911 and Mobile Phones

In my previous post I talk about how e911 services can use ANI and ALI information to know where you’re calling from when you’re on an analog circuit. Now let’s consider what happens when you call from your mobile phone instead of an analog landline.

Mobile Challenges

When you call 911 from a mobile phone, several challenges arise when trying to determine your location. Since you could be just about anywhere, it cannot be assumed that you are at your home or “billing” address. Instead, the telco needs to sort out your location. There are two different location determinations that need to be made.

Routing your call to the correct PSAP

The telco needs to connect your emergency call to the correct PSAP. This could be a simple determination if you’re in the middle of a large geographic region service by one PSAP. It could be a losing cause if you’re near a jurisdictional boundary, especially if your mobile is connected to a tower in the neighbouring jurisdiction. Two neighbouring counties may know how to re-route your call if you wind up talking to the wrong PSAP, however that may be more complicated if national boundaries are involved.

Providing your location to first responders

Once the telco has routed your call to the (hopefully!) correct PSAP, the PSAP needs to know where you’re located so that first responders can be dispatched.

PSAPs and Pizza are not the same

John Oliver has a brilliant segment on YouTube where they stand in a PSAP and order pizza using a smartphone app. The app knows exactly where they are. The PSAP call taker doesn’t get the same results.

Why the different results? When you use an app to order pizza, the app has full access to the location gizmos like GPS in your smartphone. Your mobile call to 911 doesn’t, since there’s no standards or protocols in place for your phone to report its location to the PSAP. The 911 infrastructure just isn’t advanced enough to receive that location information.

Instead, it’s up to the telco to try to establish your location. It does this by calculating the length of time it takes a signal from your phone to reach a cell tower. This tells the tower how far away from it you are, but does not give a reasonable direction. Assuming you are near multiple towers, up to 4 are used. More towers means better accuracy.

A tower will have a very rough idea of your direction. Most towers have 3 or 4 antennas pointing in different directions, and their coverage areas are far too broad to be of use.

Once the telco has calculated your latitude and longitude, this information needs to be provided to the PSAP. The methodology for this seems like a Rube Goldberg machine: the telco creates a fake phone number for you, called a Pseudo-ANI. It provides this Pseudo-ANI to the PSAP as your phone number. Then, the telco performs an emergency update of the ALI database with a Pseudo-ALI that contains your latitude, longitude, and as much civic address information as can be determined. The PSAP uses the Pseudo-ANI to lookup the Pseduo-ALI, and now they have a location for you. All of this takes time to determine, so there will a period where the PSAP doesn’t have an address for you. Typically the Telco has 45 seconds to populate the Psuedo-ALI, and must update it every 30 seconds.

Mobile 911 service is really a kludge to be “backward compatible” with exisitng 911 infrastructure designed for analog phones.

Here’s what the PSAP will see once the telco will see when the information is provided (my labels are in italics, and refer to the nearest coloured box):

PSAP terminalNext up, we’ll have a look at how VoIP Systems can use 3rd party “next generation” services and ugly workarounds to also provide location information to PSAPs.

And do make sure you watch that John Oliver segment, it’s great stuff.


1 thought on “911 and Mobile Phones

  1. Pingback: Emergency Calling Oopsies | Bump in the net

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