Question: Upon receiving an SGsAP-IMSI-DETACH-ACK from the VLR, what action does the MME take regarding timer Ts9, and what subsequent action is contingent on the detach type received from the UE in case of IMSI only or combined EPS/IMSI detach not due to switch off?
Option 1: The MME starts timer Ts9 and immediately sends a detach confirmation to the UE regardless of the detach type.
Option 2: The MME stops timer Ts9 and waits for the SGsAP-IMSI-DETACH-ACK before sending the detach confirmation to the UE, but only for IMSI only or combined EPS/IMSI detach not due to switch off.
Option 3: The MME ignores the SGsAP-IMSI-DETACH-ACK for timer Ts9 purposes and sends the detach confirmation to the UE based on a different timer.
Option 4: The MME stops timer Ts9 but sends the detach confirmation to the UE immediately after, regardless of the received detach type.
Explanation: This is correct because, upon receiving an SGsAP-IMSI-DETACH-ACK from the VLR, the MME must stop timer Ts9. For IMSI only or combined EPS/IMSI detach not due to switch off, sending a detach confirmation to the UE is contingent on first receiving the SGsAP-IMSI-DETACH-ACK message, ensuring synchronization and proper detachment process handling between the MME, VLR, and UE.