a target SN and at the same time change the SCG config- uration in UE from one SN to another. This procedure always involves signaling over MCG SRB toward the UE. For instance, an SN may decide to trigger an SN-initiated SN Change procedure when the UE reports an A3 event to the SN, if the neighboring cell becoming better than the serving cell is controlled by another SN. Figure 4.3.15 shows the message flow for an SN-initiated SN Change procedure, in the EN-DC case. 1. SgNB Change Required 2. SgNB Addition Request 3. SgNB Addition Request Acknowledge 4. RRCConnectionReconfiguration 5. RRCConnectionReconfigurationComplete 6. SgNB Change Confirm 7. SgNB Reconfiguration Complete 8. Random Access Procedure 9a. SN Status Transfer 9b. SN Status Transfer 10. Data Forwarding 11. Path Update procedure 12. UE Context Release Figure 4.3.15 SN Change - SN-initiated. (Source: Reproduced by permission of © 3GPP). 4 NG-RAN Architecture 1. If the source SN decides to initiate the SN Change procedure, it sends the X2-AP SgNB Change Required message to the MN, carrying the target SN identifier information and optionally the SCG configuration via the SgNB to MeNB Container IE. 2. If the MN accepts the request, it sends the X2-AP SgNB Addition message to the target SN requesting it to allocate resources; the message may also carry the measurement results related to the target SN received from the source SN. 3. If the target SN accepts the request, it responds with the X2-AP SgNB Addition Request Acknowledge message. 4. The MN then provides the new configuration to the UE in the RRCConnectionRecon- figuration message, including the NR RRC Reconfiguration message generated by the target SN. 5. The UE applies the new configuration and sends the RRCConnectionReconfigura tionComplete message, including the encoded NR RRC response message for the target SN, if needed. 6. The MN then confirms the release of the source SN resources by sending the X2-AP SgNB Change Confirm message, which may carry the downlink (and uplink) Forward- ing GTP 