Air Traffic Control (ATC) Function

Air traffic control is a service provided by ground-based controllers who direct aircraft on the ground and through controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. (Wikipedia)

Example that can we see is in KLIA and KLIA 2. It has two (2) control tower to manage the aircraft within for take-off and landing. The runways and buildings cover an area of 100 square kilometres. With its 75 ramp stands, it is capable of handling 120 aircraft movements at a time.