Description
Random Access-based Small Data Transmission (RA-SDT) is a mechanism defined in 3GPP Release 17 for New Radio (NR) that allows a User Equipment (UE), typically an IoT device, to transmit a limited amount of uplink data while remaining in RRC Inactive state. It leverages the existing Physical Random Access Channel (PRACH) procedure as a carrier for both the access request and the initial data payload. The core idea is to piggyback small data packets on the messages of the random access process, thereby avoiding the extensive signaling exchange required to transition to RRC Connected state, perform service request procedures, and then release the connection back to inactive.
Architecturally, RA-SDT integrates with the 3-step and 4-step contention-based random access procedures defined for NR. In the 3-step approach (MsgA), the UE combines the traditional preamble (Msg1) and connection request (Msg3) into a single transmission. RA-SDT can be implemented by allowing the UE to include a small data payload within this MsgA. The gNB's response (MsgB) then includes not only the contention resolution and uplink grant but also the acknowledgment for the transmitted data. For the 4-step approach, data can be included in Msg3. Key components enabling this are enhancements to the RRC Inactive state context, which is stored in both the UE and the network (last serving gNB and AMF), allowing the gNB to validate and process the data without full RRC setup.
How it works involves pre-configuration and resource allocation. The network broadcasts system information indicating support for RA-SDT and its associated parameters, such as the maximum transport block size for data in MsgA/Msg3. A UE in RRC Inactive state with a small amount of data to send and a valid Inactive state context can initiate an RA-SDT procedure. It selects the appropriate preamble resource configured for SDT and transmits MsgA (or Msg1) including the data. The gNB, recognizing the preamble or the content of MsgA as an SDT attempt, uses the stored UE context to authenticate, decipher the data, and forward it to the UPF via the last serving NG-RAN node. The UE may then return to Inactive state immediately after receiving a successful MsgB (or Msg4), completing the transaction with minimal state transitions.
Purpose & Motivation
RA-SDT was created to address a critical inefficiency in cellular IoT and massive Machine-Type Communication (mMTC) scenarios: the disproportionate signaling overhead and energy consumption associated with transmitting very small, infrequent data packets. Traditional procedures require a full RRC connection setup and release for even a single data packet, which can involve 10-20 signaling messages. This 'signaling storm' congests the network and drains the battery of constrained IoT devices, which are designed for years of operation on a single battery charge.
The motivation stems from the evolution of 5G to support a wider range of use cases, including ultra-reliable low-latency communication (URLLC) and massive IoT. Prior to Rel-17, mechanisms like Early Data Transmission (EDT) in LTE-M and NB-IoT addressed similar issues but were limited to those technologies. RA-SDT brings this optimization to the mainstream NR spectrum, enabling efficient support for a new class of NR-light devices (often called 'RedCap' – Reduced Capability). It solves the problem of network resource wastage and poor device battery life for applications like sensor readings, smart meter updates, or wearable health monitors that generate payloads of only a few bytes periodically.
By allowing data transmission within the RRC Inactive state, RA-SDT directly tackles the limitations of the binary connected/inactive model for sporadic traffic. It reduces latency (by skipping connection setup), minimizes control plane load on the gNB and core network, and significantly extends device battery life by avoiding the high-power operations associated with full connection states. This makes NR a more viable and efficient technology for the scaling Internet of Things.
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (33 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the foundational procedures for Random Access-based Small Data Transmission (RA-SDT) were established, including the initiation of the Random Access procedure by the MAC entity itself for events like SI request. The release introduced mechanisms for PRACH preamble and occasion selection specifically for Msg1-based SI requests, alongside enhancements for RSRP measurements to be used during random access.
- Flush HARQ buffer upon skipping a UL transmission TS 38.321CR0153
- Addition of Prioritized Random Access TS 38.321CR0166
- PRACH Preamble Selection for Msg1 based SI Request TS 38.321CR0189
- PRACH Occasion Selection for Msg1 based SI Request TS 38.321CR0302
- Correction for Random Access Back off TS 38.321CR0342
- RSRP measurements for Random Access TS 38.321CR0356
+ 1 more changes
In Release 16, the RA-SDT function introduced new mechanisms for uplink transmission prioritization and scheduling corrections. Specifically, it added clarifications and corrections for Logical Channel (LCH)-based prioritization procedures, including interactions with PUSCH skipping and PUCCH resource selection. The release also provided updates for handling HARQ Process ID selection and DRX operation with bundle transmissions for configured uplink grants.
- Correction on DRX with bundle transmission of configured uplink grant TS 38.321CR0987
- Correction to PUSCH skipping with UCI without LCH-based prioritization TS 38.321CR1062
- Clarification on prioritization of retransmission over initial transmission for HARQ PID selection in NR-U TS 38.321CR1115
- Correction on UL skipping with lch-basedPrioritization TS 38.321CR1098
- Correct on random selection TS 38.321CR1126
- Clarification of PUCCH resource in LCH-based Prioritization TS 38.321CR1141
In Release 17, the RA-SDT (Random Access-based Small Data Transmission) function was introduced as a new capability, allowing the transmission of small data packets during the Random Access procedure, specifically within Msg3 on the UL-SCH. This function was developed alongside CG-SDT (Configured Grant-based SDT) and underwent subsequent corrections and refinements to its operation in the MAC specification.
- Introduction of Small Data Transmission for MAC spec TS 38.321CR1198
- Correction to MAC spec for Small Data Transmission TS 38.321CR1243
- Corrections for TRS-based SCell activation TS 38.321CR1302
- Change to MAC spec for Small Data Transmission TS 38.321CR1357
- Correction for Simultaneous Transmission of SR and UL-SCH TS 38.321CR1404
- Correction to MAC spec for Small Data Transmission TS 38.321CR1451
+ 5 more changes
In Release 18, the RA-SDT (Random Access-based Small Data Transmission) function was enhanced with specific corrections and clarifications to existing procedures. These included a correction for the initial transmission using Configured Grant SDT (CG-SDT) and a correction related to Timing Advance (TA) validation for Sounding Reference Signal (SRS) transmission while the UE is in RRC_INACTIVE state. The updates also addressed the co-configuration of specific uplink control schemes, such as the IUC scheme-2 alongside random selection, refining the overall operation.
- Correction for SL resource pool usage for BRID/DAA transmission TS 38.321CR1743
- Correction to LTM MAC CE based CFRA with MSG1 repetition TS 38.321CR1969
- Correction on prioritization between SR and SL-PRS transmission TS 38.321CR1992
- Correction to TA validation for SRS transmission in RRC_INACTIVE TS 38.321CR2075
- Correction in TS 38.321 to support Simultaneous PUSCH and PUCCH transmissions of same priority on different inter-band cells [SimultaneousPUSCH-PUCCH] TS 38.321CR1732
- Correction on CG-SDT initial transmission TS 38.321CR1751
+ 2 more changes
In Release 19, the enhancement for RA-SDT involved a correction on UE transmission behavior specifically during Cell DRX cycles. This update clarified procedural interactions to ensure proper operation when small data transmission is initiated via the Random Access Channel (RACH). The change refined the MAC entity's handling of the ongoing Random Access procedure under these specific discontinuous reception conditions.
- Correction on UE transmissions during Cell DRX TS 38.321CR2129
Explore further
Broader topics and technologies where RA-SDT plays a role.
Defining Specifications
3GPP specifications that define or reference RA-SDT, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |