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ISDN (Integrated Services Digital Network) is an extension of the public telephone network designed to carry digitized voice and data signals from one user to another. ISDN uses the copper twisted pair cable (the common telephone wiring).


An ISDN bandwidth is divided into three channels - 'Bearer Channel' ("B-Channel"), 'D-Channel' and 'H-Channel'. The B-Channel occupies a bandwidth of 64 Kbps - some switches might limit it to 56 Kbps. It is used for carrying voice calls, circuit-switched data or packet-switched data. On the other hand the D-Channel handles the signaling at 16 or 64 Kbps depending on the type of service required by the user. It also carries control/signaling information and customer information. The H-Channel is what is used in teleconferencing, high quality audio applications or any kind of high speed data transfer running at 384 Kbps to 2 Mbps.

There are two types of ISDN services. The basic service that meets the needs of users not requiring such a huge capacity requirement is the Basic Rate Interface (BRI). This consists of two B-Channels running at 64 Kbps and one D-Channel running at 16 Kbps. Then there is the Primary Rate Interface (PRI), which consists of 23-30 B-Channels running at 64 Kbps and one D-Channel at 64 Kbps. 

ISDN has enabled multiple devices to share a single line, so that it is no longer necessary to have separate lines for your telephone, fax and video conferencing. All of these applications can be share a single ISDN line without losing either quality or 

service. This change has been made possible by using switches that support digital connections
instead of the analog ones used earlier by telephone companies. 

B-ISDN or Broadband ISDN is the latest in ISDN which enables data to be transferred at speeds grater than 2 MBPS, by using a single optical fiber cable. ¨