Comcast Fiber to the Node

Robert Ridge RRidge at
Tue Mar 9 11:59:39 MST 2010

There seems to be some confusion about how 9-1-1 calls work.  Here's how it works in Provo, which has E911 capabilities.
When a 9-1-1 call is dialed, the carrier delivers the call to the state's selective router which in Utah is managed by Qwest.  That system then forwards the call to the correct Public Safety Access Point (PSAP) for handling.  The caller ID is also sent.
If the call is from a traditional land line, the calling party's number is sent over a leased analog line at 9600 baud to Intrado which manages a database of all phone numbers and their addresses.  The address and subscribers name are returned to the PSAP and displayed to the dispatcher.
If the call is from a VoIP line, the process is similar.  The carrier sends the call to the state's selective router along with the phone number which has been registered with Intrado.
If the call is from a cell phone, the cell phone provider determines the location of the call, usually using the GPS capabilities of the phone, but sometimes using cell tower information, and determines the correct PSAP.  It then attaches a pseudo phone number to the call and send's it through to the state's router for delivery.  This number is called the Emergency Services Routing Key (ESRK).  The carrier must also send the correct location and subscriber information to Intrado, along with the ESRK.  When the PSAP gets the call, it sends the ESRK to Intrado, which then uses the ESRK to find the actual correct phone number of the cell phone, subscriber information, and the latitude and longitude of the phone.  The reason the carrier has to use the ESRK is so the state's selective router will recognize the ESRK as a phone number that belongs to a specific PSAP.  Otherwise, if I called 9-1-1 on my cell phone which is billed to my home in, say, Chicago, it would send my call to a PSAP in Chicago.  The state router must be able to recognize the ESRK as a valid phone number that belongs to some PSAP.  
Meanwhile, the cell carrier will continue to send updated location information to Intrado for the duration of the call.  The dispatcher can request a "re-bid", which will send the ESRK to Intrado again, and Intrado will send updated location information.  In this way, the dispatcher can actually "see" the cell phone's motion every few seconds.  (Some people who call 9-1-1 do not actually stop, but keep going.)
Robert Ridge
Director of Information Systems
Provo City
351 W Center
Provo, UT  84601
rridge at 

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