Figure 2-10. Intermediate altitude sectors.  Figure 2-8. Fort Worth Air Route Traffic Control Center.  Figure 2-11. High altitude sectors.  Figure 2-12. Ultra high altitude sectors.  Figure 2-9. Low altitude sectors.  2-5  The ARTCC at Fort Worth, Texas is subdivided into sectors  that are categorized as follows:  The airspace controlled by a Center may be further  administratively subdivided into smaller, manageable  pieces of airspace called sectors. A few sectors extend from  the ground up, but most areas are stratified into various  levels to accommodate a wide variety of traffic. Each sector  is staffed by a set of controllers and has a unique radio  frequency that the controller uses to communicate with  the pilots. As aircraft transition from one sector to another,  they are instructed to change to the radio frequency used  by the next sector. Each sector also has secure landline  communications with adjacent sectors, approach controls,  areas, ARTCCs, flight service centers, and military aviation  control facilities.  Figure 2-8 shows the Fort Worth, Texas Air Route Traffic  Control Center (ZFW) and the geographical area that it  covers. The Center has approximately 350 controllers. Most  are certified and some are in on-the-job training.  Sectors 