Question: What happens to the GUTI (Globally Unique Temporary Identifier) when a UE (User Equipment) performs a Tracking Area Update after manually selecting a new CSG (Closed Subscriber Group) cell with a different TAI (Tracking Area Identity)?
Option 1: The GUTI remains unchanged as it is only updated during initial attach procedures.
Option 2: The GUTI is updated with a new value reflecting the changed TAC (Tracking Area Code) portion of the TAI.
Option 3: The GUTI may be updated or remain the same, depending on the E-USS's security procedure initiation during the Tracking Area Update.
Option 4: The GUTI is always changed to a default value before being updated with a new unique identifier after the Tracking Area Update is complete.
Explanation: During a Tracking Area Update, the E-USS may initiate authentication and start integrity using the security procedure. This implies that the update of the GUTI is not strictly mandated but rather dependent on the network's response to the UE's request, which could include updating the GUTI if security procedures are re-initiated. Thus, the GUTI may be updated or remain the same based on the E-USS's actions during this procedure.