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.
PRI is the standard for providing telecommunication services to offices. It is based on the T-carrier (T1) line in the US, and the E-carrier (E1) line in Europe. The T1 line consists of 24 channels, while an E1 has 32.
T1 and E1 systems
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) and 1xD (D 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 + 1xD on an E1 2.048 Mbps. One timeslot on the E1 is used for synchronization purposes and is not considered to be a B or D channel.
PRI and BRI
The Integrated Services Digital Network (ISDN) prescribes two levels of service, the Basic Rate Interface (BRI), intended for the homes and small enterprises, and the Primary Rate Interface (PRI), for larger applications. 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.
The Primary Rate Interface (PRI) consists of 23 B-channels and one 64-kbit/s D-channel using a T1 line (North American and Japanese standard) or 30 B-channels and one D-channel using an E1 line (Europe/rest of world). A T1 Primary Rate Interface user would have access to a 1.472-Mbit/s data service. An E1 Primary Rate Interface user would have access to a 1.920 Mbit/s data service.
Larger connections are possible using PRI pairing. A dual PRI could have 24+23= 47 B-channels and 1 D-channel 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.
Fewer active B channels (also called user channels) can be used for a fractional T1. More channels can be used with more T1s, within certain design limits.
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.
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