ive services that requires registration. Once registered, the UE may need to update its registration with the network either periodically, in order to remain reachable (periodic registration update); or upon mobility (mobility regis- tration update); or to update its capabilities or renegotiate protocol parameters (mobility reg- istration update). As we discussed in Chapter 1, the mobility state of a UE in 5G core network (CN) can be either RM-REGISTERED or RM-DEREGISTERED depending on whether the UE is registered with 5GC. The registration management (RM) states are used in the UE and the Access and Mobility Management Function (AMF) to reflect the registra- tion status of the UE in the selected PLMN. In the RM-DEREGISTERED state, the UE is not registered with the network. The UE context in AMF holds no valid location or routing information for the UE; thus the UE is not reachable by the AMF. However, some parts of UE context may still be stored in the UE and the AMF to avoid performing an authentica- tion procedure in each registration procedure. In the RM-REGISTERED state, the UE is registered with the network and can receive services that require registration with the net- work [1]. Fig. 2.23 shows the UE RM states and transition between the two states. Two connection management states are used to reflect the NAS signaling connectivity status of the UE with the CN, namely, CM-IDLE and CM-CONNECTED. A UE in CM-IDLE state has no NAS signaling connection established with the AMF over N1, whereas in CM- CONNECTED state the UE has a NAS signaling connection with the AMF over N1. A NAS signaling connection uses the RRC connection between the UE and the NG-RAN to encapsulate NAS messages exchanged between the UE and the CN in the RRC messages. The NR RRC protocol states consist of three states, where in addition to RRC_IDLE and RRC_CONNECTED states, a third state has been introduced, RRC_INACTIVE, as a pri- mary sleeping state prior to transition to RRC_IDLE state in order to save UE power and to allow fast connecti