ity and power management continue to be the most important aspects of any new cel- lular standard to ensure seamless connectivity and sustainable power consumption of user terminals as well as overall energy efficiency of the network. In 4G/5G, the states of a UE with respect to mobility and connection establishment are described by NAS and RRC states. Fig. 1.81 shows and compares the EPS and 5GS/NG-RAN NAS and RRC states. There are three states shown in the EPS model, that is, EPS mobility management (EMM), EMM-DEREGISTERED RM-DEREGISTERED RM-REGISTERED ECM-IDLE CM-IDLE CM-CONNECTED RRC_IDLE RRC_IDLE EMM-REGISTERED ECM-CONNECTED RRC_CONNECTED RRC_CONNECTED EMM-REGISTERED RM-REGISTERED RRC_INACTIVE ECM-IDLE CM-IDLE RRC_IDLE RRC_IDLE EPS state model 5GS state model Figure 1.81 Comparison of mobility management states of EPS and 5GS [3]. 5G Network Architecture 165 EPS connection management (ECM), and RRC. The EMM and ECM states are managed by the core network, where the EMM state represents whether a UE is registered in the EPC and the ECM state indicates if NAS signaling connection between the UE and the MME is established. On the other hand, the RRC state is managed by E-UTRAN, and it represents whether there is a connection between the UE and the serving eNB. A UE in the ECM- CONNECTED state needs to be in the RRC_CONNECTED state, because a radio link con- nection is required in order to establish NAS signaling [3,16]. 1.5.1 Mobility Management In 5GS, the mobility management state of a UE can be either RM-REGISTERED or RM- DEREGISTERED depending on whether the UE is registered in 5GC, which is very similar to EMM-REGISTERED and EMM-DEREGISTERED states in EPC. Two connection man- agement states are used to reflect the NAS signaling connectivity of the UE with the AMF namely CM-IDLE and CM-CONNECTED. A UE in CM-IDLE state has no NAS signaling connection established with the AMF over N1. A UE in CM-CONNECTED state has a NAS signaling connection with the AMF over N1. A NAS signaling connection uses an RRC connecti