integrated services digital network (ISDN)

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A rather dated communication technology standard for simultaneous high speed baseband transmission of voice and data (including video and fax) over ordinary (twisted pair) copper telephone lines. In most of the world it has been overtaken by data compression technologies such as ADSL. It is available in two main configurations: (1) Basic rate interface (BRI) configuration, comes with three digital channels comprising of two 64 Kbps bearer (B) channels and one 16 Kbps data (D) channel that provides control and signaling for the B channels. A merchant, for example, could run credit card authorization on the D channel, use one B channel for voice, and the other for fax; or combine two B channels for 128 Kbps connection to the internet without needing a modem; (2) Primary rate interface (PRI) configuration, comes with 23 B channels and one D channel.
The newer broadband version (B-ISDN) supports transmission rates of up to 1.5 Mbps but requires fiber-optic cables (see fiber-optic transmission). Ordinary telephone equipment cannot be used with any version of ISDN.

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