Description
Mobile Originated Small Data Transmission (MO-SDT) is a feature standardized in 3GPP Release 18, designed to efficiently handle sporadic, small data transmissions from IoT and other devices. It operates by allowing a User Equipment (UE) to send a limited amount of uplink data while remaining in the RRC_INACTIVE or RRC_IDLE state, thereby avoiding the signaling overhead and latency associated with transitioning to the RRC_CONNECTED state. This is achieved by encapsulating the data within specific signaling messages, such as a modified RRC Resume Request or a new small data transmission message, which are processed by the network without fully re-activating all user plane bearers.
The architecture involves coordination between the Radio Access Network (RAN) and the Core Network (CN). When a UE in RRC_INACTIVE has data to send, it can initiate MO-SDT if the data size and other conditions (like configured thresholds) are met. The UE includes the data payload in the initial access message sent to the gNB. The gNB, upon receiving this, can forward the data to the UPF via the last serving NG-RAN node's context or a new path, depending on the implementation. The Core Network, specifically the AMF and SMF, supports this by allowing the user plane to be briefly established or by using a pre-configured small data transmission resource.
Key components include the UE, which must support the MO-SDT procedures as per 3GPP specifications; the gNB, which handles the reception of the small data and its forwarding; and the Core Network functions (AMF, SMF, UPF) that manage the context and data routing. The procedure leverages stored UE context in the RAN (for INACTIVE state) to minimize signaling. MO-SDT's role is to reduce latency, signaling load, and power consumption for devices that primarily send small, infrequent data bursts, making it a cornerstone for efficient massive Machine-Type Communication (mMTC) in 5G and beyond.
Purpose & Motivation
MO-SDT was created to address the inefficiencies of traditional connection establishment for IoT and M2M devices that generate small, sporadic data packets. Prior to its introduction, a UE needing to send data had to perform a full RRC connection setup, involving multiple signaling exchanges, even for a few bytes of data. This process consumed significant battery power and network resources, which is unsustainable for massive-scale IoT deployments with billions of devices. The limitations of previous approaches, like always keeping devices in connected state or using inefficient signaling, led to high overhead and reduced network capacity.
The motivation stems from the growth of IoT applications, such as sensors, smart meters, and wearables, which require energy-efficient and network-friendly communication. 3GPP Release 18 introduced MO-SDT as part of broader enhancements for reduced capability (RedCap) devices and IoT optimization. It solves the problem of signaling storms and battery drain by enabling data transmission without state transitions, aligning with 5G goals of supporting diverse services with varying requirements. Historically, early IoT solutions in LTE used techniques like Power Saving Mode (PSM) or extended Discontinuous Reception (eDRX), but these still incurred overhead during data transmission; MO-SDT provides a more integrated and efficient solution within the 5G NR framework.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (54 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the MO-SDT (Mobile Originated Small Data Transmission) function was newly introduced, enabling a UE in idle or inactive state to transmit small data packets directly to the network without transitioning to a connected state. This is facilitated by transmitting data within the MSGA payload of a 2-step random access procedure or within the MSG3 transmission of a 4-step random access procedure. The procedure utilizes the existing random access channel (RACH) mechanism, allowing for efficient transmission of infrequent, small data bursts with reduced signaling overhead.
In Release 16, the MO-SDT function was enhanced with specific procedures for conditional handover and mobile IAB-node operations. New capabilities included the transmission of UEAssistanceInformation after a conditional handover and support for mobile IAB-node migration procedures, including mobile IAB-MT migration via Xn or NG handover. Furthermore, the release introduced mechanisms for transmission suspension on backhaul RLC channels upon IAB-MT failure.
- Implementing confirmation of code block group based transmission TS 38.331CR1717
- Correction on uplink transmission allowed without TA TS 38.300CR0343
- Transmissions to the source that continue upon DAPS UL switching TS 38.300CR0353
- Dummifying intraFreqMultiUL-TransmissionDAPS-r16 capability TS 38.306CR0501
- CR on the Capability of PUCCH Transmissions for HARQ-ACK-38306 TS 38.306CR0521
- Correction on Capability of two PUCCH transmission TS 38.306CR0542
+ 8 more changes
In Release 17, the enhancements for Mobile Originated Small Data Transmission (MO-SDT) primarily involved corrections and refinements to its implementation. These included specific corrections to security issues related to MO-SDT and multiple corrections to the MAC layer specification for Small Data Transmission. Furthermore, the release addressed related transmission procedures with corrections for the simultaneous transmission of Scheduling Request (SR) and UL-SCH, as well as for SR and PUSCH.
- Introduction of Small Data Transmission for MAC spec TS 38.321CR1198
- Parallel PRACH and SRS/PUCCH/PUSCH transmissions across CCs in intra-band non-contiguous CA [NC-PRACH-SimulTx] TS 38.331CR3577
- Corrections for IIoT on simultaneous PUCCH and PUSCH transmission TS 38.300CR0477
- Corrections to Security Issues for MO-SDT TS 38.300CR0684
- Correction to Security Issues for MO-SDT TS 38.300CR0688
- Simultaneous PUSCH and PUCCH transmissions of same priority on different inter-band cells [SimultaneousPUSCH-PUCCH] TS 38.306CR1013
+ 13 more changes
In Release 18, the enhancements for Mobile Originated Small Data Transmission (MO-SDT) primarily involved corrections to its procedures, specifically for the CG-SDT (Configured Grant SDT) initial transmission. Furthermore, the release introduced support for RACH-less handover procedures applicable to mobile IAB-nodes, which can be utilized during mobile IAB-DU migration to improve efficiency for UEs connected via such nodes.
- Introduction of Mobile IAB TS 38.300CR0727
- Introduction of RACH-less handover for NR NTN and mobile IAB to TS 38.321 TS 38.321CR1716
- Introduction of mobile IAB TS 38.331CR4457
- Correction for SL resource pool usage for BRID/DAA transmission TS 38.321CR1743
- 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
+ 7 more changes
In Release 19, the enhancements for Mobile Originated Small Data Transmission (MO-SDT) specifically introduced support for mobile Integrated Access and Backhaul (IAB) nodes, enabling them to operate as a transmission path. This included defining the mobile IAB-node authorization and network integration procedures, and specifying that these nodes use a dedicated indicator in SIB1 while being restricted from having descendant nodes. The release also provided clarifications and corrections on UE transmission behavior during Cell DRX and for uplink duty cycles to ensure reliable small data transmission.
Explore further
Broader topics and technologies where MO-SDT plays a role.
Defining Specifications
3GPP specifications that define or reference MO-SDT, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.306 vj00 | NR UE Radio Access Capability Parameters | Rel-19 |
| TS 38.321 vj00 | NR MAC Protocol Specification | Rel-19 |
| TS 38.331 vj00 | NR Radio Resource Control (RRC) Protocol Specification | Rel-19 |