ISDN Fundamentals

Integrated Services Digital Network is based on worldwide standards developed by CCITT (the Consultative Committee for International Telephone and Telegraph) and maintained by its even more ungainly-named successor, the International Telecommunications Union - Telecommunications Standardization Sector (ITU-TSS). It is designed specifically to bring worldwide standards to digital voice and data transmissions of individual users through what is now the almost totally digital global communications network.


Two Types of Channels

Through standard telephone lines, an ISDN-capable digital switch at a local telephone company office generates two different types of "electronic channels."

Some of these channels, B channels, are designed to carry a full range of circuit-switched voice, circuit-switched data, and packet-switched data transmissions. Others, the D channels, carry call-signaling and set-up information for the network, plus a range of lower-speed packet transmissions.

There are no protocol or other restrictions for B channels. They are defined as transparent, circuit-switched, 64Kbps connections capable of carrying digital voice or digital data transmissions. B channels can also be inverse multiplexed, or "bonded" by customer equipment (or the local NYNEX switch) into single n x 64Kbps data channels.

Call signaling and set-up information traveling on the D channel is also unique, designed to travel through a separate communications network dedicated to call set-up and control. This network is called Signaling System 7 (SS7).

The primary advantage of a separate, out-of-band signaling network is the speed with which calls can be connected - about 1-3 seconds from the last digit dialed to the first ring, compared to some 10-30 seconds without it.

In addition to transmitting call signaling and set-up information, the D channel can also be simultaneously interconnected to the worldwide X.25 packet network for user messaging, small file transfer, transaction processing, remote telemetry and a host of other applications.

Using combinations of these B and D channels, a digital switch at the user's central office generates two types of ISDN interfaces. These are called the Basic Rate Interface (BRI) and the Primary Rate Interface (PRI).