Primary Rate Interface (PRI)
The Primary Rate Interface (PRI) is a standardized telecommunications service level within the Integrated Services Digital Network (ISDN) specification for carrying multiple DS0 voice and data transmissions between a network and a user. The Primary Rate user is hooked up directly to the telephone company central office.
A PRI is typically used to establish communication between a PBX, or a private branch exchange, which is a telephone exchange operated by the customer of a telephone company, and a Central Office of the telephone company or an Inter Exchange Carrier or IXC, a long distance telephone company.
The advantage of primary rate interface or PRI is that the 23 or 30 B channels can be used in various combinations for specific data transmission needs, such as a video conferencing, allowing bulk data transfer to be achieved more flexibly.
PRI and BRI
The Integrated Services Digital Network (ISDN) prescribes two levels of service:
A PRI (Primary Rate Interface) line is a form of ISDN (Integrated Services Digital Network) line which is a telecommunication standard that enables traditional phone lines to carry voice, data and video traffic, among others. A PRI circuit consists of two pairs of copper lines terminating on a modem from a service provider premises to the customer premises. It uses multiplexing/de-multiplexing techniques to carry more than one channel in a single circuit.
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 (1536 Mbit/s T1, a.k.a. "23B + D") or 30 64-kbit/s B channels (2048 Mbit/s E1, a.k.a. "30B + D").
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. Normally, no more than 2 D-channels are provisioned as additional PRIs are added to the group.