Question: What action does the Mobility Management Entity (MME) take upon receiving an SGsAP-DOWNLINK-UNITDATA message from the Visitor Location Register (VLR) for a User Equipment (UE) with non-existent or unconfirmed subscriber data by the Home Subscriber Server (HSS)?
Option 1: Ignore the received message
Option 2: Send an error response to the VLR and notify the HSS
Option 3: Establish an SGs association for the UE before processing
Option 4: Buffer the message pending subscriber data confirmation from HSS
Explanation: In cases where the MME receives a downlink unitdata message for a UE without existing or confirmed subscriber data (as validated by the HSS), the specified protocol dictates that the MME should disregard the message to maintain network integrity and prevent potential security breaches. This approach aligns with standard telecom practices for handling incomplete or unverified user information, prioritizing network stability and security.