Telephone Terminology: What Does It All Mean?

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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 played a supporting role in street boxes, automatic alarms, cellular service and, most recently, Voice over Internet Protocol (VoIP). Without the telephone, our society would certainly be different, and we might still be alerted to alarms by banging on a locomotive rim.

However, with the rise of the telephone has come the rise of myriad terms and acronyms designed to describe this ever-diversifying technology. The following is a brief description of the more important facets of telephony that we sometimes take for granted.

The heart and soul of telephony is the Public Switched Telephone Network (PSTN). This is the composite conglomerate of all the overhead and underground wires as well as the central offices and switches needed to provide dial tone and route calls. At one point, almost all of this network was owned and services were provided by AT&T and/or Bell Telephone, but on Jan. 1, 1984, a federally mandated divestiture reduced AT&T’s assets by over 75% and its employees by two-thirds.

The immediate impact was to open up long-distance competition, but over the years the effects have been more widespread. Where typically one telephone company provided service to an area, now there are many Local Exchange Carriers (LECs) that supply dial tone. Occasionally, reference will be made to the dominant LEC, as the one having the most market share, or to Alternate Local Exchange Carriers (ALECs) or Competitive Local Exchange Carriers (CLECs), but the distinction sometimes blurs. The bottom line: if you provide dial tone, you are a LEC.

Another development of divestiture was the opening of the Customer Premise Equipment market. The Regional Bell Operating Companies (RBOCs), or “Baby Bells,†now found themselves with two divisions – network and equipment – and it often took two calls to find the right person to fix the problem. For a residential customer, this eventually translated into the ability to buy telephone equipment at most any department or electronics store, rather than renting or purchasing it from the telephone company. For business and public safety, it has opened the gates to numerous third-party suppliers and competitive technology.

But 16 years earlier, another federal act also had tremendous impact upon public safety – the creation of 911. While not officially recognized as the national emergency number until the passage of Senate Bill 800, The Wireless Communications and Public Safety Act of 1999, the first 911 call was made in Haleyville, AL, in 1968. This came after a recommendation from President Johnson’s Commission on Law Enforcement and Administration of Justice the previous year that a nationwide number be established for law enforcement.

There are a variety of anecdotes as to how 911 was chosen, but it seems clear that the first official mention of those particular digits came from AT&T. The formal government proposals never identified a specific number. Some of the more likely explanations are that no such area code existed at the time, and dialing two ones was fast and simple on the rotary telephones then in vogue.

The first 911 systems provided Basic 911, meaning that emergency calls could be made for free, even from a pay telephone, and a simple-to-remember number could be used. Add-ons, such as line capture, allowed the seizure of a call to initiate a trace, automatic ring-back to recall a disconnected party and forced disconnect to terminate calls where required. Down the road, Enhanced 911 was implemented, and brought with it many important features. Automatic Number Identification (ANI) gave dispatchers immediate callback information. In our current age of caller ID this may sound trivial, but it was and still is a lifesaving feature. ANI associated with 911 calls comes from a different database than does caller ID, and unlike caller ID, it cannot be blocked.

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.

Also included in wireless considerations were FCC requirements that analog phones use any available means to complete 911 calls. Often known as the “strongest-signal concept,†this mandated that emergency requests from one carrier be handled by another if the alternate vendor could better complete the call. Uninitialized telephones, or telephones lacking active service, were also given consideration. While this was designed to protect consumers who had inadvertently allowed subscriptions to lapse or who had no roaming, it soon became a serious issue for public safety when a cottage industry of hand-me-down and expressly manufactured emergency-only telephones sprung up.

Obviously, a significant amount of work was required to enable enhanced wireless solutions. While carriers were assisted by cost recovery, which let them in some cases recover roll-out costs, municipalities often had to fend for themselves. Notable exceptions to this were funds made available through the Public Safety Foundation of America (PSFA) and by the Enhanced 911 Emergency Communications Act of 2003.

To process and dispatch 911 calls, the majority of PSAPs utilize Computer-Aided Dispatch (CAD). With the advent of Phase 2 requirements, mapping support through Geographic Information Systems (GIS) has become increasingly important. In many cases, telephone systems and CAD have been combined through the use of hybrid Computer-Telephone Integration (CTI) that allows sharing of data between applications. Automatic Call Distributors (ACDs) differentiate between emergency and non-emergency calls, routing 911 requests to the first available operator. Facilities must also be capable of processing Telecommunications Device for the Deaf (TDD) and TTY TeleTYpewriter (TTY) calls from the hearing impaired, although this is not new, and required under the Americans with Disabilities Act (ADA).

The newest challenge to public safety communicators comes from the Internet, in the form of web-based telephony. Voice Over Internet Provider lets consumers use special telephones to place calls over the web. Initially, these services did not universally support 911, and when they did, typically route calls to seven-digit non-emergency numbers. Additionally, modem-supported VoIP telephones do not work during power outages and transmissions are subject to the vulnerabilities of the Internet. Routing and ANI/ALI are also a problem. However, in May 2005, the FCC released its First Report and Order regarding E911 Requirements for IP-Enabled Service Providers that addressed significant issues.


Barry Furey, a Firehouse® contributing editor, is director of the Raleigh-Wake Emergency Communications Center in North Carolina. During his 35-year public-safety career, he has managed 911 centers and served as a volunteer fire officer. In 2002, he chaired the Association of Public-safety Communications Officials (APCO) International conference and was awarded an APCO life membership in 2005 for his work in emergency communications.

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