Tuesday, April 17, 2007

Integrated Services Digital Network (ISDN)

ISDN has been designed to replace the standard telephone system and provide greater numbers of digital services to telephone customers, such as digital audio, interactive information services, fax, e-mail, and digital video. ISDN uses asynchronous transfer mode which can handle data transmission in both connection-oriented and packet schemes. As with regular telephone lines, the user must pay a fee for use of the line. Basic rate ISDN or BRI offers two simultaneous 64 kb/s data channels as well as a 16 kb/s carrier channel for signaling and control information. The combined data rate, 128 kb/s, allows for videoconferencing capabilities. Multiple ISDN-B connections further increase the data rate and the transmission quality. Primary rate ISDN (PRI) offers 30 channels (of 64 kb/s each), giving a total of 1920 kb/s. As with BRI, each channel can be connected to a different destination, or they can be combined to give a larger bandwidth. These channels, known as “bearer” or “B” channels, give ISDN tremendous flexibility.

The original version of ISDN employs baseband transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mb/s. B-ISDN requires fiber optic cables and is not yet widely available.

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