What's the difference between SIP trunking, PRI and BRI?

Years ago, Session Initiation Protocol trunking (SIP trunking) was quite popular as it offered substantial cost benefits compared to Primary Rate Interface (PRI) and Basic Rate Interface (BRI). But what are some of the technical differences among SIP trunking, PRI and BRI, and how do they apply in today's telecommunications networks?

As part of the Integrated Services Digital Network (ISDN), both PRI and BRI have essentially died off and were dead almost from the start. The ISDN backbone carried millions of bytes of information before SIP emerged as a successor. The flexibility of SIP also means business growth opportunities, features, presence and integration with new technology capabilities arise regularly.

When SIP was introduced, its main competitor was PRI, or basically a T1 circuit. PRI and BRI technologies combine circuits to transmit digital information. Both use Bearer (B) channels at 64 Kbps for voice and data as well as Delta (D) packetized channels at 16 Kbps for signaling, voice, control and data.

PRI channels can carry a combined 30 B channels and one D channel for E-circuits or 23 B and one D for T-channels, common in the U.S. Both were part of the ISDN backbones developed to help carriers transmit more data from site to site than analog dial-up predecessors.

In their day, T1 circuits were fast, state-of-the-art backbone interfaces. A bridge was added at the perimeter to convert the ISDN to analog and back again. These circuits transmitted a whopping 128 Kbps, which is nearly nothing by today's broadband standards.

But back in the '90s, when T1 gained traction, it was faster and far more reliable than the Plain Old Telephone Service analog systems that operated in baud rates. The development of ISDN was primarily for the carrier's benefit, but it gained commercial acceptance as the equipment became more affordable and the speed benefits became necessary.

The need for less complexity, more speed, lower cost and better performance shifted the telecom industry into the Ethernet world, and SIP became dominant. Similarly, ISDN took over analog in its first iterations, going onward and upward in speeds.

When SIP gained popularity, no one could rationally dismiss the cost benefits and savings of SIP trunks over anything that traditional carriers had to offer. SIP trunks can be implemented nearly anywhere. In fact, one might argue that without SIP trunks and advancements in bandwidth infrastructure over the years, the COVID-19 pandemic would have completely impaired the business world. SIP trunks have stood the test of time and are likely to remain dominant.

Carrie Goetz is principal and CTO of StrategITcom as well as a fractional CTO at several companies. She has nearly 40 years of global experience in designing, running and auditing data centers, IT departments and intelligent buildings.