NB-CU and gNB-DU are connected via an F1 logical interface, which is a standardized interface that supports interchange of signaling informa- tion and data transmission between the termination points. From a logical point of view, F1 is a point-to-point interface between the termination points, which may be established irre- spective of the existence of a physical connectivity between the two points. F1 interface supports control-plane and user-plane separation and separates radio network and TNLs. This interface enables exchange of UE and non-UE associated information. Other nodes in the network view the gNB-CU and a set of gNB-DUs as a gNB. The gNB terminates X2, Xn, NG, and S1-U interfaces [28]. The RRM functions ensure the efficient use of the available network resources. In a gNB- CU/gNB-DU disaggregated model, different RRM functions may be located at different locations. As an example, radio bearer control function (for establishment, maintenance, and release of radio bearers) can be located either at gNB-CU or gNB-DUs, whereas inter- RAT RRM function (for management of radio resources in conjunction with inter-RAT mobility) and dynamic resource allocation and packet scheduling functions (for allocation and de-allocation of resources to user and control-plane packets) are exclusively located at gNB-CU and gNB-DU(s), respectively. Fig. 1.55 shows F1 control-plane and user-plane protocol structures. In the control plane, the TNL is based on IP transport using SCTP over IP for transfer of control messages. The application layer signaling protocol is referred to as F1 application protocol (F1AP). In the user plane, the IP-based TNL uses GTP-U over UDP/IP for transfer of data packets. User-plane PDUs GTP-U Data link layer Data link layer Physical layer Physical layer User plane Control plane Figure 1.55 User-plane and control-plane protocol structure of F1 interface [28]. 120 Chapter 1 F1AP provides a signaling conduit between gNB-DU and the gNB-CU whose services are divided into non-UE-associated (related to t