5.16.4.4 Reachability Management

Over 3GPP access, an Emergency Registered UE when its Periodic Registration Update timer expires shall not initiate a Periodic Registration Update procedure but shall enter the RM-DEREGISTERED state. For such UEs, the AMF runs a mobile reachable timer with a similar value to the UE's Periodic Registration Update timer. After expiry of this timer the AMF may change the UE RM state for 3GPP Access in the AMF to RM-DEREGISTERED. The AMF assigns the Periodic Registration Update timer value to Emergency Registered UEs. This timer keeps the Emergency Registered UE registered for Emergency Services after change to CM-IDLE state to allow for a subsequent Emergency Service without a need for a new Emergency Registration.

Over non-3GPP access, an Emergency Registered UE is only reachable in CM-CONNECTED state: since the UE may only use Emergency Services over Non-3GPP access when it is not possible over 3GPP access, 3GPP access is assumed to be unavailable for paging the UE.