Chapter 1: ISDN: A Better Way Of Working

Primary Rate Interface

The Primary Rate Interface - the PRI - typically contains 23 64Kbps B channels, coupled to one 64Kbps D channel. Transmitted through a standard, dedicated North American 1.544Mbps (megabit per second) DS1 line or trunk, the PRI is also known as a 23B+D connection. PRIs link medium and large locations directly to a telephone central office ISDN switch.


PRIs add valuable new capabilities to a standard DS1 digital line. A key benefit is call-by-call service selection, with dynamic channel allocation. This means that a PRI lets a company flexibly allocate bandwidth as demands for that bandwidth change. A telecommunications or MIS manager can adjust the inbound and outbound calling flow on the PRI's 23 B channels to respond to specific time-of-day or day-of-week needs.

A group of six B channels, for example, could be combined - bonded - by customer equipment for a studio-quality, full-color, full-motion video conference, and afterward returned to the general pool of B-channels supporting internal and external voice and data communications. These same channels might be combined again after-hours into even larger channels for high-speed file transfers to locations around the nation and around the world.

Bandwidth control can either be preprogrammed into the central office digital switch serving a location, or can be allocated on demand through a growing range of telephone and computer systems designed for the purpose. These devices include PBXs, mainframes and minicomputers, LAN and WAN gateways, multiplexers, video units and a growing breed of ISDN bandwidth-on-demand controllers.