ISDN

ISDN Abbrev. for integrated services digital network. ISDN has been developed, primarily by the PTTs, as a vehicle for the provision of a single service that carries all forms of digitally encoded traffic on a common platform. ISDN is in principle capable of carrying speech, data, and video traffic, and currently (Spring 1995) offers a range of data rates – from 64 Kbps up to 1.536 Mbps (in North America) or 1.920 Mbps (in Europe); these data rates will certainly increase with time. In general, ISDN services are presented as multiple 64 Kbps services rather than as a single higher-speed service. The service is engineered so that, for the lower data rates, existing speech-quality local loops can be used from customers' premises to an exchange supporting ISDN access. For higher speeds it is necessary to install higher-grade bearers, typically an optical fiber. The main service offering is for virtual circuits with fast call set up and clear down; the tariff structure is based on a standing charge plus a usage charge based on aggregated call duration. The presence of a call-duration component means that beyond a certain point users will find it more effective to use a permanent leased connection to a standard digital bearer.

Basic-rate ISDN (B/ISDN) is an internationally standardized service that provides digital access at 64 Kbps, referred to as a B-channel, and can carry one channel of digitized speech, or data. A D-channel operates at 16 Kbps and is intended primarily for data. A typical local loop can operate at 144 Kbps, and can support two B-channels and a D-channel.

Primary-rate ISDN (P/ISDN) is an internationally standardized service that provides digital access at 1.536 Mbps (1.920 Mbps in Europe), usually presented as 23 B-channels and one D-channel (30 B-channels and one D-channel in Europe). In addition some operators are now providing so-called N/ISDN, which offers multiple B-channels with several options on the number of channels in the range 1–30.

Broadband ISDN is offered in a variety of formats in different countries, with total bandwidths as high as those offering the service feel able to operate, up to tens or hundreds of Mbps.