Web Informant #403, 21 March 2005: 911 is not just another phone number

By Martin T. Focazio (martin@focazio.com)


[Ed. Note: In my last Web Informant, I mentioned the issues surrounding my use of Vonage and AT&T CallVantage voice over IP service. One related issue is how these services make use of the 911 emergency telephone network. I asked computer consultant Martin Focazio to share some of his thoughts and research on this subject.]

Basic telephone service is, by far, the most advanced technology that people use every day. While it seems that everyone has a cell phone and voice over IP these days, the reality is that cell phones and VoIP still have quite a way to go before they can do some of the neat tricks that ordinary switched phone service can do.

The best way to see how much more cell phones and VoIP have to learn from the "old-fashioned" telephone networks is to look at how 911 calls are handled. When you dial 911 from your home phone all sorts of cool things happen.

First of all, your call is routed to a special phone switch, one that only carries emergency service. This switch is connected to a database of the physical location of the phone wires that were used to originate the call, and the dispatcher gets the address of the caller on the screen. Sometimes there's even a map of the location, and in really fancy systems, a map and routing data is transmitted to a computer in the emergency vehicle. In many systems, the 911 operator can "seize" the line - preventing other calls from coming in, and keeping the line connected even if the caller hangs up. The 911 calls go to a Public Safety Access Point (PSAP) - the place where 911 operators and dispatchers work to assign and direct emergency service workers.

However, these Public Safety Access Points also have to deal with calls from cell phones and, more recently, VoIP calls. To say that the public safety agencies are dissatisfied with VoIP is putting it mildly: http://www.apcointl.org/about/gov/PDF/VoIPLetter021505.pdf

In both cases, these neat technologies seem to be everything "ordinary" phone service is and more. But they are not quite the same in important ways.

There are two key differences that really matter.  The first is that cell phones and VoIP calls are routed via ordinary telephone lines, through non-emergency phone switches. When you call 911 via a cell phone, your call is often run to an office telephone system that happens to be at the PSAP. While there's considerable effort and progress on handling 911 calls from cell phones effectively, they are still the bane of the dispatchers day. They often have to re-key your phone number and try to guess or figure out your location, leading to exchanges like this:

Cell phone caller: "I saw an accident on the road."
911 Operator: "What road?"
Cell phone caller: "I don't know"
911 Operator: "Where are you now?"
Cell phone caller: "In my car"

Some primitive, slightly effective cell phone location systems are being rolled out, but GPS does not work indoors, and triangulation is - at best - an iffy proposition in urban areas.

For VoIP the situation is more complex. For most VoIP carriers, 911 calling is a for-pay option. (See for example the service offered by Packet8 here:


This brings the very real possibility that a VoIP customer would not be able to dial 911 at all in the event of an emergency. However, buying 911 service on a VoIP account is not actually giving customers real 911 service.

Remember the neat little tricks that a 911 operator can do with real 911 service - like get instant address data, hold a line open and so forth? 911 calls placed on most VoIP providers is routed over ordinary phone lines to an ordinary phone system at the PSAP. In some cases, the call is not even routed to a PSAP, it goes to what amounts to a call center, where an operator will try to figure out which PSAP is supposed to handle your call. For a long series of horror stories of VoIP 911 call problems see www.911dispatch.com

VoIP is, in many ways a great example of fancy features over-riding core functionality and stability. While it's great to have 3-way calling, caller ID, call forwarding and all these neat add-ons with a VoIP service, this can't obscure the fact that neat features on top of an incomplete base of functionality can be a - literally - fatal flaw.  I'm reminded of MS-Word, which is smart enough to intercept my keystrokes and offer "help" when it detects that I'm writing a letter - but it still crashes more than I want when saving a document. Clearly, flash has outstripped function.

The point of all of this is not to say that VoIP and cell phones are a bad idea - they are not. But they are far from ready for mass deployment, despite the increasing numbers of people planning to use only cell phones and VoIP for their primary service. If a new technology comes along proposing to improve on an old one, don't forget to look at the really hard parts of implementing the old system and realize that these parts - the deep, complex and critical aspects of the technology are some of the most important features - and ones that can't be left out.


David Strom


Tom's Guides Publishing

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Westlake Village, CA 91362

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