How 9-1-1 Works

Enhanced 9-1-1 Systems

­Enhanced 9-1-1 is a very similar system, but it has some upgrades and modi­fications that make the whole process run more smoothly. There's more technology involved - in E911, there's a whole local "9-1-1 network" of collaborative databases that plays a role before the PSAP operator even picks up the call. For an E911 setup, add to the equipment list above:

  • Automatic Number Identification (ANI) The phone company knows every time you place a call from your phone number - it needs to know for billing purposes. This functionality is adapted to 9-1-1 purposes in E911: When you call 9-1-1 from your phone, the phone company recognizes the emergency number and uses the ANI system to pull up your phone number and send that data with your phone call to the 9-1-1 system.
  • Automatic Location Identification (ALI) The phone company has a subscriber database matching phone numbers to names and addresses. When your call arrives at the 9-1-1 network, the hub taps into this database to pull up the address that matches your phone number.
  • Master Street Address Guide (MSAG) The phone company and public safety agencies collaborate to create master maps that match phone numbers, addresses and cross streets to their corresponding PSAP. When you dial 9-1-1, the 9-1-1 network hub uses the MSAG to determine where to route your call.

As of 2006, 93 percent of 9-1-1 coverage is enhanced 9-1-1. Here's what the call looks like:

  1. You dial 9-1-1.
  2. The phone company computer recognizes the number, accesses the ANI to get your number and routes the call to the dedicated 9-1-1 switch that acts as a hub for the local network.
  3. The network uses your number to get your address from the ALI and uses your address to determine the proper PSAP destination from the MSAG (this is sometimes called selective routing, because the switch uses dynamic data to determine where to send your call instead of blindly routing it to a pre-determined PSAP). In most cases, this all takes a little over one second.
  4. Your phone call now carries your phone number and address along with your voice data to the nearest available PSAP. This information is displayed on the call-taker's computer when he or she takes your call.
  5. Some PSAPs simultaneously send that ANI/ALI data to the police computer dispatch network to allow for immediate access.
  6. If necessary, many PSAPs can transfer your call and your accompanying data to another PSAP.

When you're dialing from a landline, this system works really well. But when new phone technology enters the picture, a few hiccups emerge. The PSTN is not like a cell-phone network, and it's definitely not like a VoIP network. The rise of both of these types of phone service has placed demands on the 9-1-1 system to adapt in order to maintain an effective public-safety network.