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Wiktionary
primary rate interface

n. (context telecommunications English) an ISDN service delivered over T1 , E1 or J1 datalines, with delivery of connections between 1472 kilobits per second and 1984 kilobits per second (kibibits)

Wikipedia
Primary Rate Interface

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 offices. It is based on the T-carrier (T1) line in the US and Canada, and the E-carrier (E1) line in Europe. The T1 line consists of 24 channels, while an E1 has 32.

PRI provides a varying number of channels depending on the standards in the country of implementation. In North America and Japan it consists of 23xB ( B channels (bearer channels)) and 1xD ( D channel (data channel)) (23 64-kbit/s digital channels + 1 64-kbit/s signaling/control channel) on a T1 (1.544 Mbit/s). In Europe and Australia it is 30B + D on an E1 2.048 Mbit/s. One timeslot(0) on the E1 is used for synchronization purposes and is not considered to be a B or D channel. The D-channel typically uses timeslot 16 on an E1, vs timeslot 24 for a T1.