6d Procedures and flows for DRVCC Emergency Session for WLAN

23.2373GPPIP Multimedia Subsystem (IMS) Service ContinuityRelease 17Stage 2TS

6d.1 IMS Emergency origination flow for PS to CS DRVCC

This procedure is only applicable if UE uses WLAN access to EPC via s2a or s2b, see TS 23.167 [23].

Figure 6d.1-1 provides flow for an emergency session established in IMS, illustrating how the emergency session is anchored in the EATF.

Figure 6d.1-1: UE initiating an emergency session in IMS

1. The UE initiates an IMS emergency session over WLAN access to EPC as defined in TS 23.167 [23]. This involves the UE generating a SIP INVITE containing the UE’s location information and the equipment identifier.

2. The P-CSCF selects an E-CSCF and forwards the INVITE to the E-CSCF.

3. The E-CSCF sends the INVITE to the EATF.

4. The EATF (acting as a routing B2BUA) anchors the emergency session, i.e. the EATF is inserted in the signalling path which invokes a 3pcc for enablement of Access Transfers for the call as specified in clause 6.3.1.3. EATF allocates an E-STN-DR for this session. The INVITE contains information (network provided PANI header) that this call is via WLAN access and this can be used to trigger the E-STN-DR allocation. This E-STN-DR is return back to the UE in the SIP response message to #3.

5. The EATF creates a new INVITE and sends it back to E-CSCF.

6. For this optional procedure, refer to TS 23.167 [23].

7. The E-CSCF uses the routing information to format the INVITE message, and it sends it directly to the PSAP, or to the PSAP via the MGCF.

6d.2 DRVCC session transfer of IMS emergency session for PS to CS

Figure 6d.2-1 provides flow for DRVCC for IMS emergency session.

Figure 6d.2-1: IMS level Call flow for DRVCC for IMS emergency session with E-STN-DR

1. UE has an active emergency session over WLAN and have received E-STN-DR from IMS (EATF) for this session. UE determines that handover to CS RAT is needed (based on UE implementation).

2. UE performs normal CS location update if it has not yet attached to the CS domain.

3. While the IMS emergency call is on-going/active, UE starts a normal CS call setup using the E-STN-DR.

NOTE 1: It is known that this solution does not support the higher RRC level priority for CS emergency setup using CS emergency setup procedure as defined in TS 24.008 [24]. However, this is a "make before break" procedure so UE can retry on the CS domain if call fails while emergency call is maintained on the WLAN side.

Editor’s note: It is for further study if there is a need for and how to optimize the CS Setup procedure handling in the MSC.

4-5 MSC Server initiates the session transfer with the E-STN-DR and follow the same procedure as defined in clause 6c.2 with the following updates:

– Based on configuration, MSC Sever is aware the E-STN-DR is within a range of numbers that are defined for dual radio emergency session continuity procedure and can trigger priority call handling if needed.

– The EATF uses the E-STN-DR to determine that Access Transfer is requested and for correlating with the source access leg for which the Access Transfer is needed.