Question: Which of the following events triggers the SGSN to notify the HSS, which in turn notifies subscribed services like SMS over IP, according to UE Reachability function?
Option 1: When a UE initiates a voice call
Option 2: Upon detection of UE at the Radio Access Network (RAN) level by the eNodeB
Option 3: When the SGSN detects the UE at the NAS (Non-Access Stratum) layer
Option 4: After the UE completes a data transfer session
Explanation: The correct answer is Option 3 because, according to the UE Reachability function described, the trigger for the SGSN to notify the HSS (and subsequently the subscribed services like SMS over IP) is specifically when the SGSN detects the UE at the NAS layer. This indicates a change in the UE's reachability status at a level that affects packet-switched services, which is what the HSS and its subscribers are interested in. NAS layer detection is crucial for services relying on the UE's ability to communicate over packet-switched networks.