e network does not need to manage the UE's location while it is registered in the 5GC. For optimization, the core network may decide to deregister the UE after the mobile-originated data communication is finished, without transferring the UE's state into the CM-IDLE state in the RM-REGISTERED state, because most functions supported in the CM-IDLE state are not relevant to the UE in mobile-originated only mode, for example, UE location tracking and reachability management. In such cases, the UE needs to perform attach procedure whenever the mobile-originated data transmission is nec- essary to communicate with the core network. A UE receives services through a PDU session, which is a logical connection between the UE and the data network. In 5GS, various PDU session types are supported, for example, IPv4, IPv6, Ethernet, etc. Unlike EPS, where at least one default session (i.e., default EPS bearer) is always created while the UE attaches to the network, 5GS can establish a session when service is needed irrespective of the attachment procedure of UE, that is, attachment without any PDU session is possible. 5GS also supports UE establishing multiple PDU ses- sions to the same data network or to different data networks over a single or multiple access networks including 3GPP and non-3GPP access. The number of UPFs for a PDU session is not specified. The deployment with at least one UPF is essential to serve a given PDU ses- sion. For a UE with multiple PDU sessions, there is no need for a convergence point like SGW in the EPC. In other words, the user-plane paths of different PDU sessions are completely disjoint. This implies that there is a distinct buffering node per PDU session for the UE in the RRC_IDLE state. In order to ensure slice-aware mobility management when network slicing is supported, a slice ID is introduced as part of the PDU session information that is transferred during mobility signaling. This enables slice-aware admission and conges- tion control [34]. LTE-NR interworking is important for stand-a