XN Handover for a CU-DU split executes in 3 Phase:

This procedure outlines the coordination between the Source/Target gNB-CUs and DUs to maintain service continuity during UE mobility.

Phase 1: Handover Preparation

The objective is to verify that the target node can support the UE's requirements before the move is executed.

Step 1: Handover Request (XnAP): The Source gNB-CU sends the Handover Request to the Target gNB-CU over the Xn interface, providing the UE's context and QoS requirements.
Step 1a & 1b: UE Context Setup (F1AP): The Target gNB-CU initiates a UE Context Setup Request to the Target gNB-DU. The DU reserves the necessary radio resources and responds with the UE Context Setup Response.
Step 2: Handover Request Acknowledge (XnAP): The Target CU confirms the successful preparation to the Source CU.

Phase 2: Handover Execution

This phase triggers the UE to detach from the source and synchronize with the target.

Step 2a & 3a: UE Context Modification (F1AP): The Source gNB-CU sends a UE Context Modification Request to the Source gNB-DU. This notifies the DU that the handover is occurring, allowing it to prepare for the transition and stop user-plane traffic.
Step 3: RRCReconfiguration (Uu): The Source gNB-DU transmits the RRCReconfiguration (the actual Handover Command) to the UE.
Step 4: SN Status Transfer (XnAP): The Source CU sends the PDCP sequence numbers to the Target CU to ensure seamless data delivery without duplication or loss.
Step 5: Data Forwarding (Xn-U): User data is forwarded from the Source CU to the Target CU.
Step 6: Random Access Procedure: The UE performs RACH synchronization with the Target gNB-DU.
Step 7 & 7a: RRC Complete: The UE sends the RRCReconfigurationComplete message to the Target gNB-DU, which is forwarded to the Target gNB-CU via F1AP: UL RRC Message Transfer.
Step 7b & 7c: Target Context Modification (F1AP): The Target gNB-CU performs a final UE Context Modification with the Target gNB-DU to synchronize the operational state of the UE.

Phase 3: Handover Completion

Once the UE is successfully connected to the target, the network updates the core path and releases the old resources.

Step 8: Path Switch Procedure (NGAP): The Target gNB-CU initiates the path switch with the AMF/UPF to redirect the DL data flow from the core network to the new target node.
Step 9 & 10: End Marker & New Path: The Source gNB-CU sends an End Marker to the Target CU to signify the end of forwarded data. Subsequently, the New Path traffic begins flowing from the UPF to the Target gNB.
Step 11: UE Context Release (XnAP): The Target gNB-CU sends the UE Context Release message to the Source gNB-CU, signaling that the handover was successful.
Step 11a & 11b: F1 Context Release (F1AP): This is the final "cleanup" job. The Source gNB-CU sends the UE Context Release Command to the Source gNB-DU. The DU deletes the UE context and responds with UE Context Release Complete

Sequence diagram for XN Handover flow:

3gpp spec Reference: 38.300,

Oran spec Reference : O-RAN.WG5.TS.C.1-R005-v17.00

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