The most important development in the computer communications industry in the 1990s is the evolution of the integrated services digital network (ISDN) and broadband ISDN (B-ISDN). The ISDN and B-ISDN have had a dramatic impact on the planning and deployment of intelligent digital networks providing integrated services for voice, data and video. Further, the work on the ISDN and B-ISDN standards has led to the development of two major new networking technologies; frame relay and asynchronous transfer mode (ATM). Frame relay and ATM have become the essential ingredients in developing high-speed networks for local, metropolitan and wider area applications.
The ISDN is intended to be a worldwide public telecommunications network to replace existing public telecommunication networks and deliver a wide variety of services. The ISDN is defined by the standardization of user interfaces and implemented as a set of digital switches and paths supporting a broad range of traffic types and providing value added processing services. In practice, there are multiple networks, implemented within national boundaries but from the users point of view, the eventual widespread deployment of ISDN will lead to a single, uniformly accessible, worldwide network.
The narrowband ISDN is based on the use of a 64 kbps channel as the basic unit of switching and has a circuit switching orientation. The major technical contribution of the narrowband ISDN effort has been
frame relay. The B-ISDN supports very high data rates (100s of Mbps) and has a packet switching orientation. The major technical contribution of the B-ISDN effort has been asynchronous transfer mode, also known as cell relay