What is the trigger which tells UE to initiate the attach request?

Question

Does it happen immediately after camping on a cell or only when a Data Indication comes from L4 to NAS after camping on a cell?

Answer 1

If the device supports voice and SMS or only SMS or only voice, attach is triggered immediately as soon as UE camps on a cell. Otherwise, the device will not be reachable for voice calls and/or SMS.

Answer 2

As per my understanding, attach request is independent from data indication. In the lab, even without data indication , I always initiate attach procedure for UE. I found only two scenarios in which UE sends attach request to MME. These scenarios are 1) switch-on the UE 2) Aero-plane mode to active mode.