Primary Rate Interface ( PRI )

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Primary Rate Interface
The other ISDN service available is the Primary Rate Interface (PRI) which is carried over an E1 (2048 kbit/s) in most parts of the world. An E1 is 30 ‘B’ channels of 64 kbit/s, one ‘D’ channel of 64 kbit/s and a timing and alarm channel of 64 kbit/s. In North America PRI service is delivered on one or more T1s (sometimes referred to as 23B+D) of 1544 kbit/s (24 channels). A T1 has 23 ‘B’ channels and 1 ‘D’ channel for signalling (Japan uses a circuit called a J1, which is similar to a T1).
In North America, NFAS allows two or more PRIs to be controlled by a single D channel, and is sometimes called “23B+D + n*24B”. D-channel backup allows you to have a second D channel in case the primary fails. One popular use of NFAS is on a T3.
PRI-ISDN is popular throughout the world, especially for connection of PSTN circuits to PBXs.
Even though many network professionals use the term “ISDN” to refer to the lower-bandwidth BRI circuit, in North America by far the majority of ISDN services are in fact PRI circuits serving PBXs.
Data Channel
The bearer channel (B) is a standard 64 kbit/s voice channel of 8 bits sampled at 8 kHz with G.711 encoding. B-Channels can also be used to carry data, since they are nothing more than digital channels.
Each one of these channels is known as a DS0.Most B channels can carry a 64 kbit/s signal, but some were limited to 56K because they traveled over RBS lines. This was more of a problem in the past, and is not commonly encountered nowadays.

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