 transport network capabilities, etc. 4.4.1 Key Ideas 3GPP has defined a number of NG-RAN architecture options: a monolithic (i.e. single log- ical network node) gNB, split gNB-CU and gNB-DU, and an option allowing control- and user-plane separation. In the latter case, a gNB-CU is separated into two logical nodes: a control-plane node (gNB-CU-CP) and one or multiple user-plane nodes (gNB-CU-UP), connected via a standardized E1 interface. Key reasons for control/user plane separation are: independent scaling of control and user planes, centralized RRM, alignment with the SDN concept, and better support of network slicing. The gNB-CU-UP logical network node hosts SDAP and PDCP (for user plane) layers, and gNB-CU-CP hosts RRC and PDCP (for control plane) layers. 3GPP have standardized E1 interface between a gNB-CU-CP and gNB-CU-UP, which supports control-plane protocol only. The E1 interface and the E1-AP protocol 8 3GPP work item on control- and user-plane separation of EPC nodes (CUPS), (3GPP TS 23.214). 4.4 Control-User Plane Separation design generally follow the same design principles as other RAN control-plane interfaces.9 The standardized E1 interface may allow multi-vendor deployments of gNB-CU-CP and gNB-CU-UP network nodes. The E1AP supports interface management procedures and bearer context management procedures. gNB-CU-CP and gNB-CU-UP are managed separately via OAM. Certain information has to be preconfigured in both nodes by OAM. For example, a gNB-CU-UP must be precon- figured with the transport network address of a gNB-CU-CP, etc. gNB-CU-CP and gNB-CU-UP control/user-plane separation with the E1 interface by itself is not equivalent to SDN, but rather follows a similar design concept. SDN is normally characterized by additional features, such as flow-based forwarding and routing, use of OpenFlow, and others. 4.4.2 Market Drivers The idea of decoupling control- and user-plane protocols and deploying them in separate network nodes has been around for a while. It inspired the development of SDN, which 