Question: In the context of the SGs interface, which entity(ies) can initiate the tunnelling of NAS messages procedure for encapsulating UE-VLR NAS message exchange?
Option 1: Only the Visitor Location Register (VLR)
Option 2: Only the Mobility Management Entity (MME)
Option 3: Either the Serving Gateway (SGW) or the Packet Data Network Gateway (PGW)
Option 4: Both the Mobility Management Entity (MME) and the Visitor Location Register (VLR), depending on the NAS message direction
Explanation: The correct answer is because both the MME and the VLR can initiate the tunnelling of NAS messages, with the direction of the NAS message determining which entity takes the initiative, highlighting the collaborative nature of the SGs interface in managing UE communications.