Enhanced 911

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As Enhanced 911 or E911 into the is US law by the mandate E911 prescribed service called, the fully automatic transmission of geographical location information of the caller when you initiate an emergency call requires.

Basically, one makes use of an inverse number search, so that on the basis of the network operator necessarily transmitted telephone number identifier by transmitting the stored number to the address or the geographical position data in wireless telephony. The rescue control center that receives the emergency call is therefore not dependent on the verbal transmission of the location by the often panicked or injured caller.

Enhanced 911 is available in most metropolitan areas in the United States and Canada . In some places (e.g. Minneapolis / St. Paul ) it has been in use since the early 1980s.

In North America, the system only works when the emergency number 911 is dialed. Calls to other numbers, even if these are called emergency telephone numbers , may not be localized.

Outside the United States, this process is referred to caller location (to German caller location ), the implementation of the structure and the switching manner of emergency calls locally varied.

Public Safety Answering Point (PSAP)

Target of E911 emergency call is a Public Safety Answering Point (PSAP, dt. Mutatis mutandis control center ). A local exchange can be served by multiple PSAPs. Several exchanges can also be assigned to one PSAP. The catchment area of ​​a PSAP depends more on historical, administrative and organizational conditions than on technical requirements. Most PSAPs are provided with a so-called ESN , an individual identification number.

The location information of the caller is usually displayed to the dispatcher in the rescue control center on a map by means of computer systems. The closest emergency services are also visualized on that map. In the case of wired emergency calls, the address stored with the network operator is displayed; in the case of wireless emergency calls (e.g. from cell phones), geographic coordinates are transmitted. Not all PSAPs have the full functionality of the system.

Enhanced 911 for wired telephony

Based on the E911 mandate , US network operators are obliged to transmit a caller ID to the control centers. Using the caller ID, each emergency call is assigned an address and the responsible control center via the ESN . This assignment is made using a database function called ALI (Automatic Line Identification) . In principle, the maintenance of the ALI database is delegated to a private operator named ILEC (Incumbent Local Exchange Carrier) . Since no specifications are standardized for the ALI database, different standards exist for the ILECs . The ALI database is supplemented in many places by an MSAG database (Master Street Address Guide) , which contains street names, house number ranges and other relevant address elements. Operators of local exchanges are connected via at least two DS0-level dedicated lines with a PSAP or special emergency call switching equipment, which are also referred to as E911 routers , as they are connected to the PSAPs via ISUP lines using the SS7 protocol. This solution is becoming increasingly widespread as it facilitates the connection between ISUP / SS7-based exchanges and the many proprietary PSAPs .

Analog connected PSAPs usually use CAMA connections. These are similar to conventional POTS connections, but have been expanded to include the ability to transmit the ANI , a special caller ID.

PSAPs that use an older digital connection use specialized MF connections that transmit the ANI at Wink Start during the call setup.

Modernized PSAPs are connected via ISUP / SS7 lines. The ANI is already transmitted in the SS7 setup message .

Data from the ALI database is not transmitted over the telephone lines, but over a separate infrastructure.

An ALI failure is the case in which an ANI is not transmitted or the ALI database does not have a corresponding entry. If necessary, the emergency call is then routed to a PSAP whose ESN has been stored for the respective network area. Then the dispatcher in the PSAP has to ask the emergency caller about his location and, if necessary, put him through to another PSAP that is responsible. The network operators are then obliged to correct any errors in the ALI database immediately.

The (often competing) local network operators ( Competitive Local Exchange Carriers , CLEC for short ) negotiate the interconnection to the ALI database with the local ILEC and complete the ALI database on the basis of the MSAG .