Primary Rate Interface (PRI)

PRI has 24 64-kb channels in the US and Japan and 30 elsewhere in the world. PRI lines are similar to T-1 in that they both have 24 channels. However, PRI ISDN has out-of band signaling on the 24th channel. This is different from T-1 circuits, in which signaling is carried in-band, along with voice or data on each channel. On data communications, the signaling channel leaves each of the bearer channels "clear" capacity for all 64,000 bits. PRI does not require any bearer channel capacity for signaling such as call setup or teardown of signals.

PRI is used with PBXs, key systems and routers for incoming and outgoing voice and data. It is also used by ISPs (Internet Service Providers) and CLECs (Competitve Local Exchange Carrier) for dial-in modem Internet access. Each PRI supports 23 rack-mounted modems. The signaling channel carries the customer telephone number and the type of modem used. This provides billing and routing information. Moreover, the modems can handle ISDN as well as analog modem traffic.

BRI and PRI ISDN can communicate with each other for data such as videoconferencing and voice. PRI as will as BRI can make and receive voice calls to any device on the PSTN.