Our colleague Jim Cavanagh (e-mail) has been discussing in a series of newsletters the changes that VoIP is bringing to how calls to emergency services are placed (see below for links to the newsletters in this series). The articles also described the shift from phone number assignments that were strictly geographical and always resolved to a single physical address to wireless phone numbers that were assigned to an individual who could be virtually anywhere to VoIP phone numbers that are little more than "labels" and can, and will, be increasingly alphanumeric, such as james.p.cavanagh@telco.net. Today, Jim expands that discussion by examining mechanism for locating VoIP subscribers.
 
Jim says:

"In the old 9-1-1 system a phone number mapped to one and only one physical address served by one and only one 9-1-1 call center known as a Public Safety Answering Point. A VoIP subscriber, on the other hand, may plug their SIP Phone, Analog Terminal Adapter or similar VoIP-equipped hardware into any Internet connection, anytime, anywhere.

"The first step in finding you is to know your status. If you are a stationary user, such as a non-mobile Vonage user or a cable company subscriber the VoIP provider will have verified your physical location, or "registered address," at the time you signed up for the service. Increasingly VoIP providers are validating where you claim to be against Master Street Address Guides provided by 9-1-1 agencies to assure their database has your location and that they can accurately dispatch help when your call comes in.

"If you are a nomadic or mobile user, for instance, and have plugged your ATA or SIP phone into the Ethernet outlet in your hotel room, or are using your new VoIP Wi-Fi phone from the Starbucks, airport or hotel hot spot you will be harder to find. Eventually the system will be automated using some type of location technology, possibly GPS-based, which presents problems indoors, or it could be based upon the physical location of the MAC address of where you have plugged in - an approach that's just now being considered. But in the meantime, the process requires human intervention.

"For the interim, 9-1-1 calls from nomadic or mobile users will go to a 9-1-1 call center where your description of where you are will be matched to a PSAP, usually with a check of the MSAG to assure accurate dispatch, and your call forwarded along with your location information. In these instances, the PSAP database will be fooled into thinking it knows you through the insertion of a temporary record in its database, a system already in use for wireless users.

Steve Taylor is president of Distributed Networking Associates and publisher/editor-in-chief of Webtorials. Larry Hettick is a principal analyst at Current Analysis.