Concepts in PS/ALI

(This post is a continuation of a previous posting on What Annie and Alleys Have To Do With 9-1-1. I recommend at least skimming that post before taking on this one.)

In a campus environment, the ALI records associated with your service usually reflect the location where the service is physically terminated by the telco. In one case, a local university had their PBX in the lower level of their library so the ALI database would have the address for that building loaded in. Unfortunately, that’s just not adequate for many emergencies. The Telecommunicator needs to know which building, and perhaps even which floor and which part of the floor, that call is originating from. This can be done using Private Switch Automatic Location Identification (PS/ALI).

To begin with, the customer needs a connection to the telco which permits it to transmit Calling Party Number (CPN). For 9-1-1, this traditionally takes the form of a Centralized Automatic Message Accounting (CAMA) trunk or an ISDN-PRI circuit. While it may or may not be an accepted practice where you live, in my part of the world, we can also use SIP trunks for this purpose.

Next, the switch at the CO needs to be configured to use CPN as ANI for 9-1-1.  If this setting isn’t made, the main billing telephone number will be transmitted as the ANI for your 9-1-1 calls, regardless of what CPN your equipment is transmitting.  Historically, I’ve had a problem with this when the CO is equipped with a 5ESS switch because there were two separate settings in that switch, one for using CPN as ANI for routine calls, and a second for 9-1-1 calls. Technicians frequently overlooked one setting when configuring the other.

Let’s assume for just a moment that you have worked with the telco to get each and every one of your DID numbers in the ALI database. Let’s also assume you took advantage of the Location field in the ALI database to insert remarks such as “Bldg 6, Flr 1, Rm 1109.” Let’s further assume that every phone has a DID number and a correctly configured External Phone Number Mask which the 9-1-1 route patterns are configured to use. Lot of assumptions there, eh?

When a user dials 9-1-1, the route pattern is matched, the External Phone Number Mask applied, and the DID number transmitted across the PRI as the CPN to the CO. The CO sees the Called Party Number is 9-1-1 and hands the call off to the SR. The SR looks up the ANI (which is the CPN) in the ALI database, identifies the ESN, and routes the call to the PSAP handling that ESN. The Telecommunicator at the PSAP receives a “screen splash” which shows data including the Location field where your remark “Bldg 6, Flr 1, Rm 1109” shows up. In the event the person making the call is unable to speak, the Telecommunicator knows what Emergency Response Location (ERL pronounced “Earl”) to direct the first responders to. If the call is disconnected, the Telecommunicator can use the ANI, which is the DID, to call back to the person making the call.

However, what’s going to cause all of this to break down are all of those assumptions I mentioned. You might also have nomadic devices (Wi-Fi phones, soft phones, extension mobility profiles) which further complicate matters.

There is a solution though. To start off, find out just how granular your data needs to be. Check out to see what 9-1-1 legislation has been enacted in your area, and double check to see if your locality has a more stringent rule on the books. Let’s say your AHJ has determined the need to know the building, floor, and general area (not to exceed a certain number of square feet) the call is originating from. After reviewing your floor plans, you have determined you need 10 ERLs to comply. You also create a few more ERLs for wireless devices and a default ERL.

Now, take a range of DID numbers and assign at least one (preferably two or three or more) DIDs to each ERL. These DIDs are now known as Emergency Location Identification Numbers (ELINs). Work with the telco to get each of your ELINs added to the ALI database; depending upon the quantity, you’ll be given one of several options:

  • Complete a form and fax it back.
  • Complete a spreadsheet and email it back.
  • Create a CSV file formatted according to the NENA standards (aka “a NENA file”) and email it to the telco.
  • Use a secure web interface to upload your NENA file to the telco.  It will be automatically screened against the MSAG database and added to the ALI database if the records validate.

Let’s walk through how a call works now:

  1. The user places a call to 9-1-1
  2. Your equipment figures out which ERL they are in and sends an ELIN for that ERL as CPN which the CO uses as the ANI (More on this later)
  3. The CO forwards the call to the SR, which locates the ANI in the ALI, determines the ESN and sends the call to the PSAP
  4. The PSAP sees the ALI record for that ELIN, which includes a description of the ERL in the Location field
  5. The Telecommunicator dials the ELIN back
  6. Your equipment receives the call and routes it to the phone which last called 9-1-1 from the ELIN, regardless of whether the phone has a DID number or not

Sounds great, right? Except for that little tiny detail of how your equipment is going to determine which ERL the call is originating from and facilitate the call back. Hands down, the best way to do this is with Cisco Emergency Responder (CER). To oversimplify things for our discussion today, CER is told by the administrator which switch ports belong to which ERLs. When a 9-1-1 call is placed, it pinpoints the switch port which the phone is connected to, determines the associated ERL, selects an available ELIN for that ERL, and sends the call out. You could write an entire book about CER, and my editor hasn’t granted me that many column inches for this blog. So we’ll just leave it at that.

In this article

Join the Conversation