ISDN -- Integrated Services Distributed Network. ISDN is a technology that uses existing or upgraded telephone wiring and local office switching equipment to provide 144 Kbps communication. ISDN is an international standard for digital communications. A Basic Rate Interface consists of two 64 Kbps (sometimes 56K) Bearer (B) Channels and a 16 Kbps Data (D) Channel. Some providers will split the B channels to deliver 64(/56)K to two different users. There is also provision for a 1.536 Mbps Primary Rate Interface consisting of 23 B channels and a 64K D channel. In addition to higher speeds, ISDN offers quick connect to destination users.

A BRI is interfaced to the user through an NT1 Network Terminator that splits off the three channels and also provides power to any ISDN phones connected to the NT1. Existing non-ISDN ready equipment also requires a Terminal Adapter to convert the signals at an RS-232, RJ-11 or whatever connector to/from an ISDN B channel signal.

Technically, ISDN is accessed by an Service Profiler ID Number (SPID) rather than the familiar directory number. The two may (or may not) look the same to a user. ISDN availability is restricted by distance from the telephone company switch and may require new local loop (user site to telco switch) wiring. In some cases a Loop Extension may be available at extra charge to bring service to more distant customers. Pricing and availability of ISDN service vary widely between different service providers. Several Regional providers in the US are pushing ISDN aggressively, while others seem to be doing everything possible to discourage its use. ISDN is reported to be widely available (and less overpriced compared to alternatives) in Europe.

If multiple devices are used on an ISDN line, they are distinguished by Terminal Endpoint Identifiers (TEIs). TEIs are not (usually) the same as SPIDs, are dynamically assigned, and are generally invisible to the user.

The telephone company can connect ISDN calls, on a call by call basis either to the analog Public Switched Telephone Network or to the digital version of the PSTN.

Return To Index Copyright 1994-2002 by Donald Kenney.