Each center has its own computer hardware and software. A network of data lines connects each center to its neighbors, as well as approach and tower facilities in the geographic region. Voice communications lines, commonly referred to as “land lines”, allow controllers to communicate with each other between facilities and within each facility. Controllers coordinate the movement of air traffic through these land lines, and many common coordination tasks are accomplished by communications between computers via the data lines.

In addition to the voice communications you hear on the frequency, air traffic controllers are also dealing with each other on these land lines. Should a controller become so busy with land lines that it becomes difficult to communicate with the pilots on the radio, another controller will be assigned to assist. The assisting controller is called a D-side and exclusively takes on the task of handling land line communications. This frees the R-side controller to deal with the airplanes. Just because a frequency is quiet, doesn’t mean there is nothing going on. Controllers have to communicate with each other very often. They might be communicating over a line and are often times simply shouting across the room at one another.

One such common task is a handoff. Each controller is designated a geographic area of jurisdiction. The controller cannot allow traffic under his control to enter another area of jurisdiction without permission from the receiving controller. A handoff is initiated from the transferring controller via computer. The receiving controller will then accept the handoff via computer, and the airplane’s radar data tag conveys the status of the handoff to both controllers. Once the receiving controller has accepted the radar handoff, the transferring controller will switch the flight over to the next controller's frequency. A handoff can also be made manually by voice communications between the two controllers.

Each facility is broken down into smaller areas of jurisdiction called sectors. Approach control facilities typically have arrival and departure sectors, while center sectors are delegated for a geographic area. Each sector has its own lateral and vertical limits. During slow times, sectors are combined so that one controller works multiple sectors.

Handoffs must be made vertically and laterally. This is why certain altitudes are used over and over in a specific airspace or by a specific facility. The controller is climbing traffic to the upper limit or descending traffic to the lower limit of the sector. A handoff is then initiated to the sector above or below. Quite often, communications is transferred to the receiving controller and the airplane is allowed to climb or descend into the next sector without actually having to level off. But, the transferring controller cannot issue an altitude in another controller’s airspace without permission.

The vertical and lateral limits of an approach control facility are normally fairly complicated, being custom tailored for each airspace area. Center sectors are divided vertically into three stratums, which are low, high, and super high. Low altitude sectors usually extend from FL230 down to the surface or upper limit of airspace delegated to approach or tower. High altitude sectors normally extend from FL240 up to somewhere between FL290 and FL360.