Controller Pilot Data Link Communications (CPDLC) is a protocol for pilots and controllers to communicate with each other via digital means. Think of it as pilots and controllers communicating via email.

The standard method of communication between an air traffic controller and a pilot is voice radio, using either VHF bands for line-of-sight communication or HF bands for long-distance communication (such as that provided by Shanwick Oceanic Control).

One of the major problems with voice radio communications used in this manner is that all pilots being handled by a particular controller are tuned to the same frequency. As the number of flights air traffic controllers must handle is steadily increasing (for instance, Shanwick handled 414,570 flights in 2007, an increase of 5% - or 22,000 flights - from 2006, the number of pilots tuned to a particular station also increases. This increases the chances that one pilot will accidentally override another, thus requiring the transmission to be repeated. In addition, each exchange between a controller and pilot requires a certain amount of time to complete; eventually, as the number of flights being controlled reaches a saturation point, the controller will not be able to handle any further aircraft.

The CPDLC application provides air-ground data communication for the ATC service. This includes a set of clearance/information/request message elements which correspond to voice phraseology employed by air traffic control procedures. The controller is provided with the capability to issue level assignments, crossing constraints, lateral deviations, route changes and clearances, speed assignments, radio frequency assignments, and various requests for information. The pilot is provided with the capability to respond to messages, to request clearances and information, to report information, and to declare/rescind an emergency.

The sequence of messages between the controller and a pilot relating to a particular transaction (for example request and receipt of a clearance) is termed a ‘dialogue’. There can be several sequences of messages in the dialogue, each of which is closed by means of appropriate messages, usually of acknowledgement or acceptance. Closure of the dialogue does not necessarily terminate the link, since there can be several dialogues between controller and pilot while an aircraft transits the air traffic service unit (ATSU) airspace.

https://youtu.be/PweVyYqleF8