LTE: What is periodic TAU timer?

Question

Wt what situation does HSS provides the TAU subscription? If UE is providing TAU timer to MME what is the use of HSS providing subscription to MME?

Answer 1

Periodic TAU
Periodic tracking area updating is used to periodically notify the availability of the UE to the network. The procedure is controlled in the UE by the periodic tracking area update timer (timer T3412). The value of timer T3412 is sent by the network to the UE in the ATTACH ACCEPT message and can be sent in the TRACKING AREA UPDATE ACCEPT message. The UE shall apply this value in all tracking areas of the list of tracking areas assigned to the UE, until a new value is received.

Comments

is there any relation to HSS?

---

T3412 is associated with tracking area update. Tracking area update procedure comes under the mobility management things that's is not related to subscription data.
As far as I know, decision on T3412 value is solely with MME. It could be different scenarios in which T3412 value can vary from one UE to another UE. Implementation would be vendor specific.