 eNB gNB-DU gNB-DU gNB-DU gNB-DU Control plane User plane Scenario A Scenario B Scenario C Figure 5 Different deployment scenarios of NR and LTE [16]. Introduction and Background xxxix PDCP-C PDCP-U CU-CP CU-UP Collapsed gNB model Disaggregated gNB model Figure 6 Different deployment options for gNB (NR base station) [16]. PDCP-C and radio resource control (RRC) protocols whereas the CU-UP entity hosts the PDCP-U (and SDAP) protocols. In the disaggregated gNB deployment, the sepa- ration of control-plane CU-CP and user-plane CU-UP entities offers the possibility of optimizing the location of different RAN functions based on the desired topology and performance. For example, the CU-CP can be placed in a location close to the DU entities. It could also be co-located with the DU, thus providing shorter latency for the critical CP procedures such as connection (re)establishment, handover, and state transition. On the other hand, the CU-UP can be centralized in a regional or national data center, thus favoring centralized implementations and providing a central termi- nation point for the UP traffic in dual connectivity and tight interworking scenarios. An additional CU-UP can also be placed closer (or co-located) with the DU to pro- vide a local termination point for the UP traffic for applications that require very low latency (e.g., URLLC traffic). In the disaggregated gNB architecture shown in Fig. 6, it is necessary to coordinate the CU- CP and CU-UP entities. Some of the functions that may require a control-plane interface between CU-CP and CU-UP include CU-CP set-up, modification, and configuration of the data radio bearers (DRBs) in the CU-UP and when CU-CP entity configures the security keys in the CU-UP entity for RAN-level security activation and configuration. A new open interface between CU-CP and CU-UP has been defined to enable these functions, which is denoted as . The interface E is a control-plane interface, and it does not require a UP part because CU-CP and CU-UP do not exchange UP traffic. xl Intr