The backward channel is usually used for transmission of request, supervisory, acknowledgement, or error-control signals.
The direction of flow of these signals is opposite to that in which user information is being transferred.
In data transmission, it is 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.
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.
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