Enhanced 9-1-1

Enhanced 911 (E-911 or E911) is a system used in North America to automatically provide the caller's location to 911 dispatchers. 911 is the universal emergency telephone number in the region. In the European Union, a similar system exists known as E112 (where 112 is the emergency access number) and known as eCall when called by a vehicle. An incoming 911 call is routed to a Public Safety Answering Point (PSAP), which is a call center operated by the local government. At the PSAP, the call is answered by a specially trained official known as a 9-1-1 dispatcher. The dispatcher's computer receives information from the telephone company about the physical address (for landlines) or geographic coordinates (for wireless) of the caller. This information is used to dispatch police, fire, medical and other services as needed. Calls to 911 over the public switched telephone network (PSTN) are routed to a special router (known as Selective Router, or 9-1-1 Tandem). The router looks for the address associated with the caller's telephone number in a database. The caller's phone number is known as an ANI. The database relating ANIs to addresses is known as ALI (Automatic Location Identification). The router then uses the address to search in the MSAG for the Emergency Service Number (ESN) of the appropriate PSAP for that area and connects the call to it. The Master Street Address Guide (MSAG) is a database of street addresses and corresponding Emergency Service Numbers (ESNs). ESNs represent one or more emergency service agencies (e.g. fire department, law enforcement) designated to serve a specific range of addresses in a geographic area, called an Emergency Service Zone. Calls from cellular phones are received via cell towers by mobile switching centers (MSC). The switching center automatically assigns a unique identifier to each cellular 911 call, known as a "pseudo ANI". The Selective Router connects the call to a PSAP based on the cell tower's location.

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