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Integrated Services Digital Network (ISDN)

ISDN is a fully digital telecommunications system that transfers data at a rate of up to 128kbps, either as voice, data, fax or a combination thereof. It has been available for over a decade, but has only recently become generally available due to the capital cost of installation. Although it uses existing telephone wiring, ISDB requires both ends of the connection to be digital and thus requires the Internet Services Provider that also uses ISDN. To connect a PC to ISDN the system must have a hardware device called a Terminal Adapter (TA), which are either a adapter board or an external device connected to a serial port. This are often referred to as 'ISDN modems' although the phrase is somewhat incorrect as they do not engage in modulation and demodulation from analog to digital. ISDN is digital all the way! When choosing an ISDN TA, the internal versions are considered superior as often the sheer speed of ISDN exceeds the capacity of the serial port to send and receive data.

With a standard ISDN connection, bandwidth is demarcated into bearer channels (B channels) that run on 64kbps and a delta channel (D channel) that runs on either 16kbps or 64kpbs depending on the service. The B channels carries voice and data, whilst the D channel controls traffic. Further, There are two types of ISDN service - a Basic Rate Interface (BRI) and a Primary Rate Interface (PRI). The former is intended for private and home use and consists of two 64kbps B channels and one 16kbs D channel - a total of 144kbps. The latter is intended more for large business use such as PBX connections, which is either 23 B channels (United States and Japan) or 30 (Europe) and one 64 kbps D channel.

One would initially think that only having two B channels would seem rather limiting, but this isn't necessarily the case. A single BRI line can actually accommodate up to eight different ISDN devices, each assigned with their own number. The D channel provide routing services, allowing all devices to share the two channels. It is also possible to aggregate the bandwidth of multiple B channels using a protocol such as BONDING, developed by the Bandwidth on Demand Interoperability Group.

A variant of ISDN is Dynamic ISDN or Always On/Dynamic ISDN (AO/DI). Basically, it's a combination of circuit-switching that you get in a traditional telephone system with the packet-switching of computerized networks. Instead of using the full 64kbps of the B channels, the D channel is used instead for minor traffic and the high-speed connection is switched on when heavy traffic is required. In terms of cost and resource efficiency this may be the best kind of ISDN and has grown significantly in Asia and Europe.