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9-1-1 Resources / History

on November 12, 2013

The nation’s independent, locally operated 911 systems have become the backbone of emergency reporting and response, and the term “nine-one-one” has become synonymous with public safety in general. However, 911 (or 9-1-1) systems are simply telephone equipment that routes calls to the nearest public safety agency communications center, and in most cases displays the callers location and telephone to the dispatcher, thereby insuring a quick and accurate response to emergencies.

Within the past 10 years the subject of 911 has become very complex with the addition of more advanced forms of incoming communications, including wireless cellular and satellite phones, automatic crash notification systems and, most recently, Voice Over IP (VoIP) telephone service. With each new emerging technology, the nation’s local 911 systems managers must change procedures, upgrade their equipment, find more sources of funding, and adapt to how the public wants to report emergencies–not an easy task!

Behind all the technology is a very interesting development history, which I’ve researched and verified first-hand. Although the first 911 call was placed in 1968, the concept was raised much earlier…check the history page for more details.

The FCC and cellular carriers are working on implementing text-to-911 in a methodical way. In general, the service is only available at fewer than 10 communications centers as of mid-2013. The FCC has posted information about text-to-911 service and the implementation effort, along with a Quick Facts & FAQs page.

Key 911 Features
  • allow calls from any phone, including pay phones without coin
  • route call to proper emergency agency
  • display the caller’s location (ALI) and phone number (ANI)

The Network

There is no national 911 system or network. Instead, the 911 networks are created and operated locally, either by cities and towns, but more commonly by counties. In some cases the networks are administered by a government public safety team, and technically operated by a telecommunications company (the phone company). In other networks, some of the technical operations are handled by public safety personnel (usually the subscriber database and call routing information).

Most states have a method of collecting taxes or surcharges from telephone users, and funding the provision of 911 equipment used by local public safety agencies. These funds do not, however, fund the actual operation of the center. Then, state funding goes for computers, terminals, consoles and associated gear, but not to salaries.

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 that handles emergencies for the location of the call, and in an Enhanced 911 (E911) system, display the callers telephone number and address in the comm center to assist in a proper response.

The technology of performing this concept takes many forms. However, there are some basic building blocks, which is also shown here. The explanation below is mainly for the wired telephone network, but also applies to the cellular VoIP and telematics networks.

Also check a 911 reference diagram by Intrado, and a wireless 911 call processing diagram by Sprint.

First, the public telephone system already identifies the telephone number for every call placed on the network in order to properly bill the subscriber each month. When a 911 call is placed, this phone number is identified through what is known as Automatic Number Identification (ANI) and passed to the 911 network.

Second, telephone companies maintains a subscriber database for every assigned telephone number, the subscriber’s name, physical address for a wired phone account, and billing information. The address is therefore identified through what is known as Automatic Location Identification (ALI).

Third, the telcom provider and the pubilc safety agencies collaborate to create an E911 Master Street Address Guide (MSAG), a database that 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. This allows the phone company’s computer to match the caller’s location with its public safety jurisdiction, and then route the call to the appropriate communications center.

Note that “pubilc safety jurisdiction” may be different than the caller’s political jurisdiction. That is, the caller may live in Oaktown, but their 911 calls are handled by Maple City, or Elm County. The MSAG is pre-programmed to take 911 calls and route them to the proper Public Safety Answering Point (PSAP) that will then dispatch emergency personnel in response to a 911 call.

The location, administration and structure of MSAGs is different among telecom providers. Some companies maintain many local databases, while others maintain just one or two database sites for their entire, multi-state territory. In still other cases, the MSAG is maintained by the participating public safety agencies, or even by private companies. In all cases, the physical location of the MSAG is not relevant, since today’s computer systems allow extremely fast transmission and data retrieval speeds that don’t delay the routing of 911 calls.

Lastly, 911 calls travel on special telephone networks and uses special switches to provide some degree of non-interference with other phone traffic, and a degree of protection 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.

The Issues Now
  • extend E911 to every local jurisdiction in the United States [NENA map]
  • connect emerging technologies to E911 network, including text messages, photos and videos
  • adjust funding models to account for fewer landlines and more cellular phones
  • plan, fund and create a Next-Generation E911 network [see info]

Next, the call is processed–usually within milliseconds–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 transferred directly to the dispatch computer system so the information is available for dispatch.

An important additional feature of 911 systems is the ability to transfer calls to another PSAP or to any seven-digit number. Importantly, if a 911 call is transferred to another PSAP, the ANI/ALI information will be transferred along with the voice call, so the other PSAP can also view the information.

The federal Network Reliability Council has posted a general network diagram, and NENA has posted a view of the next-generation of E911.

The primary national organization devoted to 911 is the National Emergency Number Association (NENA), headquartered in the Washington (DC) area. Check their Web site for all things related to 911.

The federal government’s National 911 Program has many resources, including an on-going series of Webinars on various topics.

In early 2013 a series of SWATing calls began, targeting celebrities. Read more about the incidents, how the calls are made and possible methods for screening such malicious calls.

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