How 911 Systems Work

When a person dials 911, the telephone company's central office routes the call automatically via dedicated lines to a PSAP. In some systems, especially in smaller communities, the PSAP operator will handle all callers, regardless of the nature of their emergency needs. PSAPs in cities will typically route the caller to a secondary service answering point (SSAP), operated by the appropriate fire or emergency medical services (EMS). While the PSAP operator talks to the caller; he/she enters information pertinent to the call using a computer keyboard. The computer then routes this information to emergency service workers. The operator(s) can also speak to emergency personnel by radio.

911 systems are widely available in the United States. In 1976, only 17% of the U.S. population had access to 911. By contrast, a 1997 Bureau of Justice Statistics survey found that Enhanced 911 (E911, defined below) was operational at 83% of all American law enforcement agencies. The National Emergency Number Association (NENA) estimates that 85% of jurisdictions (which serve 93% of the U.S. population) are equipped with some form of 911, and the vast majority have access to the technologically advanced E911.

There are still jurisdictions, however, which do not offer this service. In those areas, callers must dial specific, separate 7-digit (or 10-digit) telephone numbers to reach law enforcement, fire, or EMS.

Basic 911 systems, deployed decades ago, cannot provide an address to the PSAP operator. Basic 911 may be equipped to provide the caller's telephone number to the operator, and the majority of 911 systems have this capability. E911 adds Automatic Number Identification and Automatic Location Identification to Basic 911. In E911, Automatic Number Identification is used to retrieve a street address or precise location from an Automatic Location Identification database. (Automatic Location Identification is actually a stand-alone database in its own right, and could be accessed by any Basic 911 system if the operator merely had an additional computer terminal in front of him/her.)

Emergency call centers in most medium-sized and virtually all large cities use computer-aided dispatch (CAD) systems. CAD stores and retrieves for the operator call histories for a given location or caller (e.g., it will note previous calls reporting altercations with police officers, previous calls reporting fire, previous calls for cardiac arrest, or previous calls requiring "HazMat" dispatches); the location of desired response resources, such as patrol officers, fire trucks, ambulances and special response teams; and additional notes received from responders in the field. CAD systems help the 911 operators to efficiently manage the call and resources deployed in response to the call. The operator can locate and dispatch the

911 Calls Flowchart

End Office figure 25.1 Call flow process, Basic 911. End Office, telephone company switching office local to caller; Host End Office, switching office operating 911 PSAP. (Courtesy of National Emergency Number Association.)

End Office figure 25.1 Call flow process, Basic 911. End Office, telephone company switching office local to caller; Host End Office, switching office operating 911 PSAP. (Courtesy of National Emergency Number Association.)

most appropriate responders to a call. CAD systems may also be linked to geographic information systems (GIS).

Figure 25.1 illustrates the architecture of a Basic 911 system. Industry professionals estimate that roughly half of 911 systems in the U.S. are equipped with an integrated CAD system. Figure 25.2 shows a typical E911 setup.

In 2000, callers placed 30% of the 150 million calls to 911 in the United States from cellular phones, according to NENA, and the organization estimates that in 2005, more 911 callers will use cellular phones than conventional land lines. Despite this, the infrastructure required for cellular E911 systems is only now in place. Currently, a PSAP operator receiving a 911 call can view the cellular phone number and the location of the cellular tower handling the call; this satisfies the Federal Communications Commission's (FCC) E911 Phase I requirement. Cellular tower transmission ranges vary from under 3 miles in dense urban settings, to 4 miles in suburbs, to up to 16 miles in rural settings. Taking advantage of overlapping

E911 Phase
figure 25.2 Call flow process, Enhanced 911. End Office, telephone company switching office local to caller; Host End Office, switching office operating 911 PSAP. (Courtesy of National Emergency Number Association.)

tower coverage and previous call handoffs (in cases where the caller is traveling), the PSAP can place an individual within about 2 to 3 square miles in a city. If only one tower had contact with a stationary caller, the known area encompassing the caller's location may be 20 square miles or more.

The FCC, in its E911 Phase II standard, requires cellular carriers to locate a caller automatically to within 300 meters (Federal Communications Commission, 2005b). Triangulation of a signal using cellular towers is one way to accomplish this. T-Mobile and Cingular Wireless support this scheme. One drawback is that, in many places, the phone may not be able to communicate with three towers due to topography or to coverage gaps.

The use of global positioning system (GPS) satellites is a more precise way to locate callers. The latter technique is already used by many commercial transport companies, which equip their trucks with GPS receivers. New cellular phones manufactured by Motorola are GPS-ready; the carriers who sell them to the public must place software in the phones which will access the GPS interface (Motorola Inc., 2001, Rivera, 2005). Once this occurs, the PSAP answering the 911 call will, in theory, be able to ascertain a caller's location within a few square meters; the FCC wants GPS-equipped phones to provide a caller's location with a maximum error of 400 feet.

Nextel has very recently enabled this function within its service areas in the United States, and as of this writing, Sprint and Verizon are initiating similar services, although it is unclear whether they are able to meet the FCC's requirements. The caller must be talking on a late-model cellular phone equipped with the requisite interface and software.

The FCC wants wireless carriers to complete the Phase II roll-out by December 2005, but the carriers have encountered numerous technical obstacles to meeting the FCC's Phase II requirements, and the agency has already issued a number of waivers. As of the first quarter of 2005, PSAPS in 35% of the 3,100 counties in the United States have Activated Phase II systems, those that should be able to determine the precise location of a wireless 911 caller (Hixson, 2005). However, tests to determine compliance have seen mixed results, which was not unexpected given the new technologies and methods involved.

Another limitation is that the caller may not be in the PSAP's jurisdiction; that is, the call may not be routed to the PSAP, which would have handled a call from a land line at the caller's location. In this case, the caller must supply the PSAP with the address and city requiring an emergency response; often, this requires the caller to be transferred to another PSAP that is empowered to dispatch the appropriate agency's personnel. If the caller specifies the address where emergency response is needed and the jurisdiction has CAD installed, the operator will be able to retrieve and display the reasons for previous calls from and dispatches to the address.

Lastly, Internet-based phones (voice-over Internet protocol, or VoIP) have made their debut. These phones often cannot be used to reach a PSAP directly; instead, the caller relies on operator relay. Several tragedies and near tragedies involving people in distress trying to call 911 on VoIP phones prompted the FCC to take action and in the spring of 2005, the FCC ordered all VoIP service providers to provide direct 911 connections to PSAPS; the providers must obtain the physical address of the phone's user and display this information and a call-back number to the PSAP. The FCC expects VoIP service providers to comply with this rule by the end of 2005. The degree to which the providers will be able to comply remains to be seen (Federal Communications Commission, 2005a).

If the subject of interest is a private vehicle, useful systems for locating them include Lo-JackĀ®, which, when activated, allows police to track its movements.

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