Primary rate interface
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The primary rate interface (PRI) is a telecommunications standard for carrying multiple DS0 voice and data transmissions between two physical locations.
All data and voice channels are ISDN and operate at 64 kbit/s.
North America and Japan use a T1 of 23 B channels and one D channel which corresponds to a T1 line. Europe, Australia and most of the rest of the world use the slightly higher capacity E1, which is composed of 30 B channels and one D channel.
Fewer active B channels (also called user channels) can be used for a fractional T1. More channels can be used with more T1's, or with a fractional or full T3 or E3.
See T-carrier for DS0, T1, E1, T3, E3 and other carrier communications terms.
In the Integrated Services Digital Network (ISDN), there are two levels of service: the Basic Rate Interface (BRI), intended for the home and small enterprise, and the Primary Rate Interface (PRI), for larger users. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. The Basic Rate Interface consists of two 64 kbit/s B-channels and one 16 kbit/s D-channel. Thus, a Basic Rate Interface user can have up to 128 kbit/s service. The Primary Rate Interface consists of 23 B-channels and one 64 Kbps D-channel using a T-1 line or 30 B-channels and 1 D-channel using an E1 line. Thus, a Primary Rate Interface user on a T-1 line can have up to 1.544 Mbit/s service or up to 2.048 Mbit/s service on an E1 line. PRI uses the Q.931 protocol over the D-channel. The B and D channel sizes are dependent on the Framing format of the T-1 circuit. ie. Super Frame(SF) is a 56kb channel and the Extended Super Frame (ESF) is a 64kb channel.
The Primary Rate Interface channels are carried on a T-carrier system line (in the U.S., Canada, and Japan) or an E-carrier line (in other countries) and are typically used by medium to large enterprises. The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as videoconferences. The Primary Rate user is hooked up directly to the telephone company central office.
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Sources: From Federal Standard 1037C
