-
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
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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
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
-
Copyright # 2000 by John Wiley & Sons, LtdBaffins Lane,
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Reprinted with corrections July 2001
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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
TK5105 .H429 1997004.6 — dc20 00-032094
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British
Library
ISBN 0 471 627453
Typeset in 912/1112pt Bookman by Aarontype Ltd, Easton,
Bristol
Printed and bound in Great Britain by Antony Rowe Ltd,
Chippenham, Wiltshire.This book is printed on acid-free paper
responsibly manufactured from sustainable forestry,in which at
least two trees are planted for each one used for paper
production.
4 CHAPTER TITLE
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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
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
-
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
3.6 Review Questions 72
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
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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
4.10 Review Questions 122
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
5.5 Review Questions 157
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
6.6 Review Questions 180
CONTENTS vii
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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
7.4 Review Questions 233
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
8.6 Review Questions 332
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
10.4 Review Questions 374
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
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11.2.3 Bisynchronous protocols 39511.2.4. Digital Data
Communications Message Protocol (DDCMP) 40011.2.5 Bit-oriented line
control procedures 402
11.3 Review Questions 407
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
12.3 Review Questions 445
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
13.7 Review Questions 528
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
14.4 Review Questions 575
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
15.6 Review Questions 628
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
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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
16.5 Review Questions 694
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
17.4 Review Questions 745
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
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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
18.4 Review Questions 789
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
19.4 Review Questions 821
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
20.5 Review Questions 838
Index 841
CONTENTS xiii
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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
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
-
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.
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
-
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
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
-
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
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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.
4 COMMUNICATIONS IN A MODERN SOCIETY
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
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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
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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
6 COMMUNICATIONS IN A MODERN SOCIETY
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
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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
8 COMMUNICATIONS IN A MODERN SOCIETY
-
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
10 COMMUNICATIONS IN A MODERN SOCIETY
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
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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.
12 COMMUNICATIONS IN A MODERN SOCIETY
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.
14 COMMUNICATIONS IN A MODERN SOCIETY
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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
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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,
16 COMMUNICATIONS IN A MODERN SOCIETY
-
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
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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.
18 COMMUNICATIONS IN A MODERN SOCIETY
Figure 1.11 Interactive transmission response time
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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
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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?
20 COMMUNICATIONS IN A MODERN SOCIETY
-
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
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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
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
-
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