The B channel carries ISDN Bearer Services across the network and so carries the content of call (the voice, fax or data) between users.

The B channel is a neutral conduit for bits and carries data at 64 000 bits per second (56 000 bits per second in some North American networks).

The ISDN does not need to know what the bits represent. The job of the network is to accept a stream of bits supplied by one user at one end of the B channel and to deliver them to the other user at the opposite end of the channel.

Within an interface, the B channels are numbered. In a Basic Rate Interface they are numbered 1 & 2; in a Primary Rate Interface, they are numbered 1 to 30 (or 23 in North America). When two users are connected, there is no relationship between the channel numbers used at each end. You might have one user's B channel number 17 connected with the other user's B channel number 2. The ISDN is responsible for managing this relationship.

Notice that channel number 17 would only be possible on a PRI, while channel number 2 is possible on both a BRI and a PRI. ISDN does not restrict the interconnection of B channels between the two kinds of interface.