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Primary Rate Interface: (PRI) is a standardized telecommunications service level within the Integrated Services Digital Network (ISDN) specification for carrying multiple DS0 voice and data transmissions between a 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 the E-carrier (E1) line in Europe. The T1 line consists of 24 channels, while an E1 has 32.

The Integrated Services Digital Network (ISDN) prescribes two levels of service, the Basic Rate Interface (BRI), intended for the homes and small enterprises, and the Primary Rate Interface (PRI), for larger applications. Both rates include a number of B-channels and a D-channel. Each B-channel carries data, voice, and other services. The D-channel carries control and signaling information. The Basic Rate Interface consists of two 64-kbit/s B-channels and one 16-kbit/s D-channel.

The Primary Rate Interface (PRI) consists of 23 B-channels and one 64-kbit/s D-channel using a T1 line (North American and Japanese standard) or 30 B-channels and one D-channel using an E1 line (Europe/rest of world). A T1 Primary Rate Interface user would have access to a 1.472-Mbit/s data service. An E1 Primary Rate Interface user would have access to a 1.920 Mbit/s data service.

Larger connections are possible using PRI pairing. A dual PRI could have 24+23= 47 B-channels and 1 D-channel but more commonly has 46 B-channels and 2 D-channels thus providing a backup signaling channel. The concept applies to E1s as well and both can include more than 2 PRIs. Normally, no more than 2 D-channels are provisioned as additional PRIs are added to the group.