Question: Upon sending an SGsAP-EPS-DETACH-INDICATION message to the VLR for implicit IMSI detach from EPS services, what action does the MME take regarding the SGs association state and timer Ts13?
Option 1: The MME changes the SGs association state to SGs-ACTIVE and stops timer Ts13.
Option 2: The MME keeps the current SGs association state unchanged and does not start any new timers.
Option 3: The MME moves the SGs association state to SGs-NULL but does not initiate timer Ts13 until an acknowledgement is received.
Option 4: The MME moves the state of the SGs association to SGs-NULL and starts timer Ts13 upon transmission of the message.
Explanation: According to the procedure for implicit IMSI detach from EPS services, after sending the SGsAP-EPS-DETACH-INDICATION message, the MME must update the state of the SGs association to reflect the initiation of the detachment process (SGs-NULL) and simultaneously start a timer (Ts13) to monitor the acknowledgement procedure. This ensures both a change in state and the initiation of a timer to manage potential non-responses from the VLR.