Section: 8.18.1 RACH based SDT
Content: The procedure for RACH based small data transmission in RRC Inactive is shown in Figure 8.18.1-1. Figure 8.18.1-1: RACH based Small Data Transmission in RRC Inactive state. 1. The UE in RRC Inactive sends the RRCResumeRequest message together with UL SDT data and/or UL SDT signalling. 2. The gNB-DU buffers the UL SDT data and/or UL SDT signalling. 3. The step 3 is as defined in step 4 in clause 8.6.2, including an indication of SDT access. The gNB-DU may also provide SDT assistance information. 4-5. If UE context is successfully retrieved as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2], the steps 4-5 are as defined in steps 6-7 in clause 8.9.6.2. The UL SDT data, if any, is forwarded to the gNB-CU-UP, and the UL signalling, if any, is forwarded to the gNB-CU-CP via the UL RRC MESSAGE TRANSFER message, in which any UL NAS PDU is delivered to AMF. NOTE 1: In case that full UE context is retrieved from another gNB-CU-CP as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2], the gNB-CU-CP first establishes the UE context in the gNB-CU-UP via the Bearer Context Setup procedure and F1-U UL TEIDs are retrieved before step 4. The BEARER CONTEXT SETUP REQUSET message may include an indication to suspend non-SDT bearers, and in this case, the BEARER CONTEXT MODIFICATION REQUEST message in step 6 does not include resume indication for SDT DRBs. NOTE 2: In case that only partial UE context for SDT including F1-U UL TEIDs is retrieved from another gNB-CU-CP as specified in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [2], the gNB-CU-CP uses those F1-U UL TEIDs for steps 4-5, and the subsequent steps 6-7 are not executed. The F1-U DL TEIDs received from the gNB-DU in step 5 should be forwarded to the other gNB-CU-CP, to be used for transferring of the DL SDT data. In addition, the UL SDT data, if any, is forwarded from the gNB-DU to the gNB-CU-UP of the other gNB-CU-CP for which the partial context is retrieved, and the UL signalling, if any, is forwarded from the gNB-CU-CP to the other gNB-CU-CP (the last serving gNB-CU-CP) via the XnAP RRC TRANSFER message. NOTE 3: The other gNB-CU-UP may need to buffer the UL SDT data if received before the SDT bearer(s) are resumed. 6. The gNB-CU-CP sends the BEARER CONTEXT MODIFICATION REQUEST message including an resume indication for SDT DRBs. The gNB-CU-CP also includes the F1-U DL TEIDs received from the gNB-DU in step 5. 7. The gNB-CU-UP responds with the BEARER CONTEXT MODIFICATION RESPONSE message. NOTE 4: void. Upon receiving the UE INACTIVITY NOTIFICATION message without SDT volume threshold crossed indication from the gNB-DU, the gNB-CU, if serving the UE and deciding to terminate the ongoing SDT procedure, shall transmit the UE CONTEXT RELEASE COMMAND message to the gNB-DU. 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. NOTE 5: 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. 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.