5.4.2 UE reachability in CM-CONNECTED

For a UE in CM-CONNECTED state:

• the AMF knows the UE location on a serving (R)AN node granularity.
• the NG-RAN notifies the AMF when UE becomes unreachable from RAN point of view.

UE RAN reachability management is used by RAN for UEs in RRC_INACTIVE state, see TS 38.300 [27]. The location of a UE in RRC_INACTIVE state is known by the RAN on a RAN Notification area granularity. A UE in RRC_INACTIVE state is paged in cells of the RAN Notification area that is assigned to the UEs. The RAN Notification area can be a subset of cells configured in UE's Registration Area or all cells configured in the UE's Registration Area. UE in RRC_INACTIVE state performs RAN Notification Area Update when entering a cell that is not part of the RAN Notification area that is assigned to the UE.

At transition into RRC_INACTIVE state RAN configures the UE with a periodic RAN Notification Area Update timer value and the timer is restarted in the UE with this initial timer value. After the expiry of the periodic RAN Notification Area Update timer in the UE, the UE in RRC_INACTIVE state performs periodic RAN Notification Area Update, as specified in TS 38.300 [27].

To aid the UE reachability management in the AMF, RAN uses a guard timer with a value longer than the RAN Notification Area Update timer value provided to the UE. Upon the expiry of the periodic RAN Notification Area Update guard timer in RAN, the RAN shall initiate the AN Release procedure as specified in TS 23.502 [3]. The RAN may provide the elapsed time since RAN's last contact with the UE to AMF.