In LTE networks, devices must periodically perform a Tracking Area Update (TAU), even when stationary.
This process ensures the device’s registration with the core network remains valid, allowing paging and mobile-terminated traffic to reach it.
Why is periodic TAU needed?
Maintains network knowledge of UE state
Keeps paging possible
Ensures session continuity
Helps with mobility tracking if the UE has moved.
How is it triggered?
The Timer T3412 is controlled by the network (not the device).
After T3412 expires, the UE initiates a TAU Request to refresh registration.
If the UE misses this procedure (due to poor coverage, bugs or misconfiguration), it may become un-pagable and lose sync with the network.
In this example:
The device receives Paging
Sends Tracking Area Update Request → eps_update_type = 0x3 (periodic updating)
Receives TAU Accept → RRC Release → back to Idle → registration refreshed.
Why validate this?
TAU behaviour varies by network, operators use different timers and configurations.
Some devices mishandle TAU and cause “silent offline” bugs after many hours.
Correct TAU handling is critical for long-term device reliability, especially in IoT.
A simple but vital behaviour to test and a key part of ensuring devices stay reachable in real-world networks.
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