Routing E9-1-1 Calls by Using an ELIN Gateway

Topic Last Modified: 2013-02-05

Some partners in the Unified Communications Open Interoperability Program provide qualified Emergency Location Identification Number (ELIN)-capable gateways, which can serve as an alternative to a SIP trunk connection to a qualified E9-1-1 service provider. ELIN gateways support ISDN or Centralized Automatic Message Accounting (CAMA) connectivity to public switched telephone network (PSTN)-based E9-1-1 services. For details about partners who provide ELIN gateways and links to their documentation, see https://go.microsoft.com/fwlink/p/?LinkId=248425.

Like SIP trunk connections to E9-1-1 service providers, ELIN gateways also provide the means of routing an emergency call to the caller's most appropriate Public Safety Answering Point (PSAP), but these gateways use an ELIN as the location identifier. You define ELINs for each Emergency Response Location (ERL) in your organization (for details, see Managing Locations for ELIN Gateways).

When you use an ELIN gateway for emergency calls, you use the same Lync Server E9-1-1 infrastructure that you would use for a SIP trunk connection. That is, the Location Information service database provides the location to the Lync Server client, and the location policy enables the feature and defines the routing. With an ELIN gateway, however, you need to add the ELINs to the Location Information service database and have your PSTN carrier upload them to the Automatic Location Identification (ALI) database.

When a Lync client obtains its location from the Location Information service, the location includes the ELIN. During an emergency call, the ELIN is included with the location sent to the ELIN gateway. The ELIN gateway identifies the call as an emergency call and swaps the calling party's number with the ELIN. The ELIN gateway then routes the call to the PSTN with the ELIN as the calling number. The PSTN E9-1-1 provider looks up the ELIN in the ALI database, which is a companion database to the Master Street Address Guide (MSAG) database. The PSTN then sends the call to the most appropriate PSAP based on the ALI lookup, and the PSAP sends first responders to the caller's location based on the ALI lookup. The calling number is cached on the ELIN gateway for a predefined amount of time for callbacks. During a callback, the PSAP reaches the ELIN gateway, which swaps the ELIN for the caller's direct inward dialing (DID) number.

ELIN gateways support emergency calls only from within your organization's network. They do not support emergency calls made from outside your network.

The following diagram shows how an emergency call is routed from Lync Server to the PSAP when you use an ELIN gateway.

Routing E9-1-1 calls with an ELIN gateway

1. A SIP INVITE containing the location, the caller's callback number, and the (optional) Notification URL and conference callback number is routed to Lync Server.

2. Lync Server matches the emergency number and then routes the call (based on the PSTN Usage value defined in the applicable location policy) to a Mediation Server, and from there to an ELIN gateway.

3. The ELIN gateway routes the call over an ISDN or CAMA trunk to the PSTN.

4. The PSTN identifies the call as an emergency call and routes it to an E9-1-1 router in the network. The E9-1-1 router looks up the caller's number in the ALI and MSAG databases to obtain the geographical location. The E9-1-1 router sends the call to the most appropriate PSAP based on the location information that was retrieved from the databases. If the location was manually entered by the user, the Emergency Call Response Center (ECRC) first verbally verifies the accuracy of the location with the caller before routing the emergency call to the PSAP.

5. If you configured the location policy for notifications, one or more of your organization’s security officers are sent a special Lync emergency notification instant message. This message always pops up on the security officers’ screen(s) and contains the caller’s name, phone number, time, and location.

6. If you configured the location policy for conferencing and it is supported by the PSTN E9-1-1 service provider, an internal Security Desk is conferenced into the call with either one-way or two-way audio.

7. If the call is broken prematurely, the PSAP uses the ELIN to contact the caller directly. The ELIN gateway swaps the ELIN for the caller's DID.