ISDN can be supplied by a carrier in two main forms: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). An ISDN BRI consists of two 64-kbps services (B channels) and one 16-kbps signaling channel (D channel). An ISDN PRI consists of 23 B or 30 B channels, depending on the country. In North America and Japan, a PRI service consists of 23 B channels. In Europe and Australia, a PRI service consists of 30 B channels. A signaling channel (or D channel) is used in a PRI service and is a dedicated 64-kbps channel. The B channel sends data and the D channel primarily controls signaling.
NOTE The effective throughput of a PRI service with 23 channels is 1.472 Mbps (23 x 64 kbps). With 30 B channels, the effective throughput is 1.920 Mbps (30 x 64 kbps). The International Telecommunications Union (ITU) defines the standards for ISDN. The specified standard is ITU-T Q.921.
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