face and is not used for user data forwarding. As shown in Fig. 1.52, a gNB may consist of one gNB-CU-CP, multiple gNB-CU-UPs, and multiple gNB-DUs. One gNB-DU can be connected to multiple gNB-CU-UPs under the 122 Chapter 1 Data link layer Physical layer Figure 1.56 Protocol stack for E1 interface [36]. control of the same gNB-CU-CP and one NB-CU-UP can be connected to multiple DUs under the control of the same gNB-CU-CP. The connectivity between a gNB-CU-UP and a gNB-DU is established by the gNB-CU-CP using bearer or UE context management func- tions. The gNB-CU-CP further selects the appropriate gNB-CU-UP(s) for the requested UE services. The E1 interface would support independent virtualization of the control and UPFs. It would also enable more flexible allocation of the functions of the central unit. It allows for energy and cost-efficient central processing and resource pooling for the user plane. Several functions such as security, packet inspection, header compression, and data mining could benefit from centralization. Furthermore, it would provide optimum routing of packets in case of multi-connectivity and interworking with other systems. The protocol structure for E1 is shown in Fig. 1.56. The TNL uses IP transport with SCTP over IP. The application layer signaling protocol is referred to as E1 application protocol (E1AP). 1.2.1.1.4 NG Control-Plane/User-Plane Functions and Procedures The NG control-plane interface (NG-C) is defined between the gNB/ng-eNB and the AMF. The control-plane protocol stack of NG interface is shown in Fig. 1.57. The TNL relies on IP transport; however, for more reliable transmission of signaling mes- sages, the SCTP protocol is used over IP. The application layer signaling protocol is referred to as NG application protocol (NGAP). The SCTP layer provides guaranteed delivery of application layer messages. IP layer point-to-point transmission is used to deliver the signaling PDUs. The NG-C interface further enables interface management (the functionality to manage the NG-C inter