LTE: For the CS fall back what messages are exchanged between IMS, LTE and GSM nodes to continue VoLTE call ?

Question

Sometimes due to coverage hole issue in the LTE network, the ongoing VoLTE is continued to UE by using the CS network like GSM network. This is known as SRVCC (Single Radio Voice Call continuity). What messages are exchanged between network nodes when UE moves from LTE to GSM ?

Answer 1

1. EUTRAN indicates MME of SRVCC handover after measurement;
2. MME initiates SRVCC procedure via Sv interface to MSC server;
3. MME also handles PS handover procedure for non-voice sessions
4. MSC Server reserves bearers in CS network, then initiates IMS session transfer procedure;
5. After successful bearer reservation, MSC Server indicates MME, who will then notify UE to reselect to UTRAN/GERAN;
6. UE continues the ongoing call when successfully reselecting to UTRAN/GERAN