Primary Rate Interface (PRI) is an ISDN service that uses T1 or E1 circuits to provide voice and data services to large offices and call centers. PRI is cheaper than using standard telephone lines and is provided over T1 lines in North America and Japan, and E1 lines in Europe, Australia, and much of the rest of the world. T1 and E1 lines can be combined to provide larger connections, and a High-Speed channel can be used for applications that require more bandwidth.
The Primary Rate Interface is the enterprise telecommunications service layer of the Integrated Services Digital Network (ISDN). Also known by the acronym PRI, it uses T-Carrier (T1) or E-Carrier (E1) telephone data circuits. Each four-wire T1 circuit provides 24 channels of digital data, 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 telephone solution for large companies. Full use of T1/E1 lines made the primary rate interface cheaper than using the same number of standard telephone lines. ISDN also provided a two-channel Basic Rate Interface (BRI) for homes and small businesses. Once higher bandwidth cable and Digital Subscriber Line (DSL) Internet services took hold, BRI was effectively abandoned.
In North America and Japan, Primary Rate Interface is provided over the T1 lines of 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 a “conversation” of voice or data simultaneously. The T1 line also includes a Delta (D) channel which is used for signaling and control of the other 23 channels.
Europe, Australia and much of the rest of the world implement Primary Rate Interface across E1 network lines. E1 includes a sync channel that T1 does not have, as well as 30 B channels and one D channel. E1 operates at a faster overall speed of 2.048 Megabits per second (Mbps) compared to T1.544’s 1 Mbps. However, the B-channel data rate for both E1 and T1 is 64 Kbps.
T1 or E1 trunk groups can be combined if needed to provide larger Primary Rate Interface connections. For example, two E1 lines could be coupled, 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 sufficient, 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. It is a collection of B-channels linked together to support applications that require data rates in excess of 64 Kbps. For example, a video teleconference or high-quality audio transmission may require much more bandwidth than standard PRI.
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