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Integrated Services Digital Network (ISDN) is essentially digital telephone service. ISDN envisions telephone companies providing "Integrated Services," capable of handling voice, data, and video over the same circuits. The core of the telephone network is now digital, so most ordinary telephone calls are now converted into bits and bytes, transported through digital circuits, and converted back into analog audio at the remote end. The international standard for the digital telephone network is Signaling System 7 (SS-7), a protocol suite in its own right, roughly comparable to TCP/IP. End users never see SS-7, since it is only used between telephone switches. ISDN provides a fully digital user interface to the SS-7 network, capable of transporting either voice or data. BISDN (Broadband ISDN) uses ATM instead of SS-7 as the underlying networking technology.

ISDN is a complete networking technology in its own right, providing clearly defined Physical, Data Link, Network and Presentation layer protocols. For most Internet applications, though, ISDN is regarded as a fancy Data Link protocol used to transport IP packets.

An ISDN interface is time division multiplexed into channels. In accordance with SS-7 convention, control and data signals are seperated onto different channels. Contrast this to TCP/IP, where control packets are largely regarded as special cases of data packets and are transported over the same channel. In ISDN, the D channel is used for control, and the B channels are for data. B channels are always bi-directional 64 kbps, the standard data rate for transporting a single audio conversation; D channels vary in size.

The two primary varients of ISDN are BRI (Basic Rate Interface) and PRI (Primary Rate Interface). BRI, sometimes referred to as 2B+D, provides two 64 kbps B channels and a 16 kbps D channel over a single 192 kbps circuit (the remaining bandwidth is used for framing). BRI is the ISDN equivalent of a single phone line, though it can handle two calls simultaneously over its two B channels. PRI, essentially ISDN over T1, is referred to as 23B+D and provides 23 B channels and a 64 kbps D channel. PRI is intended for use by an Internet Service Provider, for example, multiplexing almost two dozen calls over a single pair of wires.

A number of international standards define ISDN. I.430 describes the Physical layer and part of the Data Link layer for BRI. Q.921 documents the Data Link protocol used over the D channel. Q.931, one of the most important ISO standards, documents the Network layer user-to-network interface, providing call setup and breakdown, channel allocation, and a variety of optional services. Varients of Q.931 are used in both ATM and voice-over-IP. G.711 documents the standard 64 kbps audio encoding used by telcos throughout the world.

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