ISDN

Definition: Integrated Services Digital Network: digital communications service provided by telephone companies designed to carry digital data as well as capable of carrying analogue transmissions such as voice over a wide area network. * The service consists of two kinds of channels: (i) B (bearer) channels which carry up to 64Kbs of data on full-duplex which are circuit-switched (i.e. connected by phone service to one destination) (ii) D (delta or data) channels which carry up to 16Kbs of data on full-duplex which are packet-switched (i.e. can go to different destinations at once). B channels carry data or digitised voice, D channels carry voice or ancillary signals and codes. * There are three basic forms: (i) BRI (Basic Rate Interface) which offers two B channels and one D channel and (ii) PRI (Primary Rate Interface) offering up to 30 B channels, according to country, and one D channel; and (iii) B-ISDN: broad-band ISDN, capable of exceeding rates of 2Gbs (giga-bits per second). * May also be known as Integraded Services Distributed Network.

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