T1 and E1 Systems
PRI provides a varying number of channels depending on the standards in the country of implementation. In North America and Japan it consists of 23xB (B channels (bearer channels)) and 1xD (D channel (data channel)) (23 64-kbit/s digital channels + 1 64-kbit/s signaling/control channel) on a T1 (1.544 Mbit/s). In Europe and Australia it is 30xB + 1xD on an E1 2.048 Mbit/s. One timeslot on the E1 is used for synchronization purposes and is not considered to be a B or D channel.
Read more about this topic: Primary Rate Interface
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