Question: What action does the Visitor Location Register (VLR) take upon receiving an SGsAP-UPLINK-UNITDATA message for a UE with no existing or unconfirmed subscriber data, and what is the indicated cause in the response?
Option 1: The VLR processes the message normally, and responds with an SGsAP-DOWNLINK-UNITDATA message indicating 'unknown' as the SGs cause.
Option 2: The VLR ignores the message but does not send any response back to the MME regarding the issue with subscriber data.
Option 3: The VLR ignores the received message and returns an SGsAP-RELEASE-REQUEST message to the MME with an SGs cause information element indicating 'unknown'.
Option 4: The VLR creates a new SGs association for the UE and then processes the UPLINK-UNITDATA message.
Explanation: In cases where the VLR receives an SGsAP-UPLINK-UNITDATA message for a UE without existing or confirmed subscriber data, the VLR's protocol dictates ignoring the message to prevent potential security breaches or mismanagement. It then responds with an SGsAP-RELEASE-REQUEST to properly terminate the attempt and includes an 'unknown' cause indicator to inform the MME of the reason (non-existent/unconfirmed subscriber data), ensuring both ends are aware of the session's termination rationale.