ISDN from ITU

INTEGRATED SERVICES DIGITAL NETWORKS (ISDNs)
(Malaga-Torremolinos, 1984; amended at Helsinki 1993)

1 Principles of ISDN
1.1 The main feature of the ISDN concept is the support of a wide range of voice and non-voice applications in the
same network. A key element of service integration for an ISDN is the provision of a range of services using a limited
set of connection types and multipurpose user-network interface arrangements.
1.2 ISDNs support a variety of applications including both switched and non-switched connections. Switched
connections in an ISDN include both circuit-switched and packet-switched connections and their concatenations.
1.3 As far as practicable, new services introduced into an ISDN should be arranged to be compatible with 64 kbit/s
switched digital connections.
1.4 An ISDN will contain intelligence for the purpose of providing service features, maintenance and network
management functions. This intelligence may not be sufficient for some new services and may have to be supplemented
by either additional intelligence within the network, or possibly compatible intelligence in the user terminals.
1.5 A layered protocol structure should be used for the specification of the access to an ISDN. Access from a user
to ISDN resources may vary depending upon the service required and upon the status of implementation of national
ISDNs.
1.6 It is recognized that ISDNs may be implemented in a variety of configurations according to specific national
situations.
1.7 Based on these principles the I-Series Recommendations have been developed. Figure 1 provides a broad
outline of the structure of the I-Series of Recommendations and their relationship to other Recommendations. As shown
in Figures, the current structure of the I-Series documentation is subdivided into seven major parts. Other I-Series
documents may be added as the need arises. In addition, to support the implementation of the ISDN concepts,
Recommendations have been produced and others will be produced in other series by the appropriate specialist group.
2 Network evolution towards ISDN
2.1 ISDNs are based on the concepts developed for telephone IDNs and may evolve by progressively
incorporating additional functions and network features including those of any other dedicated networks such as
circuit-switching and packet-switching for data so as to provide for existing and new services.
2.2 The transition from an existing network to a comprehensive ISDN may require a period of time extending over
one or more decades. During this period arrangements must be developed for the interworking of services on ISDNs and
services on other networks.
2.3 In the evolution towards an ISDN, digital end-to-end connectivity will be obtained via plant and equipment
used in existing networks, such as digital transmission, time-division multiplex switching and/or space-division
multiplex switching. Existing relevant Recommendations for these constituent elements of an ISDN are contained in the
appropriate series of Recommendations of CCITT and of CCIR.
2.4 In the early stages of the evolution of ISDNs, some interim user-network arrangements may need to be adopted
in certain countries to facilitate early penetration of digital service capabilities. Arrangements corresponding to national
variants may comply partly or wholly with I-Series Recommendations. However, the intention is that they will not be
specifically included in the I-Series.