A UE may need to send only a small sensor report, but a normal RRC resume still involves control signalling and time in RRC_CONNECTED. Release 17 SDT lets the UE exchange eligible data while remaining in RRC_INACTIVE. Release 18 adds Mobile Terminated SDT (MT-SDT), so the network can trigger an SDT exchange when downlink data arrives.

Start with the RRC INACTIVE note if the retained UE context and suspend/resume procedure are unfamiliar. This note focuses on the Rel-17 mechanism and its Rel-18 MT-SDT extension. Current specification text is used to check the parameter definitions; later-release features are not treated as Rel-17 capabilities.

• Executive Summary
• Why use SDT instead of a normal resume ?
  • Retained context and bearer permission
  • RRC state and practical benefit
• Rel-17: How does UE start SDT ?
  • Data volume, RSRP and resource selection
  • RA-SDT: put the small payload into the access exchange
  • CG-SDT: timing alignment comes first
• Rel-18: Mobile Terminated SDT
  • Paging starts the procedure, not the data transfer
  • What changes in the access conditions ?
• How does SDT end, and where is the UE context ?
  • Completion, non-SDT traffic and failure
  • Receiving gNB versus last serving gNB
• RRC Parameters
  • Which parameters should I check first ?
  • Selected RRC ASN.1 definitions
• Reference

Executive Summary

The trigger and the access resource are two separate choices. MO/MT identifies who starts the exchange; RA/CG identifies how UE obtains the initial uplink transmission opportunity.

Why use SDT instead of a normal resume ?

For a short, occasional packet, the signalling surrounding the packet can be a substantial part of the transaction. SDT uses the context already retained during RRC_INACTIVE to reduce that overhead. It still needs resource selection, security, scheduling and a clear end to the exchange.

Retained context and bearer permission

The network first configures the UE while it has an RRC connection, then suspends that connection with RRCRelease. The UE keeps the information needed for a later resume. SDT adds permission to use selected radio bearers before returning to RRC_CONNECTED.

This permission is per bearer. A small packet on an arbitrary DRB does not automatically qualify. The dedicated sdt-DRB-List-r17 identifies enabled DRBs; sdt-SRB2-Indication-r17 can allow SRB2. AS security applies to the enabled bearers when SDT starts. SDT does not mean sending an unprotected application packet simply because the UE is inactive.

RRC state and practical benefit

An RRCResumeRequest can appear in an SDT trace even though the UE does not enter RRC_CONNECTED. The request identifies the retained context and starts the procedure; the presence of that message alone does not prove that a normal resume has completed.

SDT can save signalling and energy for short exchanges, but the saving depends on radio conditions, contention, grant timing and how long UE monitors for a response. A long burst can make ordinary connected operation more suitable. Compare the state transitions in RRC INACTIVE with the resource allocation in Configured Scheduling.

• SDT reuses the inactive context and selected bearers; it does not remove the access procedure or the need for network control.

Rel-17: How does UE start SDT ?

The UE first decides whether the pending traffic qualifies, then checks whether a usable SDT resource exists. This order matters: configuring a grant does not authorize every packet, and a small packet does not make an expired grant usable.

Data volume, RSRP and resource selection

For MO-SDT, UE checks the aggregate pending UL volume across the radio bearers enabled for SDT against the configured data-volume threshold. The condition is less than or equal to the threshold, rather than a separate allowance for every bearer.

The volume calculation follows the RRC rules and excludes the CCCH message size. UE also checks the configured RSRP condition and the MAC resource-selection conditions. If the applicable RSRP threshold is absent, that threshold check is not imposed. In a trace, check the bearer list, system information and measured reference signal before concluding that the UE ignored SDT.

RA-SDT: put the small payload into the access exchange

RACH provides an access opportunity when UE cannot simply transmit on a valid configured grant. For MO-SDT, the network can configure SDT resources for 2-step or 4-step random access in system information; contention-free random access is not supported for RA-SDT.

The sequence below shows a simplified 4-step MO-SDT exchange. Data multiplexing is subject to the available transport block and logical-channel rules. Subsequent transmissions use dynamic scheduling after random access completes. The RACH note explains the four messages, while 2-step RACH explains MsgA and MsgB.

Figure 1. Simplified successful 4-step MO-SDT; context retrieval and retransmissions are omitted.

CG-SDT: timing alignment comes first

CG-SDT avoids the initial random-access exchange by using a Type 1 grant supplied in RRCRelease. The resource is valid only in the PCell where UE received that release, and it is associated with one or more SSBs.

UE must have valid UL timing alignment. The network configures an SDT-specific timing-alignment timer, and initial access also checks the relevant SSB measurements. A configured maximum wait to the next usable CG occasion can prevent an excessively delayed transmission. Losing CG eligibility does not by itself prove that all SDT is unavailable: RA-SDT may still be an option if its conditions hold.

After the initial CG PUSCH, UE needs network confirmation before initiating subsequent UL transmission. Dynamic grants or later CG occasions can carry subsequent UL data, while DL uses dynamic assignments. Initial autonomous retransmission has its own rules; UE cannot apply the same autonomous CG retransmission behavior to subsequent UL transmissions. See Configured Scheduling for the underlying grant concept.

• Check traffic eligibility and resource validity separately. CG-SDT needs valid timing; RA-SDT needs a successful contention-based access exchange.

Rel-18: Mobile Terminated SDT

Rel-17 SDT already allows downlink traffic after a UE starts an exchange. What it lacks is the MT-SDT trigger for a new exchange initiated by pending downlink traffic. Rel-18 adds that trigger through paging and a corresponding resume cause.

Paging starts the procedure, not the data transfer

When downlink data arrives for an SDT-enabled bearer, the network can page the inactive UE with the MT-SDT indication. UE evaluates the applicable conditions and, when it initiates MT-SDT, sends RRCResumeRequest or RRCResumeRequest1 with an MT-SDT resume cause.

The paging record carries the indication, not the application payload. UE still performs an uplink access step so the network can reach it for scheduled downlink delivery. Ordinary paging remains relevant if MT-SDT is not selected. The Paging note provides the paging timing and identity background.

Figure 2. Rel-18 MT-SDT trigger and exchange; the access handshake is condensed.

What changes in the access conditions ?

MT-SDT does not depend on UE having a small pending UL packet. It uses the paging trigger and the applicable MT-SDT RSRP condition. The MT-specific RSRP threshold takes precedence when configured; otherwise the applicable MO-SDT threshold can be used.

A useful detail is the RACH resource restriction: MT-SDT initiated over RACH uses resources not configured for SDT. Do not reuse the MO-SDT resource assumption when reading an MT-SDT trace. CG access remains subject to its validity checks, with an MT-SDT maximum duration to the next CG occasion configured per UE. For MO-SDT, that maximum-wait configuration is per logical channel.

A cell can enable MO-SDT, MT-SDT or both. UE capability, dedicated bearer configuration, broadcast support and paging indication therefore need to agree. A release label by itself does not prove that a particular network has enabled the feature. Related release context is available in Release 17 and Release 18.

• Rel-18 MT-SDT adds a network-triggered start. Downlink transfer during an existing MO-SDT exchange was already possible in Rel-17.

How does SDT end, and where is the UE context ?

Sending the first payload does not finish the SDT procedure. The network can exchange more data, decide that a connected session is needed, or terminate the transaction. The receiving gNB may also need to retrieve context from the last serving gNB.

Completion, non-SDT traffic and failure

The terminating RRC message determines the next state. RRCRelease with a suspend indication can keep UE in RRC_INACTIVE, while release without suspension can lead to RRC_IDLE. RRCResume or RRCSetup can move the exchange toward RRC_CONNECTED.

If data arrives on a bearer not enabled for SDT during the procedure, UE can report the non-SDT data arrival using UEAssistanceInformation. The network can then choose an appropriate transition. A large subsequent buffer report can also influence that decision. SDT is not a requirement to keep an expanding transaction inactive indefinitely.

Failure handling is equally important. T319a provides SDT failure detection, and conditions such as cell reselection, retry limits or integrity failure can end the procedure unsuccessfully. For CG-SDT, expiry of relevant timers before a network response also matters. The unsuccessful-completion path takes UE to RRC_IDLE; it should not be drawn as an automatic successful resume to CONNECTED.

Receiving gNB versus last serving gNB

UE may perform RA-SDT through a gNB other than the one that suspended its connection. The receiving gNB then requests the retained UE context. The last serving gNB decides whether to relocate that context or support the exchange without full relocation.

With relocation, the receiving gNB takes over the context and can perform the corresponding core-network path switch. Without full relocation, the last serving gNB transfers the partial context needed for SDT handling. User-plane tunnels between the gNBs can forward DRB data, while the context anchor remains at the last serving gNB. This is why a short radio trace alone does not show all the work involved in SDT.

For troubleshooting, correlate RRC and MAC events with the context-retrieval outcome. A good RSRP and a decodable initial PUSCH do not guarantee that the network can complete the transaction. The retained-context model is described further in RRC INACTIVE.

• Separate radio access success, context handling and final RRC state when diagnosing an SDT transaction.

RRC Parameters

SDT configuration is split between dedicated UE information and cell-common information. The dedicated part identifies permitted bearers and any configured grant, while the common part supplies access conditions. Reading only one of these parts can give a misleading picture of whether SDT is usable.

Which parameters should I check first ?

Start with the earlier RRCRelease, then inspect the current cell information and the actual trigger. This connects what the network permitted before suspension with the resources and conditions that UE sees when data arrives.

Selected RRC ASN.1 definitions

The following complete IE definitions were checked against TS 38.331 v19.4.0. The r17 and r18 suffixes identify their release of introduction. These are selected definitions, not a complete RRCRelease, SIB or Paging message; referenced types and surrounding extension containers are intentionally outside this listing.

SDT-Config-r17 — TS 38.331 v19.4.0

SDT-Config-r17 ::=                  SEQUENCE {
    sdt-DRB-List-r17                    SEQUENCE (SIZE (0..maxDRB)) OF DRB-Identity                         OPTIONAL,   -- Need M
    sdt-SRB2-Indication-r17             ENUMERATED {allowed}                                                OPTIONAL,   -- Need R
    sdt-MAC-PHY-CG-Config-r17           SetupRelease {SDT-CG-Config-r17}                                    OPTIONAL,   -- Need M
    sdt-DRB-ContinueROHC-r17            ENUMERATED { cell, rna }                                            OPTIONAL    -- Need S
}

SDT-ConfigCommonSIB-r17 — TS 38.331 v19.4.0

SDT-ConfigCommonSIB-r17 ::=          SEQUENCE {
    sdt-RSRP-Threshold-r17               RSRP-Range                                                            OPTIONAL, -- Need R
    sdt-LogicalChannelSR-DelayTimer-r17  ENUMERATED { sf20, sf40, sf64, sf128, sf512, sf1024, sf2560, spare1}  OPTIONAL, -- Need R
    sdt-DataVolumeThreshold-r17          ENUMERATED {byte32, byte100, byte200, byte400, byte600, byte800, byte1000, byte2000, byte4000,
                                                     byte8000, byte9000, byte10000, byte12000, byte24000, byte48000, byte96000},
    t319a-r17                            ENUMERATED { ms100, ms200, ms300, ms400, ms600, ms1000, ms2000,
                                                      ms3000, ms4000, spare7, spare6, spare5, spare4, spare3, spare2, spare1}
}

MT-SDT-ConfigCommonSIB-r18 — TS 38.331 v19.4.0

MT-SDT-ConfigCommonSIB-r18 ::=       SEQUENCE {
    mt-SDT-RSRP-Threshold-r18            RSRP-Range                                                            OPTIONAL, -- Need S
    sdt-LogicalChannelSR-DelayTimer-r18  ENUMERATED { sf20, sf40, sf64, sf128, sf512, sf1024, sf2560, spare1}  OPTIONAL, -- Cond MT-SDT1
    t319a-r18                            ENUMERATED { ms100, ms200, ms300, ms400, ms600, ms1000, ms2000,
                                                      ms3000, ms4000, spare7, spare6, spare5, spare4,
                                                      spare3, spare2, spare1}                                  OPTIONAL  -- Cond MT-SDT2
}

PagingRecord-v1800 — TS 38.331 v19.4.0

PagingRecord-v1800 ::=              SEQUENCE {
    mt-SDT                              ENUMERATED {true}       OPTIONAL    -- Need N
}

The enumerated data-volume values should not be interpreted as a universally fixed packet-size limit. The network selects a threshold, and UE applies the specified aggregate-volume test. Likewise, an optional field must be interpreted with its presence condition and field description; absence does not always mean a numeric zero. Use TS 38.331 for the RRC decision and TS 38.321 for MAC resource selection rather than treating the ASN.1 syntax as the entire procedure.

• Match dedicated permissions, cell-common conditions and live resource validity before interpreting an SDT log.

Reference

The procedure descriptions use the Rel-17 baseline and Rel-18 extension, with current specification text used to check definitions and corrections. The latest-release references below do not imply that every feature in those documents belongs to Rel-17 or Rel-18.

• TS 38.300 V19.4.0: NR overall description; section 18, SDT and MT-SDT.
• TS 38.321 V19.4.0: NR MAC; section 5.27, SDT resource selection and related MAC procedures.
• TS 38.331 v19.4.0: NR RRC; section 5.3.13 and SDT configuration, paging and timer definitions.

• TS 38.300 V18.8.0: Rel-18 overall SDT description, including MT-SDT.
• TS 38.331 V18.6.0: Rel-18 RRC procedures and information elements.