Question: What triggers a Cell Update in a mobile radio interface, and what condition must be met for the Mobile Station (MS) to use the LLC NULL frame?
Option 1: The READY timer expires, and both the network and MS support the Cell Notification.
Option 2: A request from the low layers is received, but only if the READY timer is running.
Option 3: A request from the high layers is received, regardless of the READY timer status.
Option 4: A request from the low layers is received, and both the network and MS support the Cell Notification.
Explanation: According to the specification, a Cell Update may be requested by the low layers. In this case, if both the network and the MS support the Cell Notification, then the MS shall use the LLC NULL frame to perform the Cell Update. This indicates that the correct trigger is a request from the low layers, and the condition for using the LLC NULL frame is that both the network and MS must support the Cell Notification.