ISDN

 Integrated Services Digital Network (ISDN) has been perhaps the most important development to emerge in the field of telecommunications in the 1980s, and it will probably continue to dominate the developments in decades to come. Unlike many other developments, ISDN is a well-conceived and planned area of development in the field of telecommunications. ITU-T the then CCITT has been pioneering and guiding the efforts towards the development of ISDN. The process of digitalisation of telecommunication networks started in early 1960s when PCM was introduces for digital transmission of voice signals. With the arrival of transistorised mini computers the possibility of digital switching opened up in late 1960s.

Integrated Services Digital Network: An integrated digital network in which the same digital switches and digital paths are used to establish different services, for example, telephony and data. It is interesting to note that a study that started with the objective of exploring the use of digital technology quickly shifted its emphasis to an Integrated Digital Network (IDN) and within a period of four years even foresaw the emergence of ISDN. From then on, the progress towards ISDN has been steady and unrelenting. By mid 1970s, the concept of common channel signalling using digital technology emerged. Digital end equipments like digital telephone started appearing in the market. In 1984, a comprehensive and generic definition of ISDN was adopted as: An ISDN is a network, in general evolving from telephony IDN, that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which users have access by a limited set of standard multipurpose user-network interfaces.

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