Section: 10.8.2 MR-DC with 5GC
Content: The MN to ng-eNB/gNB Change procedure is used to transfer UE context data from a source MN/SN to a target ng-eNB/gNB. Both the cases where the source MN and the target node belong to the same RAT (i.e. they are both ng-eNBs or both gNBs) and the cases where the source MN and the target node belong to different RATs are supported. NOTE 0: Inter-system HO from ng-eNB/gNB MN to eNB is also supported. Figure 10.8.2-1: MN to ng-eNB/gNB Change procedure Figure 10.8.2-1 shows an example signalling flow for the MN to ng-eNB/gNB Change procedure: 1. The source MN starts the MN to ng-eNB/gNB Change procedure by initiating the Xn Handover Preparation procedure, including both MCG and SCG configuration. NOTE 1: The source MN may trigger the MN-initiated SN Modification procedure (to the source SN) to retrieve the current SCG configuration and to allow provision of data forwarding related information before step 1. 2. The target ng-eNB/gNB includes the field in HO command which releases the SCG configuration, and may also provide forwarding addresses to the source MN. 3. If the resource allocation of target ng-eNB/gNB was successful, the MN initiates the release of the source SN resources towards the source SN including a Cause indicating MCG mobility. The SN acknowledges the release request. If data forwarding is needed, the MN provides data forwarding addresses to the source SN. Reception of the SN Release Request message triggers the source SN to stop providing user data to the UE and, if applicable, to start data forwarding. NOTE 1a: In case the handover is a conditional handover, step 3a and step 3b are performed after the source MN receives an indication that the UE has successfully accessed one of the potential target gNB(s) as described in step 8a in Figure 9.2.3.4.2-1 in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [3], i.e., after step 6 in Figure 10.8.2-1. NOTE 1b: In case the handover is a conditional handover, the step 3c is executed right after step 2. The Xn-U Address Indication message notifies conditional handover to the source SN, for which it may decide to perform, if applicable, early data forwarding for SN-terminated bearers, together with the sending of an EARLY STATUS TRANSFER message to the source MN. Separate Xn-U Address Indication procedures may be invoked to provide different forwarding addresses of the prepared conditional handovers. In this case, it is up to the source MN and SN implementations to make sure that the EARLY STATUS TRANSFER message(s) from the source SN, if any, is forwarded to the right target destination. The Xn-U Address Indication procedure may further be invoked to indicate to the source SN to stop already initiated early data forwarding for some SN-terminated bearers if they are no longer subject to data forwarding due to the modification or cancellation of the prepared conditional handovers. If applicable, the normal data forwarding and SN STATUS TRANSFER message would follow from the source SN once it receives the SN Release Request message of the step 3a that is performed after step 6. In case the step 3c Xn-U Address Indication procedure corresponding to the conditional handover that the UE successfully accessed was rejected by the source SN, the source MN re-sends it after the step 3b that is performed after step 6. 4. The MN triggers the UE to perform HO and apply the new configuration. Upon receiving the new configuration, the UE releases the entire SCG configuration. 5/6. The UE synchronizes to the target ng-eNB/gNB. 7. If PDCP termination point is changed for bearers using RLC AM, the SN sends the SN Status Transfer message, which the source MN sends then to the target ng-eNB/gNB. 8. If applicable, data forwarding takes place from the source side. 9a. The source SN sends the Secondary RAT Data Usage Report message to the source MN and includes the data volumes delivered to and received from the UE as described in clause 10.11.2. NOTE 2: The order the SN sends the Secondary RAT Data Usage Report message and performs data forwarding with MN is not defined. The SN may send the report when the transmission of the related QoS flow is stopped. 9b. The source MN sends the Secondary RAT Report message to AMF to provide information on the used NR/E-UTRA resource. 10-14. The target ng-eNB/gNB initiates the Path Switch procedure. 15. The target ng-eNB/gNB initiates the UE Context Release procedure towards the source MN. 16. Upon reception of the UE Context Release message from the MN, the source SN releases radio and C-plane related resources associated to the UE context. Any ongoing data forwarding may continue.