Primary rate interface

In telecommunication, a primary rate interface (PRI) is an integrated services digital network (ISDN) interface standard (a) that is designated in North America as having a 23B+D channels, (b) in which all circuit-switched B channels operate at 64 kb/s, and (c) in which the D channel also operates at 64 kb/s.

Note: The PRI combination of channels results in a digital signal 1 (T1) interface at the network boundary.

Source: from Federal Standard 1037C