Editing Enhanced 911 (section)

==Location transmission==
Calls made to non-911 emergency numbers (such as the direct line to a police or fire department) might not have automatic location enabled.

Calls to 911 are answered by an operator at a PSAP. In addition to the voice transmission, the telephone network also transmits a number associated with the current call, the ANI. The 911 operator (or their computer) at the PSAP searches a database (ALI) for the ANI to find the caller's location.

The ALI record associated with the query is then returned to the PSAP, where the [[Customer-premises equipment]] (CPE) correlates that information with the call taker receiving the call, and displays the information on their computer screen.

Automatic location of the emergency is intended to be faster and more reliable than verbal communication of the location, though this is usually requested anyway for confirmation. It also makes it possible for emergency services to respond when callers cannot communicate their location, because they do not know their location, they are a child, or they are too panicked, too distracted by the ongoing emergency, or do not wish to attract the attention of the perpetrator of a crime in progress.

===Landline transmission===
For landline calls the ANI resembles the caller's phone number. The ALI stores a pre-determined address associated with the caller's telephone number. This address is typically the phone's billing address.<ref name=":2" />

===Wireless transmission===
In parallel to the actual voice call, the ALI database gets periodically updated with more precise and recent location information.<ref name=":3" /> Cellular networks can determine a more precise location of the caller's device by using triangulation from the cell towers ([[radiolocation]]). In addition to triangulation, a second source of location information may be the caller's phone itself (or other cellular device).

Many phones manufactured after 2005 have [[Global Positioning System|GPS]] receivers built in.<ref>{{Cite news|url=https://electronics.howstuffworks.com/gps-phone2.htm|title=How GPS Phones Work|date=2005-10-24|work=HowStuffWorks|access-date=2018-10-05|language=en}}</ref> 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 enable GPS function automatically (in case it has been turned off) when the user places an emergency call.<ref name=":4" />

For wireless calls, the ANI (or "pseudo-ANI") is a unique number assigned to each individual 911 call, assigned at a mobile switching center.<ref name=":3" />

===ALI database===
[[File:9-1-1 System.svg|thumb|500px|The 911 system|alt=The 911 system]]The ALI database is secured and separate from the public phone network by design. 

The ALI is maintained on behalf of local governments by contracted private third parties, generally the [[incumbent local exchange carrier]] (ILEC). Often, the contracted 3rd party further subcontracts the actual ALI database management to companies such as Intrado, [[Bandwidth (company)|Bandwidth]] and TeleCommunication Systems, Inc. The ALI database also feeds the Master Street Address Guide database which is used to route the call to the appropriate PSAP and when the call arrives, the ALI database is queried to determine the location of the caller.

Each ILEC has its own standard for the formatting of the database.

Most ALI databases have a companion database known as the MSAG, Master Street Address Guide. The MSAG describes the exact spelling of streets, street number ranges, and other address elements. When a new account is created, the address is located in the Master Street Address Guide to track the proper Emergency Service Number (ESN) that 911 calls from that phone number should be routed to. [[Competitive local exchange carrier]]s (CLEC) and other competing wireline carriers negotiate for access to the ALI database in their respective Interconnect Agreement with the ILEC. They populate the database using the ILEC MSAG as a guide.

If the phone number is not in the ALI database, this is known as ''ALI Failure''; the call is then passed to default ESN for the call's [[trunk line]] group, which is a PSAP designated for this function. The 911 operator must then ask the incoming caller for their location and redirect them to the correct PSAP. The legal penalty in most states for ALI database lookup failure is limited to a requirement that the telephone company fix the database entry.