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`Published by Chapman & Hall, 2—6 Boundary Row, London SE1 SHN, UK
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`First edition 1997
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`© 1997 Nick Burd
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`Printed in Great Britain by Cambridge University Press
`ISBN 0 412 49730 1
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`

`

`I
`
`_—I-—
`
`s
`
`i. 29‘s.. - — u—n— 'I
`
`10
`
`ISDN—the dawning of a new era
`
`0
`
`information-related services such as the World Wide Web, news, financial or
`timetable information;
`
`0
`
`processing services such as credit card processing, inter-bank money transfers
`and airline reservation systems;
`0 messaging services such as email and EDI (electronic data interchange used
`for direct order entry).
`
`Access to these services is dependent on the targeted customer. For example,
`the SWIFT system used by banks for customer
`transfers, bank transfers,
`statements and confirmations, uses encryption techniques to provide secure
`communications over leased line connections. On the other hand, services such as
`public transport timetables, news and weather services, are targeted at a much
`wider audience and therefore available via videotex or teletex across the PSTN.
`
`1.3 THE INTEGRATED SERVICES DIGITAL NETWORK (ISDN)
`
`This brings us to ISDN, whose purpose it is to support a wide range of both new
`and existing voice and data services using a limited, but well defined, set of
`connection types and interfaces between the user and the network. The main
`feature of ISDN is reflected in its name, that is to provide an end—to—end Digital
`Network capable of supporting Services which, from the user’s perspective,
`appear as though they are provided by an Integrated network by virtue of the fact
`that the services are accessed from a single connection to the network using a
`common set of well defined protocols.
`Figure 1.2 shows the arrangement of networks prior to ISDN and how access to
`them becomes integrated with the implementation of an ISDN. Access to the
`services of the separate networks is achieved through an ISDN exchange with a
`single interface to the user such that the user perceives the network to be a single
`entity. The ISDN subscriber loop and the protocols it employs are defined such
`that
`the user can readily access the wide range of services provided by the
`individual networks
`that
`lie behind the user—network interface, and are
`sufficiently flexible that new services can be added in the future.
`The definition of
`the ISDN subscriber
`loop is key to the successful
`implementation of lSDN, because it must:
`
`0
`
`provide the user with a flexible digital interface with access to a wide variety
`of present and future services;
`
`

`

`r———'
`
`The integrated services digital network
`
`11
`
`allow the evolution of the individual network services to progress towards
`achieving an ISDN according to the different strategies of the networks
`operators .
`
`Information
`plus
`signalling
`
`voice
`
`
`circuit-
`
`switch
`
`exchange
`
`
`
`
`data
`
`packet-
`switch
`
`
`exchange
`
`/
`
`PUBLIC NETWORKS
`
`telephone network
`
`packet-switched data
`network
`
`telex network
`
`
`
`
`voice
`circuit-
`
`switch
`exchange
`
`
`
`
`
`
`
`
`(a)
`
`ISDN
`subscriber
`
`ISDN
`
`(II
`
`-
`
`loop
`
`subscriber
`
`iSDN
`exchange
`
`packet-switched data
`network
`
`telex network
`
`ISDN
`exchange
`
`Fig. 1.2 (a) Separate access to networks prior to ISDN; (b) access integration
`provided by ISDN.
`
`With many of the world’s public networks evolving in different directions and
`at different paces, it is necessary to define a common starting point for ISDN. This
`
`