Controllers certified to direct traffic in an area can work traffic in any one of its sectors. If traffic demand decreases—such as during nighttime hours—a controller may be assigned to handle traffic in contiguous sectors that are combined or the entire area may be combined into one large sector. As traffic demand increases, the sectors will be uncombined and additional controllers may be added to each sector. Repeatable traffic patterns due to scheduled commercial flights aid in the planning of such sector assignments on a daily and seasonal basis.
Each sector is typically positioned with one or two controllers. A radar controller, or “R-side” controller, is the lead, responsible for radio communications with aircraft, monitoring the radar screen to maintain safe separation, and communicating with other controllers. All open sectors are staffed with one R-side controller. When two controllers work a sector, the second is an associate controller, known as a data, or D-side, controller. The D-side controller typically receives flight-plan information and helps plan and organize the flow of traffic within the sector. In the absence of a D-side controller, the R-side controller must handle these D-side responsibilities along with R-side responsibilities. During exceptionally busy periods, a third controller (T-side) may be assigned to the team, although three-member teams are not typically planned for.