Definition 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. CIRCUIT SWITCHING 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.
PACKET SWITCHING
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.
FRAME RELAY 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.
