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A back-channel in a telecommunications system typically is a low-speed or less-than-optimal transmission channel opposite to the main channel's direction. The system is asymmetric in that the communication is not the same speed or capacity in both directions - the back-channel being the subservient or lower speed/capacity direction.

In many cases the back-channel is used mostly for acknowlegementss of the validity of the forward-channel's data (i.e. that the forward-channel's data passes validity tests of some sort.)


ADSL where "A" stands for "asymmetric", and the channel from the subsciber to the supplier, is slower and may be of less capacity than the channel from the supplier (ISP) to the subscriber - the channel from the subscriber to the supplier is the back-channel.