ISDN: Basics

For a long time the telephone system has been the primary communication infrastructure. But it was designed to transmit analog voice data und therefore inadequate for other (digital) needs of data transmission, for example fax or video. The major goal of the new Integrated Services Digital Network (ISDN) is the integration of voice and non-voice services via a full digital network, which would replace the telephone system (and therefore is a point-to-point Wide Area Network) step by step. The idea is not to offer the highest possible speed, but a universal connectivity. If higher capacity for one application is needed, several independent connections can be combinded to create a single, high-speed end-to-end connection.
A posssible ISDN configuration may look like this:

The basic idea behind ISDN is the digital bit pipe, an abstract pipe between customer and carrier through which bit flows. It does not matter what kind of source produced the bits. In terms of the OSI model, ISDN provides a physical layer service onto which layers 2 to 7 can be built. ISDN uses the out-of-band signaling concept, which is quite different from how Local Area Networks work. They use the same cable with interleaved actual data and control data. ISDN transmitts actual data and control data in two different ways (normally, on so called B- and D-channels) at data rates up to 2 Million bits per second (most common are two B- and one D-channel, which is 144 Thousand bits per second).

A comprehensive summary of ISDN is available. A special ISDN page contains links to many sources about ISDN, in particular to lists of ISDN products.