T1PRI

T1PRI, or Primary Rate Interface on a T1 circuit, is the North American ISDN PRI implementation that provides multiple voice channels over a single DS1/T1 line. A T1 carries 24 time slots of 64 kbps each, totaling 1.544 Mbps. In T1PRI, 23 of the time slots (B channels) carry user traffic, while the 24th slot (D channel) carries signaling and control information used to set up and tear down calls.

The 23 B channels are dedicated for voice or data, allowing up to 23 simultaneous conversations on a single T1 connection. The D channel, at 64 kbps, supports ISDN signaling (for call setup, teardown, and feature control) and, in North America, is associated with Q.931-style call signaling within the ISDN framework. This arrangement provides predictable, circuit-switched performance with relatively stable latency and jitter suitable for traditional telephony and certain data applications.

T1PRI circuits are commonly provisioned to connect a private branch exchange (PBX) or unified communications gateway to the public switched telephone network (PSTN). They can include features such as caller ID, call forwarding, and toll-free service, depending on the carrier and service plan. Management typically includes circuit provisioning, performance monitoring, and failover considerations for business continuity.

For context, PRI functionality exists elsewhere as E1PRI in many regions, which uses an E1 line with 32 timeslots (30 B channels, 1 D channel, and framing). E1PRI supports up to 30 simultaneous calls, reflecting regional differences in ISDN PRI implementations. SIP trunking has emerged as a flexible alternative to fixed T1PRI for many organizations.