The long form of ISDN is Integrated Service Digital Network. It is a wide area network becoming widely available. A user has a access to ISDN by means of a local interface to a “digital pipe” of a certain bit rate. Such digital pipes of various sizes are available to satisfy different needs. The data rate of the pipe will vary depending on the need of application. The user will be charged according to the capacity of the ISDN interface and not according to the connection time.

As shown in the figure ISDN supports the telephone service, data terminals, private telephone exchanges (PBX), local area network (LANs) etc. This is the meaning of ‘integrated services’. The customer ISDN interface is then connected to ISDN central office via digital pipe. The digital pipes are also called as ISDN lines. The ISDN central office is connected to the Integrated Digital Network(IDN). The IDN consists of packet switched network, circuit switched networks, data bases etc. The ISDN provides a variety of services which include the existing voice and data applications as well as some additional services such as: Facsimile, Teletext, Videotext etc. A number of standard organizations are involved in different aspects of ISDN. The development of ISDN is governed by a set of recommendations issued by CCITT.