The Primary Rate Interface (PRI) is a telecommunications interface standard used on an Integrated Services Digital Network (ISDN) for carrying multiple DS0 voice and data transmissions between the network and a user.

​

PRI is the standard for providing telecommunication services to enterprises and offices. It can be based on T-carrier (T1) or E-carrier (E1) transmission. The T1 line consists of 23 bearer (B) channels and one data (D) channel for control purposes, using a total bandwidth of 24x64-kbit/s or 1.544 Mbit/s. The E1 carrier provides 30 B- and one D-channel, using total bandwidth of 2.048 Mbit/s.  In other words, T1 supports 23 concurrent calls while E1 supports 30 concurrent calls.

 

The T1/E1 circuit, though physically look like a RJ45 connector, needs a special telephonny card for connection.