911 — The Untold Story
The concept of 911 is very simple—recognize when the numbers 9-1-1 are dialed from any telephone, allow no-coin calls from pay telephones, route the call to the jurisdiction where the call is originated, and in an Enhanced 911 (E911) system, display the callers telephone number and address in the comm center.
The first call on 911 was made from a telephone in Haleyville (Ala.) on February 16, 1968.
The technology of performing this concept takes many forms. However, there are some basic building blocks. In foreign countries, in fact, they’ve adopted other digits for reporting emergencies. As of 1992, the European Union has mandated that its member states adopt 112 as their emergency number, but allowed participating countries to operate that number simultaneously with their previous three-digit emergency number. [EU legislation]
First, telephone companies already maintain a subscriber database listing every assigned telephone number, the subscriber’s name, address and billing information.
Second, the telephone system already identifies the telephone number for every call placed, in order to properly bill the subscriber each month. This is known as Automatic Number Identification (ANI).
An E911 system adds a third component to the system—a Master Street Address Guide (MSAG). This database cross-references every assigned telephone number, subscriber’s address and the block number ranges for every street, in every jurisdiction served by the telephone company.
The location, administration and structure of the MSAG is different among the seven regional Bell companies and the many independent telephone companies. Some companies maintain local databases, while others maintain just one or two database sites for their entire, multi-state territory. In either case, the ability to send data at the speed of light means that transactions occur very quickly.
Fourthly, most telephone company have built special switches and networks to carry 911 traffic, so that other telephone company traffic will not intefere with 911 operations, and 911 traffic is protected from power failures and other system problems.
Now, when a caller dials 911, the call is recognized by the telephone company central office switch and routed to the 911 network. The ANI (telephone number) information is decoded through a subscriber database to obtain the caller’s address and other information.
Next, the call is processed—sometimes simultaneously—through the MSAG to obtain the ID code of the agency that should handle the call. The 911 network then routes the voice and ANI/ALI information to the correct agency. The ANI/ALI information is displayed when the call-taker answers and, at some agencies, the call information is printed out when the call is completed.
At most agencies, E911 also allows an agency to transfer a 911 call to another PSAP or to any seven-digit number. If a 911 call is transferred to another PSAP, the ANI/ALI information is transferred along with the voice call, so the other call-taker can also view the information.