need to move within a large airport can be substantial. It is common for airports to provide moving walkways, buses, and rail transport systems. Some airports like Hartsfield–Jackson Atlanta International Airport and London Stansted Airport have a transit system that connects some of the gates to a main terminal. Airports with more than one terminal have a transit system to connect the terminals together, such as John F. Kennedy International Airport, Mexico City International Airport and London Gatwick Airport. Airport operations There are three types of surface that aircraft operate on: Runways, for takeoff and landing Taxiways, where planes "taxi" (transfer to and from a runway) Apron or ramp: a surface where planes are parked, loaded, unloaded or refuelled. Air traffic control Air traffic control (ATC) is the task of managing aircraft movements and making sure they are safe, orderly and expeditious. At the largest airports, air traffic control is a series of highly complex operations that requires managing frequent traffic that moves in all three dimensions. A "towered" or "controlled" airport has a control tower where the air traffic controllers are based. Pilots are required to maintain two-way radio communication with the controllers, and to acknowledge and comply with their instructions. A "non-towered" airport has no operating control tower and therefore two-way radio communications are not required, though it is good operating practice for pilots to transmit their intentions on the airport's common traffic advisory frequency (CTAF) for the benefit of other aircraft in the area. The CTAF may be a Universal Integrated Community (UNICOM), MULTICOM, Flight Service Station (FSS), or tower frequency. The majority of the world's airports are small facilities without a tower. Not all towered airports have 24/7 ATC operations. In those cases, non-towered procedures apply when the tower is not in use, such as at night. Non-towered airports come under area (en-route) control. Remote and virtual tower (RVT) is a system in which ATC is handled by controllers who are not present at the airport itself. Air traffic control responsibilities at airports are usually divided into at least two main areas: ground and tower, though a single controller may work both stations. The busiest airports may subdivide responsibilities further, with clearance delivery, apron control, and/or other specialized ATC stations. Ground control Ground control is responsible for directing all ground traffic in designated "movement areas", except the traffic on runways. This includes planes, baggage trains, snowplows,