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Content for  TS 38.401  Word version:  18.4.0

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8.18 Overall procedure for Small Data Transmission during RRC Inactive  8.18.1 RACH based SDT  8.18.2 CG based SDT  8.18.3 RA-SDT or non-SDT with CG-SDT configuration  8.18.4 MT-SDT 
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8.18  Overall procedure for Small Data Transmission during RRC Inactive |R17|  p. 122

8.18.1  RACH based SDT  p. 122

The procedure for RACH based small data transmission in RRC Inactive is shown in Figure 8.18.1-1.
Reproduction of 3GPP TS 38.401, Fig. 8.18.1-1: RACH based Small Data Transmission in RRC Inactive state
Figure 8.18.1-1: RACH based Small Data Transmission in RRC Inactive state
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Step 1.
The UE in RRC Inactive sends the RRCResumeRequest message together with UL SDT data and/or UL SDT signalling.
Step 2.
The gNB-DU buffers the UL SDT data and/or UL SDT signalling.
Step 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.
Step 4-5.
If UE context is successfully retrieved as specified in TS 38.300, 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.
Step 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.
Step 7.
The gNB-CU-UP responds with the BEARER CONTEXT MODIFICATION RESPONSE message.
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.
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, or by sending the RRCRelease message with resume indication to trigger the UE to initiate RRC Resume procedure to move to RRC_CONNECTED state as specified in TS 38.300.
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.
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 as specified in TS 38.300.
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8.18.2  CG based SDT  p. 123

The procedure for CG based small data transmission in RRC Inactive is shown in Figure 8.18.2-1.
Reproduction of 3GPP TS 38.401, Fig. 8.18.2-1: CG based Small Data Transmission in RRC Inactive state
Figure 8.18.2-1: CG based Small Data Transmission in RRC Inactive state
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Step 1.
The gNB-CU decides to move UE into RRC_INACTIVE state.
Step 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).
Step 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.
Step 4.
The gNB-CU-CP sends the BEARER CONTEXT MODIFICATION REQUEST towards the gNB-CU-UP, with the suspend indication.
Step 5.
The gNB-CU-UP sends the BEARER CONTEXT MODIFICATION RESPONSE towards the gNB-CU-CP.
Step 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.
Step 7.
The gNB-DU sends the RRCRelease message to UE.
Step 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, CG-SDT-CS-RNTI, and which bearers are CG-SDT bearers.
After a period of time of the UE being in RRC_INACTIVE state.
Step 9.
The UE decides to perform CG based SDT procedure, it sends the RRCResumeRequest message together with UL SDT data/UL NAS PDU.
Step 10.
The gNB-DU sends the UL RRC MESSAGE TRANSFER message including the RRCResumeRequest message to indicate the access due to CG-SDT.
Step 11-12.
If UE context is successfully retrieved as specified in TS 38.300, the gNB-CU-CP initiates the BEARER CONTEXT MODIFICATION procedure to resume SDT DRBs.
Step 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.
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.
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.
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.
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 as specified in TS 38.300.
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8.18.3  RA-SDT or non-SDT with CG-SDT configuration  p. 125

The procedure for the case where the UE has CG-SDT resource configurations but decides to perform RACH based small data transmission in RRC Inactive or to perform RACH procedure to transit to RRC Connected (see clause 5.27 of TS 38.321) is shown in Figure 8.18.3-1.
Reproduction of 3GPP TS 38.401, Fig. 8.18.3-1: RA-SDT or non-SDT with CG-SDT configuration
Figure 8.18.3-1: RA-SDT or non-SDT with CG-SDT configuration
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Step 1.
The UE in RRC Inactive sends RRCResumeRequest message. If the UE decides to perform RACH based SDT procedure, it also sends UL SDT data and/or UL SDT signalling.
Step 2.
The gNB-DU buffers the UL SDT data and/or UL SDT signalling.
Step 3.
The gNB-DU sends the INITIAL UL RRC MESSAGE TRANSFER message to the gNB-CU-CP, including a new gNB-DU UE F1AP ID, and in case of RACH based SDT access, the gNB-DU provides an indication of SDT access and may also the SDT assistance information.
Step 4.
If UE context is successfully retrieved as specified in TS 38.300, the gNB-CU-CP sends the UE CONTEXT SETUP REQUEST message with the stored (or retrieved from the last serving gNB) F1 UL TEIDs and the new gNB-DU UE F1AP ID received in step 3.
In case that the gNB-DU is the one that sent the RRCRelease message with CG-SDT resource configurations to the UE, the gNB-CU-CP also includes the old gNB-DU UE F1AP ID and the old gNB-CU F1AP UE ID within the Old CG-SDT Session Info IE of the UE CONTEXT SETUP REQUEST message.
In case that the gNB-CU-CP is the one that generated the RRCRelease message with CG-SDT resource configurations but the gNB-DU is not the old gNB-DU that sent the RRCRelease message to the UE, the gNB-CU-CP initiates the UE Context Release procedure by sending the UE CONTEXT RELEASE COMMAND message to the old gNB-DU.
In case that the UE accesses a gNB other than the last serving gNB, upon receiving the RETRIEVE UE CONTEXT REQUEST message from the receiving gNB-CU-CP, the last serving gNB-CU-CP initiates the UE Context Release procedure by sending the UE CONTEXT RELEASE COMMAND message to the last serving gNB-DU.
Step 5.
The gNB-DU sends the UE CONTEXT SETUP RESPONSE message with the new gNB-DU UE F1AP ID. In case the old gNB-DU UE F1AP ID is received within the Old CG-SDT Session Info IE in step 4, the gNB-DU retrieves the stored CG-SDT resource configurations and UE context based on the Old CG-SDT Session Info IE, if any, and associates them with the new gNB-DU F1AP UE ID.
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8.18.4  MT-SDT |R18|  p. 126

The procedure for mobile terminated small data transmission in RRC Inactive is shown in Figure 8.18.4-1.
Reproduction of 3GPP TS 38.401, Fig. 8.18.4-1: Mobile Terminated Small Data Transmission in RRC Inactive state
Figure 8.18.4-1: Mobile Terminated Small Data Transmission in RRC Inactive state
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Step 1.
During the setup or modification of the bearer context as specified in clause 8.9.2, the gNB-CU-CP requests the gNB-CU-UP to provide MT-SDT information.
Step 2a-0.
The gNB-CU-UP receives DL data for the UE in RRC Inactive on NG-U interface.
Step 2a-1.
The gNB-CU-UP sends DL DATA NOTIFICATION message to the gNB-CU-CP. If determining that DL data packets are only mapped to SDT bearers, as requested in step 1, the gNB-CU-UP includes the MT-SDT information in the DL DATA NOTIFICATION message.
Step 2b.
The gNB-CU-CP receives DL NAS signalling or RAN PAGING REQUEST message over NGAP.
Step 2c.
The gNB-CU-CP receives the MT-SDT information in XnAP RAN PAGING message.
Step 3.
After 2a or 2b or 2c, the gNB-CU-CP sends PAGING message to the gNB-DU. The MT-SDT indication may be included in the PAGING message.
Step 4.
The gNB-DU sends the Paging message to the UE. In case the MT-SDT indication is received in step 3, the gNB-DU includes the MT-SDT indicator in the Paging message.
Step 5.
If the UE has been successfully reached, it initiates the RRC connection resume procedure as described in clause 8.6.2 or clause 8.9.6.2, or initiates the SDT procedure as described from step 1 in clause 8.18.1 or from step 9 in clause 8.18.2 or from step 1 in clause 8.18.3 with the following difference:
  • In case SDT procedure is initiated, the UE may indicate MT-SDT in the RRCResumeRequest, which may be without UL data.
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