B-channels are digital 64 kbps circuits intended to provide access to network services. An ISDN’s ability to provide a variety of services over a single access line is dependent on the B-channel’s ability to carry the various voice and data services. The B-channels provide a point-to-point connection between two users or a user and a network service. B-channels do not carry any signaling information and are therefore 64 kbps “clear” channels.
A rate of 64 kbps for digitized voice was “state of the art” when the initial work on ISDN standards began. Since then, alternate techniques allowing lower speed voice have been developed. ISDN CPE may support the further demultiplexing of B-channels into several lower speed, derived circuits, but this is transparent to the ISDN and the B-channel.
It should be noted that on the 1.544 and 2.048 Mbps interfaces, some ones density mechanism (i.e., B8ZS in T-carrier, HDB3 in CEPT) is required to allow the B-channel to operate at the full 64 kbps. If this requirement is not met, the B-channel will only support 56 kbps. On the low-speed interface (144 kbps), the Physical Layer ensures sufficient changes in state.
B-channels are time division multiplexed over the interface to the serving switch, where they may connect to a number of different subnetworks within the ISDN. These include the circuit switched voice network or a packet switched network.
B-channels only provide a dedicated path from the user to the LE, and onward to another user. Any protocol above the Physical Layer employed on this channel is an issue between the user and the end connection; the ISDN connection may be between two users or between a user and a packet switched network. In either case, the B-channels do not assure the compatibility or the reliability of the information transferred.