Section: 4.3.12.3a Reachability Management for UE in ECM-IDLE state
Content: An emergency attached UE when its periodic TAU update timer expires shall not initiate a periodic TAU procedure but enter EMM-DEREGISTERED state. For emergency attached UEs the MME runs a mobile reachable timer with a similar value to the UE's periodic TAU timer. Any time after expiry of this timer the MME may change the EMM state of an emergency attached UE to EMM-DEREGISTERED. The MME assigns the periodic TAU timer value to emergency attached UE. This timer keeps the UE emergency attached after change to EMM-IDLE state to allow for a subsequent emergency service without a need to emergency attach again.