Reverse channel

In communications systems that use star topologies, the return channel (also reverse channel, back channel or return link) is the transmission link from a user terminal to the central hub.

Return links are often, but not always, slower than the corresponding forward links. Examples where this is true include ADSL (where the "A" stands for "asymmetric"), cable modems, cellular Internet access facilities (e.g. 3G) and satellite internet access (e.g. ASTRA2Connect).

The return channel need not use the same medium as the main channel. For example, some "hybrid" Internet access services use a one-way cable television system for the forward channel and a dial-up modem and telephone line for the return channel.

Even when the return and forward channels use the same medium, their differences often dictate the use of very different data modulation and coding techniques. For example, in a star radio network, only the central hub transmits on the forward link, so multiple access contention is a consideration only on the return link.

The "forward/return" terminology is also used for spacecraft command and telemetry links. Because the return link carries telemetry, often including imagery, it is often orders of magnitude faster than the forward link that transmits only a few predefined spacecraft commands.

"Return" and "forward" links are distinct from, and should not be confused with, uplinks and downlinks in satellite communication systems. For example, satellite Internet access with conventional bent pipe spacecraft transponders requires a total of two uplinks and two downlinks. One uplink and downlink pair are used for the forward link from the central ground hub through the satellite to the user terminal, and another uplink/downlink pair are used for the return link from the user terminal to the central hub.

See also

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