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.1.2 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.