Technology Reference Guide

	Technology Reference Guide 7. Wide-Area Networks C. ISDN

What is it?
ISDN (Integrated Services Digital Network) is a relatively new carrier service. It is designed to carry voice, data, graphics and video. ISDN can be purchased as dial-up or dedicated circuit options. ISDN circuits can accommodate wide-area protocols such as X.25, PPP (point-to-point protocol) and others. ISDN was designed to eventually replace the analog phone network now in use, and provides service options for both large businesses, as well as small business and residential circuits.

What does it do?

ISDN offers two common types of service. The first type is called basic rate (BRI). BRI consists of two B channels (bearer) and one D channel. The B channels are 64 Kb/sec bandwidth and are designed to carry the data. The D channel is a 16 Kb/sec line used for signaling information. It can also be used for low-speed packet-switched data. This class of service was designed for residential and small business use.

The second class of service is termed primary rate (PRI) and consists of 23 B channels (64 Kb each) and a 16 Kb D channel. This was designed to service the needs of larger organizations with higher throughput requirements. The maximum throughput of this service is 1.5 Mb/sec, achieved by utilizing all the channels simultaneously.

The interface to an ISDN service is termed an NT1. This stands for Network Terminator Type 1, and performs functions similar to a modem. ISDN can provide a permanent virtual circuit connection, or a dial-up service. The channels can be assigned to different virtual circuits, or combined for greater bandwidth to one location.

How is it used?
ISDN is an emerging technology, which is only now becoming available. It offers the data capacity to handle LAN-to-LAN traffic, as well as high bandwidth applications such as video. It is also intended to provide digital voice transmission as a replacement for the current analog telephone network.

Where do I get more information?
Contact your local CBV Office.