How Does E911 Work?

September 9, 2015

Todd Piett

Over the past few months, I’ve found myself describing how E9-1-1 wireless call routing works. I’ve struggled to find a simple way to describe it. Looking at the ENP study guide didn’t help simplify things any. After searching around, I found a nice description on an obscure page on the Sprint web site. I modified it (hopefully keeping it accurate) to include commonly used acronyms.

I hope this helps those of you who just need a simple overview. For those of you experts out there that have improvements (or corrections!), feel free to comment below. And thanks to Sprint for the great start!

user_places_call1. The user places a wireless call, which transmits the caller’s voice and his or her number to ”best” tower, which is often, but not necessarily, the closest tower (which can impact the routing of the call. See # 4, below).

msc2. The tower transmits the caller’s voice signal, the wireless phone’s callback number, and the tower’s ID code to a mobile switching center. The mobile switching center assigns a temporary unique 10-digit routing number to ID the call (pANI), and groups the information into different electronic packets sent to different parts of the 911 system.

PDE3. Notified of the emergency, the position-determining system uses one of a few different types of technology to find the caller’s location. Some technology is based on the location of the wireless tower or antenna orientation (known as “Phase I” location), other technologies such as GPS, A-GPS, or AFLT attempt to directly locate the wires caller’s phone (known as “Phase II” location). The pANI and associated latitude/longitude location data is sent to a 9-1-1 location information database (ALI Database). This process happens while the voice call is being handled by the local exchange carrier. Typically this initial location data is calculated via Phase I technologies, but under optimal conditions, a Phase II location may be provided.

LEC4. The local exchange carrier (LEC) receives the voice signal and the pANI. The local exchange carrier’s switch is notified of the emergency call, and the switch (a.k.a selective router) decides automatically, based on the caller location discovered through the pANI, which emergency call center is most appropriate and routes the call.

PSAP5. At the appropriate emergency call center, (known as a Public Safety Answering Point or PSAP), the call taker receives the 9-1-1 voice call from the LEC and the call taker’s equipment (CPE) receives the call’s pANI. Using the pANI, the CPE searches the ALI database and retrieves the caller’s wireless number (Call back number), the tower information, carrier ID, and any available Lat/Long for the caller’s location. The call taker can check for updates to the caller’s location (in hope of acquiring more accurate Phase II location data, or to determine if the caller has moved). This is referred to as the re-bid process (and may be either automated or manual). This re-bid provides any updated location information acquired by the Position Determining System since the call was routed and answered at the PSAP. This ALI data can also be made available to the Computer Aided Dispatch (CAD) system, via the CAD or ALI Spill.

responders6. The call is either handled by a telecommunicator who handles both the call taking and dispatching (blended role) or by a team of call taker and dispatcher who tag team the incident. In some cases, the call may be transferred from the primary answering PSAP to a secondary PSAP if the caller is determined to be located in a jurisdiction covered by a different PSAP, or if a secondary PSAP handles specific call types. The former is most common with wireless calls, and the latter with medical and fire emergencies.

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