Description
The Service Description Table (SDT) is a key signaling table used in 3GPP broadcast and multicast service delivery frameworks, such as Multimedia Broadcast Multicast Service (MBMS) and 5G Media Streaming (5GMS). It is a structured dataset that provides descriptive metadata about the services available within a broadcast stream. The SDT lists each service, assigns it a unique identifier, and provides information necessary for a receiver to access and interpret that service, such as the service name, service type, and references to other essential tables or components.
Architecturally, the SDT is generated by the broadcast service provider or network and is multiplexed into the transport stream (e.g., FLUTE/ALC session in MBMS) or delivered via an application layer protocol. It works in conjunction with other tables like the Program Map Table (PMT) or Media Presentation Description (MPD) in DASH-based streaming. Key components of an SDT entry include the service_id, service_descriptors (which can contain text names, provider info, and service type classification), and pointers to other descriptors that may indicate the location of electronic service guide (ESG) data or component streams. The receiver parses the SDT to build a user-presentable list of available services.
Its role is to act as the primary service discovery mechanism within a broadcast/multicast session. Without the SDT, a receiver would only see a raw data stream without knowledge of what services (e.g., TV channel, radio station, file delivery session) it contains or how to decode them. It is therefore fundamental for user interaction, enabling channel surfing, service selection, and ensuring the receiver configures the correct decoders for audio, video, or data components associated with the chosen service.
Purpose & Motivation
The SDT was created to solve the problem of service discovery and identification in IP-based broadcast and multicast systems. In traditional digital TV broadcasting (like DVB), similar tables (e.g., DVB-SI) serve this purpose. As 3GPP developed MBMS to deliver multimedia over cellular networks, a standardized, efficient method was needed to describe services within an IP multicast flow, motivating the adoption and adaptation of the SDT concept.
The core problem it addresses is informing the user equipment about 'what is available' in a broadcast service area. Prior to its standardization, proprietary or non-interoperable methods would have hindered the widespread adoption of broadcast services, as each vendor's receiver might need custom logic to find services. The SDT provides a universal, well-defined format that ensures any compliant receiver can discover all services offered by any network operator.
It overcomes the limitations of pure IP multicast, where a receiver might join a multicast group but have no inherent information about the content's nature or how to present it. The SDT adds this essential service layer metadata, making broadcast services user-friendly and interoperable. Its evolution through releases reflects the expansion of broadcast use cases from MBMS/TV services to include public warning, automotive, and 5G broadcast, requiring more sophisticated service descriptions and integration with streaming protocols like DASH.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (331 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-5, normative work from Rel-15.
In Release 15, the specification introduced several corrections and clarifications to the Service Description Table (SDT) function. These updates primarily focused on ensuring consistent and complete descriptions for key 5G procedures, including 5GSM and 5GMM operations, QoS flow descriptions, and LADN-related structures. Additionally, the release addressed missing or erroneous descriptions for aspects like power saving, bandwidth adaptation, and active BWP.
- Dynamic Routing indicator update description TS 24.501CR0395
- Protection of initial NAS messages – overall description TS 24.501CR0424
- Adding EPLMN list related descriptions TS 24.501CR0102
- Corrections on inconsistent descriptions for 5GSM and 5GMM TS 24.501CR0108
- Missing general description on sub-clause 9.10 TS 24.501CR0128
- Correction on QoS flow description IE TS 24.501CR0522
+ 40 more changes
In Release 16, the SDT function was enhanced with new general descriptions for several control mechanisms, including small data rate control, serving PLMN rate control, and Service Gap control in 5GS. Additionally, the release added a general description for the Restriction on use of enhanced coverage and introduced the general description of RACS (Rate Adaptation and Control Services). These updates provided a more comprehensive framework for managing service parameters and network resource controls.
- Small data rate control, general description TS 24.501CR0970
- Serving PLMN rate control, general description TS 24.501CR0972
- Service Gap control in 5GS, general description TS 24.501CR0974
- General Description for Restriction on use of enhanced coverage TS 24.501CR0980
- Adding general description of RACS TS 24.501CR1355
- Add Power, Energy and Environmental (PEE) measurements and related use case description TS 28.552CR0111
+ 55 more changes
In Release 17, the key new SDT (Service Description Table) function introduced support for RACH-based SDT (RA-SDT) and CG-SDT procedures, as detailed in new Base Line Change Requests (BLCRs) for multiple technical specifications. This release formally introduced SDT support itself, extending its capabilities beyond the previous release. The updates also included enhancements to the handling and description of QoS flow parameters within the SDT framework.
- Add the SMF shall provide the QoS flow description(s) for the PDU sessions used for relaying TS 24.501CR3595
- Extension of SNN description for NSWO TS 24.501CR4123
- SDT support TS 24.501CR4362
- Add Filter and Filter naming description TS 28.552CR0296
- Introduction of SDT TS 38.300CR0357
- Introduction of SDT TS 38.331CR2937
+ 109 more changes
In Release 18, the SDT function was enhanced with the introduction of Mobile-Terminated SDT (MT-SDT) and several improvements for Configured Grant SDT (CG-SDT). Key enhancements included support for oversize uplink data arrival, a defined maximum time duration to initiate CG-SDT, and the introduction of beam failure recovery procedures for RA-SDT. Furthermore, the release specified an RRCRelease message with a resume indication specifically for SDT scenarios.
- General description to support PRUs TS 23.273CR0244
- Functional description update for UP positioning TS 23.273CR0387
- General description of PDU set handling TS 24.501CR5649
- Protocol description support TS 24.501CR5973
- Introduction of MT-SDT TS 37.483CR0054
- Introduction of MT-SDT in Stage-2 TS 38.300CR0711
+ 86 more changes
In Release 19, the enhancements to the Service Description Table (SDT) function specifically focused on correcting and clarifying measurement details. These updates included corrections to the descriptions and units of specific SDT measurements, ensuring greater accuracy in service performance monitoring. Additionally, technical descriptions for parameters like `maxnoofUEReports` and the handling of unavailability information were refined to improve implementation clarity.
- Update the Use Case description on Monitoring of NWDAF data collection TS 28.552CR0635
- Rel-19 CR 28.552 Add use case descriptions on monitoring of NWDAF data collection TS 28.552CR0683
- Updates to AI/ML-based positioning general descriptions TS 23.273CR0642
- Correction on description of the slice deregistration inactivity timer in 4.6.2.9 TS 24.501CR6404
- Modified description regarding UE's handling of the unavailability information TS 24.501CR6610
- Clause subject and general description updates for support of NTZ TS 24.501CR6691
+ 10 more changes
In Release 20, the SDT function was updated to enhance the purpose descriptions of performance measurements specifically related to the SSB beam. This provided a more comprehensive and precise description of the service capability for this key radio resource. The changes refined how this service category is defined within the management framework.
- Rel-20 CR TS 28.552 Update purpose descriptions of performance measurements related the SSB beam TS 28.552CR0756
Explore further
Broader topics and technologies where SDT plays a role.
Defining Specifications
3GPP specifications that define or reference SDT, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.273 vj50 | 5G Location Services Stage 2 Architecture | Rel-19 |
| TS 23.887 vc00 | Architectural enhancements for MTC and mobile data | Rel-12 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 25.705 vd00 | UMTS Small Data Transmission Enhancements Study | Rel-13 |
| TR 26.917 vj00 | TV Service Enhancements over 3GPP | Rel-19 |
| TS 28.552 vk10 | 5G Performance Management Measurements | Rel-20 |
| TS 37.483 vj10 | E1 Application Protocol (E1AP) | Rel-19 |
| TS 38.300 vj00 | NG-RAN Overall Description | Rel-19 |
| TS 38.304 vj00 | UE RRC_IDLE and RRC_INACTIVE Procedures | Rel-19 |
| TS 38.305 vj00 | NG-RAN UE Positioning Stage 2 | 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 |
| TS 38.401 vj10 | NG-RAN Architecture Specification | Rel-19 |
| TS 38.423 vj10 | Xn Application Protocol (XnAP) specification | Rel-19 |
| TS 38.473 vj10 | 5G F1 Application Protocol (F1AP) | Rel-19 |
| TS 38.523 vj20 | 5G NR UE Conformance Testing: Idle/Inactive | Rel-19 |