Emergency Services for Mobile

There are two most important components in emergency services handling -

PSAP - Public Safety Answering Point, the call center that answers emergency calls.

URN - Uniform Resource Name

From the point of view of the PSAP, three essential elements characterize an emergency call:

1. The call is routed to the most appropriate PSAP, based primarily on the location of the caller.

2. The PSAP must be able to automatically obtain the location of the caller with sufficient accuracy to dispatch a responder to help the caller.

3. The PSAP must be able to re-establish a session to the caller if for any reason the original session is disrupted.

An emergency call can be distinguished from any other call by a unique Service URN that is placed in the call set-up signaling when a home or visited emergency dial string is detected. Because emergency services are local to specific geographic regions, a caller obtains his location prior to making emergency calls. To get this location, either a form of measuring, for example, GNSS is deployed, or the endpoint is configured with its location from the access network's Location Information Server (LIS) using a Location Configuration Protocol (LCP). The location is conveyed in the SIP signaling with the call. The call is routed based on location using the LoST protocol which maps a location to a set of PSAP URIs. Each URI resolves to a PSAP or an Emergency Services Routing Proxy (ESRP) that serves as an incoming proxy for a group of PSAPs. The call arrives at the PSAP with the location included in the INVITE request.

Example -

* The phone "boots" and connects to its access network.

* The phone gets location via a Location Configuration Protocol (LCP), for example from the DHCP server in civic and/or geo forms, a HELD server or the first level switch's LLDP server.

* The phone obtains the local emergency dial string(s) from the LoST server for its current location. It also receives and caches the PSAP URI obtained from the LoST server.

* Sometime later, the user places an emergency call. The phone recognizes an emergency call from the dial strings and uses the "urn:service:sos" URN to mark an emergency call.

* It refreshes its location via DHCP and updates the PSAP's URI by querying the LoST mapping server with its location.

* It puts its location in the SIP INVITE request in a Geolocation header and forwards the call using its normal outbound call processing, which commonly involves an outbound proxy.

* The proxy recognizes