An ISDN "bearer" channel.

Back haul is a verb. A communications channel is back hauling when it takes traffic beyond its destination and back. One reason to do this is economics. It's cheaper to send calls that way. Another reason is to accommodate changes in your calling or staffing patterns. You may have an ACD in Omaha and one in Chicago. A call from New York may come into your Omaha ACD, but when it gets there you may discover that there are no agents available to handle the call. So it may now make sense to back haul the call to the Chicago ACD, where an agent is available.

The card inside the ACD on which you attach other cards. It is exactly the same as the motherboard in a PC. In fact, increasingly an ACD is in fact a souped-up server PC, as often as not running UNIX or NT. In that case, the backplane is the motherboard.

A signal sent in the direction from the called to the calling station, or from the original communications sink to the original communications source. The backward signal is usually sent in the backward channel and consists of supervisory, acknowledgement or error control elements.

(1) Back in the old days, real WATS (Wide Area Telephone Service) had real bands. Today most services offer virtual bands on virtual WATS. Real WATS split the US into six bands...