ISDN-BRI divides the telephone line into three digital channels: two bearer channels, commonly referred to as B channels, and one D channel. All three channels can be used simultaneously.
The B channels can be used to send voice, circuit-switched data, or packet-switched data at rates of 56 to 64 kilobits per second. The maximum available transfer rate is determined by your local telephone company. The B channels function independently of one another. They can be used simultaneously for separate calls or combined to provide a total bandwidth of 128 kilobits per second.
The D channel can transfer data at 16 kilobits per second and is used for signaling and control information sent between the computer and the telephone office. D channel signaling provides functions such as call setup, call monitoring, call termination, and enhanced telephony features.
When using ISDN-BRI, it is possible to perform several types of connections. For example, it is possible to have a voice conversation on one B channel, a circuit-switched data call on the second B channel, and a packet-switched data call on the D channel. The D channel can simultaneously complete all necessary signaling and call control functions.