911 Background and Basics

Emergency calling is a part of every SfB deployment I’m part of, and yet it seems to be an area that most people don’t have a good background on. This makes understanding capabilities and limitations a bit of a challenge. Over the next handful of posts, I’ll cover the background behind 911, some basics on how 911 works with boring analog home phones, traditional TDM business phones, mobiles, and then we’ll through SfB into the mix.

Let’s start by talking about an analog home phone, or a single business analog line like for a fax machine. When you sign up with your telco for this line, you provide them with an address for service, and that address is where the line terminates. There is no option for you to move the line, so the telco can build a large list of names, addresses, and phone numbers, and be very confident in the static nature of that list, and thus its accuracy.

I won’t address MLTS, or Multi-Line Telephone Systems, aka a traditional PBX connected to the telco via PRIs. For the purposes of how 911 works, they’re largely treated as a collection of virtual analog lines, and there’s not much different about how that’s handled – you assign locations to numbers, and the telco enters that information into their databases.

In a 911 (vs e911) scenario, when you call 911, no address or telephone number information is provided. The caller has to provide this information. Once your location is established, that operator has to transfer your call to the correct PSAP – Public Safety Answering Point – local to you. That’s never fun when you’re in distress, so in most regions of North America we’ve progressed to e911 – e for enhanced – where your information can be automatically passed on.

With e911, the address that you provided to the telco is put into a database along with your phone number. Now when you call 911, the telco can use this information to route you to the correct PSAP, and the operator at the PSAP can see your information on their screen. As a safety measure, they’ll almost always verify your address with you. When your number is automatically display at the PSAP, it’s referred to as the ANI, or Automatic Number Identification. . The PSAP then takes your ANI and performs a database lookup to retrieve the civic address. This is called the ALI, or Automatic Location Identification.

Though very similar, ANI and Caller ID aren’t the same thing. Caller ID is for customers and can be blocked. ANI is for telco use. You can block your Caller ID, you cannot block ANI.

This system works very well with static analog systems, where a pair of wires physically terminates at the address you provided. It falls apart completely when the phone becomes mobile, either VoIP or cellular. In my next post, we’ll consider how cellular/mobile e911 can work (and fail!) and why the pizza company knows more about your location than your PSAP.

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