What is the Primary Rate Interface?

The Primary Rate Interface is the enterprise telecommunications service level of the Integrated Services Digital Network (ISDN). Also known by the acronym PRI, it utilizes T-Carrier (T1) or E-Carrier (E1) telephone data circuits. Each four-wire T1 circuit provides 24 digital data channels, while each E1 includes 32 channels. PRI is used by many large offices and call centers to provide voice and data services to their employees. It is also used by Internet Service Providers to connect their modem banks to the telephone network.

Before Voice Over Internet Protocol (VoIP) became widely available, PRI was usually the best phone solution for large corporations. Fully utilizing T1/E1 lines made Primary Rate Interface more cost-effective than using the same number of standard phone lines. ISDN also provided a two-channel Basic Rate Interface (BRI) for homes and small businesses. Once the higher-bandwidth Cable Internet and Digital Subscriber Line (DSL) services caught on, BRI was effectively abandoned.

In North America and Japan, Primary Rate Interface is provided through T1 lines in the telephone network. Each T1 includes 23 data channels at 64 Kilobits per second (Kbps), also known as Bearer (B) channels. Each of these can carry one "conversation" of either voice or data at once. The T1 line also includes one Delta (D) channel that is used for signaling and control of the other 23 channels.

Europe, Australia and most of the rest of the world implement Primary Rate Interface through E1 network lines. E1 includes a synchronization channel which T1 does not have, as well as 30 B channels and one D channel. E1 operates at a faster rate overall of 2.048 Megabits per second (Mbps) compared to T1's 1.544 Mbps. The B channel data rate for both E1 and T1 is still 64 Kbps, however.

Groups of T1 or E1 lines can be combined if needed to provide larger Primary Rate Interface connections. For example, two E1 lines can be paired, providing 60 B channels and two D channels at 64 Kbps. The second D channel in this case would be used as a backup, rather than an active control channel. Two D channels are usually all that are needed, even if more than two T1 or E1 lines are combined.

T1 or E1 lines can also be combined into a Primary Rate Interface High-Speed (H) channel, also known as multi-rate ISDN. This is a collection of B channels bonded together to support applications which need a data rate faster than 64 Kbps. For example, a video teleconference or a high-quality audio transmission may need much more bandwidth than the PRI standard.