Question: Upon reception of an IDENTITY REQUEST message with Identity type IE set to 'SUCI', what action does the UE take if timer T3519 is already running, in relation to storing and sending SUCI (Subscriber Concealed Identifier)?
Option 1: The UE generates a fresh SUCI, sends it in an IDENTITY RESPONSE message, and updates the stored SUCI value.
Option 2: The UE sends an IDENTITY RESPONSE message with the previously stored SUCI without modifying the stored value or generating a new one.
Option 3: The UE stops timer T3519, generates a new SUCI, and then sends this new SUCI in an IDENTITY RESPONSE message.
Option 4: The UE ignores the request until timer T3519 expires and then generates a fresh SUCI for the next IDENTITY REQUEST.
Explanation: According to the specified behavior, if timer T3519 is already running when the UE receives an IDENTITY REQUEST with Identity type IE set to 'SUCI', the UE responds by sending an IDENTITY RESPONSE message containing the SUCI that is currently stored. This action does not involve generating a new SUCI or updating the stored value at this point, ensuring continuity and security as defined in telecom standards for subscriber identity management.