A PRI (Primary Rate Interface) is 1 of the 2 flavors of ISDN. PRIs, like the BRI (Basic Rate Interface), consist of D and B — or Bearer — channels (BRIs contain 2 B channels for carrying voice, video and data). While PRIs also contain just a single D channel for signaling, it has 23 B channels to be used for voice, data and video transmission. The PRI is based on T-carrier transmission, which is used in the U.S., Japan and Canada—we’ve all heard of T-1s, right? T-1s provide bandwidth totaling 1.544 Mbps (in Europe and Australia, PRIs are based on E-carrier, which totals 2.048 Mbps. Apparently, they want to one-up us. As a result, their PRIs contain 30 B channels.
Alright, let’s do the quick math—each channel, both the single D and the 23 Bs of a PRI, is 64 kbps. If you multiply 64 by 24 (total number of B and D channels), you get 1.536. You may be wondering why that doesn’t add up to 1.544, the bandwidth of a T-1. Well, that additional 8 kbps is used as a framing bit that is added to each group of 24 channels in a T-1. Maybe we’ll get to framing bits down the road.