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.

Integrated Services Digital Network (ISDN) High-speed telephone lines that carry digital information. The various grades of ISDN can carry information more than a thousand times faster than conventional analogue voice lines. ISDN lines connect directly to a computer and do not need a modem.

ISDN See Integrated Services Digital Network

integrated services digital network See ISDN.

ISDN Computing, Telecom. Integrated Services Digital Network