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(Primary Rate Interface) An ISDN service that provides 23 64 Kbps B (Bearer) channels and one 64 Kbps D (Data) channel (23B+D), which is equivalent to the 24 channels of a T1 line. The advantage of the D channel is that it sends control signals that can dynamically allocate any number of B channels for different applications. For example, one channel can be used for voice, while another can be used for data, while six more can be used for a videoconferencing channel and so on. PRI lines typically use four wire pairs. PRI lines are often designated as PRI/T1 or T1/PRI lines, but they are dial-up PRI lines, not T1 lines, which are point-to-point. See ISDN.