Enhanced 911
Enhanced 911 is a system used in North America to automatically provide the caller's location to 911 dispatchers. 911 is the universal emergency telephone number in the region. In the European Union, a similar system exists known as E112 and known as eCall when called by a vehicle.
An incoming 911 call is routed to a Public Safety Answering Point, which is a call center operated by the local government. At the PSAP, the call is answered by a specially trained official known as a 9-1-1 dispatcher. The dispatcher's computer receives information from the telephone company about the physical address or geographic coordinates of the caller. This information is used to dispatch police, fire, medical and other services as needed. The planned replacement service is NG911.
Call routing
Landline routing
Calls to 911 over the public switched telephone network are routed to a special router. The router looks for the address associated with the caller's telephone number in a database. The caller's phone number is known as an Automatic number identification. The database relating ANIs to addresses is known as Automatic Location Identification. The router then uses the address to search in the Master Street Address Guide for the Emergency Service Number of the appropriate Public Safety Answering Point for that area and connects the call to it.MSAG
The Master Street Address Guide is a database of street addresses and corresponding Emergency Service Numbers. ESNs represent one or more emergency service agencies designated to serve a specific range of addresses in a geographic area, called an Emergency Service Zone.Wireless routing
Calls from cellular phones are received via cell towers by mobile switching centers. The switching center automatically assigns a unique identifier to each cellular 911 call, known as a "pseudo ANI". The Selective Router connects the call to a PSAP based on the cell tower's location.Location transmission
Calls made to non-911 emergency numbers might not have automatic location enabled.Calls to 911 are answered by an operator at a PSAP. In addition to the voice transmission, the telephone network also transmits a number associated with the current call, the ANI. The 911 operator at the PSAP searches a database for the ANI to find the caller's location.
The ALI record associated with the query is then returned to the PSAP, where the Customer-premises equipment correlates that information with the call taker receiving the call, and displays the information on their computer screen.
Automatic location of the emergency is intended to be faster and more reliable than verbal communication of the location, though this is usually requested anyway for confirmation. It also makes it possible for emergency services to respond when callers cannot communicate their location, because they do not know their location, they are a child, or they are too panicked, too distracted by the ongoing emergency, or do not wish to attract the attention of the perpetrator of a crime in progress.
Landline transmission
For landline calls the ANI resembles the caller's phone number. The ALI stores a pre-determined address associated with the caller's telephone number. This address is typically the phone's billing address.Wireless transmission
In parallel to the actual voice call, the ALI database gets periodically updated with more precise and recent location information. Cellular networks can determine a more precise location of the caller's device by using triangulation from the cell towers. In addition to triangulation, a second source of location information may be the caller's phone itself.Many phones manufactured after 2005 have GPS receivers built in. When the cellular phone detects that the user is placing an emergency call, it begins to transmit its location to a secure server, from which the PSAP can retrieve it. Cellphone manufacturers may program the phone to enable GPS function automatically when the user places an emergency call.
For wireless calls, the ANI is a unique number assigned to each individual 911 call, assigned at a mobile switching center.
ALI database
The ALI database is secured and separate from the public phone network by design.The ALI is maintained on behalf of local governments by contracted private third parties, generally the incumbent local exchange carrier. Often, the contracted 3rd party further subcontracts the actual ALI database management to companies such as Intrado, Bandwidth and TeleCommunication Systems, Inc. The ALI database also feeds the Master Street Address Guide database which is used to route the call to the appropriate PSAP and when the call arrives, the ALI database is queried to determine the location of the caller.
Each ILEC has its own standard 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. When a new account is created, the address is located in the Master Street Address Guide to track the proper Emergency Service Number that 911 calls from that phone number should be routed to. Competitive local exchange carriers and other competing wireline carriers negotiate for access to the ALI database in their respective Interconnect Agreement with the ILEC. They populate the database using the ILEC MSAG as a guide.
If the phone number is not in the ALI database, this is known as ALI Failure; the call is then passed to default ESN for the call's trunk line group, which is a PSAP designated for this function. The 911 operator must then ask the incoming caller 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.
Location determination
The way location is determined varies by the type of originating device or network.Landline location
Landline or wireline calls originate from a device connected to a known and fixed location connection to the PSTN. These locations are stored in the Automatic Location Information database. This is permitted by special privacy legislation.Location information is not passed along by the public phone network; only the calling party's phone number is known to the receiver.
Wireless location
The billing address associated with a cell phone is not necessarily considered the location to which emergency responders should be sent, since the device is portable. This means that locating the caller is more difficult, which resulted in the second phase of the Enhanced 911 service, which relates to locating wireless or mobile telephone devices.To locate a mobile telephone geographically, there are two general approaches. One is to use some form of radiolocation from the cellular network; the other is to use a Global Positioning System receiver built into the phone itself. Both approaches are described by the Radio resource location services protocol.
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 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 works like GPS using multilateration, except that it is the networks that determine the time difference and therefore distance from each tower.
- Location signature uses "fingerprinting" to store and recall patterns which mobile phone signals are known to exhibit at different locations in each cell.
Code division multiple access networks tend to use handset-based radiolocation technologies, which are technically more similar to radionavigation. GPS is one of those technologies. Alltel, Verizon Wireless, T-Mobile 3G, and Sprint PCS use Assisted GPS.
Hybrid solutions, needing both the handset and the network include:
- Assisted GPS allows use of GPS even indoors
- Advanced Forward Link Trilateration
- Timing Advance/Network Measurement Report
- Enhanced Observed Time Difference
The 3GPP specified protocol for handset geolocation in GSM networks is called Radio Resource Location Protocol.
911 address
The term 911 address refers to a format for specifying where a 911 call originated from.The 911 address contains a uniform number, the street name, direction, and the city. The uniform number is usually assigned by the grid of the existing community. Each county usually has their own policy on how the addressing is done, but for the most part NENA guidelines are followed. These guidelines are expressed by the Master Street Address Guide. The exact 911 addresses and associated phone numbers are put into the ALI database.