PRI is also called T1 PRI because in the USA, Canada and Japan it is provisioned on a T1 carrier trunk. But, wait. Isn't there also a service called T1? Yes, indeed. PRI might be considered just another flavor of T1 line service. Here's how they work.
The classic T1 transmission format carries 24 digitized telephone conversations. That's called channelized T1. You get 24 channels of 8 bits each per T1 line. But, you also need signaling for each phone call. To accomplish this, one bit from each channel is "robbed" for this purpose, leaving 7 for the actual call. That's called robbed bit signaling, in-band signaling or channel associated signaling.
Another way to format the T1 line is use the PRI scheme. In this case, you get 23 channels per line for phone or modem calls with the 24th being used for signaling to support the other 23. With PRI, each voice or "bearer" channel keeps all 8 bits for the digitized call. This separate data channel, called out of band signaling or clear channel signaling, has a lot more capability than the robbed bit scheme. With PRI you can get Caller ID or ANI automatic number identification. The out of band signaling arrangement also results in faster call setups and tear downs, which is important to call centers and ISPs. PRI channels can be configured as inbound only, outbound only or two-way.
So, how do you choose between channelized T1 and PRI? It depends on your application and the capability of your equipment. If having caller information is important for computer telephone integration (CTI), ISP billing, or displaying caller ID on desk phones, you'll want PRI service. If those aren't important features and you'd prefer to have all 24 channels available for phone calls, or if you have a PBX system that doesn't support PRI, then channelized T1 service may be your better choice.
