Many residential customers are choosing VoIP as a primary line replacement for traditional residential wireline phone service. There are many attractive aspects to residential VoIP service--it's cost-effective, allows for subscriber mobility and subscribers may be assigned telephone numbers outside their local rate centers. However, the very features that make VoIP a compelling alternative to traditional wireline service also present challenges when it comes to delivering emergency calls.

With E911 (Enhanced 911) service, the call taker at a PSAP (public safety answering point) can automatically view the call back number (ANI, or Automatic Number Identification) and street address (ALI, or Automatic Location Identification) when receiving a 911 call.

This article describes strategies for providing comprehensive E911 services to VoIP subscribers within the existing 911 infrastructure, without requiring major investment from VoIP providers, 911 service providers or PSAPs.

The existing 911 network and provisioning infrastructure was designed regionally, with specific 911 selective routers and ALI databases serving telephone numbers in distinct geographic areas. This existing regional infrastructure accommodates residential VoIP customers under the following conditions:

1. The VoIP service provider must have access to the regional 911 selective routers that funnel calls to the PSAPs.

2. The VoIP customer must be associated with a geographically relevant telephone number recognized within that rate center.

3. The VoIP customer must be located at a known, static address within the rate center.

If these three conditions are met, the VoIP customer's 911 call can use existing 911 provisioning and call flow processes and will appear as a standard wireline call at the PSAP. The 911 call arrives over dedicated 911 trunks to the PSAP, and the PSAP automatically receives the VoIP caller's ALI and ANI.

When the conditions above are not met, the 911 call cannot move as needed through the existing infrastructure. If the service provider does not have access to the 911 selective router, the PSAP cannot receive ANI/ALI. If the caller does not have a geographically relevant telephone number, the selective router will not route the 911 call. Finally, if the caller changes location--that is, places the call from outside the identified geographic location--the location information will either be unavailable to the PSAP or will be incorrect on the 911 call taker's display screen. VoIP has the potential to disrupt the 911 network in each of these three areas because a VoIP subscriber's nationwide reach contrasts with the geographically regional 911 network call processing in place today.

A true VoIP E911 solution requires that VoIP networks be able to route 911 calls to the appropriate regional 911 selective router based on the caller's location. This enables the PSAP to receive the call on dedicated 911 trunks and view the ANI and ALI information for each 911 caller. In addition, the receiving PSAP needs to have access to the VoIP caller's ALI information, regardless of where that ALI is stored. Today, the 911 network does not accommodate this requirement.

Unlike traditional wireline, a VoIP subscriber's assigned telephone number does not have to match the area codes and other number conventions for the rate center where they are located. A user in Boston may choose to have a non-native 212 New York City telephone number. Or a user may move to a different state but keep the phone number associated with a previous address.

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The issue of location is further exacerbated because VoIP subscribers can place and receive calls from their VoIP telephone number from any broadband Internet connection. This mobility feature allows subscribers to take their phone number with them wherever they can plug into a high-speed data connection, whether that location is a coffee shop in their neighborhood, an airport in another state or a hotel in another city.

To be effective and comprehensive, VoIP E911 solutions must dissolve the implied linkage between a telephone number and its assumed location so the network can determine the caller's current location, route the call to the correct PSAP, and display the correct ALI and ANI.

Rather than today's geographically localized E911 service, VoIP E911 requires a national footprint that will allow the 911 network to seamlessly follow mobile VoIP users and retain the ability to route calls and data to any PSAP in the country. However, it is possible to extend the existing infrastructure to provide VoIP E911.

VoIP E911 Solution

Industry stakeholders are currently developing solutions that will enable E911 for VoIP with nationwide coverage. These solutions allow calls--regardless of the caller's physical location--to be routed to the appropriate PSAP using the 911 selective router and enable delivery of accurate ANI and ALI information to the PSAP.