face), UE context management (the functionality to manage the UE context between NG-RAN and 5GC), UE mobility 5G Network Architecture 123 User-plane PDUs GTP-U Data link layer Data link layer Physical layer Physical layer User plane Control plane Figure 1.57 NG-U and NG-C protocol stack [16]. management (the functionality to manage the UE mobility in connected mode between NG-RAN and 5GC), transport of NAS messages (procedures to transfer NAS messages between 5GC and UE), and paging (the functionality to enable 5GC to generate paging messages sent to NG-RAN to allow NG-RAN to page the UE in RRC_IDLE state). It further enables PDU session management (the functionality to establish, manage, and remove PDU sessions and respective NG-RAN resources that are made of data flows carrying user-plane traffic) as well as configuration transfer (the functionality to trans- fer the NG-RAN configuration information, e.g., transport layer addresses for establish- ment of Xn interface, between two NG-RAN nodes via 5GC). The NGAP protocol consists of elementary procedures where an elementary procedure is a unit of interac- tion between the NG-RAN node and the AMF. These elementary procedures are defined separately and are used to create complete sequences in a flexible manner. Unless otherwise stated by the restrictions, the EPs may be invoked independent of each other as stand-alone procedures, which can be active in parallel. An EP consists of an initiating message and possibly the corresponding response message. Two types of EPs referred to as class 1 and class 2 are defined, where class 1 EP consists of mentary procedures with response (success and/or failure) and class 2 EP comprises elementary procedures without response. The NG user-plane (NG-U) interface is defined between a NG-RAN node and a UPF. The NG-U interface provides non-guaranteed delivery of user-plane PDUs between the two nodes. The user-plane protocol stack of the NG interface is also shown in Fig. 1.57. The TNL relies on IP transport and use of GTP-U over UDP