The other ISDN service available, along side BRI, 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.