e protocols will be further discussed. 1.2.1 Access Network 1.2.1.1 Reference Architecture: Network Entities and Interfaces The overall reference architecture of next-generation network comprises the entities associ- ated with the radio access network (NG-RAN) and the core network (5GC) and their corre- sponding interfaces that terminate the protocols. In this section, we will describe the access network reference architecture. The NG-RAN architecture consists of a set of gNBs con- nected to the 5GC through the NG interface (see Fig. 1.51). The interface between the NG-RAN and 5GC is referred to as N2 and N3 depending on the termination point in 3GPP 5G Network Architecture NG-RAN Central entity gNB-CU gNB-DU gNB-DU Distributed entity Logical NG-RAN node Figure 1.51 Overall NG-RAN architecture [15]. system architecture specifications. Furthermore, gNBs are interconnected through the Xn interface. In a C-RAN architecture, a gNB may be further disaggregated such that some lower layer protocol functions are implemented in the DUs and the remaining upper layer protocol functions are implemented in the edge cloud and as part of the CU(s). In that case, the gNB would consist of a gNB-CU and gNB-DUs as shown in Fig. 1.51. The gNB-CU and gNB-DU entities are connected via F1 logical interface. Note that one gNB-DU is con- nected to only one gNB-CU. In some deployment scenarios, one gNB-DU may be con- nected to multiple gNB-CUs. In NG-RAN reference architecture, NG, Xn and F1 are logical interfaces [16]. The traditional architecture of a base station where all protocol layers and functionalities were concentrated in a single logical RAN entity has evolved into a disaggregated model in 5G network architecture. In a disaggregated gNB architecture, the NG and Xn-C interfaces are terminated at the gNB-CU. In an LTE-NR dual connectivity (EN-DC) scenario, the S1- U and X2-C interfaces for a disaggregated gNB terminate at the gNB-CU. It must be noted that the gNB-CU and its associated gNB-DUs are seen as a gNB to other gNBs and t