Question: Upon receiving a Detach Request from a UE with an existing SGs association, indicating 'Combined EPS/IMSI detach', what action does the MME take regarding the VLR and the SGs association state?
Option 1: The MME sends an SGsAP-LOCATION-UPDATE-REQUEST to the VLR and changes the SGs association state to SGs-ACTIVE.
Option 2: The MME sends an SGsAP-IMSI-DETACH-INDICATION to the VLR indicating 'Explicit UE initiated IMSI detach from non-EPS services' only.
Option 3: The MME sends an SGsAP-IMSI-DETACH-INDICATION to the VLR indicating 'Combined UE initiated IMSI detach from EPS and non-EPS services' and then moves the state of the SGs association to SGs-NULL.
Option 4: The MME ignores the Detach Request if the UE has an existing SGs association.
Explanation: When a UE indicates 'Combined EPS/IMSI detach' in a Detach Request and there's an existing SGs association, the MME must inform the VLR about the detachment from both EPS and non-EPS services using an SGsAP-IMSI-DETACH-INDICATION message. Following this, to reflect the updated status accurately, the MME changes the state of the SGs association to SGs-NULL, signifying no active connection. This ensures synchronization between the MME's records and the UE's detachment status across different service types.