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System that provides a callers location to emergency dispatchers

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).

Calls from cellular phones are received via cell towers by mobile switching centers (MSC). When the cellular phone detects that the user is placing an emergency call, it begins to transmit its location to a secure server, from which the PSAP can retrieve it. Cellphone manufacturers may program the phone to automatically enable GPS functionality (if disabled) when an emergency call is placed, so that it may transmit its location. This is permitted by special privacy legislation. Location information is not passed along by the public phone network; only the calling party's phone number is known to the receiver.

In 2009, the Canadian Radio television and Telecommunications Commission (CRTC) required implementation of Phase II Stage 1 for wireless carrier by 1 February 2010, in areas that provide landline E911. Many Canadians now have access to Phase II service.

The final destination of an E911 call (where the 911 operator sits) is a public safety answering point (PSAP). There may be multiple PSAPs within the same exchange or one PSAP may cover multiple exchanges. The territories (Emergency Service Zone) covered by a single PSAP is based on the dispatch and response arrangements for the fire, police, and medical services for a particular area. All primary PSAPs have a regional Emergency Service Number (ESN), a number identifying the PSAP. The Caller Location Information (CLI) provided is normally integrated into emergency dispatch center's computer assisted dispatch (CAD) system. Early CAD systems provided text display of the caller's address, call history and available emergency response resources. In 1994, working in cooperation with the emergency response agencies of Covington, KY, 911 Mapping Systems, Inc. founded in 1992 by Robert Graham Thomas Jr., implemented the first real time on screen E911 street map display to highlight the caller's position, nearest available emergency responders and other relevant information such as fire hydrants, hazardous materials and/or other data maintained by the city. Shortly thereafter, integrated mapping became a standard and integral part of all CAD systems and continues to evolve alongside 911 response technology. For Wireline E911, the location is an address. For Wireless E911, the location is a coordinate. Not all PSAPs have the Wireless and Wireline systems integrated.

Each telephone company (local exchange carrier, or LEC) has at least two redundant DS0 level (that is, 64 kbit/s, or voice quality) trunks connecting each host office telephone switch to each call center. These trunks are either directly connected to the call center or they are connected to a telephone company central switch that intelligently distributes calls to the PSAPs. These special switches are often known as 911 Selective Routers. Their use is becoming increasingly more common as it simplifies the interconnection between newer ISUP/SS7 based host office switches and the many older PSAP systems. If the PSAP receives calls from the telephone company on older analog trunks, they are usually Pulse driven circuits. These circuits are similar to traditional telephone lines, but are formatted to pass the calling party's number (Automatic Number Identification, ANI). For historical reasons, the PSAP will refer to these as CAMA circuits even though Centralized Automatic Message Accounting (CAMA) is actually a reference to the call log. If the PSAP receives calls on older style digital trunks, they are specially formatted Multi Frequency (MF) trunks that pass the calling party's number (ANI) only. Some of the upgraded PSAPs can receive calls on ISUP trunks controlled by the SS7 protocol. In that case, the calling party's number (ANI) is already present in the SS7 setup message. The Charge Number Parameter contains the ANI.

Multi line Telephone System (MLTS) pertains to the location of callers dialing 911 from within the private telecommunications networks used by large organizations. A Multi line Telephone System (MLTS), often referred to as a private branch exchange, is a telecommunications switching system used by large organizations to process calls between employees within the organization and with parties external to the organization. An MLTS may serve a single building, segments of multi tenant buildings, a group of buildings on a campus or even a number of buildings separated by geography. New communications technologies are making it possible for single MLTS systems to serve locations at far distant places that may span multiple governmental jurisdictions even distant countries. The challenge of Enhanced 911 for the MLTS is that information about the location of callers is only available to the extent that the private organization discloses the information. For the organization the challenges of collecting and reporting the information can be significant. Today’s highly mobile work forces and technologies that allow users to relocate without the intervention of an administrator place significant responsibilities on the MLTS owner or operator.

In addition to upgrading communications systems, some counties and communities in the United States have established ordinances (e. g. IRC section R319.1) requiring property owners to standardize the display of house numbers on buildings and along streets and roadways, to allow emergency personnel to more easily identify a given address day or night, even in poor weather. These are normally composed of reflective characters, at least 3 to 6 inches high, on a contrasting reflective background. It is necessary for the address number to be affixed to the building or to a separate structure such as a post, wall, fence, or mailbox, provided that such separate structure is located in front of the building and on the building's side of the street. Compliant signage systems are often advertised as being "E911 compliant".

The FBI CAST Cellular Analysis & Geo Location Field Resource Guide reveals that wireless carriers in the U. S. routinely activate E911 functionality on their customers' mobile phones for the purposes of tracking their locations without their permission, as of March 2019. Telecommunications companies then provide this location data to various government agencies that request it, making it clear that use of E911 functionality is not limited to emergencies and that the technology is often implemented in such a way that tracking can be remotely activated without the user's knowledge or consent.

After a national test of Wireless Emergency Alerts (live since 2012) on 3 October 2018, a number of rumours and false statements spread on social media. Among them a tweet by John McAfee that went viral, in which McAfee claimed that the "Predentials alert" involved the E911 system, and that smartphones have a "E911 chip" capable of giving the government access to the phone's location and microphone. The Electronic Frontier Foundation reported that there is "no such thing as an E911 chip".