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 user's 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.