In telecommunication a data link is the means of connecting one location to another for the purpose of transmitting and receiving data. It can also be an assembly, consisting of parts of two data terminal equipments (DTEs) and the interconnecting data circuit, that is controlled by a link protocol enabling data to be transferred from a data source to a data sink. In aviation, a data link system (known as Controller Pilot Data Link Communications) is used to send information between aircraft and air traffic control when an aircraft is too far from the ATC to make voice radio communication and radar possible. Such systems are used for aircraft crossing the Atlantic and Pacific oceans. One such system, used by NavCanada and NATS over the north atlantic uses a 5 digit data link sequence number which is confirmed between air traffic control and the pilots of the aircraft before the aircraft proceeds to cross the ocean. This system uses the aircraft's flight management computer to send location, speed and altitude information about the aircraft to the ATC. ATC can then send messages to the aircraft regarding any necessary change of course. In unmanned aircraft or vehicles, the data link is used to send command and control (C2) signals and receive telemetry.
