ack split. A gNB-CU hosts all the upper-layer protocols and functionality: RRC, SDAP, and PDCP. A gNB-DU hosts all the lower-level protocols and functionality: RLC, MAC, and PHY. This is illustrated in Figure 4.2.2. Note that for brevity Figure 4.2.2 does not illustrate the F1 protocol stack, which is used to carry PDCP Protocol Data Units (PDUs) between gNB-CU and gNB-DU. Another point worth mentioning is that even though according to the gNB split architecture the RRC layer resides in the gNB-CU, certain functions with tight timing requirements, such as System Information Broadcast, which otherwise could be considered as part of the RRC layer, are actually implemented in the gNB-DU. In other words, the layer separation is not perfect. A gNB-CU is connected to potentially multiple gNB-DUs via the standardized F1 inter- face; a gNB-DU is connected to only one gNB-CU. A gNB-DU (and by extension a gNB-CU) may control multiple cells; however, a single cell is always controlled by a single gNB-DU. 3GPP decided not to allow a single cell to span multiple gNB-DUs for simplicity of stan- dardization and deployment. The F1 standardized interface between a gNB-CU and gNB-DU(s) supports the user-plane protocol (F1-U) and control-plane protocol (F1AP). The protocol design of both the user and the control plane generally follow the same principles as other RAN interfaces, as we show in more detail below. More details about the overall NG-RAN architecture can be found in 3GPP TS 38.401. 4.2.3.1 F1 Control-Plane Protocol The control-plane part of the F1 interface (F1-C) uses the F1AP. The F1AP protocol design is similar to other application protocols defined for RAN in 5G and LTE (e.g. X2-AP and Xn-AP). As with most other RAN control protocols (e.g. Xn-AP described later in the chapter), it relies on SCTP, which provides reliable transport for in-sequence delivery of the control-plane messages. The F1-C protocol stack is illustrated in Figure 4.2.3. FIAP procedures² can be categorized as follows: Interface management procedure