General

Recommendation I.120 gives the conceptual principles on which an ISDN should be based. The main feature of an ISDN is the support of a wide range of service capabilities, including voice and nonvoice applications, in the same network by offering end-to-end digital connectivity.

A key element of service integration for an ISDN is the provision of a limited set of standard multipurpose user-network interfaces. These interfaces represent a focal point both for the development of ISDN network components and configurations and for the development of ISDN terminal equipment and applications.

An ISDN is recognized by the service characteristics available through user-network interfaces, rather than by its internal architecture, configuration or technology. This concert plays a key role in permitting user and network technologies and configurations to evolve separately