Enhanced 911 or E911 service is a North American telephone network (NANP) feature of the 911 emergency-calling system that automatically associates a physical address with the calling party's telephone number. List of VOIP 911 Service Providers.

This is generally done by a form of reverse telephone directory that is supplied by the telephone company as a computerized file. Computer software makes an association between the callers' line and a street address. This provides emergency responders with the location of the emergency without the person calling for help having to provide it. This is often useful in times of fires, break-ins, kidnapping, and other events where communicating your location is difficult or impossible.


A pioneering system was in place in Chicago by the mid-1970s providing both police and fire departments access to the source location of emergency calls. Enhanced 911 is currently deployed in most metropolitan areas in the United States and Canada.

The system only works in North America if the emergency telephone number 911 is called. Calls made to other telephone numbers, even though they may be listed as an emergency telephone number may not permit this feature to function correctly.

Outside the United States this type of facility is often called caller location, though its implementation is dependent on how the telephone network processes emergency calls.

Public Safety Answering Point (PSAP)

The final destination of a E911 call (where the 911 operator sits) is a Public Safety Answering Point (PSAP). There may be multiple PSAPs within the same exchange or one PSAP may cover multiple exchanges. The territories covered by a single PSAP is based more on historical and legal police considerations rather than telecommunications issues. Most PSAPs have a regional ESN, a number identifying the PSAP.

The location information provided is normally integrated into emergency dispatch center's computer-assisted dispatch or CAD system, to provide the dispatcher with an on screen street map that highlights the caller's position and the nearest available emergency responders. For Wireline E911, the location is an address. For Wireless E911, the location is a coordinate. Not all PSAPs have the Wireless and Wireline systems integrated.

Wireline Enhanced 911

There is special privacy legislation that permits emergency operators to obtain the caller's information. This information is gathered by mapping the calling phone number to an address and ESN in a database. This database function is known as ALI, Automatic Location Identification. The database is generally maintained by the Incumbent Local Exchange Carrier (ILEC) under contract by the PSAP. Each ILEC has their own standards for the formatting of the database. Most ALI databases have a companion database known as the MSAG, Master Street Address Guide. The MSAG describes the exact spelling of streets, street number ranges, and other address elements.

Each telephone company (LEC) has at least two redundant DS0-level (that is, 64 kbit/s, or voice quality) trunks connecting each host office telephone switch to each PSAP. These trunks are either directly connected to the PSAPs or they are connected to a telephone company central switch that intelligently distributes calls to the PSAPs. These special switches are often known as 911 Selective Routers. Their use is becoming increasingly more common as it simplifies the interconnection between newer ISUP/SS7-based host office switches and the many older PSAP systems.

If the PSAP receives calls from the telephone company on older analog trunks, they are usually CAMA circuits. These circuits are similar to regular telephone lines, but are formatted to pass the calling party number.

If the PSAP receives calls on older-style digital trunks, they are specially formatted Multi-Frequency (MF) trunks that pass ANI only.

Some of the upgraded PSAPs can receive calls on ISUP trunks controlled by the SS7 protocol. In that case, the calling party number is already present in the SS7 setup message.The Charge Number Parameter contains the ANI.

The PSAP trunking does not pass address information along with the call. Instead, only the calling party number is passed. The PSAP uses the calling party number to look up the address in the ALI database. The ALI database is secured and separate from the public phone network by design.

ALI Failure is when the phone number is not passed or that the phone number is not in the ALI database. If this happens, the call is passed to the trunk group's default ESN, which is a PSAP designated for this function. The PSAP operator must then ask the incoming call for their location and redirect them to the correct PSAP. The legal penalty in most states for ALI database lookup failure is limited to a requirement that the telephone company fix the database entry.

Competitive local exchange carriers (CLEC) and other competing wireline carriers negotiate for access to the ALI database in their respective Interconnect Agreement with the ILEC. They typically populate the database using the ILEC MSAG as a guide.

Wireless Enhanced 911

A second phase of Enhanced 911 service is to allow a wireless or mobile telephone to be located geographically using some form of radiolocation from the cellular network, or by using a Global Positioning System built into the phone itself.

Radiolocation in cellular telephony uses base stations. Most often, this is done through triangulation between radio towers. The location of the caller or handset can be determined several ways:

  • angle of arrival (AOA) requires at least two towers, locating the caller at the point where the lines along the angles from each tower intersect
  • time difference of arrival (TDOA) works like GPS using multilateration, except that it is the networks that determine the time difference and therefore distance from each tower (as with seismometers)
  • location signature uses "fingerprinting" to store and recall patterns (such as multipath) which mobile phone signals are known to exhibit at different locations in each cell

The first two depend on a line of sight, which can be difficult or impossible in mountainous terrain or around skyscrapers. Location signatures actually work better in these conditions however. TDMA and GSM networks such as Cingular and T-Mobile use TDOA.

CDMA networks such as Verizon Wireless and Sprint PCS tend to use handset-based radiolocation technologies, which are technically more similar to radionavigation. GPS is one of those technologies.

Hybrid solutions, needing both the handset and the network include:

  • Assisted GPS (wireless or TV) allows use of GPS even indoors
  • Advanced Forward Link Trilateration (A-FLT)
  • Timing Advance/Network Measurement Report (TA/NMR)
  • Enhanced Observed Time Difference (E-OTD)

The purpose of any of these in mobile phones is twofold: first, the wireless system must know to which PSAP it should route the call, and second, the PSAP that answers the call should know where the caller is and exactly where to send emergency services.

Mobile phone users may also have a selection to permit the location information gathered to be sent to other phone numbers or data networks, so that it can help people who are simply lost or want other location-based services. By default, this selection is usually turned off, to protect privacy.

VoIP Enhanced 911

Initial implementations of Voice over IP telephone systems were not integrated with the 911 system at all, meaning that customers could not even dial 911 in the event of an emergency. However, the Federal Communications Commission has mandated all VoIP providers to provide 911 service, including the E911 feature.

On June 3, 2005, the FCC adopted rules requiring providers of VoIP services that connect with the traditional telephone network to supply E911 capabilities to their customers. The E911 hookup may be directly with the Wireline E911 Network, indirectly through a third party such as a competitive local exchange carrier (CLEC), or by any other technical means. The FCC explained that they felt compelled to issue this mandate because of the public safety concerns. (FCC, Notice of Proposed Rulemaking, WC Docket No. 05-196 available at [1]. Last visited September 18, 2005).)

There are, however, complicated technological problems with implementing E911 with VoIP, which providers are attempting to solve. For example, Vonage, has encouraged its customers to register their locations from which their 911 calls could be routed to the local public safety answering point. (Phil Weiser, Digital Crossroads, 2005, 222.) The FCC had continued to add more requirements and mandate a more sophisticated 911 function.

In some cases, VoIP providers are attempting to connect customers to E911 services through the traditional fixed-line telephone network, but the network is controlled by telecom carriers who are their economic competitors. (Grant Gross, FCC extends VoIP E911 deadline, August 26, 2005 available at [2] last visited September, 18 2005.)

In March 2005, Texas Attorney General Greg Abbott filed a lawsuit against Vonage for deceptive marketing practices by not making it clear that VoIP users had to actually sign up for E911 service. Then in May, the FCC ordered VoIP providers to offer E911 service by late November. (Grant Gross, FCC extends VoIP E911 deadline, August 26, 2005 available at [3]. Last visited September, 18 2005.)

In June, 2005, the FCC announced that customers must respond to the E911 VoIP warning and those who do not have their service cut off on August 30, 2005. The FCC extended the deadline to September 28, 2005. (Gross - Ibid.) As of November 29, 2005, some VoIP providers were significantly out of compliance with the order. The FCC threatened to prevent these companies from marketing their services or signing up new customers in non-compliant areas. [4]

There are also other proposed features that are intended to allow telephone callers from large corporate telephone networks, on both traditional and VoIP PBXs, to be located down to the specific office on a particular floor of a building.

See Also

Emergency Calls
E911 Support on Asterisk® Open Source PBX - Information about E911 as it relates to the Asterisk PBX platform.

Companies that provide 911/E911 services (please put in alphabetical order)

  • 911Broadcast - provides emergency 911 calling systems and services.
  • 911Enable Provide your customers with E911 VoIP service at the lowest cost!!
  • 911ETC E911 Fully Managed Service for VoIP
  • Bandwidth e911 - Easily add e911 capabilities to phones, products or apps for fast, accurate emergency routing. Get started for free.
  • Bulk911 E911 provisioning with simple and upfront billing, rates starting at $0.72 per TN per month. No contract required.
  • Broadvox is a leading wholesale VoIP service provider that delivers reliable VoIP solutions for domestic and international businesses. Get better e911 service for your customers with text-to-911 and provision your on-net and off-net phone numbers.

Created by: lejoneric, Last modification: Wed 28 of Feb, 2018 (15:31 UTC) by khill
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