Section: 10.8.1 EN-DC
Content: The Master Node to eNB Change procedure is used to transfer context data from a source MN/SN to a target eNB. Figure 10.8.1-1: Master Node to eNB Change procedure Figure 10.8.1-1 shows an example signalling flow for the Master Node to eNB Change procedure: 1. The source MN starts the MN to eNB Change procedure by initiating the X2 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 before step 1. 2. The target eNB includes the field in HO command which releases SCG configuration, and may also provide forwarding addresses to the source MN. 3. If the allocation of target eNB resources 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 SgNB 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 eNB(s) as described in step 11a in Figure 10.1.2.1a-1in TS 36.300[ Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 ]  [2], i.e,. after step 6 in Figure 10.8.1-1. NOTE 1b: In case the handover is a conditional handover, the Data Forwarding Address Indication procedure is executed right after step 2. This Data Forwarding Address Indication procedure 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 Data Forwarding 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 Data Forwarding 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 SgNB Release Request message of the step 3a that is performed after step 6. 4. The MN triggers the UE to apply the new configuration. Upon receiving the new configuration, the UE releases the entire SCG configuration. 5/6. The UE synchronizes to the target eNB. 7. For SN terminated bearers using RLC AM, the SN sends the SN Status Transfer message, which the source MN sends then to the target eNB. 8. If applicable, data forwarding takes place from the source side. 9a. The source SN sends the Secondary RAT Data Usagee Report message to the source MN and includes the data volumes delivered to and received from the UE over the NR radio for the related E-RABs. 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 bearer is stopped. 9b. The source MN sends the Secondary RAT Report message to MME to provide information on the used NR resource. 10-14. The target eNB initiates the S1 Path Switch procedure. 15. The target eNB initiates the UE Context Release procedure towards the source MN. 16. Upon reception of the UE Context Release message, the SN releases radio and C-plane related resources associated to the UE context. Any ongoing data forwarding may continue. NOTE 3: Inter-system HO from E-UTRA with EN-DC configuration to NR or to E-UTRA connected to 5GC is also supported.