• What is the purpose of the SRVCC Handover indication in the handover request message from eNB to MME?
• What is the difference between STNSR and CMSISDN in the SRVCC PS to CS Request Message?
• How does the MSC Server initiate the session transfer with the IMS network?
• What are the advantages and disadvantages of SRVCC compared to other solutions for voice continuity?
• How does the UE perform cell selection and handover detection in the target UTRAN/GERAN cell?
?????? ????? ????? ???? ?????????? (?????)
SRVCC is a call transfer method or handover which is implemented in a simplified and reliable manner used . When an LTE user has an active voice session in IMS and is moving to areas which have 2G/3G coverage and does not have LTE coverage.
????? ???????? ??????????? & ?????????/?????????? ?????:
1. ??? ????????? ???????? ???????:
eNB sends handover request to MME with SRVCC Handover indication for CS + PS handover.
2. ?????? ????????? ?? ?????? ???:
Source MME splits voice bearer from other PS bearers. Initiates relocation of PS bearers towards MSC Server and SGSN.
3. ?? ?? ???????? ?????????:
MME sends SRVCC PS to CS Request Message to MSC Server. Contains critical information: IMSI, Target ID for Voice Call, STNSR, CMSISDN, etc.
4. ???????? ?? ?????? ??????????:
MME sends Forward Relocation Request to target SGSN.
5. ???????? ?????????? ??????? ?? ???:
Both MSC and SGSN request resource allocation for CS and PS from the RNC in the 3G network.
6. ??? ?? ??? ?????:
After receiving Relocation Handover Request Ack from RNC, MSC sets up CS leg with IMS network.
7. ???????? ??????:
SGSN updates HLR that now it will handle data traffic for UE.
8. ??? ????????? ??????? ????????:
MSC initiates session transfer using STNSR with IMS network. Sends New Call ISUP IAM message toward IMS with STNSR as destination.
9. ????????????? ?? ????? ????:
Newly established call between MSC and IMS network is used to carry voice once handover is completed.
10. ???????? ??????? ?? ??:
MME sends Handover Command message to ENB. ENB sends Handover Command message to UE, instructing it to switch to 3G radio.
11. ?????????? ?? ?? ?? ???????? ?????????:
PS CS handover procedure is completed, and voice communication is now established on the 3G network.
12. ?? ????? ?? ?????? ?????/????? ????:
UE performs cell selection and tunes to the target UTRAN/GERAN cell. Handover detection occurs at the target RNC/BSS.
13. ?? ????? ?????????? ???????? ????????:
UE sends a Relocation Handover Complete message via the target RNS/BSS to the target MSC.
14. ????? ?? ?? ?? ???????? ????????????/???:
MSC Server sends SRVCC PS to CS Complete Notification message to MME. MME acknowledges the information.
15. ????? ?????? ???????????? ?? ???:
At this point, MME deactivates the voice bearer towards S-GW/P-GW, indicating the completion of the SRVCC handover.
