It can also refer to a set of electronics assemblies, consisting of a transmitter and a receiver (two pieces of data terminal equipment) and the interconnecting data telecommunication circuit.
There are at least three types of basic data-link configurations that can be conceived of and used: In civil aviation, a data-link system (known as Controller Pilot Data Link Communications) is used to send information between aircraft and air traffic controllers for example when an aircraft is too far from the ATC to make voice radio communication and radar observations possible.
Such systems are used for aircraft crossing the Atlantic, Pacific and Indian oceans.
One such system, used by Nav Canada and NATS over the North Atlantic, uses a five-digit data link sequence number confirmed between air traffic control and the pilots of the aircraft before the aircraft proceeds to cross the ocean.
In unmanned aircraft, land vehicles, boats, and spacecraft, a two-way (full-duplex or half-duplex) data-link is used to send control signals, and to receive telemetry.