Overview
ISO 4426:2021 - Intelligent transport systems - Lower layer protocols for usage in the European digital tachograph defines the communication foundations for the Smart Digital Tachograph (SDT) as identified by EU Regulation 2016/799. The standard specifies the SDT Communications (SDTC) stack - covering the physical layer at 5.8 GHz, the data link layer (DLL), the application layer, and coherent SDTC profiles. Normative annexes provide implementation-relevant details and test guidance to support design, development and conformance of SDTC equipment.
Key topics and technical requirements
- Physical layer (L1): RF band allocation and carrier frequency considerations at 5.8 GHz, antenna and polarization aspects, incident power limits and E.I.R.P. masks, wake‑up triggers and communication zone definitions.
- Data link layer (L2): Medium access, logical link control and framing (preambles, trailing bits), bit rates, data coding (e.g., Manchester/FM0 types referenced), and bit error ratio targets.
- Application layer (L7): Message structures and services to support SDT interrogation and exchange between roadside units (RSUs) and on‑board units (OBUs).
- SDTC profiles: Bundled sets of communication tools and parameter sets for common SDT applications to ensure interoperable deployments.
- Test methods and conformance: Normative annexes define testable parameters and test procedures to validate performance and interoperability.
- Definitions and normative references (e.g., ISO 15628 and HDLC procedures) ensure consistent terminology and alignment with existing DSRC standards.
Practical applications
- Enabling vehicle-to-infrastructure interrogation of digital tachographs for inspection, enforcement and fleet management.
- Guiding manufacturers of OBUs (on‑board units) and RSUs (roadside units) to implement compliant lower‑layer protocols for SDT.
- Supporting test laboratories and certification bodies with test parameters and procedures needed for conformity assessment.
- Assisting transport authorities and system integrators in designing interoperable roadside systems and deployment profiles.
Who should use ISO 4426:2021
- Hardware and firmware engineers designing DSRC/SDTC radio and transponder units
- System integrators and solution providers deploying digital tachograph interrogation stations
- Test houses and conformity assessors validating SDT communication performance
- Regulators, transport authorities and large fleet operators planning compliant inspection infrastructure
Related standards
ISO 4426 builds on and references existing DSRC standards and EN specifications (physical, DLL, application layers) and aligns with ISO 15628 and relevant ETSI/EN test methods to ensure interoperability with the wider DSRC ecosystem.
Keywords: ISO 4426:2021, intelligent transport systems, digital tachograph, Smart Digital Tachograph, SDTC, DSRC, 5.8 GHz, data link layer, application layer, RSU, OBU, SDTC profiles.