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System Information

Vermont's 911 System

It seems so simple—Dial three numbers, and like magic, help is on the way. As the famous author, Arthur C. Clark, so aptly stated, “Any sufficiently advanced technology is indistinguishable from magic.” 911 uses very advanced technology to make sure that callers get the help they need. This memo illustrates how Vermont’s 911 system connects a caller with the appropriate help.

The Vermont Enhanced 911 Board is one of the first to implement a next generation statewide 911 system that delivers 911 calls from the Public Switched Telephone Network (PSTN) to a Public Safety Answering Point (PSAP) using Voice Over Internet Protocol (VoIP) over its Emergency Services Network (ESINet).

Currently the State of Vermont 911 System consists of six diverse locations collectively serving as one statewide 911 system. 911 calls are initially routed to one of these answering points based on the originating caller's location. The map below depicts the PSAP's coverage areas.

Click on the map for a larger view:

Call flow through the system

Vermont has an Enhanced 911 system. Enhanced means that the system is usually able to locate the caller. To do this it uses an Automatic Location Information (ALI) database. The ALI database contains the location of every wired telephone in Vermont. Cellular phones are not in the ALI database; they are located through a third-party location service. Once the system knows the caller’s location, it routes the call to the correct Public Safety Answering Point (PSAP). The following diagram illustrates how the 911 system determines where a caller is located and where to send the call:

Flowchart of e911 Call PathIt looks complicated because it is. To better illustrate the call flow, let’s follow a typical call through the system.

  1. Susan is driving with her husband Bill on I-89 near exit 10 when Bill complains of chest pain. Susan pulls over to the side of the road and then shortly afterward, Bill passes out. Susan grabs her cell phone and dials 911.
  2. All phone service providers are required to have dedicated phone connections into the 911 system. This is so 911 is accessible even when circuits are very busy, such as on Mother’s Day. These circuits route Susan's call directly into the 911 system.
  3. Computers running specialized software receive Susan’s call. Very quickly, they determine that Susan is using a cell phone.
  4. Had Susan been at home or work, she might have used a wired phone. If that were the case, the 911 computers would have looked up her location in the ALI database.
  5. Because Susan called 911 with a cell phone, the 911 system has to work a little harder to find out where she is. A Global Positioning System (GPS) computer chip in her phone sends her location to a location service provider, located in Colorado. A computer system at the service provider records her location, cross-referenced with her phone number. Meanwhile, Vermont’s 911 system contacts the same service provider, and using Susan’s phone number, checks to see if the service provider knows the location of Susan’s phone. The location service queries its database, and replies with the latitude and longitude of Susan’s phone.
  6. The 911 system sees that Susan is on I-89 in Waterbury, Vermont. The system checks one of its databases and determines that the Vermont State Police 911 answering point in Williston, Vermont, should take this call.
  7. The 911 system checks to see if there is an available call taker in Williston. It finds that Agent 44 is logged in and ready to take calls in Williston.
  8. If all of the Williston call takers were busy, the system would have checked the call takers at other answering points to see who was free. It would have sent the call to the first available call taker.
  9. The system sends the call to Agent 44’s workstation. It also sends Susan’s location so a map on Agent 44’s workstation can show the call taker where the emergency is. To save time, the system checks to see which agencies respond to police, fire and emergency medical calls on I-89 in Waterbury; it sends that information to Agent 44’s workstation too. Agent 44’s workstation alerts her about an incoming call. Less than 5 seconds have passed since Susan pressed 911 on her phone when Agent 44 answers the call.
  10. Agent 44 confirms Susan’s location, and then asks her what kind of emergency she is calling about. Once it is clear that an ambulance is necessary, Agent 44 uses a single mouse click to add the correct ambulance service to the call. Agent 44 tells the ambulance service dispatcher where the emergency is. Paramedics are soon on their way. Agent 44 stays on the phone with Susan and coaches her through giving Bill CPR, until help arrives.
  11. Because this is a cell call, the location service continuously tracks Susan’s location. If she kept driving, Agent 44 would be able to track her and keep the paramedics informed.  Thanks to a sophisticated computer system, and a host of well-trained and dedicated professionals, Bill was able to get help quickly. His doctors expect a complete recovery. I hope this memo has given you a greater appreciation of Vermont’s 911 system. For someone like Bill, in a life or death emergency, 911 can truly be indistinguishable from magic.