IT Technology: Basic Rate Interface ( BRI ) versus Primary Rate Interface ( PRI )

<< Basic Rate Interface >>
The entry level interface to ISDN is the Basic Rate Interface (BRI), a 144 kbit/s service delivered over a pair of standard telephone copper wires. The 144 kbit/s rate is broken down into two 64 kbit/s bearer channels ('B' channels) and one 16 kbit/s signalling channel ('D' channel).

BRI is sometimes referred to as 2B+D
The Interface specifies three different network interfaces:

• The U interface is a two-wire interface between the exchange and the Network Terminating Unit which is usually the demarcation point in non-North American networks. 
• The T interface is a serial interface between a computing device and a Terminal Adapter, which is the digital equivalent of a modem. 
• The S interface is a four-wire bus that ISDN consumer devices plug into; the S & T reference points are commonly implemented as a single interface labeled 'S/T' on an NT1 
• The R interface defines the point between a non-ISDN device and a terminal adapter (TA) which provides translation to and from such a device.

BRI-ISDN is very popular in Europe but is much less common in North America.

<<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."


Reference:
Integrated Services Digital Network
http://en.wikipedia.org/wiki/Integrated_Services_Digital_Network