What is the difference between periodic tracking area update v/s non-periodic tracking area update?

Question

There are two ways for tracking area update procedure to get triggered from UE to network . One is known as periodic and other one as non-periodic.

Answer 1

Periodic tracking area update is used to notify the availability of the UE to the network(EPC) periodically.
For this purpose the UE maintains a periodic tracking area update timer (T3412).

The value of T3412 is sent by the network to the UE in the ATTACH ACCEPT/TRACKING AREA UPDATE ACCEPT message. The UE shall apply this value in the list of tracking areas assigned to the UE, until a new value is received.

Network(MME) can also deactivate/disable this procedure by setting the timer value as zero in the ATTACH ACCEPT/ TRACKING AREA UPDATE ACCEPT message after which the UE shall not perform periodic tracking area updating procedure.

Now Non Periodic TAU procedure is initiated -
1. If the Tracking Area changes.
2. If the UE transits from ECM Idle state to ECM Connected state.

Suppose a UE is in idle state & receives an incoming call or tries to access internet, all the released tunnels are re established (either using paging or service request) & the UE is moved to the connected state. So now after coming back to the connected state, the UE needs to update its whereabouts/location to the network, so it sends a TAU request message.

Hope this helps!