In data transmission, a secondary channel in which the direction of transmission is constrained to be opposite to that of the primary, i.e., the forward (user-information) channel. Note: The direction of transmission in the backward channel is restricted by the control interchange circuit that controls the direction of transmission in the primary channel. 2. In a data circuit, the channel that passes data in a direction opposite to that of its associated forward channel. Note 1: The backward channel is usually used for transmission of supervisory, acknowledgement, or error-control signals. The direction of flow of these signals is opposite to that in which user information is being transferred. Note 2: The backward-channel bandwidth is usually less than that of the primary channel, i.e., the forward (user information) channel.