ISDN provides end-to end digital connectivity among user devices of different types of functions. Devices and functions include telephones, computers, video signals, audio signals, telephone etc. ISDN integrates voice, video, audio and data transmission in a single communication link. ISDN works over copper wiring and a loop is designed to carry digital signals. A central office has to be equipped with ISDN capable peripherals. For the transmission of information, a subscriber loop composed of channels named D, B, H etc. is used. B channel is bearer channel, a basic user channel at 64 kbps. D channel is data channel which carries signaling information to control circuit switching calls at the user interface at 16 or 64 kbps. H channel is higher bit rate channel for multimedia applications. The two types of ISDN service interface are basic rate interface (BRI) and primary rate interface (PRI). BRI is designed for residential subscribers which operate over existing local loop wiring. PRI is designed for business subscribers and requires a wide band communication medium.
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