Basic Rate Interface

The Basic Rate Interface - the BRI - consists of two 64Kbps B channels, and one 16Kbps D channel. Often called 2B+D connections, they link the end user directly to either a telephone company switch, or to a private branch exchange (PBX) or other ISDN call controller, which is itself linked to the telephone company's central office.

The Basic Rate Interface represents ISDN in its simplest form - a dialed, high-speed digital connection capable of carrying virtually any type of voice, data, video, image, sound or other transmission.

It should be noted that in some areas of the country where Signaling System 7 is not yet fully operational, D channel signaling information is carried in-band, that is within the main flow of voice or data. The result is that B channels are reduced to a speed of 56Kbps on calls from one telephone company central office to another. Nonetheless, D-channel interconnections to the X.25 packet network are still possible.