Section: 10.9.2 MR-DC with 5GC
Content: The ng-eNB/gNB to MN change procedure is used to transfer UE context data from a source ng-eNB/gNB to a target MN that adds an SN during the handover. Only the cases where the source node and the target MN belong to the same RAT (i.e. they are both ng-eNBs or both gNBs) are supported. Figure 10.9.2-1: ng-eNB/gNB to MN change procedure Figure 10.9.2-1 shows an example signalling flow for ng-eNB/gNB to MN change: 1. The source ng-eNB/gNB starts the handover procedure by initiating the Xn Handover Preparation procedure. 2. The target MN sends SN Addition Request to the target SN. 3. The target SN replies with SN Addition Request Acknowledge. If data forwarding is needed, the target SN provides forwarding addresses to the target MN. NOTE 0: Void. 3a. For SN terminated bearers using MCG resources, the target MN provides Xn-U DL TNL address information in the Xn-U Address Indication message. 4. The target MN includes within the Handover Request Acknowledge message the SN RRC reconfiguration message to be sent to the UE that includes the SCG configuration to perform the handover, and may also provide forwarding addresses to the source ng-eNB/gNB. 5. The source ng-eNB/gNB triggers the UE to perform handover and apply the new configuration. 6/7. The UE synchronizes to the target MN and replies with MN RRC reconfiguration complete message including the SN RRC reconfiguration complete message. 8. If configured with bearers requiring SCG radio resources, the UE synchronizes to the target SN. NOTE 1: The order the UE performs Random Access towards the target MN (step 6) and performs the Random Access procedure towards the target SN (step 8) is not defined. 9. If the RRC connection reconfiguration procedure was successful, the target MN informs the target SN via SN Reconfiguration Complete message. 10. For bearers using RLC AM, the source ng-eNB/gNB sends the SN Status Transfer message, which the target MN forwards then to the target SN, if needed. 11. Data forwarding from the source ng-eNB/gNB takes place. 12-15. The target MN initiates the PDU Session Path Switch procedure. NOTE 2: If new UL TEIDs of the UPF are included, the target MN performs MN initiated SN Modification procedure to provide them to the target SN. 16. The target MN initiates the UE Context Release procedure towards the source ng-eNb/gNB.