RIC 1002 (Part 1 of 2)
`IPR of U.S. Pat. No. 7,986,426
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`0002
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`XEROX NETV\_IORK SYSTEMS ARCHITECTURE
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`GENERAL INFORMATION MANUAL
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`XEROX
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`xusc 068504
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`April 1985
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`0003
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`Copyright 9 1985 by Xerox Corporation. All rights reserved.
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`XEROX 0, XNS, NS, Interpress,
`CORPORATION
`Printed in U.S. A.
`
`lnterscript, Clearinghouse. 8010. and 860 are trademarks of XEROX
`
`Text and graphics prepared on the Xerox 8010 Information System, and printed on a Xerox laser printer
`using the lnterpress electronic printing standard.
`
`Xerox Corporation
`Office Systems Division
`2300 Geng Road
`Paio Alto, California 94303
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`0004
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`0004
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`

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`TABLE OF CONTENTS
`
`1. Introduction
`
`Xerox Network systems: the key to integration
`
`1
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`3
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`Network architectures and distributed systems
`4
`
`Realization of Xerox Network Systems
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`XNS architecture is open
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`2. Xerox Network Systems Overview
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`Xerox‘ goal in office automation
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`5
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`6
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`7
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`7
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`Xerox Network Systems objectives
`8
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`Qualities of Xerox Network Systems
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`/ Xerox NetworkSystemsconceptsandfacilities
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`ISO Open Systems Interconnection Reference Model
`XNS structural overview
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`/
`I
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`10
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`11
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`11
`14
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`Network devices and terminology
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`Servers and services
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`17
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`18
`
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`Clients
`19
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`3. Communications
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`Names
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`20
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`21
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`
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`Ethernet
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`Ethernet benefits
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`21
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`23
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`/ SynchronousPoint-to-PointProtocol
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`Ethernet architecture
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`The Internet
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`
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`24
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`26
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`28
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`_
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`_/
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`‘
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`Internet architecture
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`Datagrarns
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`Source and destination addresses
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`Internet delivery and routing
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`29
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`30
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`31
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`33
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`
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`Message integrity
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`Other Internet protocols
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`Internetwork Routing Service
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`Dedicated and switched circuits
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`)(.25 public data networks
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`34
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`36
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`37
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`39
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`39
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`Clusternet communication
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`Network management
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`40
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`41
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`XEROX NETWORK SYSTEMS GENERAL INFORMATION MANUAL
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`4. Remote Procedures: Courier
`43
`/ The internal Courier layers
`44
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`Bulk data transfer and third parties
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`46
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`5. Application Support Environment
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`Clearinghouse
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`/.
`/ Time
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`Authentication
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`Object names and addresses
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`Character code and fonts
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`Character code standard
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`Font architecture
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`6. Gateway access
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`Gateway Access Protocol
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`49
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`49
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`53
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`50
`57
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`57
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`59
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`60
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`65
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`65
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`Gateway services
`66
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`~\_
`""-
`,/
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`I
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`. /
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`\'\_‘_.
`X
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`External Communication Service
`Interactive Terminal Service
`ssorsso Gateway Service
`Remote Batch Service
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`7. Filing
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`Filing Protocol
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`_/ File Service
`8. Document interchange
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`Document compatibility
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`Interscript
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`67
`68
`70
`70
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`71
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`71
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`75
`77
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`77
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`78
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`Interscript as a document interchange standard
`79
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`interscript base language
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`Document Interchange Service
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`9. Mailing
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`80
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`82
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`33
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`Mailing standards
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`Mail Service
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`Mail Transport Protocol and Inbasket Protocol
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`Mail format standard
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`84
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`85
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`87
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`88
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`External Mail Gateway and Tel etex Gateway Service
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`90
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`10. Printing
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`Electronic printing
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`lnterpress
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`Interpress as an industry standard
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`91
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`91
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`93
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`95
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`lnterpress language
`97
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`ii
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`XEROX NETWORK SYSTEMS ARCHITECTURE GENERAL INFORMATION MANUAL
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`TABLE OF CONTENTS
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`
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`Utility programs
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`Printing Protocol
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`Print Service
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`Electronic printing service
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`Facsimile printing service
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`Print Service Integration Standard
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`101
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`101
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`103
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`104
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`104
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`105
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`11. Scanning
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`107
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`Raster Encoding Standard
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`Scanning services
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`Electronic reprographics
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`107
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`111
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`114
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`12. XNS applications
`Office information systems
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`Engineering information systems
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`Programming and knowledge-based systems
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`117
`117
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`121
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`123
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`‘
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`Xerox Development Environment
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`123
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`Artificial intelligence applications
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`Electronic publishing
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`Xerox and electronic publishing: baseline products
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`125
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`126
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`129
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`Xerox and electronic publishing: potential future extensions 132
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`
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`Appendices:
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`_
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`Index
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`A. Objectives of network architectures
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`B. XNS and other network architectures
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`C. XNS and industry standards
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`D. Examples of XNS protocol usage
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`E. Glossary
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`F. Annotated Bibliography
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`137
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`141
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`14?
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`149
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`155
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`169
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`179
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`-wmw
`XEROX NETWORK SYSTEMS GENERAL INFORMATION MANUAL
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`XEROX NETWORK SYSTEMS ARCHITECTURE GENERAL INF-ORMAT|ON MANUAL
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`LIST OF FIGURES
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`1-1 Structural relationship of architectu re. standards,
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`and products
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`6
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`2-1 Return on information assets
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`2-2 Layers in the ISO Model
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`2-3 Protocols and interfaces in the ISO Model
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`7
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`11
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`13
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`2-4 Overview of Xerox Network Systems
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`2-5 Network devices
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`15
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`17
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`2-6 Portion of a user's desktop
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`3-1 A local area network
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`20
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`3-2 Ethernet frame
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`Poi nt-to-Poi nt Protocol
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`26
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`3-3
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`lnterfacility communication using the Synchronous
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`3-4 Exam pies of internetworking configurations
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`29
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`3-5 The XNS Internet layer
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`30
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`3-6 Internet packet or “datagram"
`31
`3-7 A sequenced packet protocol packet allows
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`successive transmission of internet packets
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`35
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`3-8 A packet exchange protocol packet simply
`
`transmits a request and receives a response 36
`3-9 Two local area networks interconnected by a
`communication line
`38
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`3-10 Internet Routing Service (IRS) with )(.25 links
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`3-1 1 Internet Routing Service (IRS) with cl usternet feature
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`40
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`41
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`4-1 The Courier remote procedure call model
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`4-2 The layers within Courier
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`4-3 Example of Courier usage
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`43
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`45
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`46
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`47
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`48
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`53
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`4-4 Third-party bulk data transfer (receiver active)
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`4-5 Immediate bulk data transfer (initiator the receiver)
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`5-1 The Clearinghouse Service
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`5-2 Strong authentication model
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`5-3 Simple authentication model
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`
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`5-4 Layout of data field of Time Service response packet
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`5-5 16-bit character code
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`58
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`60
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`S-6 Xerox Character Set allocation 61
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`5-7 A font service
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`6-1 Gateway access torfrom non-XNS systems
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`7-1 Transmitting documents between a workstation
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`63
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`67
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`72
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`73
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`78
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`and a file service
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`7-2 Filing Protocoi structure
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`8-1 Document interchange using Interscript
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`8-2 A document's constituent parts
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`8-3 Interscript base language grammar
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`9-1 CCI'lT XA00 architecture model
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`79
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`82
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`84
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`9-2 The Inbasket Protocol provides an internal
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`interface for user convenience
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`9-3 Envelope and content of a mail message
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`85
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`86
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`9-4 Heading and body of a mail message
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`10-1 Electronic printing
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`10-2
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`Interpress used for printing or archiving
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`10-3 Example of an lnterpress master
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`10-4 Preparing two-up signature pages
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`11-1 Example of raster image file
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`11-2 Two models for scanning service
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`87
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`92
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`94
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`99
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`102
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`109
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`1 1-3
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`Intelligent scanning system converts hardcopy to text
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`1 13
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`12-1 XNS applications in office information systems
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`I 12-2 XNS applications in engineering information systems
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`12-3 XNS applications in artificial intelligence and
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`software development
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`118
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`122
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`124
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`12-4 Traditional document publishing process
`128
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`12-5 Electronic publishing process 129
`12-6 Electronic publishing using Xerox professional
`workstations
`130
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`12-7 Electronic publishing using the 700 Xerox Publishing
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`System
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`12-8 Extended integration in the Xerox electronic
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`publishing applications
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`B-1
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`XNS and other architectures
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`131
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`135
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`141
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`(3-1
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`Clearinghouse request and response
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`13-2 Remote printing
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`149
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`13-3
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`Filing with authentication
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`D4
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`Sending mail
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`D5
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`Receiving mail
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`151
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`152
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`VI
`XEROX NETWORK SYSTEMS ARCHITECTURE GENERAL INFORMATION MANUAL
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`I.
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`INTRODUCTION
`
`For nearly three decades Xerox Corporation has played an
`important rote in improving the productivity of office workers.
`in addition to its widely—recognized line of plain paper copiers,
`Xerox is manufacturing and marketing word processors,
`facsimile transceivers, data terminals, electronic typewriters,
`professional workstations, electronic printers, and a host of
`related products. These advanced products are being used
`throughout the world to improve personal efficiency and
`
`productivity in handling physical and electronic documents in
`the office.
`
`The size and vitality of the office systems industry is a direct
`result of the substantial gains in productivity users have been
`able to realize through modern computing and communica-
`tion technology- As a result of those gains, work can flow more
`smoothly, data can be processed more accurately and quickly,
`and information can be distributed more efficiently. in turn,
`management can be more effective and responsive in its
`decision-making.
`
`The early office machines were basically “point products,“
`which means that their benefits were derived essentially from
`functions wholly performed within the product
`itself- For
`example, a point product such as a word processor made it
`possible for a secretary to be much more productive in many
`routine tasks, without requiring that the product be connected
`to anything else. In this fashion many of the more obvious tasks
`in offices were automated and significant gains in productivity
`were realized. Xerox was part of this initial wave of office
`automation with its word processing systems. Moreover, many
`of the most important subsequent developments in office
`automation have come from basic research work carried out at
`
`the Xerox Palo Alto Research Center.
`
`One result of that research was a recognition that further
`major productivity gains would be made when point products
`could easily communicate and share work with each other.
`These new requirements for intercommunication and integra-
`tion led Xerox to announce, with its partners, Intel Corporation
`and Digital Equipment Corporation, that Xerox’ proprietary
`local area networking technology, Ethernet, would be made
`avaiiable to the business and academic communities. Xerox‘
`
`intent was to promote widespread acceptance and use of local
`area network technology to provide a basic, generaI—purpose
`
`Productivity through systems
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`Productivitythrough networks
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`XEROX NETWORK SYSTEMS GENERAL INFORMATION MANUAL
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`data transmission "highway" within a facility such as an office
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`building, laboratory, or factory.
`
`The importance of local area networks in such applications as
`office automation cannot be overemphasized. They are one of
`the most important means by which individual devices-work-
`stations, word processors. electronic typewriters, filing and
`printing subsystems, etc.-—communicate directly with each
`other. Being able to intercommunicate is a necessary first step
`toward the functional integration of these devices: the coor-
`dinated cross-functioning between products that will deliver
`the next wave of effectiveness and efficiency improvements in
`offices and related environments. (Note, however, that a local
`
`area network, or any other communication system, by itself
`
`does not guarantee integration; more is required as we will
`see-)
`
`As an example of functional integration, when the Ethernet
`announcement was made, Xerox introduced a series of inte-
`
`grated office products called the 8000 Network System, which
`included the Star Information System along with a number of
`shared—resource "servers-" Star pioneered such concepts as
`
`"bit-mapped" (or "ail points addressable“) displays, "win-
`dows" for simultaneously interacting with multiple processes,
`and a "mouse" for efficient operator control- The 8000 Net-
`
`work System provided not only the functions that were avail-
`able with the oider point products, but an entirely new set of
`functions such as coordinated document creation, centralized
`
`printing, and filing. These functions were possible only because
`of the high degree of integration among the members of the
`8000 family.
`
`More recently Xerox has broadened the integration of its
`product line, continuing to offer additional productivity im-
`provements. Gradually drawn into the integrated community
`
`were such products as word processors, personal computers,
`very fast electronic printers, an input scanner, electronic
`typewriters, and special systems for electronic engineers.
`In
`each of these cases, integration means that all products work
`effectively with other products, exchanging data, sharing
`resources, and building applications, thereby leading to new
`
`levels of productivity.
`
`Xerox’ development philosophy has emphasized taking maxi-
`mum advantage of what has been developed before and of
`the efforts in the international standards groups that are
`
`leading to robust, durable standards. The openness of ‘the
`Xerox approach works both ways: as new developments have
`matured at Xerox, a conscientious effort has been made to
`
`share these developments with the rest of the information
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`systems world. Xerox is committed to an open systems
`approach to the development of its information systems.
`
`Productivity through integration
`
`Commitment to open systems
`
`2
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`XEREX EETWORK SYSTEMS ARCHITECTURE GENERAL INFORMATION MANUAL
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`0012
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`INTRODUCTION
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`
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`Xerox Network Systems: the key to integration
`
`Network architecture
`
`Long-range planning
`
`local area networking technology
`Although the Ethernet
`makes it Palssible for intelligent devices to intercommunicate
`efficiently, simply having that facility does not ensure the
`devices will be integrated. All it guarantees is that they will be
`able to exchange data back and forth. Simply put, the benefits
`of an integrated information system can be obtained only
`when the individual element work together. That,
`in turn,
`depends on what those elements do with the data they
`exchange.
`
`The rules governing the exchange of information among net-
`worked devices, and specifying the processes through which
`work on that information is to be done, are collectively known
`as a network architecture. There are many different types of
`network architectures, varying with respect to their application
`orientation, the emphasis placed on local vs. wide area com—
`munications, etc. But they all define a structured approach to
`the exchange and handling of information in a network, and
`they all encompass greater functional scope than the compar-
`atively straightforward matter of data transmission provided
`by" the network itself.
`In fact, a general purpose network
`architecture is capable of employing a number of different
`transmission techniques as required: a locat network here, a
`wide area network there, along with satellite links, public data
`networks, etc.
`‘
`'
`
`An analogy might be drawn between network architectures
`and city planning. At first glance, a “city plan" could be
`thought of as a map of the city's streets. But in order to have a
`completely integrated city plan, provisions must be made for
`various city services, the delivery of utilities, the extent of land
`use restrictions, the establishment of public activities such as
`
`education and recreation, and the integration of various
`transportation systems, of which the city street system is an
`important part. but only a part. The city streets are analogous
`to a local area network and the overall city plan is analogous to
`the network architecture.
`
`As is the case with a comprehensive city plan, a good network
`architecture is conceived with the understanding that not all
`eventual uses of
`the architecture can be anticipated in
`advance. But it accommodates known elements successfully, is
`faithful to long-range goals, and provides for future _growth
`and change in ways that do not require the architects period-
`ically to return to their drawing boards to start over.
`
`The network architecture underlying Xerox’ focus on inte-
`grated systems is called Xerox Network Systems (XNS)-
`
`XNS provides a conceptual framework for accomplishing all the
`functions required in a general-purpose information system.
`
`
`XEROX NETWORK SYSTEMS GENERAL INFORMATION MANUAL
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`
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`Integrated office systems
`
`Like the city plan that uses city streets as one of its building
`blocks, XNS uses Ethernet (along with other networking tech-
`niques); but XNS is not synonymous with Ethernet- Indeed, XNS
`imposes extensive and stringent requirements on those pro-
`ducts designed according to its rules, requirements far more
`elaborate than those applicable to devices that simply connect
`to Ethernet.
`
`Following the ruies and specifications of XNS, it is possible to
`design highly integrated information systems, using hardware
`and software elements designed by different groups using
`different technologies. XNS brings to this process a broad
`range of facilities and functions, tested in a variety of imple-
`mentations over a long development period. This makes XNS
`one of the most thorough and robust of the extant network
`architectures, particularly among those intended for commer-
`- cial, office-—oriented applications.
`
`XNS is one of the major reasons why Xerox products are as
`capable, reliable," user-friendly, and obsolescence-resistant as
`they are. These qualities are specific objectives of the archi-
`
`tecture design and reflect directly on the products that the
`architecture supports. Office products not designed within a
`framework such as XNS provides will likely be lacking in one or
`more of these important attributes.
`
`
`
`Network architectures and distributed systems
`
`A network architecture provides the conceptual framework for
`the design of the many functions that are necessary for net-
`work elements to work together. It also provides a series of
`specifications for the common functions that must be agreed
`upon by the network community. These functions usually take
`the form of protocols that provide for accomplishing specific
`tasks such as the transfer or storage of information. In such
`instances the network architecture can be thought of inform-
`ally as a collection of protocols, each of which is usually iden-
`_- tified with the service to which it corresponds.
`
`Traditional architectures are centralized
`
`in addition to XNS, a number of other network architectures
`
`have emerged within the computer and office equipment
`industries- Two examples are IBM's Systems Network Archi-
`tecture, which is oriented to the interconnection of mainframe
`
`computers and terminals, and Digital Equipment Corporation's
`Digital Network Architecture, which supports the intercon-
`nection of that company's small- to medium-sized computers.
`
`An important distinction exists between XNS and the typical
`
`computer-manufacturer network architecture. To one extent
`or another, the latter tend to be hierarchical in organization,
`
`intended for applications where one or more computers domi-
`
`XEROX NETWORK SYSTEMS ARCHITECTURE GENERAL INFORMATION MANUAL
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`0014
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`

`
`INTRODUCTION
`
`nate the resulting information system and its users. In such
`systems other elementsrwsatellite processors, terminals. work-
`
`stations. etc.-are clearly subordinate to the large computers.
`In various ways the network architectures underlying these
`systems are designed to create and reinforce this relationship.
`
`There is a place for such arrangements. Traditional mainframe
`computing is usually organized in such a way that information
`naturaiiy flows to and from major computing centers. The
`operation of other system elements is intended to support the
`processes taking place in the centralized machines- in many
`respects this view of computing reflects traditional data
`processing implementation.
`
`Centralized systems lack flexibility Many networking applications, however, are not well served
`by this model. in particular, many of the processes and activities
`in modern offices are essentially autonomous.
`i.e-,
`initiated
`and concluded by individuals at their own pace, with only
`occasional references to external resources. This is particularly
`true with the creation, editing, storage, retrieval, and printing
`of documents, the common currency of the office. In most
`cases document management requires dealing with a large
`series of autonomous processes, for which the centralized
`mainframe-oriented model of information flow and processing
`is often not very effective and a potential bottleneck.
`
`XNS architecture is distributed
`
`For this reason XNS——and network architectures similar to it-
`
`are designed to support autonomous processes, implemented
`by distributed, rather than centralized. processors. One of the
`important results of this is that XNS makes it possible for
`networks to grow incrementally through replication of the
`individual
`system elements and data bases. XNS is
`a
`distributedlreplicative network architecture.
`'
`
`A more complete discussion of network architectures and the
`relationship of XNS to other architectures and industry
`standards is provided in the Appendices.
`
`
`
`Realization of Xerox Network Systems
`
`This manual describes the architecture of Xerox Network
`
`information on the standards and
`It provides
`Systems-
`protocols that comprise the architecture. Detail specifications
`and specific hardware and software products are not
`described, but a description of the services is provided to
`illustrate how the protocols are used and how the network
`architecture integrates products to form systems. Fig. 1-1 shows
`the relationship between the XNS architecture, standards, and
`
`products.
`
`The architecture is at the top of the hierarchy. it establishes the
`general structure and functioning of the network. The specifics
`
`XEROX NETWORK SYSTEMS GENERAL INFORMATION MANUAL
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`5
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`0015
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`
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`XNS
`
`Architecture
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`XNS
`
`Standards
`
`XNS
`
`' ' ' ' ' ' ' ° '
`
`Implementors
`
`Group
`
`ware Products
`
`XNS
`
`Releases
`
`xerox Hard-
`
`Non-Xerox Hard-
`
`ware and Soft-
`
`"‘ "' "" ware and Soft-
`
`ware Products
`
`XNS architecture is open
`
`_ Figure 1-1 Structural relationship of architecture, standards.
`and products
`
`of the architecture are contained in the various standards
`
`documents. The Xerox Network Systems releases specify which
`Xerox hardware and software products conform to the
`standards, and their level of mutual integration.
`
`There is a well-defined path for other companies who choose
`
`to offer products in conformance with XNS standards (see Fig-
`1-1)- Xerox offers a variety of help to companies wishing to
`adopt any or all of the XNS standards. This help includes
`publication of standards and guides, assistance in implemen-
`tation through the XNS Implementors Group and the XNS
`Institute, and a variety of joint intercompany arrangements.
`For further information please contact:
`
`Xerox Corporation
`Xerox Network Systems Institute
`2300 Geng Road
`Paio Alto, Ca 94303
`(41 5}-496-6088
`
` 6
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`XEROX NETWORK SYSTEMS ARCHITECTURE GENERAL INFORMATION MANUAL
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`'
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`0016
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`

`
`mznux NHWORK SYSTEMS
`OVERVIEW
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`
`
`Xerox’ goal in office automation
`
`
`Xerox’ goal in office automation is to provide its customers
`with products, systems, and services to maximize return on
`their information assets. Information assets come in two forms:
`
`people and hardvvarelsoftware products. increasing the return
`on information worker assets is
`increasing effectiveness;
`increasing the return on information product assets is increas-
`ing efficiency. Fig. 2-1 graphically displays the concept of
`return-on-information assets (ROIA). The boxes at the bottom
`
`of the diagram show some of the ways in which the return can
`be increased and the assets decreased.
`
` Return on Information Assets
`
`Efficiency
`
`Effectiveness
`
`Return on
`Information Worker
`Assets
`
`
`
`I Better decision making
`I More effective communication
`
`I More persuasive communication
`I Higher th roughput (people)
`I Less information in transit
`I Higherjoh satisfaction
`
`support‘
`.............;..
`
`Direct personnel costs
`indirect personnel costs
`
`
`
`
`I High quality CRT interfaces
`I Typeset-quality primed output
`I Integration of paper and
`electronic documents
`
`I Ease of growth and change
`I Higher throughput (processing)
`I High-speed networks
`
`Cost of acquisition and support of
`hardvvarefsoftwa re
`
`Return on
`Hardware and Software
`ASSEB
`
`Figure 2-1 Return on information assets
`
`Typeset quality
`
`Consider an electronic printer which can provide typeset quali-
`ty directly. Not only can documents be printed on fewer pieces
`of paper, thereby reducing material. storage. and delivery
`costs. but higher-quality documents can support more effective
`communication and decision making-
`
`.
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`XEROX NETWORK SYSTEMS GENERAL INFORMATION MANUAL
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`7
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`0017
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`Documents and document management
`
`People come together to accomplish information related tasks
`in offices because these tasks are generally highly interactive.
`The means of interaction is usuaiiy either through verbal
`
`means, including physical cues, or through the creation and
`reading of documents.
`
`A clear understanding of the concept of a document is essen-
`tial to understanding the objectives of XNS. A document is a
`structured organization of information designed to communi-
`cate effectively with people- A document may be rendered by a
`printer, which produces a paper document from an electronic
`original, or it may be rendered by a workstation, which makes
`an electronic document visible. Alternate forms of documents
`
`include voice documents and video records. In the future it will
`
`be possible to integrate these various document forms; for
`example, a text document may be annotated by digitized
`voice.
`
`As most documents are stored at one time or another, it is easy
`to confuse documents with stored information. Not all stored
`
`information is in document form. A computer data base, for
`
`example, is not a document since it is organized to provide
`efficient storage and access to elements of information, not to
`communicate with people.
`
`The general objectives of XNS is therefore to increase the ROIA
`by facilitating the creation, capture, storage. communication,
`printing, and replicating of electronic or paper documents
`
`within the office, especially at the work group and depart-
`mental levels. This is what Xerox calls document management.
`
`Xerox Network Systems objectives
`
`Systems that fulfill specific short—range objectives are always
`possible, and may appear to have a lower initial cost. The real
`test of a system's quality, however, is if it can be used over a
`long period as applications change and new technology is
`introduced.
`In the long run,
`systems
`that
`lack proper
`architecturai support are seldom the most cost effective. They
`become obsolete in a short time and have to be replaced. often
`resulting in costly disruption for users. Xerox Network Systems
`Architecture, with its long-range view, is for those users and
`
`su ppliers who want to do it right the first time.
`
`The XNS objectives include:
`
`Formal definition of standards
`
`The creators of XNS knew that this architecture would be used
`
`as a basis for the design of a wide variety of products. The
`architecture. therefore, would have to be powerful and open-
`
`ended- The products supported by the architecture would be
`
`designed by a number of Xerox and non-Xerox organizations.
`
` B
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`XEROX NETWORK SYSTEMS ARCHITECTURE GENERAL INFORMATION MANUAL
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`0018
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`

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`ACHUA nu: I WUHK SYSTEMS OVERVIEW
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`Document compatibility
`
`Product performance
`
`Some of these organizations would be as distantly removed
`
`from the group responsible for the architecture as if they were
`outside of Xerox. For this reason, the architecture had to be
`
`formally defined and its individual elements subject to strict
`rules of standardization and configuration control.
`
`One of the key issues in the design of an information system is
`the preservation of compatibility for the users‘ work products
`(documents of all kinds). Once a document is created, it must
`
`be capable of being operated upon at some point in the future
`
`and at another part of the system (even in another part of the
`world where a different language is used).
`If at all possible,
`architectural changes that "dead-end“ already-created docu-
`ments must be avoided.
`'
`
`The architecture must be designed so the products it supports
`can be engineered in accordance with it.
`Using proven
`technology. the products must still meet their performance,
`reliability, and cost goals. This objective separates the
`theoretical from the practical as each architectural provision
`must be implemented with real hardware and software at a
`
`competitive cost. An appreciation of this must
`architectural design work.
`
`influence
`
`Product evolution
`
`The architecture must make it possible for products and
`services to evolve in two directions: toward greater breadth
`(new areas of applications, user categories, operating environ-
`ments, etc.), and toward greater depth (new functions).
`
`Interconnection with other systems
`
`Industry standards
`
`interface and interact with computer
`Xerox systems must
`products and specialized systems from a large number of
`suppliers. Because Xerox’ autonomous distributedireplicative
`_system must be closely integrated with hierarchical, main-
`frame—oriented systems, some interfaces become very complex
`and create profound challenges for the architectural designer.
`Despite the difficulty, these challenges have been met- The end
`user and the industry are not well served by approaches that
`omit interconnection with external mainframe systems.
`
`Xerox has a strong commitment to the development and use of
`industry standards. Xerox personnel have participated actively
`in standardization efforts sponsored by groups such as IEEE,
`ANSI, ISO, ECMA, and CClTT. XNS architecture will either adopt
`
`or be compatible with all the important relevant standards.
`Many aspects of XNS have yet to be the subject of external
`standardization since in many cases Xerox’ work on XNS tends
`to lead official standards formulation and adoption processes
`by several years. Nevertheless, Xerox anticipates making XNS-
`derived contributions to such efforts when they are finally
`undertaken just as it has with past and present standards
`projects- A summary of
`the XNS relationship with key
`international standards appears in appendix C.
`
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`XEROX NETWORK SYSTEMS GENERAL INFORMATION MANUAL
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`0019
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`Qualities of Xerox Network Systems
`
`The layers and functions of XNS bring together an important
`set of qualities on behalf of the products they support.
`
`Together these qualities ensure that a system designed with
`the support of XNS will be among the most powerful. cost-
`effective, and obsolescence—resistant systems available.
`
`The XNS qualities include:
`
`Maturity
`
`XNS is more than just a theoretical construct; Xerox has been
`working on it for over a decade. Thousands of man-years of
`effort have gone into its design, implementation, testing, and
`refinement. It is a practical system with well-established per-
`formance and functional characteristics. XNS is a key support
`element in Xerox‘ distributed information systems.
`
`Distributivefreplicative
`
`Xerox has designed XNS to support distributed processing in
`which autonomous devices are interconnected in a network
`
`Completeness
`
`that permits simple, low-cost, incremental expansion. Because
`it is easy to replicate workstations, file and print servers, and
`
`other network resources as a user’s needs grow, a user will be
`able to benefit from the system for a long time—unlike
`mainframe—centered architectures-
`
`XNS is one of the most complete architectures available. It
`provides two separate information exchange techniques: one
`for moving data and the other for invoking (and responding
`to) remote processes. It also provides a broad spectrum of self-
`contained application processes.
`Including these processes
`within the architecture rather than leaving them for user
`development means that an XNS—based system is more immedi-
`
`ately useful to the user and ensures uniformity and robustness
`in all the important network functions.
`
`Growth and expandability
`
`XNS is not a static architecture- One of its strengths is that new
`facilities can continue to be added without major disruption to
`
`the product hardware and software it supports.
`
`Transparency to the user
`
`Global scope
`
`Despite its many functions and its potentially worldwide scope,
`XNS is transparent to the user. Information is exchanged, re-
`sources are added to and subtracted from the network, remote
`
`procedures are invoked, and all activities defined by the
`architecture take place while the user remains free to do his job
`without concern for what is making that job possible.
`
`Whether the physical scope of a user's network is limited to a
`single building or whether it spans the world, XNS provides a
`single, consistent set of services that automatically adjust to the
`network's scope. Each user sees one logical network, even
`though that network may consist of many physical local area
`networks interconnected by a web of wide area telecommuni-
`cation circuits.
`
`‘I0
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`XEROX NETWORK SYSTEMS ARCHITECTURE GENERAL lNFORMATlON MANUAL
`
`0020
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`

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`Ar:r<UJk Nl:TW'ORK SYSTEMS OVERVIEW
`
`An "open" system
`
`XNS systems are rnultivendor systems. As new, specialized
`
`hardware and software are introduced for use in office
`applications, these facilities will be capable of integration
`within an XNS-based system. Xerox began this process in 1981
`with the public disclosure of the non-application layers of the
`XNS architecture. The application layers are also being fully
`