The CPDLC is a FANS application that consists of transmitting datalink formatted messages between the pilot (the aircraft) and the ATC controller or the Company (the ground). A message sent by the aircraft to the ground is called a downlink message, the opposite is called an uplink message. Datalink messages may concern various requests such as route modifications, speed changes, voice contact requests (ex: transfer of ATC center), emergency messages (MAYDAY), crossing constraints, pre-departure clearances, ATIS, oceanic clearances, etc.