Section: 8.18.2 CG based SDT
Content: The procedure for CG based small data transmission in RRC Inactive is shown in Figure 8.18.2-1. Figure 8.18.2-1: CG based Small Data Transmission in RRC Inactive state. 1. The gNB-CU decides to move UE into RRC_INACTIVE state. 2. The gNB-CU-CP decides to configure CG-SDT, it sends UE CONTEXT MODIFICATION REQUEST message including a query indication for CG-SDT related resource configuration associated with the information of SDT Radio Bearer(s). 3. The gNB-DU sends the UE CONTEXT MODIFICATION RESPONSE message including the CG-SDT related resource configurations for the requested SDT Radio Bearer(s) within the DU to CU RRC Information IE. 4. The gNB-CU-CP sends the BEARER CONTEXT MODIFICATION REQUEST towards the gNB-CU-UP, with the suspend indication. 5. The gNB-CU-UP sends the BEARER CONTEXT MODIFICATION RESPONSE towards the gNB-CU-CP. 6. The gNB-CU-CP sends the UE CONTEXT RELEASE COMMAND message to the gNB-DU including an RRCRelease message to the UE with the CG-SDT information within suspend configuration. The gNB-CU notifies the gNB-DU to keep the SDT RLC config, F1-U tunnels, F1AP UE association, and store the CG resource for SDT when the UE is entering RRC_INACTIVE state with an explicit CG-SDT kept indicator. 7. The gNB-DU sends the RRCRelease message to UE. 8. The gNB-DU sends UE CONTEXT RELEASE COMPLETE message. The gNB-DU keeps the SDT RLC config, F1-U tunnels, F1AP UE association, and stores the CG resource for SDT when the UE entering RRC_INACTIVE. The gNB-DU also stores the C-RNTI, CS-RNTI, and which bearers are CG-SDT bearers. After a period of time of the UE being in RRC_INACTIVE state. 9. The UE decides to perform CG based SDT procedure, it sends the RRCResumeRequest message together with UL SDT data/UL NAS PDU. 10. The gNB-DU sends the UL RRC MESSAGE TRANSFER message including the RRCResumeRequest message to indicate the access due to CG-SDT. 11/12. If UE context is successfully retrieved as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2], the gNB-CU-CP initiates the BEARER CONTEXT MODIFICATION procedure to resume SDT DRBs. 13 – 13a. The gNB-DU sends the UL SDT data, if any, to the gNB-CU-UP, and/or sends the UL signalling, if any, to the gNB-CU-CP via the UL RRC MESSAGE TRANSFER message, in which any UL NAS PDU is delivered to AMF. NOTE 1: void. Upon receiving the UE INACTIVITY NOTIFICATION message without SDT volume threshold crossed indication from the gNB-DU and deciding to terminate the ongoing SDT procedure, the gNB-CU shall transmit the UE CONTEXT RELEASE COMMAND message to the gNB-DU. NOTE 2: Upon receiving BSR from the UE, in case that UL SDT data size in the BSR is larger than the threshold configured from the gNB-CU-CP, the gNB-DU sends the UE INACTIVITY NOTIFICATION message with the SDT volume threshold crossed indication to the gNB-CU-CP. Upon receiving such indication, the gNB-CU-CP may terminate the ongoing SDT procedure, by sending the RRCResume message to move the UE to RRC_CONNECTED, or by sending the RRCRelease message to move the UE to RRC_INACTIVE. If CG-SDT is re-configured, the gNB-CU may request the gNB-DU to keep CG-SDT configuration and resources in the UE CONTEXT RELEASE COMMAND message. Upon receiving non-SDT data, the gNB-CU-UP shall send the DL DATA NOTIFICATION message to the gNB-CU-CP. The gNB-CU-CP shall terminate the ongoing SDT procedure as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2]. If the amount of the received DL SDT data is above the data size threshold configured by the gNB-CU-CP, the gNB-CU-UP shall send the DL DATA NOTIFICATION message with the SDT data size threshold crossed indication. The gNB-CU-CP may terminate the ongoing SDT procedure.