Editing Enhanced 911 (section)

==Public-safety answering point (PSAP)==
{{Main|Public safety answering point}}
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 [[telephone exchange|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 (logistics)|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.<ref>{{cite web|url=http://www.911mapping.com/web/index.html|title=911 Mapping Systems, Inc|publisher=911mapping.com|archive-url=https://web.archive.org/web/20060323213607/http://www.911mapping.com/web/index.html|archive-date=2006-03-23|url-status=dead|access-date=2010-11-18}}</ref> founded in 1992 by Robert Graham Thomas Jr.,<ref>{{cite web|date=2002-12-25|title=Obituary: Robert Thomas Jr., 911 Mapping CEO|url=http://www.enquirer.com/editions/2002/12/25/loc_otherobit25.html|url-status=dead|archive-url=https://archive.today/20120729203007/http://www.enquirer.com/editions/2002/12/25/loc_otherobit25.html|archive-date=2012-07-29|access-date=2010-11-18|publisher=Enquirer.com}}</ref> 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.

===Interconnection details===
Each telephone company ([[local exchange carrier]], or LEC) has at least two redundant [[Digital Signal 0|DS0]]-level (that is, 64&nbsp;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 dialing|Pulse driven]] circuits. These circuits are similar to [[Plain old telephone service|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 [[ISDN User Part|ISUP]] trunks controlled by the [[Signalling System No. 7|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.