A.19 Signalling flows for vSRVCC
24.2373GPPIP Multimedia (IM) Core Network (CN) subsystem IP Multimedia Subsystem (IMS) service continuityRelease 17Stage 3TS
A.19.1 Introduction
The signalling flows in this subclause demonstrate how a session consisting of active speech and video media components is transferred from PS to CS using vSRVCC procedures.
A.19.2 Session transfer for active call with speech and video using vSRVCC procedure: PS to CS
In the example flow in figure A.19.2-1, the SC UE A has an ongoing session anchored at SCC AS, with only active speech and video media components and the SC UE has an ongoing session anchored at the SCC AS with only inactive speech media component. Based upon measurement reports sent from the UE to E-UTRAN, the source E-UTRAN decides to trigger a Single Radio handover from PS to CS access.
Figure A.19.2-1: PS-CS vSRVCC for an ongoing speech and video session
1. SC UE A has an active speech and video session and a held speech-only session
The UE has an session containing only active speech and video media components, and a session containing only inactive speech media component. The corresponding multimedia PS bearer extends between UE A and UE B via intermediate IM CN entities.
2. Interaction between UE, E-UTRAN, MME and MSC
UE A sends the measurement reports to E-UTRAN, and the source E-UTRAN decides to trigger a Single Radio handover from PS to CS access (as specified in 3GPP TS 23.216 [5]). The MME sends a PS to CS Request message, containing an vSRVCC indication and STN-SR, via Sv to the MSC.
3. SIP OPTIONS request (MSC server to intermediate IM CN subsystem entities) – see example in table A.19.2-1
The MSC has no prior knowledge whether the most recently made active session is a session with audio and video media or a session with only audio media. The receipt of the vSRVCC indication over the Sv interface is the trigger for the MSC server to send a SIP OPTIONS request for the purpose of determining the session that was made active most recently.
Table A.19.2-1: SIP OPTIONS request (MSC server to intermediate IM CN subsystem entities)
OPTIONS tel: +1-237-555-3333 SIP/2.0
Via: SIP/2.0/UDP mgcf1.visit1.net;branch=z9hG4bk731b87
Max-Forwards: 70
P-Asserted-Identity: <tel:+1-237-555-1111>
From: <tel:+1-237-555-1111>;tag=171828
Call-ID: cb03a0s09a2sdfglkj490334
Contact: sip:msc1.visit1.net
Accept: application/sdp
Content-Length: 0
4. SIP OPTIONS request (intermediate IM CN subsystem entities to SCC AS)
The SIP OPTIONS request is routed towards the SCC AS.
5. Determination of session that was made active most recently
The SCC AS determines the session that was made active most recently, based on the C-MSISDN in the P-Asserted-Identity header field.
6. SIP 200 (OK) response (SCC AS to Intermediate IM CN subsystem entities)
The SCC AS sends a SIP 200 (OK) response. with an SDP body containing "m=" lines for audio and video, as the session that was made active most recently is a session with both audio and video media. The contents of the SIP 200 (OK) allow the MSC server to determine how to populate the SDP Offer in the SIP INVITE for STN-SR (see Step 8).
Table A.19.2-2: SIP 200 (OK) response (SCC AS to intermediate IM CN subsystem entities)
SIP/2.0 200 OK
Via: SIP/2.0/UDP mgcf1.visit1.net;branch=z9hG4bk731b87
From: <tel:+1-237-555-3333>
To: <tel:+1-237-555-1111>;tag=314159
Call-ID: cb03a0s09a2sdfglkj490334
Content-Type: application/sdp
Content-Length: (…)
m=audio 3456 RTP/AVP 97 96
m=video 3400 RTP/AVP 98 99
7. SIP 200 (OK) response (Intermediate IM CN subsystem entities to MSC server)
The intermediate IM CN subsystem entities forward the SIP 200 (OK) response to the MSC server.
8. SIP INVITE (MSC server to Intermediate IM CN subsystem entities)
The MSC server sends a SIP INVITE due to STN-SR with the following parameter settings:
Request-URI: contains the STN-SR
SDP: contains only a speech media component and a video media component with default codecs for speech and video (as specified in 3GPP TS 26.111 [69])
Contact: contains the address of the MSC server.
9. SIP INVITE (Intermediate IM CN subsystem entities to SCC AS)
The SIP INVITE is forwarded to the SCC AS with parameters listed in step 8 unchanged.
10. Update of the remote leg
The SCC AS performs SIP signaling (re-INVITE) for the purpose of updating the remote leg (UE B) with new contact information (for signaling and media).
11. and 12. SIP 200 (OK) response (SCC AS via Intermediate IM CN subsystem entities to MSC server)
The SCC AS acknowledges the receipt of the INVITE received in step 8. The SIP acknowledgement is forwarded to the MSC server.
13. and 14. SIP ACK (MSC server via Intermediate IM CN subsystem entities to SCC AS)
The MSC server sends a SIP acknowledgement to the Intermediate IM CN subsystem entities; it is forwarded to the SCC AS.
15. and 16. SIP BYE request (SCC AS via intermediate IM CN subsystem entities to UE)
The SCC AS releases the source access.
17. and 18. SIP 200 (OK) response (UE to SCC AS via Intermediate IM CN subsystem entities)
The UE acknowledges the release of the source access leg.
19. UE attaches in CS and sets up a data call
The UE attaches in the CS domain and establishes a data call based on a BS30 bearer, as described in 3GPP TS 24.008 [8]. The UE replaces the session with active speech and video media components with the newly established CS video call.