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Narrow Band and Broadband ISDN

Published on Dec 12, 2015


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.


The circuit switching is the dominant technology for both voice and data communications. Communication via circuit switching implies that there is a dedicated communication path between two stations. That path is a connected sequence of links between network nodes. On each physical link, a channel is dedicated to the connection. The three phases involved in a communication via circuit switching are circuit establishment, information transfer and circuit disconnect.


In a typical data connection much of the time the line is idle. Thus circuit switched approach is inefficient. In packet switching data are transmitted in short packets. Each packet contains a portion of the user's data plus some control information. The control information, at a minimum, includes the information that the network requires to be able to route the packet through the network and deliver it to the intended destination. At each node enroute, the packet is received, stored briefly, and passed on the next node. The advantages of packet switching are line efficiency is greater, data rate conversion is possible and priorities can be used.


With modern, high-speed telecommunication systems, the overhead in error control is unnecessary and counter productive. To take advantages of the high data rates and low error rates of contemporary networking facilities, frame relay was developed. Whereas the original packet switching networks were designed with a data rate to the end user of about 64 kbps, Frame relay networks are designed to operate at user data rates of up to 2 Mbps. The key to achieving these high data rates are to strip out most of the overhead involved with error control.

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