Version 6.6 339 Mediant 2000
User's Manual 26. Configuring Supplementary Services
26.5.2.1 About E9-1-1 Services
E9-1-1 is a national emergency service for many countries, enabling E9-1-1 operators to
automatically identify the geographical location and phone number of a 911 caller. In E9-1-
1, the 911 caller is routed to the nearest E9-1-1 operator, termed public safety answering
point (PSAP) based on the location of the caller. Automatic identification of the caller's
location and phone number reduces the time spent on requesting this information from the
911 caller. Therefore, the E9-1-1 service enables the PSAP to quickly dispatch the relevant
emergency services (for example, fire department or police) to the caller's location. Even if
the call prematurely disconnects, the operator has sufficient information to call back the
911 caller.
The figure below illustrates the routing of an E9-1-1 call to the PSAP:
Figure 26-2: Call Flow of E9-1-1 to PSTN-Based Emergency Services Provider
1. The VoIP user dials 9-1-1.
2. The call is eventually sent to the PSTN through a PSTN Gateway.
3. The PSTN identifies the call is an emergency call and sends it to an E9-1-1 Selective
Router in the Emergency Services provider's network.
4. The E9-1-1 Selective Router determines the geographical location of the caller by
requesting this information from an Automatic Location Identification (ALI) database
based on the phone number or Automatic Number Identifier (ANI) of the 911 caller.
Exact location information is also supplied by the Master Street Address Guide
(MSAG) database, which is a companion database to the ALI database. Phone
companies and public safety agencies collaborate beforehand to create master maps
that match phone numbers, addresses and cross streets to their corresponding PSAP.
This MSAG is the official record of valid streets (with exact spelling), street number
ranges, and other address elements with which the service providers are required to
update their ALI databases.
5. The E9-1-1 Selective Router sends the call to the appropriate PSAP based on the
retrieved location information from the ALI.
6. The PSAP operator dispatches the relevant emergency services to the E9-1-1 caller.
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