Also known as 23B plus D T1. A digital transmission system that clusters 24 trunk lines into a single two-way phone/data line from the CO to the destination. A T1-PRI line is made up of 1 D-channel and 23 B-channels. The D-channel carries the call information for all channels and the actual calls are carried on the B-channels. Therefore, one T1-PRI line can simultaneously transmit 23 calls.