Section: 8.2.1.1 Inter-gNB-DU Mobility
Content: This procedure is used for the case when the UE moves from one gNB-DU to another gNB-DU within the same gNB-CU during NR operation. Figure 8.2.1.1-1 shows the inter-gNB-DU mobility procedure for intra-NR. Figure 8.2.1.1-1: Inter-gNB-DU Mobility for intra-NR 1. The UE sends a MeasurementReport message to the source gNB-DU. 2. The source gNB-DU sends an UL RRC MESSAGE TRANSFER message to the gNB-CU to convey the received MeasurementReport message. 2a. The gNB-CU may send an UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU to query the latest configuration. 2b. The source gNB-DU responds with an UE CONTEXT MODIFICATION RESPONSE message that includes full configuration information. 3. The gNB-CU sends an UE CONTEXT SETUP REQUEST message to the target gNB-DU to create an UE context and setup one or more data bearers. The UE CONTEXT SETUP REQUEST message includes a HandoverPreparationInformation. In case of NG-RAN sharing, the gNB-CU includes the serving PLMN ID (for SNPNs the serving SNPN ID). 4. The target gNB-DU responds to the gNB-CU with an UE CONTEXT SETUP RESPONSE message. 5. The gNB-CU sends a UE CONTEXT MODIFICATION REQUEST message to the source gNB-DU, which includes a generated RRCReconfiguration message and indicates to stop the data transmission for the UE. The source gNB-DU also sends a Downlink Data Delivery Status frame to inform the gNB-CU about the unsuccessfully transmitted downlink data to the UE. NOTE: In case of DAPS Handover, the UE CONTEXT MODIFICATION REQUEST message in step 5 may indicate to stop the data transmission only for the DRB(s) not subject to DAPS Handover or may not indicate to stop the data transmission at all. Instead, the DL RRC Message Transfer procedure can be used to carry the handover command to the UE. The UE CONTEXT MODIFICATION REQUEST message that indicates to stop the data transmission for the UE is sent to the source gNB-DU once the gNB-CU knows that the UE has successfully accessed the target gNB-DU, for which the source gNB-DU sends a DDDS frame about the unsuccessfully transmitted downlink data to the gNB-CU. 6. The source gNB-DU forwards the received RRCReconfiguration message to the UE. 7. The source gNB-DU responds to the gNB-CU with the UE CONTEXT MODIFICATION RESPONSE message. 8. A Random Access procedure is performed at the target gNB-DU. The target gNB-DU sends a Downlink Data Delivery Status frame to inform the gNB-CU. Downlink packets, which may include PDCP PDUs not successfully transmitted in the source gNB-DU, are sent from the gNB-CU to the target gNB-DU. NOTE: It is up to gNB-CU implementation whether to start sending DL User Data to gNB-DU before or after reception of the Downlink Data Delivery Status. 9. The UE responds to the target gNB-DU with an RRCReconfigurationComplete message. 10. The target gNB-DU sends an UL RRC MESSAGE TRANSFER message to the gNB-CU to convey the received RRCReconfigurationComplete message. Downlink packets are sent to the UE. Also, uplink packets are sent from the UE, which are forwarded to the gNB-CU through the target gNB-DU. 11. The gNB-CU sends an UE CONTEXT RELEASE COMMAND message to the source gNB-DU. 12. The source gNB-DU releases the UE context and responds the gNB-CU with an UE CONTEXT RELEASE COMPLETE message.