Integrated Services Digital Network (ISDN) is comprised of digital transmission and data-transport services offered by telephone carriers. ISDN involves the digitization of the telephone network, which permits voice, data, text, graphics, music, video, and other source material to be transmitted over existing telephone wires. Home and business users who install an ISDN adapter replacing a telephone modem receive surfing speed up to 128 Kbps compared with the maximum 56 Kbps rate of a modem connection.

ISDN requires adapters at both ends of the transmission so your access provider also needs an ISDN adapter. ISDN is generally available from your phone company in most urban areas. ISDN applications include high-speed image applications, additional telephone lines in homes to serve the telecommuting industry, high-speed file transfer, video conferencing and voice service. In many areas where DSL and broad band services are now offered, ISDN is no longer as popular an option as it was formerly.

There two types of services associated with ISDN service. These two levels of service are known as the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. BRI and PRI both interfaces include a number of B-channels and D-channels. Each D-channel carries control and signaling information and each B-channel carries data, voice, and other services.

ISDN BRI Service
The ISDN Basic Rate Interface (BRI) service offers two B channels and one D channel (2B+D). BRI B-channel service operates at 64 kbps and is meant to carry user data; BRI D-channel service operates at 16 kbps and is meant to carry control and signaling information, although it can support user data transmission under certain circumstances. Thus, a Basic Rate user can have more than 128 Kbps service.

ISDN PRI service
ISDN Primary Rate Interface (PRI) service offers 23 B channels and 1 D channel in North America and Japan, yielding a total bit rate of 1.544 Mbps. ISDN PRI in Europe, Australia, and other parts of the world provides 30 B channels plus one 64-kbps D channel and a total interface rate of 2.048 Mbps.

ISDN in concept is the integration of both analog or voice data together with digital data over the same network. The broadband ISDN (B-ISDN) is capable of providing all these integrated services (voice, data, video, etc.) through a single interface just like ISDN was supposed to but it will do it a lot faster than ISDN could. B-ISDN is intended to support transmission from 2 Mbps up to much higher, moving faster than ISDN, but it is still quite a ways off.

ISDN has been very slow in coming. In fact, many people consider them to be out of date already. ISDN is available now in many places, but it is not widely used. Further most of the products and services that people have forecast for ISDN are still not available. For this reason many people say that ISDN also stands for "It Still Does Nothing".

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