ISDN (Integrated Services Digital Network)

ISDN (Integrated Services Digital Network) is a set of CCITT/ITU standards for circuit-switched transmission of data over various media, including ordinary telephone-grade copper wire. 

There are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates divide their capacity across a number of channels:

  • B-channels carry payloads (e.g., data or voice streams)
  • D-channels carry control and signaling information.

A BRI connection consists of two 64 Kbps B-channels and one 16 Kbps D-channel. Thus, a BRI delivers up to 128 Kbps of data.

In the United States, a PRI connection consists of 23 B-channels (1,472 Kbps total) and one 64 Kbps D-channel. In Europe a PRI connection consists of 30 B-channels (1,920 Kbps total) and 1 D-channel.

ISDN was slow to achieve standardization and was rapidly overtaken and surpassed in both speeds possible and breadth of deployment by packet-switched technologies.

Broadband ISDN (BISDN) extends the integration throughout the rest of an end-to-end path at higher data rates -- for example, using fiber optic or radio media. ISDN can combine both analog/voice data and digital data on the same network link. Most video conferencing services used in the 1990s and early 2000s were delivered primarily via ISDN services.

ISDN can deliver speeds up to 128 Kbps over home phone lines. Where it is still available for residential users, it -- like acoustic modems --  typically serves as a last-resort technique for low-end data connectivity, used only when DSL and cable modem services are not available. 

This was last updated in March 2016

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