While many assume that enhanced-911 (E-911) systems follow a uniform and comprehensive standard, state legislatures determine the legal requirements. In fact, only 14 states currently legislate E-911 compliance for businesses. This means that for most businesses operating a private branch exchange (PBX), an E-911 solution is really more a best practice than a legal requirement.

Still, the states that do have requirements have some similarities among them. For example, an emergency call should include location information. Few states specify how accurate this location should be — at a minimum, it should identify a street address. Many large organizations prefer to give a building identification (if on a campus), floor and even geographic corner of the building from which the call originated.

There also must be a valid call-back number, so if the call is dropped, the public-safety dispatcher can dial back and reach the caller. This requirement is trivial for domestic phone lines, but can be challenging for PBX switches.

When a call arrives at the public-safety answering point (PSAP), it must have a valid answering number identification (ANI). This is the usual calling number indication, but it means that the call must leave the enterprise on a T1 PRI (primary rate interface) line — or CAMA (centralized automatic message accounting) line, but these are nearly extinct — voice line. Residences and small businesses don't require this if their phone number is registered to their address, but larger businesses with many locations and phone numbers will need PRI.

The PSAP uses the ANI as a lookup in the automatic location information (ALI) database, which designates a physical location for each ANI phone number. The local ALI is populated by the phone company when a domestic line is installed, but businesses using E-911 must perform this function using a private switch/ALI service. When calls are received, dispatchers can read a location on screen in master street address guide (MSAG) format.

At this point the emergency server has done its work, except to distribute internal notifications and to ensure that should the call be dropped, the dispatcher can dial the ANI and reach the caller.

IP PBX vendors and a number of third parties have developed solutions for E-911, setting up an adjunct server linked to the PBX to specially handle emergency calls. While these were designed for wired VoIP phones, they also are capable of supporting Wi-Fi clients. In fact, the architecture used by IP PBXs with Wi-Fi phones is followed by nearly all E-911 IP PBX solutions today.

The first stage in a deployment is to decide how accurately the emergency call should be located, by designating a number of zones on the floorplan. A zone can cover an entire building or a single floor. Each zone is mapped in the E-911 server to a dummy PBX extension number, sometimes known as the emergency location indication number (ELIN). Subsequently, the installer populates the local PSAP's ALI database, mapping each ELIN to the street address and location description of its emergency response location, or ERL.

Whenever a Wi-Fi phone joins the network, it is allocated an IP address. The WLAN matches each device to an available VLAN. When the phone subsequently registers with the IP PBX, the emergency server receives notification of its IP address. In the emergency server database, the subnet identifies the zone in which the phone is located — hence it maintains a mapping of Wi-Fi phones to their current zone.

No further activity is required until the phone makes an emergency call. Now the IP PBX notifies the emergency server of the call. The server intervenes, mapping the phone's zone to its ERL and substituting the ERL's dummy extension number for the usual DID ANI as the 911 call is dialed. When it receives the call, the local PSAP maps the dummy number to the pre-configured address in its ALI database, and the correct location pops up on the dispatcher's screen.

Meanwhile the emergency server notifies local first-responders. Enterprises normally prefer emergency services to be met at the campus entrance and guided to the incident, regardless of E-911 location process. Many organizations have internal security and first-responder teams, who are notified first via pager, message service or pop-up screen when 911 is dialed.

If the call is dropped and the PSAP dispatcher calls back, the dummy extension number is recognized in the IP PBX and the emergency server directs it to connect to the original 911 caller.

The details of emergency call-handling are not understood widely. Few states today have comprehensive requirements applicable to VoIP and Wi-Fi phones, so the impetus for compliance is more often the enterprise wanting to follow best practices than it is legal compliance. But the lack of legislation should not make a good E-911 system optional, nor should it allow Wi-Fi telephony to be excused from such a solution. Location of the caller to the correct street address, building and floor is more than sufficient accuracy for E-911 purposes.

A WLAN can be configured to work in conjunction with IP PBXs and emergency call servers to deliver comprehensive E-911 services. There are some aspects of the architecture that could be improved: using the IP address of the caller to map to a location is somewhat cumbersome, and better solutions can be expected over time, but Wi-Fi phones can be made E-911-compliant today with relatively simple configuration. n

Peter Thornycroft is a technology analyst at Aruba Networks with primary interests in wireless technology, voice-over-WLAN and e-FMC solutions. He has experience with a variety of wireless, carrier and voice technologies. Thornycroft holds an MA in electrical sciences from Cambridge University, and an MBA from Santa Clara University.


RedSky, an independent provider of enterprise E-911 solutions, has a library of relevant legislation at www.redskytech.com/e911_information_center/e911_legislation/.

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