arge number of gNB-CU-UPs covering a large geographical area for improved radio performance and resource utilization. Better support for network slicing, as centralized RRM makes it easier to provide slice-level isolation as well as improved resource utilization. Moreover, with the emergence of big data analytics and artificial intelligence, a central- ized control plane opens the door to applying these cutting-edge technologies for RAN optimization. One example is the work conducted in the O-RAN Alliance to leverage emerg- ing deep learning techniques to empower an intelligent RAN controller (O-RAN). Despite numerous benefits of control- and user-plane separation, there are also chal- lenges. For example, network maintenance complexity and cost may increase due to the introduction of new logical nodes. While some deployment options improve latency, gen- erally, the implementation of control- and user-plane functionalities in separate network nodes with a network interface between them carrying signaling messages (as opposed to an internal interface in a co-located scenario) introduces additional delay. These issues should be carefully addressed during deployment planning. 4.4.3 Functional Description In the NG-RAN architecture with control-and user-plane separation, the gNB-CU is further split into control plane (gNB-CU-CP) and user plane (gNB-CU-UP) logical network nodes. It generally follows the same design principle as the gNB split into gNB-CU and gNB-DU, that is - deployments with and without control- and user-plane NG-RAN separation shall be indistinguishable from the point of view of a UE, a 5GC, or another gNB. Furthermore, a monolithic gNB-CU and split gNB-CU-CP/gNB-CU-UP shall be indistinguishable from the perspective of a gNB-DU. A gNB-CU-CP hosts the RRC and PDCP (for control-plane) protocols and a gNB-CU-UP hosts SDAP and PDCP (for user-plane) protocols. A gNB-CU-CP is connected to poten- tially multiple gNB-CU-UPs via the standardized control-plane E1 interface, as illustrated by Figure 4.4.2. Figu