Overview of E-9-1-1

I was recently asked to provide a parlor brief on Wireless E-9-1-1.

Wireless E-9-1-1 allows a handset to be located.

There are three phases of E-9-1-1:

  • Phase 0 provides the call-back number only, no location information.
  • Phase I provides the location of the cellular tower from which the 911 call was made.
  • Phase II provides the most accurate location, often within a few meters, depending upon the location technology deployed by the service provider. If for some reason a phase II “fix” is not possible during a given call, the system should fallback to Phase I location.

Location accuracy depends on which phase your service provider and the local Public Safety Access Provider (PSAP) have implemented. If phase II has been implemented the accuracy also will depend upon the wireless network technology (such as GSM or CDMA) and on the location technology used by the wireless service provider (such as TDOA, E-TDOA, NAGPS, AOA, etc.  Wikipedia has a good article.) PSAP generally are police departments, fire departments, and other emergency services.

When the PSAP and the service provider each deploy E-9-1-1 the lattitude and longitude of the cellular towers is configured in PSAP location software and equipment. The PSAP will add street address information, and usually both are overlaid on a map of the covered area.

All of the PSAP in a given area agree where the boundary between each PSAP should be drawn on the map. This information is shared with the service provider, or, more commonly, is shared with a wireless location service provider such as TCS or Intrado, who provides location services under contract to the wireless service provider.

At the time an E-9-1-1 call is made, the wireless service provider’s switch (the switch has many names: MSC, MTSO, MTX, Central Office Exchange, there are others) signals the wireless location service provider using SS7 (Signaling System 7, which someone with a poor grip on reality called “The Intelligent Network”.) This signaling provides a way for equipment at the wireless location service provider to determine the location of the caller, which is then compared to the previously agreed map of PSAP boundaries.

When the proper or most likely PSAP is determined then the wireless location service provider signals the switch to route the voice portion of the call to that PSAP, where a call taker will answer and the location of the caller is displayed on the map, and along with other information about the call.

There is much more. But this is just a high-level overview.

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