In the Integrated Services Digital Network (ISDN), the D-channel is the channel that carries control and signalling information. (The "D" stands for "delta" channel.) The B-channel ("B" for "bearer") carries the main data.
In ISDN, there are two levels of service: the Basic Rate Interface, intended for the home and small enterprise, and the Primary Rate Interface, for larger users. Both rates include a number of B- (bearer) channels and a D- (delta) channel. The B-channels carry data, voice, and other services. The D-channel carries control and signaling information.
The Basic Rate Interface consists of two 64 Kbps B-channels and one 16 Kbps D-channel. Thus, a Basic Rate Interface user can have up to 128 Kbps service. The Primary Rate Interface consists of 23 B-channels and one 64- Kpbs D channel in the United States or 30 B-channels and 1 D-channel in Europe.