iled definition of the F1 Setup Request message can be found in 3GPP TS 38.473, clause 9.2.1.4. 4.2.3.1.2 UE Context Management Procedures UE context management procedures are arguably the most important aspect of the F1 inter- face. With these procedures, a gNB-CU can establish, modify, or release a UE context in the gNB-DU. These procedures are also used to admit a new UE in a cell, for example, during an initial access, Secondary Node Addition in DC or EN-DC or a handover. These procedures UE Context Setup/Modification/Release UE Inactivity Notification Notify procedure. Generally, a UE context management is controlled by a gNB-CU, which is the node that initiates most of the UE context management procedures. In certain cases, when a proce- dure needs to be triggered by a gNB-DU, the gNB-DU sends an indication to the gNB-CU, which triggers the relevant procedure. The UE Context Setup Request procedure is used by a gNB-CU to establish a UE context in the gNB-DU. This procedure can only be fully understood in the context of general initial access, handover, or secondary cell addition procedures (involving air interface and also core network), which are described in Chapter 3. Here we only describe the F1 aspects of these procedures, assuming that the reader is familiar with the overall concept. We further illustrate the F1 UE context management described above by the UE mobil- ity procedure, which involves the UE Context Setup, Modification, and Release messages, shown in Figure 4.2.5. We then focus on the detailed description of the message establish- ing a UE Context in the gNB-DU and we encourage the reader to study further details about F1 maintenance procedures in 3GPP TS 38.473. 0. At the beginning, it is assumed that a UE is connected to the network and sends/ receives uplink/downlink traffic. 1. Furthermore, the UE is assumed to be configured to report measurements to the net- work; when a measurement report is triggered, the UE sends a Measurement Report message to the source gNB-DU. 2. Since the mobili