Readback : A procedure whereby the receiving station repeats a received message or an appropriate part thereof back to the transmitting station so as to obtain confirmation of correct reception.
Aeronautical telecommunication agency: An agency responsible for operating a station or stations in the aeronautical telecommunication service.
Secondary frequency: The radiotelephony frequency assigned to an aircraft as a second choice for air-ground communication in a radiotelephony network.
Controller-pilot data link communications (CPDLC): A means of communication between controller and pilot, using data link for ATC communications.