Telephone Terminology: What Does It All Mean?

When Alexander Graham Bell invented the telephone, he did more than create an object of amusement. He created a tool that has become the primary means of alerting public safety. In addition to traditional telephony, telephone lines and technology have...


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A companion to ANI is Automatic Location Information (ALI). For the first time, emergency services personnel received an accurate address of an emergency automatically. ANI and ALI are supported by a file known as a Master Street Address Guide (MSAG) that correlates every telephone number with an appropriate address within a jurisdiction. MSAGs may be maintained by the telephone company or by the Public Safety Answering Points (PSAPs) themselves.

Because telephone exchange boundaries rarely conform directly to the boundaries of public safety jurisdictions, Selective Routing assured that exchanges could be split so that calls would be delivered properly. This is accomplished through the cross referencing of all conventional telephone numbers to an Emergency Service Number (ESN) that indexes the appropriate combination of fire, EMS, and law enforcement responders to the corresponding address. This is accomplished by the Tandem, or a network hub switch in a telephone company central office. Telco central offices (COs) contain switchgear and provide dial tone and other services for a geographic area and selected exchanges.

The Tandem office serves a larger region, typically receiving and routing 911 calls from multiple local COs over 911 trunk lines, which receive a priority status over traditional calls and circuits. Even the smallest community will have at least two 911 trunks to allow for multiple emergencies. The central office where the call originates is often termed the End Office.

All of the items previously discussed relate to the provision of local emergency and non-emergency conventional telephone service, with the latter commonly referred to as Plain Old Telephone Service (POTS). However, the picture is much larger than that. The North American Numbering Plan, instituted in 1951, coordinates assignments of three-digit area codes to the U.S., Canada and 17 other countries. While area codes used to be adjoining geographic areas that served major regions or entire states, the demand for telephone numbers created by cell phones, pagers, fax machines and Personal Digital Assistants (PDAs) created a new animal known as the area code overlay, which rests atop existing area codes in an effort to provide a sufficient amount of available numbers. And, while in the past, telephone numbers were assigned to specific carriers, the edict of number portability in some cases lets a consumer take his or her telephone number to another provider. All of this is a far cry from the days of a single telephone company.

A far cry, too, from those days, is cellular telephone service. It is now commonly used as both a means of reporting emergencies and by the emergency crews themselves for coordination. The Cellular Telephone Industry Association (CTIA) estimates that over 72 million wireless distress calls were made in the U.S. alone during 2003. As a comparison, in 1985 – the first year for which statistics are published – fewer than 195,000 were made. Obviously, these demands have had an impact on communications systems. In 1996, the Federal Communications Commission issued Report and Order 94-102, establishing guidelines for enhanced wireless 911.

When it comes to wireless, 911 service is categorized by phases. Phase 0 is equivalent to basic conventional 911. The call is delivered and little more. Phase 1 provides the caller telephone number, as well as the number and location of the receiving cell site. The cell site telephone number is known as Pseudo Automatic Number Identification (P-ANI). Phase 2 delivers location information based on FCC guidelines, which are specific to the technology used by the carrier.

Network-based solutions rely on a system akin to triangulation, while handset-based solutions use Global Positioning System (GPS) chips in the telephones themselves. Wireless location is based on the X/Y coordinates of longitude and latitude, but it does not provide a Z coordinate or relative elevation. While this is of lesser concern in rural areas, it can be problematic in an urban high-rise environment. However, problems with automatically obtaining a correct address were not limited to cellular calls. Even today, issues exist with conventional Post Branch Exchange (PBX) switches that serve corporate campuses or companies with dispersed facilities that are served by a centralized phone system. While the ANI/ALI received will indicate that the call is coming from the corporate headquarters, the real emergency may exist many miles away.