The Primary Rate Interface (PRI) is a telecommunications interface standard used on an Integrated Services Digital Network (ISDN) for carrying multiple DS0 voice and data transmissions between the network and a user.
PRI is the standard for providing telecommunication services to offices. It is based on the T-carrier (T1) line in the US and Canada, and the E-carrier (E1) line in Europe. The T1 line consists of 24 channels, while an E1 has 32.
PRI provides a varying number of channels depending on the standards in the country of implementation. In North America and Japan it consists of 23xB (B channels (bearer channels)) and 1xD (D channel (delta channel)) (23 64-kbit/s digital channels + 1 64-kbit/s signaling/control channel) on a T1 (1.544 Mbit/s). In Europe and Australia it is 30xB + 2xD on an E1 2.048 Mbit/s. One timeslot on the E1 is used for synchronization purposes and is not considered to be a B or D channel.
Fewer active B channels (also called bearer channels) can be used for a fractional T1. Bearer channels may also be known as user channels. More channels can be used with more T1s, within certain design limits.
PRI and BRI
The Integrated Services Digital Network (ISDN) prescribes two levels of service:
- BRI, the Basic Rate Interface: two 64-kbit/s B channels and one 16-kbit/s D channel, intended for the homes and small enterprises
- PRI, the Primary Rate Interface for large organisations, with one 64-kbit/s D channel and 23 (1.536 Mbit/s T1, a.k.a. "23B + D") or 30 ,64-kbit/s B channels (2.048 Mbit/s E1, a.k.a. "30B + 2D").
Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. Larger connections are possible using PRI pairing. A dual T1-PRI could have 24 + 23 = 47 B-channels and 1 D-channel (often called "47B + D"), but more commonly has 46 B-channels and 2 D-channels thus providing a backup signaling channel. The concept applies to E1s as well and both can include more than 2 PRIs. When configuring multiple T1's as ISDN-PRI's, it's possible to use NFAS (non-facility associated signalling) to enable one or two D-channels to support additional B-channels on separate T1 circuits.
The Primary Rate Interface channels are typically used by medium to large enterprises with digital PBXs to provide them digital access to the Public Switched Telephone Network (PSTN). The 23 (or 30) B-channels can be used flexibly and reassigned when necessary to meet special needs such as video conferences. The Primary Rate user is hooked up directly to the telephone company central office.
PRI channels and direct inward dialing are also common as a means of delivering inbound calls to voice over IP gateways from the PSTN.