Mobile phone coverage areas indicate the geographical region where a particular carrier's phones may be used. To provide service, a carrier must either install its own towers in an area or make an agreement with another carrier to transmit its signals for subscribers.
Carriers typically provide maps detailing their approximate coverage areas so potential customers can judge whether or not service is available and reliable in their area. These maps are an approximation, however, and actual signal coverage depends on geographic features and other sources of interference such as TV or radio towers.
In some cases, carriers may offer multiple tiers of service, with speeds decreasing the further a subscriber travels from prime coverage areas.