D.2 ECS routes the emergency call to the gateway with record route

23.1673GPPIP Multimedia Subsystem (IMS) emergency sessionsRelease 17TS

Figure D.2

This flow is supported by the procedures in clause 7.3, where the E‑CSCF need not enquire the LRF for location information.

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 re-issues an Invite to an appropriate MGCF/MGW with the ESRN/LRO, ESQK and a record route indication. or the call to be routed to PSAP the P-Asserted-Identity contains ESQK, A P-Asserted-Identity field may be inserted in the INVITE message, f for the call to be routed to other emergency answering centre the P-Asserted-Identity contains the CBN.

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

5) The emergency session establishment signalling continues.

6) The PSAP retrieves location from the ECS.

7) Either the caller or PSAP initiates the call termination signalling.

8) The E‑CSCF or MGCF/MGW forwards the hang-up message to the ECS. At this time, the ESQK allocated to the emergency session can be released.

9) The ECS sends an OK to the E‑CSCF or MGCF/MGW.

10) The call termination signalling continues.

Annex E (informative):
Emergency support in different IP-CANs

Support for emergency services in the IP Multimedia Core Network can be provided by certain IP-CANs according to table E.1, which shows the level of possible emergency support in this version of the specification. The UE may be able to use also other IP-CANs that are not included in table E.1 to access emergency services in the IP Multimedia Core Network, but the support of such access technologies is out of scope of this specification.

Table E.1: Support for IMS Emergency Services by Different IP-CANs

IP-CAN	Normal Access	Emergency support	Insufficient Security Credentials
GPRS (UTRAN)	X	X	X
Fixed Broadband	X	X	X
cdma2000 HRPD/PDS	X	X	X
cdma2000 HRPD/EPC	X	X	X
EPS (UTRAN and E-UTRAN)	X	X	X
WLAN access to EPS	X (NOTE 1)	X	–
5GS (NG-RAN)	X	X	X
non-3GPP access to 5GC	X (NOTE 1)	X	–
NOTE 1: Emergency Sessions use a normal access to EPC/5GC only in the case of emergency sessions not detected by the UE and nevertheless served by the IMS network.

The term "normal access" in table E.1 means that the IP-CAN is not made aware of the UE intent to establish an emergency services session and therefore provides no special emergency support e.g. call priority.

The term "emergency support" in the table means that the IP-CAN is made aware of the UE intent to establish an emergency services session, or at least of the priority nature of the access, so that the access network is able to handle the emergency call with appropriate priority.

The term "insufficient security credentials" in the table means that the IP CAN is able to allow a UE with no UICC card or a UICC card with insufficient credentials to obtain IP bearer access for emergency services in the IP Multimedia Core Network.

Annex F (normative):
IMS Emergency Services Using HRPD/PDS Network