Primary Rate Interface

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The access structure for the Primary Rate Interface (PRI) is a digital carrier system using time division multiplexing (TDM). In North America this system is the T-1 carrier system. The channel structure on the primary rate is variable. Depending on the tariff and the provisioning options, the PRI can contain B-channels, D-channels, and H-channels. In T-carrier systems the PRI supports 24 time slots, which are assigned on an as-needed basis for the various channel types. The most common configuration is a 23B+D configuration comprising 23 B-channels and one D-channel (for signaling). If multiple PRIs are deployed to the same device, the D-channel of one PRI can provide signaling support for the B-channels of another PRI. It is common to have at least 2 D-channels in such a group of PRI, but the remaining PRIs can be deployed as 24B comprising 24 B-channels and no D-channel.

The European PRI follows the Conference of European Postal & Telecommunications (CEPT) standard for Level 1 (E-1) digital time division multiplexing. The European Telecommunications Standards Institute (ETSI) defines a 32-time-slot digital carrier system in which time slot 0 is used for framing and synchronization, and time slot 16 is used for signaling. The other 30 time slots are used as traffic carrying channels.

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Primary Rate Interface Applications

Primary Rate Interfaces (PRI) are deployed for applications requiring either numerous B-channels, or higher bandwidth than just the two B-channels of a Basic Rate Interface (BRI). These applications, in addition to mainframe access, Internet access, and videoconferencing are supported on CPE that can terminate these higher speed interfaces.

Examples of such CPE include:

Private branch exchanges (PBX): A PRI allows flexible calling groups and calling line identification (CLID), coupled with high-speed call setup and clearing.