ctions that otherwise could be considered as part of RRC are actually implemented in the gNB-DU. These are generally time-critical functions, the implementing of which in a gNB-CU would have affected the timing of sending them over the air because of the F1 interface latency. For example, MIB and SIB1, which need to be transmitted period- ically with precise timing, are generated in gNB-DU - sending these from gNB-CU would be suboptimal. Therefore, even though RRC is considered a part of a gNB-CU, quite a few RRC procedures involve both gNB-DU and gNB-CU, and thus require F1AP signaling support, as these are not completely transparent to gNB-DU. We further illustrate the concepts described above by the RRC inactive to RRC con- nected states transition procedure (for details about RRC states, see Chapter 3), which involves most of the RRC-related F1 messages: F1 Paging, Initial UL RRC Message Transfer, DL RRC Message Transfer, and UL RRC Message Transfer. This is illustrated in Figure 4.2.6. We then focus on the detailed description of the F1 Paging message and we encourage the reader to study further details about F1 RRC-related procedures in 3GPP TS 38.473. 1. When downlink data are received from the 5GC, the gNB-CU sends the F1AP Paging message to the gNB-DU with the paging-related information (most of which it receives from the 5GC), such as paging DRX, paging priority, and list of cells in which to page a UE. 2. Using the information received from the gNB-CU, the gNB-DU constructs the RRC paging message and sends it to a UE over the air interface. If a UE receives the RRC paging message triggering the UE to resume the RRC connec- tion, it sends the RRC Resume request to the gNB-DU. As the RRC functionality is implemented in the gNB-CU, the gNB-DU includes the RRC Resume request received from the UE in a non-UE-associated F1AP Initial UL RRC Message Transfer message and transfers it to the gNB-CU. 5. To establish an association for that UE over the F1 interface, the gNB-CU allocates gNB-CU UE F1AP ID and sends