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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.