A.17.4 User answers in PS domain; Handover to CS successful
24.2373GPPIP Multimedia (IM) Core Network (CN) subsystem IP Multimedia Subsystem (IMS) service continuityRelease 17Stage 3TS
In the example flow in figure A.17.4-1, SC UE A has an incoming session with speech media component which is anchored at SCC AS. The session is in alerting phase. Based upon measurement reports sent from the UE to E-UTRAN, the source E-UTRAN decides to trigger a PS to CS SRVCC handover to CS access. However the user answers the call in E-UTRAN and the SC UE sends a SIP 200 (OK) response to the SCC AS. It this scenario the handover to CS is successful.
Figure A.17.4-1: SIP 200 (OK) response from SC UE received by SCC AS: Handover to CS successful
NOTE 1: For clarity, the SIP 100 (Trying) responses are not shown in the signalling flow.
1 SC UE A has received an incoming call and is in Ringing State
The incoming call has been anchored at the SCC AS of SC UE A. Both ends have reserved the resources and SC UE A has sent a SIP 180 (Ringing) response.
2-5 MSC server sends session transfer request. SCC AS sends SIP UPDATE to update the remote end
These steps are identical to steps 3-6 in subclause A.17.2.
6 User answers the call when the UE is still in the source E-UTRAN access
7-8 SIP 200 (OK) response (SC UE to intermediate IM CN subsystem entities to SCC AS)
The SCC AS performs no additional actions on receipt of the SIP 200 (OK) i.e. the SCC AS does not confirm reception of the SIP 200 (OK) response with SIP ACK request and performs no actions on dialogs with UE B and with MSC server.
9-21 Continuation of procedure for PS to CS SRVCC in Alerting Phase
These steps are identical to steps 7-19 in subclause A.17.2.
22 UE receives H/O command from source E-UTRAN
23 UE retunes to 3G
24 CC CONNECT message from SC UE A to MSC server
The SC UE A sends the CC CONNECT message as it did not receive a SIP ACK to the SIP 200 (OK) sent in step 7.
25-37 Continuation of procedure for PS to CS SRVCC in Alerting Phase
These steps are identical to steps 21-33 in subclause A.17.2.
38-39 SIP ACK request (SCC AS to intermediate IM CN subsystem entities to SC UE)
The SCC AS confirms reception of the SIP 200 (OK) response received in message 8.
40 Release original SIP dialog
The SCC AS releases the SIP dialog towards the SC UE.
NOTE: Step 39 is performed only if the SC UE A uses Gm after the PS-CS access transfer in alerting phase is completed; otherwise, the SC UE A and the network release the source access leg locally, without any signalling between the SC UE A and the network