What Annie and Alleys Have to do with 911

Designing for emergency calls is an important part of implementing a Unified Communications system.  In Canada and the United States, a system has been implemented to assist Emergency Telecommunicators (“dispatchers”) handling emergency calls. The system is called Enhanced 9-1-1 (e911).  This system allows dispatchers to identify where the emergency call is originating from.  For the remainder of this blog, I’ll be focusing on the mechanics of how the system is deployed in the US (Get ready for acronym city!).

Each telephone is connected to the telephone company (“telco”) central office (CO), which in this case, is referred to as the “End Office.”  When a 9-1-1 call is placed, the switch at the CO recognizes it, and sends the call to the Tandem Office, sometimes utilizing trunks dedicated for this purpose. A call setup message is transmitted from the End Office to the Tandem Office which contains the Calling Party Number (CPN) which becomes the Automatic Number Identification (ANI, pronounced “Annie”).

The device at the Tandem Office that these emergency trunks are connected to is called the Selective Router (SR), though I’ve also heard them referred to as Selective Access Routers (SAR). Once upon a time, a CO switch such as Nortel DMS-200 would be used for this, but most modern tandems use special-purpose equipment from vendors like EADS.

Upon reaching the SR, the ANI is looked up in the Automatic Location Identification (ALI, pronounced “alley”). The ALI database is populated by the telco, and shows the service address for each ANI. To ensure the addresses are valid, the telco usually has a procedure which screens the address against the Master Street Address Guide (MSAG) when an order for new service is taken. If the address provided by the customer cannot be found in the MSAG, the order should be rejected.

The MSAG is maintained by the local Authorities Having Jurisdiction (AHJ), which is usually the local 9-1-1 coordinator. Sometimes the local planning department, along with address database services from the US Postal Service, are used to maintain the quality of data in the MSAG. Another important piece of the MSAG is that it identifies which Emergency Services Number (ESN) the address is a part of.  The ESN

is a three to five digit number representing a unique combination of emergency service agencies (Law Enforcement, Fire, and Emergency Medical Service) designated to serve a specific range of addresses within a particular geographical area, or Emergency Service Zone (ESZ). The ESN facilitates selective routing and selective transfer, if required, to the appropriate PSAP and the dispatching of the proper service agency(ies).”

~ NENA Master Glossary Of 9-1-1 Terminology

In other words, the ESN tells the SR which Public Safety Answering Point (PSAP) takes calls for your location.

So let’s summarize what happens up to this point:  When you call 9-1-1, your end office forwards the call to the 9-1-1 SR, which looks up your ANI in the ALI to determine which ESN you’re associated with, and which PSAP your call should be directed to.

In a future post, I’m going to talk about how you, yes you, can create multiple entries in the ALI database so the dispatcher can determine where in the building/campus the call is originating from. This process is called Private Switch Automatic Location Identification (PS/ALI) and you might be required to do it by law according to the AHJ.


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