e gNB-DU, are sent from the gNB-CU to the target gNB-DU.³ 3 It is up to gNB-CU implementation whether to start sending downlink user data to gNB-DU before or after reception of the Downlink Data Delivery Status. 4.2 High-Level gNB-CU/DU Split 9. The UE completes the Random Access procedure with an RRCConnectionReconfigura- tionComplete message. 10. The target gNB-DU sends an Uplink RRC Transfer message to the gNB-CU to convey the received RRCConnectionReconfigurationComplete message. Downlink packets are sent to the UE. Also, uplink packets are sent from the UE, which are forwarded to the gNB-CU through the target gNB-DU. 11. The gNB-CU sends an UE Context Release Command message to the source gNB-DU. 12. The source gNB-DU releases the UE context and responds to the gNB-CU with an UE Context Release Complete message. The UE Context Setup Request message, used by a gNB-CU to establish a context for a new UE in the gNB-DU, carries UE identifiers on the F1 interface, information about cells for the UE, Signaling Radio Bearers (SRBs), and Data Radio Bearers (DRBs) for the UE, and others. While the explanation of all the IEs in that message is beyond the scope of this chapter, we focus on a few select IEs that are important to illustrate the concept of UE context establishment. Specifically: gNB-CU UE F1AP ID and gNB-DU UE F1AP ID are the identifiers of the UE on the F1 interface. Special Cell (SpCell) ID and a list of candidate SpCells (in non-multiconnectivity oper- ation, SpCell refers to Primary Cell [PCell], which operates in the primary frequency and in which the UE performs initial connection establishment). CU to DU RRC Information, which is a collection of RRC containers carrying infor- mation defined in 3GPP TS 38.331, such as measurement configuration and handover preparation information. Discontinuous Reception (DRX) Cycle. Secondary Cell (SCell) to be setup list (in the case of Carrier Aggregation [CA]). A list of SRBs to be set up. A list of DRB to be set up. Inactivity monitoring request to trigger (opt