LTE: What is the significance of time alignment timer for an UE and what actions are initiated when it expires ?

Question

Is TTI bundling enabled by default or on a specific condition enabled ?

Answer 1

TimeAlignmentTimer is the duration when UE is considered as uplink synchronized. TimeAlignmentTimer is configurable at cell level as well as dedicated level i.e. UE specific. At the cell level timeAlignmentTimer is configured in SIB2 and at the UE level it is communicated within the MAC-MainConfig.
If UE receives Timing Advance Command from eNodeB, it restarts the timer again but if it does not receive any Timing Advance Cmd from eNodeB until expiry of timer, UE does RACH procedure again.

Please correct me if my understanding is incorrect.