based client–server computing model which formed the basis for office automation throughthe mid-1980s.During the 1980s the ubiquitous office typewriter was rapidly replaced

by the personal computer. At first, a lack of application programs resulted inthe PC being used as a dumb terminal in an office environment, with client–server computing continuing to resemble the illustration shown in Figure 1.6.In fact, the access to IBM’s mainframe-based Office Vision calendaring andelectronic mail system previously illustrated in Figure 1.2 occurred throughthe use of a PC acting as a dumb terminal. While some organizations continueto use mainframe centric computing, other organizations elected to distributecomputing applications based upon the use of PCs.


Mainframecomputer

Controlunit

Controlunit

Controlunit

Terminals TerminalsTerminals

Figure 1.6 Mainframe-based client^server computing model

The rapid increase in the processing power of personal computers soonresulted in the development of a variety of office automation software toinclude word processing, electronic spreadsheets, visual presentations, data-base creation and retrieval and other programs. The expansion in the use ofpersonal computers was accompanied by a requirement to share informationbetween personal computer users. This requirement was primarily satisfiedby the development of local area networks (LANs). Through the use of LANssmall corporate departments within an organization, as well as companiesthat could not afford the expense associated with operating a mainframecomputer, were able to establish their own client–server computing opera-tions. In large corporations islands of workstations on individual LANs beganto rapidly appear during the late 1980s, changing the corporate client–servermodel from a hierarchical mainframe centric model to a distributed com-puting environment with individual LANs connected to one another viaspecialized communications devices, as well as maintaining one or moreconnections to the corporate mainframe. This modern client–server model isillustrated in Figure 1.7.In comparing the mainframe based client–server model illustrated in

Figure 1.6 to the modern client–server model shown in Figure 1.7 the dif-ferences in potential network structures are apparent. The mainframe-basedmodel communicated with dumb terminals, and it was difficult if notimpossible to establish multiple routes for the transmission of information.In comparison, the modern client–server model is based upon the use of intel-ligent computers, as both workstations connected to a LAN as well asspecialized communications devices that have routing capabilities. Thismakes it possible to use different topological structures to interconnect LANsas well as to support multiple communications paths between LANs.

1.1 APPLICATIONS 13

Mainframecomputer LAN

LAN

LAN

LAN

LAN

Figure 1.7 The modern client^server model

Although the centrally managed mainframe-based client–server model iseasier to manage, its ability to adjust to organizational change is limited. Incomparison, the modern client–server model is much more flexible inadjusting to a changing organizational structure, since LANs can easily besubdivided (a process known as segmentation) to accommodate growth or achanging user environment. Unfortunately, it is much more difficult tomanage as an entity all of the LANs within an organization, a processcommonly referred to as Enterprise network management, which can beviewed as the price paid for obtaining an increased flexibility to support therequirements of an organization. Readers should note that the process ofdownsizing or moving applications off the mainframe onto the corporate LANresults in a client–server model similar to the one illustrated in Figure 1.7,with the mainframe removed due to the effect of the downsizing effort.In addition to being used in computers, the microprocessor has been incor-

porated into numerous office automation products which significantlyimprove worker productivity. Today pagers, inventory control scanners, andeven the supermarket bar code reader are all based upon the use of micro-processors. Those small silicon chips interpret sequences of digital pulses togenerate characters on a pager’s display, convert the vertical lines scannedfrom a can of chicken soup into digits that a distant computer can use todetermine the price of the product, and perform other operations that havesignificantly improved our lifestyles.

1.1.8 Electronic commerce

The growth in the Internet makes it possible for consumers and businesses totake advantage of electronic commerce opportunities. As a consumer you canliterally check different merchants for product availability and price throughsimple point and click operations.To illustrate the role of electronic commerce consider Figures 1.8 and 1.9.

In Figure 1.8 I used my browser to access the Barnes & Noble World Wide Webhome page. From this page I entered my name to check the price of books Iauthored. Because I gave away my complimentary copies of one book, I neededto order another copy. A portion of the simple electronic order process isillustrated in Figure 1.9. Note the dialog box named ‘Security Information’displayed in the foreground of the screen. The Netscape browser is similar toother browsers in that it will automatically encrypt transmission to enablesecure communications required to put the consumer’s mind at rest whenordering products and providing credit card numbers over the Internet.The growth in electronic commerce conducted over the Internet has literally

exploded over the past few years. From a few sales of books, records andassorted items that may have reached $100 million during 1996, by the newmillennium electronic commerce over the Internet was estimated to haveexceeded $20 billion. Today you can purchase airline tickets, shop for a car,and buy insurance, flowers or perform your weekly food shopping, all literallyat the click of a cursor.


Figure 1.8 Using the Netscape browser to view the Barnes & Noble home page

Figure1.9 Ordering products over the Internet results in the browserencrypting informa-tion transmitted as a mechanism to protect credit card data

While electronic commerce provides a considerable benefit for consumers,it also provides benefits for businesses. Today companies run auctions forsuppliers to bid on their requirements as well as allow potential employees topost their résumés. Thus, electronic commerce fosters competition, whichis one of the reasons inflation was probably tamed during the latter part ofthe 1990s.In addition to the Internet there are two other types of networks that are

periodically used to reference electronic commerce: extranets and intranets.An extranet references the connection of a private network to the Internetand can indeed be used for electronic commerce. An intranet represents aprivate network based upon the use of communications methods associatedwith the Internet. If an intranet is connected to the Internet it can be usedfor electronic commerce. However, if the intranet is restricted to providing acommunications capability for one organization, it is difficult to envision itsuse for electronic commerce, unless it is used by employees of the organiza-tion to purchase products manufactured or services sold internally.

1.1.9 Satellite transmission

One of the things many people take for granted is the ability to obtain anewspaper on the day of its publication. Without the use of satellitetransmission, this minor event would be an impossibility in many areas ofthe world.Today, satellite transmission and newspaper publications are closely linked

to one another. Such publications as USA Today, The New York Times andThe Wall Street Journal are printed simultaneously at several locationsthroughout the United States and overseas due to the use of satellitetransmission where the editorials, articles and advertisements prepared atone location can be rapidly transmitted to several locations for simultaneousprinting and delivery. In fact, through the use of satellite transmission,journalists in one location are now able to write articles and columns that canbe transmitted to other locations for inclusion in different editions of apublication tailored for a specific market.A second use of satellite transmission facilities which greatly enhances the

rapid dissemination of news to include text and pictures involves wireservices. Until the late 1970s, most wire services used message switchingsystems and facsimile transmission to distribute text and pictures. Today, theuse of satellites permits wire services to distribute information to newspaperssubscribing to their services much more rapidly. Pictures that required 10 to20 minutes to transmit during the 1970s can now be transmitted in a matterof seconds.

1.2 CONSTRAINTS

The development of communications-based applications which are thefoundation of our modern society involves many trade-offs in terms of theuse of different types of communications facilities, types of terminal devices,


hours of operation and other constraints. Four of the key constraints asso-ciated with the development of communications applications are throughput,response time, bandwidth and economics.

1.2.1 Throughput

Throughput is a measurement of the transmission of a quantity of data perunit of time, such as the number of records, blocks or print lines transmittedduring a predefined interval. Throughput is normally associated with batchsystems where the transmission of a large volume of data to a distant locationoccurs for processing, file updating or printing. As this is typically anextension of batch processing, and since it occurs remotely from a datacenter, the device that transmission is from or to is referred to as a remotebatch or remote job entry device.Although many readers may not realize it, every time you download or

upload a program through a browser, use the file transfer protocol (ftp) or per-form a similar operation, you are performing a batch transmission. Thus,your personal computer can function as a batch terminal.In most batch transmission systems, a group of data representing a record,

block or print line is transmitted as an entity. Its receipt at its destinationmust be acknowledged prior to the next grouping of data being transmitted.Figure 1.10 illustrates the operation of a batch transmission system by time,with the waiting time indicated by shaded areas. Since the throughputdepends upon the time waiting for acknowledgements of previously trans-mitted data, one method used to increase throughput is to transmit more dataprior to requiring an acknowledgement.A second method to increase throughput can be obtained by acknowledging

a group of blocks instead of on an individual basis. For example, acknowl-edging block n could signify that all blocks through block n were receivedcorrectly and the receiver now expects to receive block nþ1. The number ofblocks that can be outstanding prior to receiving an acknowledgement is

1.2 CONSTRAINTS 17

THROUGHPUT ¼ TOTAL RECORDS; BLOCKS OR PRINT LINESTOTAL TRANSMISSION TIME

Figure 1.10 Batch transmission and throughput

referred to as a window. Later in this book we will examine the effect ofdifferent window size settings upon throughput.

1.2.2 Response time

Response time is associated with communications where two entities interactwith one another, such as a terminal user entering queries into a computersystem. Here each individual transaction or query elicits a response and thetime to receive the response is of primary importance.Response time can be defined as the time between a query being

transmitted and the receipt of the first character of the response to thequery. Figure 1.11 illustrates interactive transmission response time.The optimum response time for an application is dependent upon the type

of application. For example, a program that updates the inventory could havea slower response time than an employee badge reader or an airline reser-vation system. The reason for this is that an employee entering informationfrom a bill of lading or other data which is used to update a firm’s inventorywould probably find a 5 or 10 s response time to be satisfactory. For a badgereader system where a large number of workers arrive and leave duringa short period of time, queues would probably develop if the response timewas similar. For airline reservation systems, many potential customersrequire a large amount of information concerning discount prices, alternativeflights and time schedules. If the airline reservation clerk experiences a slowresponse time in scrolling through many screens of information to answer acustomer query, the cumulative effect of a 5 s response time could result inthe customer hanging up in disgust and calling a competitor. For otherinteractive communication applications, such as automated teller machines,competitive advertising has made slow response almost an issue involving theviolation of a user’s fundamental rights. In certain locations, it is quitecommon today to see banks battling against one another in advertisementsover who has the fastest teller machines, yet another example of the use ofcommunications to gain a competitive position.

1.2.3 Bandwidth

From a technical perspective bandwidth represents a range of contiguousfrequencies, a concept that we will examine in some detail later in this book.


Figure 1.11 Interactive transmission response time

The range of frequencies is an important consideration for communications,since the maximum amount of data that can be transmitted per unit time isproportional to the bandwidth of transmission media. For example, fiber-opticcable, which has a relatively high bandwidth since it transports light, providesthe ability to simultaneously transport thousands of telephone calls. Incomparison, the twisted wire copper cable which forms the basis of mostbusiness and residential telephone service is limited to supporting only one ora few simultaneous telephone calls.

1.2.4 Economics

Similar to other technologies there are a range of economic trade-offsassociated with the use of different types of communications. Some types ofcommunications represent services for which users are billed on a per minutebasis. Other types of communications involve leasing of a circuit for a fixedmonthly fee regardless of use. Although a per minute service is less costlythan a leased circuit when usage is minimal, as usage increases the situationcould change and the leased line may be more economical.While the preceding is an over-simplification of the economics associated

with the use of communications, it illustrates an important concept. Thatconcept is the fact that you should compare alternative means of communica-tions as well as the cost of equipment required to support different com-munications methods. Doing so will provide you with the ability to select acost-effective communications method required to satisfy your communica-tions requirement.

1.3 EMERGING TRENDS

Through the 1970s communications was a highly regulated industry thatprovided customers with a limited choice of products and services. Thedivestiture of AT&T in the United States of its operating subsidiaries, theprivatization of British Telecom and the sale of stock in other nationalcommunications carriers resulted in the emergence of a competitive marketfor communications services as well as a significant growth in the numberof hardware and software vendors marketing communications products.In addition, telecommunications reform legislation in the United States andabroad are removing artificial barriers which limited the ability of local andlong distance telephone companies and cable TV to compete with oneanother. Eventually, you can expect the distinctions between cable, local andlong distance telephone services to diminish or even disappear.In addition to changes in legislation, advances in technology are forming

the basis for a profound change in the manner by which communicationsservices are provided. The original communications infrastructure through-out the world was designed to transport voice. Although well-suited forcarrying voice conversations, that infrastructure could not directly carrydigital signals. The evolving conversion of the infrastructure of communica-tions carriers to digital technology and the increased use of fiber-optic cable to

1.3 EMERGING TRENDS 19

interconnect buildings within cities and carrier offices in one city to offices inanother city is having a profound effect upon the ability to merge voice, dataand video, a process commonly referred to as multimedia.The transport of voice requires an infrastructure that provides a minimal

delay time. In comparison, the transport of images and data can toleraterelatively long delays. Recognizing the differences between optimum trans-mission methods, a technology known as Asynchronous Transmission Modewas developed to facilitate the merging of voice, data and video so thatmultimedia can be transported on local and wide area networks. At the sametime ‘fiber to the home’ trials were in progress that extended fiber technologyand its large bandwidth to residential customers, while the use of the Internetwas being tested as a mechanism to transport digitized voice conversations.In the first decade of the new century it is quite possible that products and

services in limited use or not even presently offered will be commonlyavailable as a result of advances in communications technology. Instead ofvisiting a library you will probably telecommunicate with your library andread a book on your home computer. Instead of simply listening to a personduring a telephone conversation you will be able to see the person you aretalking with. Similarly, research, business, finance and other functions can beexpected to radically change as advances in communications unlock barriersand facilitate the interchange of information.

1.4 REVIEW QUESTIONS

1. Assume that your organization is considering the installation of badge readersto collect time and attendance data. Discuss how the time and attendance datacan be used by management as well as serving as input for automation of otherorganizational functions.

2. Discuss the operation of a transaction processing system with respect to adatabase accessed by the system.

3. What effect do you expect electronic commerce to have upon the ability ofpersons to purchase securities, airline tickets, and other products?

4. Assume that you plan a trip that includes an airline flight from New York toSan Francisco, the use of a rental car to drive to San Mateo and a week’s stay atthe San Mateo inn. Discuss the type of communications application you wouldprobably use to plan your trip.

5. What is the function of a personal identification number (PIN) when enteredinto a bank automated teller machine (ATM)?

6. Why is time sharing considered as a predecessor to desktop computingobtained through the use of a personal computer?

7. What was a primary disadvantage of early message switching systems?

8. Why is message switching commonly referred to as a store and forwardsystem?


9. What is client–server computing?

10. Discuss the differences between early and modern client–server models withrespect to their operation and network infrastructure.

11. Describe an example of electronic commerce assisting the consumer andan example of how it can help a business.

12. What are three of the key constraints associated with the development ofcommunications applications?

13. What does the term downsizing mean with respect to computer applications?

14. If the transmission of 5280 records required 2 minutes 80 seconds, what isthe throughput?

15. Discuss the use of throughput and response time measurements with respectlo remote batch and interactive systems.

16. What is bandwidth and why is it an important consideration for transmis-sion?

17. What is the term used to describe the merging of voice, data and video?

1.4 REVIEW QUESTIONS 21

2BASIC TELEGRAPH AND

TELEPHONE OPERATIONS

The foundation of modern communications can be traced to the developmentof telegraph and telephone operations during the nineteenth century. Thetelegraph can be considered as the forefather of the automatic teleprinter andits use was based upon the development of an elementary code to conveyinformation which is still in use today. The telephone has grown in use to thepoint where it is truly ubiquitous, with over 99.9% of homes and businesses inNorth America and Europe having one or more instruments. The developmentof telephone networks resulted in a structure used for the distribution of callsthat remains in use over one hundred years after its initial development.Thus, both telegraph and telephone communications provided the foundationfor modern communications, even though their operation and utilization haveconsiderably changed over the past one hundred years.In this chapter, we will first examine the evolution of communications from

simple signaling by fire to early telegraph systems. In our examination oftelegraph systems, we will focus attention upon the use of codes to conveyinformation and two areas of technological development that were required toautomate communications. This will be followed by an examination of theoperation of the telephone, the routing of calls between telephone stations andthe switching hierarchy established for the routing of long distance calls.From the information presented in this chapter, you will obtain an apprecia-tion of the evolution of modern communications as well as why the operationand constraints of twentieth and twenty-first century communications can betraced to prior developments during the nineteenth century.

2.1 EVOLUTION OF COMMUNICATIONS

Man’s method of communicating between diverse locations can be consideredto form an index of our technological development. The first known methodsof signaling were Greek and Roman signal fires which were limited in theirinformation contents to the occurrence or non-occurrence of predefinedevents. In the mid 1600s, Portuguese explorers returning from Africa reported



upon the use of jungle drums which transmitted messages between villages.Their use disseminated more information than fires, since the beat of thedrum could be changed to convey different information. With the emergenceof the Industrial Revolution, the requirement for timely and accuratemechanisms for information distribution grew, resulting in the developmentof machines that communicate with one another. In fact, much of our modernsociety is based upon the communication of messages whose informationcontent is generated by or through the use of machines. Foremost amongthose machines are the telegraph and telephone, whose development can beconsidered as the foundation of modern communications systems.

2.2 TELEGRAPHY

Although Samuel F. B. Morse is credited by most persons as the man whoinvented the telegraph, in actuality the American physicist Dyer operated asingle wire telegraph in 1828 based upon electrostatic electricity and whichused litmus paper as a signal indicator. This telegraph operated over adistance of 10km on a racecourse in Long Island and was in operation almost16 years prior to the first telegraph line established to link two cities together.Modern technology, which can be considered as the predecessor of other

methods of electronic communications began in 1832 when Samuel Morseinvented his telegraph alphabet, now known as the Morse code. By 1844, thefirst telegraph line had been constructed in the United States, linkingWashington and Baltimore. On May 24, 1844, Morse transmitted the nowfamous phrase ‘What hath God wrought’ from the old Supreme CourtChamber in the United States Capitol to his partner Alfred Vale in Baltimore.

2.2.1 Operation

The Morse telegraph system is similar to all communications systems in thatits operation requires a transmitter, a transmission medium and a receiver.The transmitter used in the first Morse telegraph system was the telegraphkey, which was a switch with a knob or handle, which, when pressed down,resulted in the closure of an electrical circuit. The power for the circuit wasprovided by a battery or another source of direct current.Morse’s first telegraph receiver used wire coils wound around metal to form

an electromagnet with a moving armature which was used to draw an inkedline on a moving strip of paper. Morse soon observed that the noise of thereceiver could be ‘read’ by a trained ear and modified the telegraph receiver.The modified receiver replaced the moving strip of paper with a thin piece ofmetal that would click on a contact due to the induced magnetism in thearmature caused by the closure of the key at the transmitter. This type ofreceiver is also known as a Morse sounder.Figure 2.1 illustrates the circuitry of a one-way telegraph system where the

term simplex is used to denote the transmission of information in onedirection. When the original Morse receiver was used to draw a line on amoving strip of paper, a mark was made on the paper when a pen attached tothe armature was attracted to the coiled metal. Since a marking condition was

24 BASIC TELEGRAPH AND TELEPHONE OPERATIONS

caused by the closure of a key which resulted in current flowing through theresulting circuit, the term marking state has evolved to denote the flow ofcurrent in a line. Similarly, the opening of the telegraph key caused a break inthe circuit which precluded the flow of current. This action caused the pen tobe moved off the paper, resulting in a space. Hence, the term space or spacingstate has evolved to denote a condition in which no current is flowing in a line.Although Morse didn’t realize it, he had created a binary state machine. Thatis, a telegraph operates in one of two states – current flowing or current notflowing. As we will note later in this book, all modern communicationssystems are based upon binary operations. For example, the ability to com-municate via a fiber optic cable is based upon the transmission of digitizedvoice conveyed as a series of light and absence of light pulses.Since the telegraph system illustrated in Figure 2.1 was capable of trans-

mitting in only one direction, it was soon modified to permit operators at eachend of a telegraph line to communicate with one another. This modificationresulted in the placement of a Morse sounder and key at each end of thecircuit, as shown in Figure 2.2. In this configuration, the key at each stationwas provided with a switch to close the circuit when the station is receivingdata. When neither end is transmitting, the line is in an idle state, bothswitches are closed, both sounders are operated and current is continuallyflowing in the resulting circuit.When an operator has data to transmit, he or she first opens the key

shorting switch, then depresses the key for varying short periods of time toproduce the dots and dashes that make up the Morse code for each characterto be transmitted. Since the sounder clicks when the operator presses the key,each operator hears the Morse code as he or she keys it. Once a message iscompleted, the operator shorts his or her key, enabling the operator at theopposite end of the line to begin transmission.

2.2 TELEGRAPHY 25

Figure 2.1 Asimplex telegraphcircuit.Ina simplex (oneway) telegraph circuit, the closureof the key causes a circuit to be formed, permitting current to flow. The flow of cur-rent around metal forms an electromagnet which causes the thin metal strip to strike the‘Mark’contact

The circuit illustrated in Figure 2.2 is called half-duplex. This type of circuitpermits an operator to transmit and receive data, however, only one functioncan be performed at a time. A circuit which is capable of supporting thesimultaneous transmission and reception of data is called full-duplex.One obvious question you may have while examining the telegraph circuit

illustrated in Figure 2.2 is how one operator can inform the other operator thathe or she has data to send. If neither operator is transmitting data, the firstoperator to open his or her switch and begin keying could be considered tohave seized control of the line. If the other operator desired to break-in, thatoperator could stop the transmission of the first operator by opening their keyshorting switch. This would cause an open in the circuit, causing the soundersat both ends to stop. It would also serve as a signal to the transmitting operatorto close their switch and listen for an urgent message. Since one operator,in effect, is breaking into the transmission of the other operator, the process ofopening a key shorting switch is also known as a break-in operation.

2.2.2 Morse code

The code that Morse developed to transmit information resulted in theassignment of a series of short (dot) and long (dash) key depressions torepresent characters. Legend has it that Morse visited a typesetter and countedthe number of letters in each of the typesetter’s letter drawers to obtain a basisfor the assignment of a code to each character. Through his examination of thetypesetter’s stock of letters, Morse assigned short duration codes to frequentlyused characters and longer duration codes, consisting of more dots anddashes to less frequently used characters. Based upon this assignment, theletter E which is the most frequently occurring character in the Englishlanguage is represented by a dot in Morse’s code. The second most frequentlyoccurring character, the letter T, is represented by a dash, and so on. Figure 2.3lists the International Morse code for characters transmitted via telegraph.

26 BASIC TELEGRAPH AND TELEPHONE OPERATIONS

Figure 2.2 Half-duplex telegraph circuit. In ahalf-duplex telegraph circuit, both operatorscan transmit data, however, onlyone can do so at a time

If you enjoy old movies and rented ‘D-Day’, you probably remember the sig-nal sent to the French Resistance. With music in the background, theforeground sound of ’dot, dot, dot, dahh’ represents the letter V in Morse. Notonly was this a signal that the invasion was on, it also represented the goal ofthe Allied forces and represents perhaps the best known use of Morse code.The first telegraph line which connected Washington, DC, to Baltimore

was soon extended to New York. Within a few years, additional lines wereinstalled throughout the United States and Europe. In the United States, thetelegraph was initially used to convey a large volume of train dispatchinginformation, resulting in a close collaboration between communications com-panies and railroads for the sharing of a ‘right of way’ that has been extendedand expanded upon by other transportation companies. As communicationsevolved, several railroads and pipeline companies sold or leased the use oftheir ’right of way’ to telephone companies. Those companies constructedmicrowave towers that at one time formed the backbone of the long distancetelephone network. Beginning during the 1970s, the rights of way of railroadsand pipeline operators were again used, this time for the construction of afiber optic cable infrastructure that is now used for a majority of long distancecommunications in North America, Western Europe and Japan.

2.2.3 Morse code limitations

Although the telegraph revolutionized communications, until the early 1900sits use was limited to hand-keyed Morse code. This restricted the telegraph to

2.2 TELEGRAPHY 27

Figure 2.3 International Morse code.The dot (*) represents a short closure of the tele-graph key, while the dash (^) represents a longer depression. A sequence of dots anddashesora dot ordashby themselves are used to define unique characters

a transmission rate between 30 and 60 words per minute when a pair ofexperienced operators were on each en

understanding data communications.dഀ漀read.pudn.com/downloads70/ebook/250852/wiley -...

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