ISDN is an abbreviation for Integrated Services Digital Network. It is a a high-speed communications network developed to provide all-digital service over the existing telephone network. ISDN was designed to carry not only voice but data, images, and video. ISDN is available in two forms, known as BRI (Basic Rate Interface) and PRI (Primary Rate Interface). BRI consists of two B (bearer) channels that carry data at 64 Kbps and one D (data) channel that carries control and signal information at 16 Kbps. In North America and Japan, PRI consists of 23 B channels and 1 D channel, all operating at 64 Kbps; all elsewhere, PRI consists of 30 B channels and 1 D channel.
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