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Content for  TS 38.401  Word version:  17.1.0

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8.9.4  Inter-gNB handover involving gNB-CU-UP changeWord‑p. 62

Figure 8.9.4-1 shows the procedure used for inter-gNB handover involving gNB-CU-UP change. Overall inter-gNB handover procedure is specified in TS 37.340.
Reproduction of 3GPP TS 38.401, Fig. 8.9.4-1: Inter-gNB handover involving gNB-CU-UP change
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Step 1.
The source gNB-CU-CP sends HANDOVER REQUEST message to the target gNB-CU-CP. In case of Conditional Handover, the target gNB is regarded as a candidate gNB which is only accessed by the UE when the CHO condition(s) are fulfilled.
Step 2-4.
Bearer Context Setup procedure is performed as described in clause 8.9.2.
Step 5.
The target gNB-CU-CP responds the source gNB-CU-CP with an HANDOVER REQUEST ACKNOWLEDGE message.
Step 6.
The F1 UE Context Modification procedure is performed to send the handover command to the UE, and to indicate to stop the data transmission for the UE.
Step 7-8.
Bearer Context Modification procedure (gNB-CU-CP initiated) is performed to enable the gNB-CU-CP to retrieve the PDCP UL/DL status and to exchange data forwarding information for the bearer.
Step 9.
The source gNB-CU-CP sends an SN STATUS TRANSFER message to the target gNB-CU-CP.
Step 10-11.
Bearer Context Modification procedure is performed as described in clause 8.9.2. The target gNB-CU-CP does not transfer the PDCP UL/DL status carried from the SN STATUS TRANSFER message to the target gNB-CU-UP if the PDCP status does not need to be preserved (e.g. full configuration). In case of DAPS Handover or Conditional Handover, the COUNT related info carried by the EARLY STATUS TRANSFER message is provided to the target gNB-CU-UP.
Step 12.
Data Forwarding may be performed from the source gNB-CU-UP to the target gNB-CU-UP.
Step 12a.
In case of DAPS Handover or Conditional Handover, the target gNB-CU-CP sends the HANDOVER SUCCESS message to the source gNB-CU-CP to inform that the UE has successfully accessed the target cell.
Step 12b.
In case of DAPS Handover or Conditional Handover, the F1 UE Context Modification procedure is performed to indicate to stop the data transmission for the UE.
Step 12c-12d.
In case of DAPS Handover or Conditional Handover, the Bearer context modification procedure (gNB-CU-CP initiated) is performed to indicate the source gNB-CU-UP to stop packet delivery and also to retrieve the PDCP UL/DL status.
Step 12e.
In case of DAPS Handover or Conditional Handover, the source gNB-CU-CP sends the SN STATUS TRANSFER message to the target gNB-CU-CP.
Step 12f-12g.
In case of DAPS Handover or Conditional Handover, the Bearer context modification procedure is performed to provide the PDCP UL/DL status to the target gNB-CU-UP only if the PDCP status needs to be preserved as described in TS 38.300.
Step 13-15.
Path Switch procedure is performed to update the DL TNL address information for the NG-U towards the core network.
Step 16.
The target gNB-CU-CP sends an UE CONTEXT RELEASE message to the source gNB-CU-CP.
Step 17 and 19.
Bearer Context Release procedure is performed.
Step 18.
F1 UE Context Release procedure is performed to release the UE context in the source gNB-DU.
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8.9.5  Change of gNB-CU-UPWord‑p. 64

Figure 8.9.5-1 shows the procedure used for the change of gNB-CU-UP within a gNB.
Reproduction of 3GPP TS 38.401, Fig. 8.9.5-1: Change of gNB-CU-UP
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Step 1.
Change of gNB-CU-UP is triggered in gNB-CU-CP based on e.g., measurement report from the UE.
Step 2-3.
Bearer Context Setup procedure is performed as described in clause 8.9.2.
Step 4.
F1 UE Context Modification procedure is performed to change the UL TNL address information for F1-U for one or more bearers in the gNB-DU.
Step 5-6.
Bearer Context Modification procedure (gNB-CU-CP initiated) is performed to enable the gNB-CU-CP to retrieve the PDCP UL/DL status and to exchange data forwarding information for the bearer.
Step 7-8.
Bearer Context Modification procedure is performed as described in clause 8.9.2.
Step 9.
Data Forwarding may be performed from the source gNB-CU-UP to the target gNB-CU-UP.
Step 10-12.
Path Switch procedure is performed to update the DL TNL address information for the NG-U towards the core network.
Step 13-14.
Bearer Context Release procedure (gNB-CU-CP initiated) is performed as described in clause 8.9.3.
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