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Zntegrated Services Digital Network (ISDN) Most public telecommunication operators throughout the world have now at least commenced the modernization of their networks by introducing digital transmission and digital switching into the public switched telephone network (PSTN). Simultaneously many operators are converting their networks to integrated services digital networks (ISDN) which will allow customers access to a variety of services while reducing the cost of provision of these services both to the administration and to the customer | Networks and Telecommunications Design and Operation Second Edition. Martin P. Clark Copyright 1991 1997 John Wiley Sons Ltd ISBNs 0-471-97346-7 Hardback 0-470-84158-3 Electronic PART 2 MODERN TELEPHONE NETWORKS Networks and Telecommunications Design and Operation Second Edition. Martin P. Clark Copyright 1991 1997 John Wiley Sons Ltd ISBNs 0-471-97346-7 Hardback 0-470-84158-3 Electronic 10 Integrated Services Digital Network ISDN Most public telecommunication operators throughout the world have now at least commenced the modernization of their networks by introducing digital transmission and digital switching into the public switched telephone network PSTN . Simultaneously many operators are converting their networks to integrated services digital networks ISDN which will allow customers access to a variety of services while reducing the cost of provision of these services both to the administration and to the customer. So what is an ISDN What is its value This chapter answers these questions and goes on to explain the technical detail of how ISDN operates and the user benefits that can be gained. THE CONCEPT OF ISDN The internationally agreed definition of ISDN is a network evolved from the telephony integrated digital network IDN that provides end-to-end digital connectivity to support a wide range of services including voice and non-voice services to which the users have access by a limited set of standard multi-purpose customer interfaces . The above definition of ITU-T makes a number of points. First that ISDN requires a digital network. Second that this digital network is not one between exchanges but it extends to the customers. Third it provides not only telephony but a variety of services. Fourth a customer does not require a separate interface for each service provided by ISDN but can use all services via one access point or at worst via a limited set of standard multi-purpose customer interfaces . Figure illustrates these principles. ISDN .