Emergency Location Identification Number

Another method for providing detailed address information is through the use of Emergency Location Identification Numbers. Consider an organization that has multiple floors in a single building, and a single PRI circuit for phone calls. When an emergency call is placed, the PSAP can see the physical address for the organization based on the circuit, but no information is passed about which floor the emergency responders should go to. The PSAP has a main number that can be called back, but that number might route to a receptionist who has no idea someone in the building has tried calling emergency services.

An ELIN is a unique phone number that identifies a particular Emergency Response Location (ERL). An ERL could be a floor, a wing, or another arbitrary location within a building. Each ERL is then assigned an ELIN so that if the PSAP sees a call from a particular ELIN, the PSAP should know exactly what area or floor of building has requested help.

Within the LIS database the ELIN is actually entered in the Company field. When the gateway receives the call and the PIDF-LO information, it places a call using the discovered ELIN as the called party, which is then routed to the PSAP and answered, as shown in Figure 32.13.

Figure 32.13. ELIN E911 routing.

The other feature of ELIN is that it provides a mapping back to the original caller. The PSAP can be connected to the original caller through that ELIN. The media gateways that support ELIN keep a temporary table of callers who dialed each ELIN so that when a call comes back from the PSAP it can be properly routed.