D.1 ECS redirecting IMS emergency call

23.1673GPPIP Multimedia Subsystem (IMS) emergency sessionsRelease 17TS

Figure D.1

This flow is supported by the procedures in clause 7.3, where the E‑CSCF need not enquire the LRF for location information. Additional steps defined here are standard SIP methods, but not defined in this specification.

Detailed description of the procedure:

1) An IMS emergency call is initiated.

2) The E‑CSCF sends an Invite message with 911 or other well-known emergency number as the dialled number, the UE’s location information in a Location Object (LO) if available, and the UE’s media capabilities encapsulated in a SDP payload, to the ECS.

3) Based on the received Location Object (LO), the ECS will determine to which PSAP/EC the call should be routed and allocate an ESQK from the ESQK pool associated with that particular PSAP/EC. The ECS then will format a SIP response with the retrieved ESRN/ESQK in the Contact fields to redirect the emergency call.

4) The IMS Core uses the ESRN/ESQK received in the call redirect message to format an INVITE message properly, and sends it to the MGCF/MGW. A P-Asserted-Identity field may be inserted in the INVITE message, it contains either an ESQK or the CBN.

5) The emergency call setup continues with the PSAP/EC.

6) The ECS initiates a subscription at the IMS Core to request a notification of call termination of the emergency call.

7) An acknowledgement is returned.

8) The emergency session establishment signalling continues.

9) The PSAP retrieves location from the ECS.

10) The emergency session is released.

11) The IMS Core sends an Event Notification message to the ECS with an Event indicating that the 911 call has been terminated. At this time, the ESQK allocated to the emergency session can be released.

12) An acknowledgement is returned to the IMS Core.