911's database contains the physical location address of all registered telephone numbers in North America, and is coordinated with a Master Street Address Guide maintained by state and local governments contracting with private developers. Mobile telephone numbers are registered by the physical address given when registering for the service, which may be checked against the Geographic Information System location showing on the dispatcher's screen.
911's database of physical address locations enables First Responders to determine call locations in order to respond immediately, even if the caller cannot give accurate information. The public telephone system automatically routes land line 911 calls to nearby Public Safety Access Points staffed by dispatchers. Dispatchers receive a physical address through an Automatic Location Information system, validate the displayed location against the local MSAG for accuracy and relay the information to police, fire or emergency medical services.
911 also maintains a database of addresses registered to cellular phone owners. Cellular phone carriers require addresses when the cell phone owners register with the service and provide them to 911. Cellular phones automatically relay 911 calls to the nearest PSAP, as required by Federal Communications Commission law, and feature Geographic Positioning System hardware for tracking, via triangulation, but dispatchers may also check calls against registered addresses to determine the caller's identity.