Uplink

An uplink (UL or U/L) is the portion of a communications link used for the transmission of signals from an Earth terminal to a satellite or to an airborne platform. An uplink is the inverse of a downlink. An uplink or downlink is distinguished from reverse link or forward link (see below).

• Pertaining to data transmission from a data station to the headend.
• Pertaining to GSM and cellular networks, the radio uplink is the transmission path from the Mobile Station (Cell Phone) to a Base Station (Cell Site). Traffic and signalling flows within the BSS and NSS may also be identified as uplink and downlink.

Downlink

A downlink (DL) is the link from a satellite to a ground station.

• Pertaining to cellular networks, the radio downlink is the transmission path from a Base Transceiver Station (Cell Site) to the Mobile Station (Cell Phone). Traffic and signalling flows within the BSS and NSS may also be identified as uplink and downlink.

Forward link

A forward link is the link from a fixed location (e.g., a base station) to a mobile user. If the link includes a communications relay satellite, the forward link will consist of both an uplink (base station to satellite) and a downlink (satellite to mobile user).

Reverse link

The reverse link (sometimes called a return link) is the link from a mobile user to a fixed base station.

If the link includes a communications relay satellite, the reverse link will consist of both an uplink (mobile station to satellite) and a downlink (satellite to base station) which together constitute a half hop.

Sources

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