Enhanced 911

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 as required by the Wireless Communications and Public Safety Act of 1999. This is generally done by a form of reverse telephone directory that is supplied by the telephone company as a computer file, used to associate the caller's phone number with a physical street address. It 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 one's 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 Emergency Service Number, a number identifying the PSAP.

The location information (CLI) provided is normally integrated into emergency dispatch center's computer-assisted dispatch (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 (Local Exchange Carrier, or 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.

Emergency Service Zone/Number (ESZ/N)
The ESZ is a geographic region which is associated with unique emergency responders (Police, Fire, EMT), and associated with a unique PSAP. A PSAP may have multiple ESZ's associated with it, but all ESZ's will only be associated with a single PSAP. The ESZ serves two functions. It is used to update the 911 selective router with the correct routing information and is used to associate English Language Translation (ELT) to the record in the ALI database.

Master Street Address Guide (MSAG)
The Master Street Address Guide is a system used to associate a telephone number from a customer subscriber to an Emergency Service Zone (ESZ). Customers telephone number records are submitted to the ALI provider and, if MSAG validation exists, will match the customers address information to an MSAG entry containing the ESN.

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

Hybrid solutions, needing both the handset and the network include:
 * Assisted GPS (wireless or television) 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 &mdash; 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
As VoIP technology matured, service providers began to interconnect VoIP with the public telephone network and marketed the VoIP service as a cheap replacement phone service. The problem was, however, that the VoIP service was not a perfect replacement; it was not directly interconnected with the 911 network and could not complete a 911 call. VoIP services tried to improvise, such as routing 911 calls to the administrative phone number of the Public Safety Answering Point, but this was an imperfect solution. In time, this problem reached the headlines of newspapers as individuals were unable to place emergency calls with their VoIP phones.

Initially, the Federal Communications Commission (FCC) took a hands off approach to VoIP in order to let the service mature and also to facilitate competition in the telephony market. When the problems with 911 service became acute, the FCC acted. In 2005, the FCC required that VoIP services that interconnect with the public telephone network begin to provide 911 service and provide notice to their consumers concerning the 911 limitations (note that if a VoIP service did not interconnect with the public telephone network, this requirement was not imposed). 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.

There are, however, complicated technological problems with implementing E911 with VoIP, which providers are attempting to solve. VoIP phones are on the Internet and nomadic; the geolocation of the individual placing the 911 call can be very difficult to determine. Service providers are attempting to phase in solutions through the I1, I2, and I3 phases. During I1, the 911 call was routed to the 911 administrative telephone lines without location information. During I2, VoIP services would participate in the public telephone networks location database for the location that is identified with that telephone number. During the I3 solution, VoIP service providers would have a true IP interconnection with Public Safety Answering Points and would be able to provide even more valuable information than the legacy 911 system. Where VoIP phones are mobile, geolocation has additional problems; VoIP service providers are seeking access to mobile phone location databases. These solutions are being developed through the cooperation of the Voice on the Network Coalition and the National Emergency Number Association. 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. The FCC had continued to add more requirements and mandate a more sophisticated 911 function.

VoIP services have noted an obstacle to full 911 interconnection; in order to interconnect with the Public Safety Answering Point, the VoIP service providers must interconnect with the 911 telephone trunk, which is owned and controlled by their competitors, the traditional fixed-line telephone carriers.

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.

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.

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.

VoIP & 911 issues are also relevant to Telecom Relay Services utilized by individuals with disabilities.

Address signage standards
In addition to upgrading communications systems, many counties and communities in the U.S. have implemented ordinances requiring property owners to standardize address numbers on buildings and along streets and roadways, to assist emergency personnel in locating callers day or night, even in poor weather. These generally consist of reflective characters, at least 4 inches high, on a contrasting reflective background. Compliant signage systems are often advertised as being "E911 compliant".