By Jeffrey Krauss,
President of Telecommunications
and Technology Policy
Enhanced 911 (E911) requires the phone company to deliver the 911 caller's location. But if a Voice-over-IP (VoIP) subscriber calls 911, what location shows up on the public safety call taker's screen? For some flavors of VoIP, particularly nomadic VoIP users, there does not seem to be a way to determine the caller's location. VoIP is fundamentally different from wired and wireless telephone service. There are no dedicated physical wires, as with wireline service. There is no cellphone with a dedicated phone number, as with wireless. As I wrote in my April 2003 column, at least one flavor of VoIP service (Free World Dialup, or FWD) is a form of Internet instant messaging.

Just like with AOL Instant Messaging, with FWD you can communicate with anyone else who is connected to the Internet and who subscribes to the FWD service. The network glue is the Internet Session Initiation Protocol (SIP). For this flavor of VoIP, you need either a special SIP-enabled phone or a computer with SIP software. When you connect to the Internet, your SIP phone or your software logs you in to your VoIP vendor's "presence" database. Maybe you have a "screen name," like with AIM, or maybe your user name looks like a phone number. The database links your user name to your then-current IP address, so that someone can call you. And you get a Universal Identifier with a format like: sip:+13015551234; or

VoIP services need a gateway to the public switched network, to place calls to non-VoIP wireline or wireless subscribers. And the service vendor might decide to build the SIP capability into a device like a cable set-top box, which is always located at the subscriber's home, and with a dedicated phone number. For PacketCable, delivering the caller's location to the 911 call taker might not be a problem, because set-tops don't move around.

But VoIP isn't limited to set-top boxes at fixed locations. Anywhere you have Internet access, you can use SIP to make VoIP calls. Now that hotels are starting to provide free broadband Internet access, you can expect businessmen to start carrying small SIP-enabled devices so they can make free phone calls over the Internet as they travel. The implications for E911 are serious.

E911 has three additional requirements besides determining and displaying the location of the 911 caller. First is the call taker's ability to call back, in case the emergency call gets disconnected. SIP devices do not have dedicated phone numbers–they have dynamically assigned IP addresses. But there is work going on in this area. The Internet Engineering Task Force has a Telephone Number Mapping group that has defined a protocol, RFC 2916, to map phone numbers to Internet Domain Names and IP addresses. So RFC 2916, plus a gateway to do the translation, should give the public safety call taker a way to call a phone number and get connected to the VoIP subscriber with the emergency.

The second requirement is to make emergency calls independent of the country. In other countries, the emergency number might not be 911, but may be 000 or 999 or 112. A Columbia University professor, Henning Schulzrinne, prepared a proposal on the subject, and maintains a Web page at with some relevant links. He suggests that instead of 911, VoIP calls to public safety agencies use a Universal Identifier like:; or brigade

sip:sos.rescue@domain.comambulance (rescue)

sip:sos.police@domain.compolice (law enforcement).

The third E911 requirement is to connect the VoIP user to the nearest public safety answering point. If I live in Maryland but call 911 from my hotel in Denver, it doesn't do much good to route the call to the Maryland officials. This is easy if you know the calling party's location, but nearly impossible if you don't.

So let's get back to the location issue. When I dial 911 from my SIP device, how can the call taker determine where I'm calling from? In some cases, the network port being used might have a physical identity. If the hotel room has a wired Ethernet network, it might be possible to interrogate the port for its location. But the millions of deployed Ethernet networks don't have this capability today. Anyway, this would not be sufficient for nomadic wireless users, such as those on WiFi networks. Moreover, by using your laptop or SIP phone on a commercial airplane equipped with Connexion by Boeing, or on a private jet using ARINC's SKYLink, you can make VoIP phone calls while zooming along in the sky.

Determining the location of nomadic VoIP users seems to be a real problem. Install GPS receiver chips in SIP devices? This might work for dedicated portable SIP phones, but don't expect to find them in computers where the SIP capability is contained in software. Require a user to log in for each session, and require the user to supply a physical location? Pretty lame, I'd say. Require the millions of deployed WiFi access points to somehow know their own location? Too late for the current technology, although it may be feasible for the next generation. In other words, at least for now, nobody has the answer.

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