Air Traffic Control Officer (Air Traffic Controller) as a function of aircraft altitude, speed and direction. The pilot must follow from flying up from the origin airport Or start entering the responsibilities (Entering the country) until landing at the destination airport. The responsibilities or be (Out of the country) safely. Planes in the air at all time. Can not float in the air brake. And the period just 1 minute, then ATC is not just a single control plane. For example, try to visualize it. Now has 20 aircraft and wanted to come off at the airport ATC to manage those machines respectively - the latter for the Tower is a space charge less. The average aircraft came down in the area of Tower will have about 3 minutes to decide everything, so be fast and safe.