ISDN is the use of digital connectivity to facilitate the provision of integrated access to a wide range of high speed, high bandwidth services with minimal interfaces. Using a standard interface, ISDN provides high-speed simultaneous digital transmission of voice, data and video with universal connectivity to public telephone network. Each ISDN line is made up of separate 64-Kbps 'channels' for sending and receiving calls, plus a channel that is used primarily for signaling.

Standard ISDN Basic Rate Interface (BRI) lines have 2 channels, while more powerful ISDN Primary Rate Interface (PRI) lines have 23 channels. Each channel can be used separately for any communications task, including voice calls, faxes and data transmission. The channels can also be combined for information intensive applications such as video conferencing.

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