What is it all about?
NENA, the National Emergency Number Association estimates that approximately 240 Million calls to 9-1-1 are made to each year in the US. 9-1-1 is a critical component to any communications solution, and careful thought and planning needs to be addressed to ensure employee safety in the enterprise for PBX users. Before any solution can be deployed, a better understanding of 9-1-1 and how it works in the Public Switched Telephone Network is required.
How does my call get to the Public Safety Answer Point or PSAP?
How do I send the location information for a particular caller to the PSAP?
How a 911 call gets to the PSAP
These are all important questions that we will address here. Let’s break a 9-1-1 call into smaller pieces to understand the actual call flow behind the scenes.
- A user behind a PBX or call server dials 9-1-1 on their telephone.
- The PBX then sends this call out over normal PSTN trunk lines to the Local Exchange Carrier or LEC.
- When the LEC receives the call, it translates on the dialled digits of 9-1-1 and immediately passes this call to a specialized component in the network Called a 9-1-1 Tandem or Selective Router.
- The Selective Router is a specialized Central Office switch usually dedicated to handle 9-1-1 traffic. When a call arrives, the Caller ID of the caller is examined and matched against a special database called the Master Street Address Guide or MSAG. Based on this database lookup, the PSAP responsible for servicing this number is returned based on the provisioned street address and number.
- The Selective Router then sends this call to the appropriate PSAP down specialized E911 trunks for further handling where they receive both the original caller ID as well as the voice of the caller.
What the 911 call taker sees
The PSAP then uses specialized equipment to extract the caller ID from the call and performs another database query to the Automatic Number Identification database usually maintained by the LEC. The LEC then returns the result of the lookup with the matching Automatic Location Identification or ALI record information that is in turned displayed on the call takers terminal screen. Along with the location information, additional information is provided to the call taker that will assist in dispatching the call.
Additional information such as the responsible Police, Fire and EMS agencies are noted where a PSAP services multiple communities, and in the case of Cellular callers specific X Y coordinates are returned representing the location of the caller.
Caller ID is the Key
You can plainly see that the key piece of information provided by your PBX is the caller ID. If you send the wrong information, you may end up at the wrong PSAP for your location, or you may cause the wrong information to be displayed to the caller.
A Terrible Tragedy
Let’s take a terrible tragedy that occurred back in April of 2006. The Washington Post reports that “Kaafee Billah walked into his Gaithersburg office shortly before 9 a.m. Tuesday. The 39-year-old sales representative, who had recently started working at the medical company MedImmune Inc., made the first call from his desk at 9:02. Roughly 40 minutes later, he called 911 to say he needed medical help.”
Unfortunately he was unable to speak and clearly communicate his location to the 9-1-1 call taker. EMS did respond however, to the address that appeared on their screen which was 35 W. Watkins Mill Road, one of several MedImmune buildings in the area. They searched the building looking for Mr. Billah, but were unable to find any issues at that address. The horror was realized 10 hours later when a cleaning person found Mr. Billah’s body on the floor of his office. The only problem was, his office was at 25 W. Watkins Mill Road, an adjacent building to the 35 W. Watkins Mill Road address that was used for 9-1-1 reporting.
How did this happen? It was reported as a “phone system glitch” in the media, but as I was taught 30 years ago, computers do not make mistakes; they do exactly what they are told to do. What happened is that MedImmune recently installed a new VoIP solution, and provided telephone service to the many buildings they have in the area from a single location 35 W. Watkins Mill Road. It appears that all of the PSTN trunks were located at that location as well, or at least the ones used for 9-1-1 calls from the system.
Everything worked as designed. The call went to the PSTN, the LEC passed the call to the 9-1-1 tandem, and the caller ID and ALI appeared on the 9-1-1 call takers screen exactly as it should have.
What could have been done?
Now that you understand how 9-1-1 works in the PSTN here are the logical choices that could have been implemented to prevent the disaster:
- Use PRI or CAMA trunks that can send a specific Caller ID based on the location of the station.
- Provision specific ranges of Caller ID to specific buildings or areas in the buildings and use those numbers as caller ID for 9-1-1 calls only.
- Route the call out of a PSTN trunk actually in the facility where the caller is located instead of the main site
- Deploy a simple On Site Notification capability, found in most PBXs today, to alert local security of any 9-1-1 call that is placed.
- Any of these simple steps could have saved a life.
Yes, the PSTN 9-1-1 architecture is 30 years old and very little change has happened in that time. It was built for a hard wired society, where number mobility was non-existent. With the world’s acceptance of VoIP, and the mobility a user can now enjoy, 9-1-1 can be more problematic than in the past.
Don't Get Complacent
Don’t get complacent if you haven’t deployed VoIP though. TDM is just as susceptible to the caller ID problem as VoIP. Many systems today do not have the 9-1-1 properties properly configured, or even worse, are not provisioned at all! In either case, all 9-1-1 calls are affected whether they are TDM of VoIP based, it is not the endpoint technology that is the root cause of the problem, it is the reporting number in most cases. VoIP just makes the problem worse.
A video version of this KNOL as well as other topics on E911 are available at YouTube.on Nortel's E911 Solutions Channel.
Your insights and comments are ALWAYS welcome!
Fletch
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Anonymous
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very good info
You did very well for us
www.egyptdoor.com
Bo Gowan
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I always thought e911 problems were related to VoIP
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