February 5, 2014

Location is an important concept in the way that the Enhanced 9-1-1 system works. Location determination depends upon the Automatic Location Information (ALI) database which is maintained on behalf of local governments by contracted private third parties generally the Incumbent Local Exchange Carrier (ILEC). The ALI database is used to both route the call to the appropriate PSAP and when the call arrives, the ALI database is used to determine the location of the caller.

The 911 system
The 9-1-1 call arrives at the appropriate PSAP after it is routed across the Public switched telephone network (PSTN) to a special facility called a Class 4 telephone switch or Tandem Office. The Tandem Office uses a technology referred to as Selective Routing to query the SRDB Selective Routing Database and MSAG (Master Street Address Guide) using the originating telephone number as a search key to match the origination of the call to the network location or Emergency Service Number (ESN) of the appropriate PSAP. When the call is delivered to the PSAP, a second query is made to the Automatic Location Information (ALI), again using the originating telephone number as a search key. 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.
There is a difference between the ways location is determined for different types of calls based upon the type of originating device or network. For each of these categories please see the sections below for the appropriate categories to learn more about location determination for each of the following types of calls. There are hardwired or Wireline enhanced 911 calls which originate from a device connected to a known fixed point of connection to the PSTN. Wireless enhanced 911 are calls that originate for mobile devices such as cellular telephones. VoIP enhanced 911 pertains to communications originating from various commercial services provided by companies that send telephone calls across the commercial internet using specialized devices and software applications. Multi-line Telephone System (MLTS) pertains to the location of callers dialing 9-1-1 from within the private telecommunications networks utilized enterprise style organizations. via Wikipedia

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