CM-CONNECTED Terminal (UE) Reachability

In 5G networks, the network has two CM (Connection Management) connection states for terminals: CM-Idle and CM-CONNECTED. The CM-CONNECTED state is crucial for achieving seamless data flow and supports low-latency, large-scale IoT, and smart city applications. The reachability of the terminal (UE) in the CM-CONNECTED state is defined by 3GPP in TS 23.501 as follows:

I. Reachability in CM-CONNECTED state specifically includes:

• The AMF knows the UE's location at the serving (RAN) node granularity;
• When the UE becomes unreachable from the RAN's perspective, the NG-RAN notifies the AMF.

II. RRC Inactive State Terminal (UE):

• For terminals (UEs) in the RRC Inactive state, the radio access network (RAN) uses UE RAN reachability management (see TS 38.300 [27]).
• The location of the terminal (UE) in the RRC Inactive state is determined by the RAN at its (RAN) notification area granularity.
• Terminals (UEs) in the RRC Inactive state are paged in the cells of the RAN notification area assigned to that UE. The RAN notification area can be a subset of the cells configured in the UE's registration area, or all cells configured in the UE's registration area.
• A UE in the RRC Inactive state performs a RAN notification area update when it enters a cell that does not belong to the RAN notification area assigned to that UE.

• Radio Access Network (RAN) Communication Area: In the 5G system, the RNA (Radio Access Network Notification Area) is a geographical area located within the 5GC registration area; this area consists of one or more cells belonging to one or more gNBs; where:

• When the UE transitions to the RRC Inactive state, the RAN configures a periodic RAN notification area update timer value for the UE, and the timer in the UE restarts with this initial timer value. After the periodic RAN notification area update timer in the UE expires, the UE in the RRC Inactive state performs a periodic RAN notification area update as specified in TS 38.300 [27].

To assist in UE reachability management in the AMF, the RAN uses a guard timer, whose value is longer than the RAN notification area update timer value provided to the UE. In the RAN, after the periodic RAN notification area update protection timer expires, the RAN should initiate the AN release procedure as specified in TS 23.502 [3]; the RAN may provide the AMF with the time elapsed since the RAN last contacted the UE.