E911 services primer: Wireless and VoIP phones in emergency situations

E911 provides services for wireless and VoIP callers during emergency situations, but determining a caller's location can be challenging.

Enhanced 911, or E911, is an emergency response system that supports wireless and VoIP callers who dial 911 during emergencies.

Established in the U.S. in 1968, the nationwide 911 system has undergone few upgrades, but support was extended to callers using wireless and interconnected VoIP phones in 2005. Now, a next-generation upgrade to shift the entire 911 system to an IP network is being deployed -- with completion targeted at some time during the next decade.

What are E911 services for wireless and VoIP telephony?

Regulated by the Federal Communications Commission, all wireless and interconnected VoIP service providers are required to support E911 access to emergency services. An interconnected VoIP carrier is simply one that sends and receives calls from other phones and uses the public switched telephone network. Vonage is an interconnected carrier, for example, while Skype isn't.

"To provide 911 access, VoIP service providers use a database of addresses that its subscribers provide. This information, in turn, is used to decide which public safety answering point -- the 911 call center -- the emergency call gets sent to," explained Brian Rosen, E911 expert and distinguished engineer at Neustar.

Two special types of service providers typically assist with this process: a voice-positioning center and an emergency services gateway.

A voice-positioning center maintains the database of subscribers' self-reported addresses and determines where to route the emergency call. When queried by the public safety answering point, the voice-positioning center supplies these locations.

An emergency services gateway converts Session Initiation Protocol (SIP)-based VoIP signaling to the time division multiplexing signaling used by the 911 system. An emergency services gateway connects to special 911 tandem switches, called selective routers, which are the path into the public safety answering point. "All calls entering a public safety answering point come in through the selective router, which is typically operated by the incumbent local exchange carrier," Rosen said.

Next-generation E911 (I3) services

One of the most interesting aspects of the long-awaited next-generation E911, I3 (which is currently being deployed), is that the design center is built on an IP network.

The next-generation system comprises emergency services IP networks, IP-based software services and applications, and databases and data management processes that are interconnected to the public safety answering point premise equipment, according to the National Emergency Number Association (NENA). The system provides location-based routing to the appropriate emergency entity.

It can also incorporate other available data elements and business policies to augment public safety answering point routing, and delivers geodetic and civic location information and the call-back number.

"We're back retrofitting in wireline, because wireline is slowly but surely disappearing," Brian Rosen said. "But we built it around how wireless and voice work, so it'll have better results for those kinds of services. A key change is that it can supply much more data. The current system is extremely limited in the amount of data you can receive -- your phone number and location are the only data 911 operators have access to today. We're fixing that to enable access to more data -- such as building floor plans and health information."

A big improvement is that in the midst of disaster situations, such as when a call center is three feet underwater and completely without power, "calls can be answered at a call center across the country, and they can dispatch your fire fighters, bring them in from nearby or help provide an appropriate emergency response," Rosen said. "Emergency calls will never go unanswered."

Next-generation 911 is designed to support the transfer of calls to other call centers or other authorized entities, according to NENA, because this is a critical feature that's been missing. The new system achieves this via standardized interfaces for call and message services that can process all types of emergency calls -- including nonvoice (multimedia) messages -- while also acquiring and integrating other data useful to call routing and handling for appropriate emergency responses.

One catch is that the next-generation 911 system isn't being deployed uniformly nationwide because of funding challenges. Nationwide deployment is targeted for completion within the next decade.

How do E911 services work?

There are three types of carriers: wireline, wireless and VoIP. All three have trunks connected to a selective router. The 911 call centers are also connected to a selective router.

"Wireline carriers enter their subscribers' addresses into an automatic location identification database, called ALI, which is maintained and synced to the selective router and keyed by the telephone number of the subscriber. When entering a location into the ALI database, it's validated against a master street address guide, which is a list of all known addresses served by the public safety answering point," Rosen explained.

When an emergency call comes in, the selective router switches it to the correct public safety answering point, which then queries ALI with the caller's phone number to determine their location.

The E911 system was designed around this model. "Wireless phones came along later and are a challenge to the system because the phone number isn't necessarily related to the location, the location can change, and the location is provided in latitude/longitude rather than a physical street address," Rosen said.

A huge hack was invented to maneuver around this problem. "When a call is sent to the public safety answering point, it passes through a mobile positioning center, which gets the cell and sector ID from the wireless carrier and then decides where to send the call," Rosen said.

"A number is allocated that looks like a telephone number, but isn't to the call. This number, which we call a pseudo automatic number identifier, or PANI, is put in the calling party telephone number field for the signaling by the mobile positioning center. It's chosen from a pool of PANIs that have entries preloaded in the ALI to route the call to the right public safety answering point," Rosen explained. "By choosing the PANI from the right pool, the mobile positioning center ensures that the selective router sends the call to the call center that serves that sector."

When the public safety answering point gets the call, it queries ALI with that PANI. The ALI has been augmented to recognize that it's a PANI and steers the ALI query to the mobile positioning center that allocated it, according to Rosen. "The mobile positioning center knows which call was assigned that PANI, and it asks the mobile network for the actual location of the caller. It then responds to the steered ALI query with the latitude/longitude of the caller, which is displayed on a map at the public safety answering point," he added.

The VoIP system works in a similar way. The voice-positioning center allocates a PANI based on the location in the self-reported location database and sends that as the calling party telephone number with the call. The voice-positioning center routes the call to the appropriate emergency services gateway, which routes the call to the selective router based on the SIP signaling fields. The selective router routes it to a public safety answering point based on the PANI. The public safety answering point queries ALI and is steered to the voice-positioning center, which responds with the location from the database.

Biggest E911 wireless and VoIP challenges?

To dispatch an appropriate response team, 911 call center operators need two critical pieces of information: the caller's phone number and location. Determining an accurate location can be a challenge with wireless and VoIP users, since they're mobile and tend not to have a fixed location.

"Roughly 70% to 80% of 911 calls come from mobile phones. Most people aren't aware that the location provided to the system by mobile phones is often inaccurate -- especially if they're calling from indoors," Rosen said. "Most 911 calls come from indoors, and GPS doesn't work well on mobile phones indoors because the antennas need to pick up satellite signals. Calling from a landline gives a more accurate location to the 911 system."

But the biggest challenge is that "self-reported location information is unreliable," Rosen said. "And voice-positioning centers and emergency service gateways aren't cheap to hire, so service providers often attempt to avoid meeting their obligation or do weird things to lower their cost."

Another huge challenge is finding funding to upgrade the 911 system. "Historically, 911 upgrades have been funded by Congress putting out a grant fund, and then state and local governments came up with matching funds. Trying to get a grant fund out of Congress at this point -- it's just not possible. No one has come up with a good alternative, which is a problem," Rosen said.

How is E911 set up on a wireless or VoIP telephony system?

This is a frequently asked question, but E911 isn't found in the phone -- it's a service the wireless or interconnected VoIP service provider is required to handle. "They're able to recognize the dialed digits 911 and give the call special treatment," Rosen said.

How do leading vendors deliver E911 services?

The vendors responsible for delivering E911 services are the VoIP service providers or carriers, so "there are three voice-positioning centers and two emergency services gateway networks, and the VoIP service provider contracts with one of each of them to handle E911," Rosen said. "It's the responsibility of whoever is supplying the telephone and 911 service -- not [of] the phone maker."


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