DETAILED CONCEPT OF INTEGRATED SERVICES DIGITAL NETWORK

Integrated Services for Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of data, voice, video, and other network services over the traditional circuits of the Public Switched Telephone Network (PSTN). It was first defined in 1988 in the CCITT red book. Before ISDN, the telephone system was viewed as a way to transport voice and some special services available for data. The main feature of ISDN is that it integrates speech and data on the same lines thus adding features that were not available in the classic telephone system. There are various kinds of access interfaces to ISDN defined as Basic Rate Interface (BRI), Primary Rate Interface (PRI), Narrowband ISDN (N-ISDN), and Broadband ISDN (B-ISDN). In a broad sense ISDN can be considered a suite of digital services existing on the layers 1, 2, and 3 of the OSI model. ISDN is designed to provide access to voice and data services simultaneously.