PREFACE:
The Integrated Services Digital Network (ISDN) requires special switch equipment at the telephone switch. Only two wires (same as a regular analog telephone line) are required. A two wire ISDN line is also referred to as an ISDN Basic Rate Interface (BRI) line. ISDN requires special devices to connect to a regular analog telephone line called a Network Termination unit -1 (NT-1).
The NT-1 converts the two wire ISDN BRI line, also called the U Bus, into the signals needed by the digital ISDN adapter (typically 6 or 8 wires) and voice telephone. The 8 wire connection after the NT-1 conversion is called the S/T Bus.
An ISDN BRI line with the NT-1 equipment integrates three logical digital channels into two wires. These 'logical' digital channels are termed the D channel and two bearer 'B' channels called B1 and B2. Typically, the B1 channel provides for Circuit Switched Voice and Data (CSV/D), the B2 provides for data only. The D channel provides 16kps and is used for setting up calls and the use of the B channels. Each B channel carries 64kbps for digital data for a combined total of 128kbps.
ISDN will connect the B2 channel for additional data bandwidth as needed. This means that you can transfer data at 64kbps on the B1 channel AND use a voice telephone on the B2 channel at the same time all on one POTS copper pair.
There are two typical configurations of an ISDN BRI line (note: a D channel is always required). The first configuration is one B channel which will provide for either voice or 64kps data bandwidth. The second is BOTH B channels which will provide for either 128kbps data bandwidth or 64kps data bandwidth with simultaneous voice.
