 Xn data transport. Available at: WWW .3gpp.org (accessed May 29, 2020). 3GPP Technical Specification 38.425 (2019). NG-RAN; NR user plane protocol. Available at: www.3gpp.org (accessed May 29, 2020). 4 NG-RAN Architecture Control-User Plane Separation Feng Yang Intel Corporation, China In the Section 4.2, we described the high-level gNB functional split architecture (using the terminology introduced in Section 4.1), consisting of a gNB-CU and gNB-DU, wherein the gNB-CU consists of both the control-plane functions and user plane functions of the upper-layer protocols, including RRC and RRM (for the control plane) and PDCP and SDAP (for the user plane). While the previously described gNB-CU/gNB-DU split architecture is a viable and beneficial deployment option, 3GPP have additionally specified a more fine-grained split architecture, in which the gNB-CU is further split into control-plane (gNB-CU-CP) and user-plane (gNB-CU-UP) logical network nodes. This architecture option is inspired by the popular software-defined networking (SDN) concept, which is in large part based on the idea of control- and user-plane separation. 3GPP applied the separation principle to EPC in 4G for the first time,8 which was later extended to NG-RAN in 5G, specifi- cally to the gNB-CU. Standardized control/user-plane separation can bring numerous benefits, including independent scaling of control and user planes, enabling multi-vendor interoperability, and centralized RRM. The gNB-CU described in Section 4.2 can be deployed as separate control-plane and user-plane logical network nodes with a standardized E1 interface between them, which is the topic of the present chapter. With this enhancement, a gNB can be deployed as a single network node, or split into gNB-CU and gNB-DU nodes, or further split into gNB-CU-CP (centralized control-plane node), gNB-CU-UP (centralized user-plane node), and gNB-DU. These are deployment options an operator can choose from to suit their needs best, depend- ing on, for example, network capacity requirements,