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`Page 1
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`
`
`The Architecture
`
`of Videotex
`
`Systems
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`
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`THE ARCHITECTURE
`
`OF VIDEOTEX
`
`SYSTEMS
`
`Jan Gecsei
`
`Université de Monrréa!
`
`Prentice-Hall, Inc.
`
`Engiewood Cliffs, New Jersey 07632
`
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`Library of Congress Cniaioging in Publication Data
`Gecsah Jan (date)
`The architecture of Vldaoiex systems.
`Bibliography: p.
`Includes Index.
`‘I. Videotex {Data ltransmission system}-
`“(5105.643 1933
`384
`8343158
`lSBN 0-15-044?76v5
`
`I. Title.
`
`Editorialiproduction supenrision
`and interior design by Kathryn Goir'r‘rr Marshak
`Cover design by Ben Samara
`Manufacturing buyer: Gordon Osboume
`
`Cover photograph courtesy of ihe Department of Communications,
`Government of Canada
`
`© 1983 by Premice-Hali. |nc.. Englewood Cliffs, NJ 07632
`
`Ail rights resenred. No part of this book
`may be reproduced in any iorm or by any means
`without permission in writing from the publisher.
`
`Printed in the United Staies of America
`
`10967654321
`
`ISBN El‘l3~i]'-ili??iav-5
`
`Prentice-Hall Internafionai, lno. London
`Prentice-Hall of Australia Pty Umited. Sydney
`Ediiora PrenticeHall do Brasil, Lida, Rio de Janeiro
`Prentice-Hall Canada Inc., Toronio
`Premioe-Hail oi India Private Limited, New Delhi
`PrenlioeHall of Japan, Inc., Tokyo
`Prentice-Hall ol Southeasi Asia Pie. Ltd., Singapore
`Whitehall Books Limited, Welir'rrgron, New Zear'arrd
`
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`
`
`To Alice, Korynna, and Dora,
`who were patient and fed me
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`Preface
`
`Part
`
`I
`
`BACKGROUND
`
`Chapter 1
`
`Introduction
`
`1.1 The Realm of Videotex
`
`3
`
`7
`
`1.2 Components and Terminology
`User Terminals, 8
`Computers, 8
`Service~Prov£der Terminals, 9
`Communication Nemarks, 9
`Tenninology, 9
`
`1.3 Teletext and Viewdata
`
`10
`
`1.4 Evolution
`
`11
`
`Key Comriburiom. 12
`
`xv
`
`1
`
`3
`
`vi!
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`viii
`
`Chapter 2 AOpttcations and Complementary Systems
`
`Contents
`
`14"
`
`2.1 Videotex Applications
`Diversion M
`A Gross Classification, 16
`Application Areas, 16
`
`14
`
`18
`2.2 Complementary Systems
`Home Computer and Home Terminal Nemorks, 18
`Electronic Mail and Teletex, 20
`
`Office Automation, 20
`Time-Sharing and 0n~Line Data Retrieval, 2}
`Computer-Aided Instruction, 21
`Private and Videotex~DerivaIive Systems, 21
`
`Chapters Videotex Architecture
`
`3.1 Computer Networks
`
`23
`
`3 .2 The 081 Reference Model
`
`25
`
`Standardizing the Protocols, 28
`
`3.3 Videotex in 081 Perspective
`Architel Protocols, 30
`
`28
`
`Part
`
`II
`
`THE COMMUNICATlONS LEVEL
`
`introduction, 33
`Classification, 33
`Delivery Protocols, 34
`
`Chapter 4 Delivery Media
`
`4.1 The Telephone Network
`Modem, 3?
`Data Rates, 38
`
`36
`
`4.2 Integrated Services Digital Networks
`Digital Caniers, 39
`Service Integration, 40
`
`39
`
`23
`
`33
`
`36
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`ix
`
`58
`
`Contents
`
`4.3
`
`4.4
`
`4.5
`
`41
`Cable Systems
`Physical Structure, 4}
`Two-Way Operation, 43
`Splitting the Bandwidth, 43
`Transmission Techniques, 44
`Characteristics of Cabie~Based Videotex Delivery System, 44
`
`Fiber Optics
`
`45
`
`Data Transmission over the TV Video Signal (DOV)
`Principles of TV Transmission, 46
`Data over Video, 47
`
`46
`
`Data~Line Assignment, 49
`Transmission Rates, 50
`
`Error Handling in DOV, 51
`Technical Criteria Used to Assess Reception Quality, 52
`Error Detection and Correction, 54
`General Data Format in DOV, 55
`
`Chapter 5 Teletext
`
`5.1
`
`History: Synchronous Versus ASynchronous Systems
`Synchronous Transmission, 59
`Asynchronous Transmission: the Didon Concept, 60
`
`58
`
`5.2
`
`6]
`UK. Teletext
`Enhanced UK. Teletext, 63
`
`Gearing, 63
`
`5.3
`
`5.4
`
`5.5
`
`5.6
`
`Didon—Antiope
`
`64
`
`The Franee-Canada«CBS Standard Proposal (FCSP)
`OS] Layers, 66
`Channel Numbers, Page Numbers. and Muinplexing, 68
`
`65
`
`Standardization 'Ln Teletext
`
`69
`
`Short Summary of Teletext Evolution
`
`70
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`x
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`Chapter 6 Topoiogy of Videoiex Networks
`
`Contents
`
`71
`
`6.1
`
`Packct~Swdtclfing Networks
`Public Newarks, 72
`
`72
`
`Packer Switching, 72
`Applications ofPSN, 74
`Access to PSN, 74
`
`6.2 Elements of Videotex Networks
`Access Machines, 76
`
`75
`
`77
`6.3 Network Types
`Centraiized and Repiicated Databases, 77
`inciusicin of Packer-Switching Networks, 79
`Hybrid Nemarks, 81
`
`82
`6.4 Network Examples
`Presiel Advanced Newark Design Architecture (PANDA), 82
`Biidschinntext Nemork, 83
`
`The Vélizy Fieid Trial, 83
`Captain Network Architecture, 88
`INDAX Interactive Cabie Newark, 88
`The IDA Trial, 90
`
`Part III
`
`THE PRESENTATION LEVEL
`
`Chapter 7 Principies of Presentation Coding
`
`93
`
`95
`
`7.1 The Television Set as Display
`The Dismay Area, 95
`Vertical and Horizontal Resoiution, 96
`Character Cells, 98
`
`95
`
`Character Rounding, 98
`Number of Characiers per Row, 99
`Number of Rows per Page, 101
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`Contents
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`xi
`
`7.2
`
`ImageLCoding Options
`Mosaic Coding, 102
`Geometric Coding, 104
`Photographic Coding, 104
`
`102
`
`'13
`
`Code Tables and Code Extension
`
`107
`
`Structure (f Code Tables, 107
`Code Extension Standard. 108
`Control .S'et Functions, 111
`
`7.4
`
`112
`Coding of Display Attributes
`Handling of Control Codes in the Terminal, 1 13
`Serial Attributes in Ftinted-Format Teletext, £15
`Applicabiity ofAttributes, 11 7
`Reconciling Serial and Parallel Attributes, 119
`Stack Model, 120
`Coding of Colors and Gray Scales, 122
`
`Chapter 8
`
`Presentation Coding in Practice
`
`T24
`
`8.1
`
`8.2
`
`8.3
`
`Major Presentation Systems: An OVerview
`Mosaic Systems: Prestel and Antiope, 124
`Picture Prestel, 125
`
`124
`
`Captain, 125
`Teiidon: The Geometric Option. 127
`Computer Graphics, 127
`P1315, 129
`
`Alternatives to Geometric Coding, 130
`
`Standards and Unified Presentation Systems
`3.100, 131
`CEPTRecommendation, 132
`CEPT Terminal, 134
`
`131
`
`The U .K . Multilevel Presentation System, 134
`North American PLPS, 135
`
`Compatibility and Standards
`The Nature of Compatibility, MI
`PLPS and CEPT Standards, 142
`
`141
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`xii
`
`Chapter 9 Terminals
`
`Contents
`
`144
`
`9.1 Classification and Features
`
`144
`
`Types of User Terminals, I44
`Multistana'ard Terminals, I46
`
`Intelligent Tenninals, 146
`Component Television, M6
`
`9.2
`
`Internal Structure of Terminals
`
`149
`
`Data Reception, 50
`Data Selection, l50
`Control Processor, 152
`Display Memory and Generator, 152
`Page Memories, 153
`Bit Plane Memories, 154
`
`Reference Tenninal, 15 7
`[SI Technology in Terminals, 158
`
`9.3
`
`ServicewProvider Terminals (SPT)
`Features, 159
`VIPS {Videotex lry‘ormation-Provider System), I61
`
`159
`
`Part IV
`
`THE APPLICATION LEVEL
`
`Chapter to Databases for Videotex
`
`10.1 A Brief Review
`
`167
`
`165
`
`167
`
`168
`10.2 Page-onenth Databases
`Accessing the Iry’omiation, l 70
`Problems with the Tree Structure, 1 71
`
`10.3 The User Inteiface
`F .300, l 73
`
`173
`
`174
`10.4 Databases for Teletext
`Transmission Sequence, 175
`Tree Structures, I76
`
`Captioning, 177
`
`Chapter 11 Gateways and Data Distribution
`
`179
`
`11.1 DatabaSe Distribution
`
`179
`
`180
`11.2 Access to Extemal Computers
`Compatibility with External Computers, l8]
`Gateway Alternatives, 182
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`Contents
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`11.3 Examples
`184
`The BildschirmtexrlPrestel Gateway, 185
`WET, 1‘88
`
`Cabieshare View-data Gateway, .1 92
`
`Chapter 12 Videorex Computers and Systems
`
`12.]
`
`Features of Access Machines
`
`194
`
`12.2
`
`12.3
`
`12.4
`
`12.5
`
`12.6
`
`The Telidon Cornputer of the DOC
`
`195
`
`The Bildschirmtext Videotex Center
`
`198
`
`200
`Steria—VideOpac
`Database Mamgenwm, 202
`Accessing Pages, 204
`
`The French Electronic Directory
`
`204
`
`207
`Teletext Computers
`Data Exchange between Teletext Systems, 208
`
`Part V
`
`COMPLEMENTARYISSUES
`
`Chapter 13 The User Interface
`
`13.1
`
`13.2
`
`13.3
`
`13.4
`
`13.5
`
`13.6
`
`214
`Question-Answer Interfaces
`Enhancenwnts ofMenu Selection System, 215
`
`217
`Keyword—Based Interaction
`Pre- and Post~CoordEnare Processing, 217
`
`Keywords in Videotex
`Design Issues, 219
`fistulas-ides Between Menus and Keywords, 220
`Post-Coordinate Search in TéléteI, 221
`
`218
`
`221
`Query Languages
`Artificial! Languages, 223
`Namra£«Language Interfaces, 224
`
`226
`Browsing in Data Spaces
`Spatial Management ofData, 227
`Interface Languages for Videotex. 230
`
`Towards a User Psychology
`
`230
`
`xiii
`
`194
`
`211
`
`213
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`xiv
`
`Chapter 14 Distributed Processing in Videotex
`
`14.1
`
`Intelligent Tenninals and Telesoftware
`Telesofiware Protocols and Standards, 235
`
`234
`
`14.2 The Austrian Approach to Telesoftwme
`
`238
`
`14.3 Toward Distributed Processing in Videotex Networks
`
`240
`
`Chapter 15 Efficiency and Perfonnence in Wdeotex
`
`243
`15.1 Queuing Models
`Open and Closed Models, 244
`Sample Calcukuions, 247
`
`15 .2 Access Model for Tree~Structured Databases
`
`252
`
`Markov Chain Model ofPage Access, 253
`
`15.3 Optimization of Teletext Transmission Sequences
`
`257
`
`. References
`
`index
`
`Contents
`
`233
`
`243
`
`262
`
`269
`
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`
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`Preface
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`Videotex is one of the recent developments combining advances in computers, telecom—
`munications technology, and consumer electronics. The ambition of videotex is no less
`than to upgrade today‘s mass communication media into computerized mass information
`utilities.
`
`It might seem that videotex is suffering from an identity crisis; indeed, there are
`numerous closely related developments, such as electronic mail, on»line information ser-
`vices, and home computing, to name only a few. But in spite of the lack of a clean
`academic definition, a number of well«established systems (such as Prestel, Telidon,
`. Bildschirmtcxt, and Télétel) are operational and under development in many countries,
`backed by technical expertise and sizeable investtnents from business interests. Underly-
`ing these systems, there is a considerable amount of accumulated lelowledge, experi-
`ence, and open research problems, most of which are highly teclmical and videotex—
`Specific. However, so far- there are few publications attempting to explain systematically
`these issues and to relate them to the traditional disciplines of computer networks, com-
`munications, and database technology. This may be caused, at least partly,- by the encr-
`mous rate of change witnessed by videotex, and the ensuing danger of rapid obsoles-
`cence of any book on the subject. However, these very same reasons Call for such an
`undertaking in order to clarify the basic concepts behind the maze of systems,
`approaches, standards, proposals, and counter-proposals flooding the desks'cf videotex
`managers, designers, engineers, and students.
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`‘
`
`xvi
`
`'
`
`"
`
`Preface
`
`The emphasis of this book is thus on the technical aspects of television-based video—
`tex. The subject matter is divided into five parts. Part I provideS an introduction to video~
`tex and background material about similar systems and computer networks. Parts II, III,
`and IV are the core of the book, and they loosely follow the architectural layers ofthe ISO
`Open Systems Interconnection model (described in Chapter 3). Part II treats ISO. layers 1
`through 5, here collectively called the communicationslevel. The subjects involved are
`the physical media used to transmit and deliver videotex information, the underlying pro-
`tocols, and the communication structure of videotex networks. Part III is concerned with
`the presentation level (ISO layer 6), treating in detail the image-coding options in current
`use, their impact on tenninal design, and the related problems of. national and interna-
`tional standardization. Part IV——the application level—deals with databases for videotex
`and teletext, gateways, sendCe computers, and service providers‘ equipment. The three
`chapters of Part V touch upon themes important for the future- of videotex: alternative
`methods of interfacing with the user, telesoftware (seen as the key to distributed process-
`ing in videotex) and methods of perfonnanoe evaluation.
`Readers familiar with the basic notions involving computers, communications, and
`databases, or having some experience in videotex, should have no difficulty in following
`the text. (Chapter 15 is a possible exception.) Most concepts used in the text are defined
`or explained, although not necessarily at their first occurrence.
`A few words are in order on what the book is not about. The limitations imposed
`by our book’s scope and extent have excluded the treatment of a number of areas closely
`related and vitally important to videotex technology. Some of these areas are: social
`impact, legal and regulatory issues, financial and marketing aSpects, field trial and user
`penetration statistics, the dynamics of service provider activity, and the esthetic and
`psychological aspects of page creation. For an in-depth treatment of these (and other)
`issues the reader is referred to a reoent book by Tydeman et al- (1982) and'to the care
`fully prepared and informative Videotex Report Series published by Butler Cox &
`Partners Ltd. in London.
`
`ACKNOWLEDGEMENTS
`
`The author is indebted to many persons for their assistance in the preparation of this text.
`Dr. Mlke Sablatash, the owuer of probably the best collection of literature on videotex in
`Canada, kindly opened up his files and provided invaluable help in documentation. Dr.
`Walter Ciciora, Professor Herman Maurer, Professor Frank Tampa, Andy TcnneSens,
`Bob Baser, Professor Stan Selkow, Professor Gregor von Bochmann and Dr. Mike
`Sablatash read the manuscript and suggeSted a large number of significant improvements
`in both style and substance. Many people from many organizations in many countries
`contributed their valuable time to interviews with the author. Special thanks go to Deniso
`St~Michel for her expert work on the preparation of the numerous versions of the _
`manuscript. The author gratefully acknowledges the support of Université de Montreal
`and of the Natural Sciences and Engineering Research Council of Canada during a sab-
`batical leave that pemiitted the writing of this book.
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`Preface
`
`xvii
`
`of
`figures 5.1 and 7.4, and Plates 1, 2, and 3 were provided by Dr. W.
`Zenith Corporation; Plates 4, 5, 8 by CCE'IT Rennes; Figure 7-2 by W. C. Treurniet and
`Figure 7.? by S. Shlien, both of the Government of Canada Depart]:th of Communica-
`' tions (DOC); and Figure 9.10 by Bell-Northern" Research. Plate 7 is reproduced courtesy
`of the Doc. Copyright permissions for materials included in Chapters 6, 10, 12 and 13
`are acknowledged to IEEE, Press Pordépie, and DOC.
`_
`_
`The specific sequences in all lists of people, countries, systems, etc., in this book
`should be considered as arbitrary in terms of relative significance. The word “he” used
`in a nonspecific sense should be interpreted throughout the text as “she or he.”
`
`Jan Gecsei
`
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`
`
`The Architecture
`of Videotex
`Systems
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`Part I
`
`BACKGROUND
`
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`I The purpose of the chaptets in Part I is twofold. Chapters 1 and 2 give a “first pass"
`assessment of the nature, stt‘uctme, terminology, and applicability of videotcx systcms‘
`Chapters 2 and 3 provide background information on related topics, such as complemen-
`tary videotex—ljke information systems, computer networks, and the Open Systems Inter-
`connection reference model.
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`
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`1.1 'ms REALM OF woeorsx
`
`Costs of mass~produced digital devices declined in the mid-19705 to the critical level at
`which forms of this technology came within the reach of the average oonsmner. Digital
`watches and pocket calculators (a “mass-computation medium”) were among the first
`manifestations of this new situation.
`'
`
`Videotex, a new digital mass-communication medium based on a blend of televi—
`sion, communications, and computer technologies, is another development driven by the
`same cost trend. Just as calculators continue to evolve towards models with more come
`
`plex features and towards home computers, so is videotex moving rapidly from its initial
`wope (as a simple means of information retrieval for use in the home), to incorporate
`advanced applications known so far only in systems designed for specialized users.
`Therefore, it is becoming increasingly difficult to ask—.or rather to answer—the ques—
`tion: What is videotex'?
`_
`Numerous authors have offered definitions ranging from crisp, factual technical
`description to almost philosophical statements (that in etfect amount to a refusal to
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`4
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`Background
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`Part I
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`define) in which they argue that the technology is not yet mature and stable enough to
`support a durable definition.
`'
`To illustrate the first type of definition, we include three formal definitions of
`videotex:
`.
`
`1. Videotex consists of “systems for the widespread dissemination of textual and
`graphic information by wholly electronic means for display on low-cost terminals
`(often suitably equipped television receivers) under the selective control of the
`recipient, using control prooedrnes easily understood by untrained users” (see
`Tyler, 1919, in the reference section at the back of the book);
`“Videotex is a medium for transmitting text and simple graphics. The usual
`display is a color TV receiver. The information is digitally encoded for transmis-
`sion. The information is organized into pages" (see Ciciora et al., 1979).
`‘
`. “Videotex is the generic name used for electronic systems that use a modified TV
`set 'to display conmuter—based information. Interactive systems using, typically, the
`TV set and the telephone line, are called telephonebased or interactive videotex.
`Broadcast services are called broadcast videotex (teletext)’ ’ (see Winsbury, 1979).
`
`As an example of the more cautious approach to definition we quote Plummer
`(1979):
`'
`“Given this prenatal Stage in the evolution of the field, we think it most appropri-
`ate to consider teletcxt and videotex as a phenomenon—mot just technologies.’ ' He goes
`on to define seven basic dimerrsions of the “phenomenon”: technology, system design,
`content, users, service providers, economics of system operation, and regulatory and
`policy environment.
`_
`-
`'
`The early date of this observation does little harm to its timeliness. On the con—
`trary: today, when the potential for new applications and technology alternatives is more
`fully appreciated, and with the emergence of parallel home informatiOn systems,
`' videotex becomes increasingly harder to characterize than it was in 1979.
`Instead of trying to draw a sharp boundary between videotex and the rest of the
`world, it is more realistic to imagine videotex as a fuzzy set represented by a number of
`concentric circles, as in Figure 1.1. Besides serving as illustration and reference, defini-
`tions 1-3 above reflect well what is at the hard core of the set: low-cost and simple-to-
`use information services for the general public, employing the television as display. The
`degree of membership in the fuzzy set (or “videotcx—ness”) is highest in the center and
`decreases towards the periphery as videotex blends into similar and related systems and
`applications (present and planned for the future).
`The fuzziness of the set has been accentuated by the advent of gateways that
`enable links to be made between videotex and virtually any information or computing
`service. It might, after all, be less frustrating and more useful to think of videotex in an
`application~independent manner, as,
`for example, a new communication medium,
`value-added television, value-added telephone, or simply a_ milestone on the way
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`Chap.1
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`Introduction
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`-
`
`5
`
`Gene‘amed “mason was
`
`C°mPuting for an, wired new“
`
`information
`retrieval
`
`TV set as
`
`Figure 1.1 Videotex as a may set: ‘ ‘videotexmess” decreases mm the potiphet'y.‘
`
`nation.” One can even hear descriptions of fideotex as “the
`towards creating the
`cheap computer network,” “friendly time-sharing with pictures,” or “just another
`temtinal.”
`
`However, the above observations apply mostly to two~way videOtex, which tends
`to corwerge with other forms of data prooessing. Teletext, due to its unique one~way
`transmission scheme, is less prone to such loss _of identity.
`As a communication channel, videotex can be roughly characterized in terms of
`the size of the audience and the time to disseminate htformation (see Tanabe, 1981).
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`6
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`Dissemination
`
`Background
`
`Part I
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`
`
`
`
`Monthly and
`weekly journals
`
`Daily
`newspapers
`
`Radio,
`TV
`
`1
`
`10
`
`100
`
`
`J__
`|
`|
`1000
`10,000
`105
`
`' Size of audience
`P330115
`
`Figure 1.2 Comparison of oomunication media.
`
`This is shown in Figure 1.2, in which several conununication media are plotted, Interac-
`tive videotex fills a gap, as indicated.
`From a technical point of view, videotcx is a special case of computer networks. It
`is not a brand-new procedure based on some glamorous breakthrough; its originality is
`rather in its combination of existing technologies. Perhaps the most unique aspect of
`videotex is that the display characteristics available on television sets (namely colOr
`capabilities and limited resolution) are fully taken into account and anchored in a number
`of presentation standards.
`I
`A seldom-mentioned but important consequence of the fact that videotex is largely
`aimed at the general public and at computer~naive professionals is the prospect (or hope)
`of planting into the minds of these people practical notions “of information technology,
`disguised as additional functions of the familiar television technology. This strategy, if it
`works, can lead to tremendous user penetration, proportional to the number of TV sets in
`use (about 160 million in the US. in 1982). Predictions for the percentage of US. TV
`households subscribing to some form of videotex service in 1990 vary between 5% and
`90%. A similar strategy aims at gradually supplementing every telephone set in France
`with an electronic directory terminal. It is such psychological strategy factors, and not
`the various particularities of coding, display, and pOSsibIe services,
`that distinguish '
`videotex from similar information systems.
`The prospective user of an information network such. as The Source has to make a
`major decision in becoming a subscriber. In order to make this decision to subscribe, he
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`Introduction I
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`7
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`has to be wefl-infonned about the benefits of the available services, and will probably be
`sufficiently motivated to subscribe (and buy a ternu'nal or home computer) only if the
`offered applications match his needs. With videotex, the decision is less dramatic; it is
`rather like adding a cable converter or a remote control unit to one’s television, or buy-
`ing a more expensive model with new built~in features. This is esyecially the case with
`teleteXt, which requires neither the additional manipulation of a telephone nor an ongo-
`ing charge.
`
`1.2 compousms AND TERMINOLOGY
`
`A typical two~way videotex system (if such a thing exists) is Outlined in Figure 1.3. It
`has four (groups of) main physical components:
`'
`
`0 user terminals
`
`0 computers
`
`0 service~provider terminals and systems
`a communication networks to interconnect with other components.
`
`User terminal
`
`(telephone)
`
`Communication
`network
`
`network
`
`I
`
`AM: Access machine (service computer)
`DE: Database
`EC: External computer
`SPT: Service-provider terminal
`PSN: Packet switching network
`
`Figure 1.3 'I‘ypical two-way videotex system.
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`8
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`T Background
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`Part |
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`Roughly speaking, these components are. administered andfor owned by the fol-
`lowing organizational entities: users (service consumers), who are often the owners of
`terminal equipment; videotex system operators, who run some of the service computers;
`Service providers, who supply and maintain the contents of databaSes and other applica-
`tions; and common carriers, who operate the communication networks. 'I'heactual situa»
`tion is often more complex than this. For example, some computers are owned and
`operated by independent (“third-party” or “external”) organizations, and can also be
`used for purposes other than videotex. (This would be true in the case of a department
`store’s inventory computer that can process teleshopping transactions initiated by
`videotex users.) Further, communication netv'vorks are often under the mixed jurisdiction
`of the system operators and the carriers.
`From a fimctional point of view, a basic videotex system can be Seen either as a
`data retrieval system or as a communication medium among users and sertrice providers.
`Experience shows that, on the whole, the latter View is more appropriate because of the
`increasing importance of transactional applications and user—trmser messaging, and
`because the largest demand in database applications is for fast~changiug, “hot” data
`pages, and not for seldom— or never-updated encyclopedic information.
`
`User Ternunats
`
`As already mentioned, most user terminals are ordinary TV sets upgraded with addi-
`tional components: a decoder (controller) and a keypad or keyboard. The controller
`serves to receive pages of digitally encoded data (text or graphics) and to generate a syn
`thetic image to be displayed on the screen. The user interacts with the system (e.g., to
`select an appropriate page) via commands entered through the keypad. Most transac—
`tional applications require the use of alphanumeric keyboards. '
`
`Computers
`
`The computers found in centemporary systems are used in a great variety of ways.
`Among them, access machines (also called service computers) are of central importance.
`An access machine can best be seen as an intelligent interface placed between users and
`the rest of the network. Its main functions are handling of dialogs with users and
`supervising ulteractions. with other computers. Access machines are usually owned and
`operated by the videotex operator. They may contain lecal databates and other applica-
`tions, as well as control functions, such as password verification and billing, that enable
`them to operate in stand—alone mode (typical for smaller systems).
`Application computers are machines dedicated to particular applications (databaso,
`banking, etc). Frequently these applications are designed, owned, and operated by
`enterprises independent of videotex. In such came they are called enema! cooputers.
`There is much current interest in interfacing with extemal computers through gateways.
`Other types of computers in a videotex network may he dedicated to system control and
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`Part 1
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`Background
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`9
`
`monitoring, service. provider support, gateways, or traffic concentration. More details0n
`network components are found in Chapter 6.
`
`Service-Provider Tenninais
`
`These terminals serve mainly for the editing of informationpages to be attached to
`videotex databases. The equipment varies in functionality from simple text-editing inte1~
`ligent terminals to image composition and filing facilities often implemented as stands
`alone computer systems.
`Service providers (also called information providers) can be divided into two large
`groups. In the first are those individuals or firms marketing their own information and
`services (e.g., a travel agency advertising various package tours.) The second group con«
`sists of specialiZed information brokers acting as middlemen between clients and the
`database.
`
`Communication Networks
`
`Virtually all types of links and networks can be. employed to build a videotex system. A
`fundamental distinction can be made according to whether the network delivering infor~
`mation to the users is a one~way or a ova-way system. In omway systems, also called
`broadcast videotex, or teietext, the data base is continuously and cyclically transmitted
`and available to all users, much like radio or television programs. Selected pages are
`captured (‘igrabbed”) by the decoder, locally stored, and presented on the display. The
`user has the impressiou of interacting with the system (by issuing commands); however,
`the interaction is on the same level as the selection of a TV channel—that is, it does not
`exceed the limits of the user’s local equipment. Hence the term pseudo~interactive is
`often used to describe this type of system. Television transmission (over the air or cable)
`is the typical communication medium.
`In two-my systems, also termed interactive videotex or viewdata, the user’s com-
`mands are actually forwarded to the service computer. There, the desired information is
`retrieved and sent back to the terminal.
`
`Terminology
`
`It is important to remember some idiosyncrasies of terminology. First, videorex generally
`(and in this book) has two connotations: it is uSed as a generic term for both one~way and
`two-way TV—based systems, and also as a specific term fer two»way systems. This dour-
`ble usage is unfortunate, but the specific meaning is usually clear from the context. The
`term viewdata is commonly used for two-way videotex in Bumps.
`A second idiosyncraSy of terminology involves the words teietaxt and relatex.
`Teletex, although dangerously close in spelling to teletext, is definitely different from it,
`and involves an enhanced form of text communication (and is a succesor of telex). We
`should also mention videography as a general term covering all digital techniques for text
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`1o
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`' Background
`
`Part I
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`and image transmission. A related term is the French téléman'que (fi-om the words tét‘é«
`communications and informatique), often used even in English for videotex—like systems.
`In addition to being referred to by the above generic terms, particular systems and
`implementations also have brand names. Here are a few examples, including mostly
`European names that are often used with definitional connotation (that is, to denote par-
`ticular transmission or coding schemes):
`.
`
`Antiope: Image coding system used in teleteitt and viewdata in France; also used to
`designate the French teletext service
`Didbn: French system for broadcast of teleteitt and other data over the