ISDN was developed, primarily by the PTTs, as a vehicle for the provision of a single service that carries all forms of digitally encoded traffic—speech, data, and video—on a common platform. 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 fibre. 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.

ISDN has been superseded by ADSL for Internet access, but its high quality of service makes it popular for verbal communication. It remains a core technology in the telephone network.