Overview
ISO 21806-6:2020 defines the Data Link Layer for Media Oriented Systems Transport (MOST) networks used in road vehicles. The standard specifies the technical requirements and behaviors that each node must implement to participate in a MOST network. A MOST network consists of two or more nodes connected over a physical layer; nodes are time-synchronised with one node acting as the TimingMaster and others as TimingSlaves. The node’s timing role determines the data link tasks it performs.
Key topics and technical requirements
ISO 21806-6:2020 covers the following core data link layer subjects:
- Service interface to the network layer - data types, parameters, event indications, and action requests between the data link and upper layers.
- Network frame structure - administrative and source data areas, indicators, and frame layout.
- Channel definitions - allocation, control, protected system, timestamp, packet, synchronous, and isochronous channels and their frame structures.
- Flow control mechanisms - pre-emptive acknowledge byte, early ending, low-level retries and other mechanisms to manage data flow.
- Arbitration methods - load-adaptive arbitration and round-robin arbitration, including downstream/upstream specifics and examples.
- Addressing options - 16-bit and 48-bit addressing types (logical, group, broadcast, node position, debug, etc.).
- Error detection and acknowledgement - multiple cyclic redundancy checks (CRCs), CRC usage rules and CRC acknowledge behavior.
- Frame indicators and timing behavior - indicators for source-drop recognition, allocation/de-allocation, and timestamp channel behavior.
(The document also specifies parameter lists, event indications such as L_EVENT.INDICATE, control/packet/streaming data primitives, and channel-specific behaviors.)
Practical applications and who uses this standard
ISO 21806-6:2020 is intended for organizations developing, integrating, testing, or certifying MOST-based in-vehicle networks, including:
- Automotive OEMs and Tier‑1/2 suppliers designing infotainment, multimedia and system communication solutions
- Embedded software and firmware engineers implementing the MOST data link layer
- System architects and test engineers validating timing, arbitration and flow-control behavior
- Tool and test-house vendors creating conformance and diagnostic tools for MOST networks
Adopting the standard ensures interoperability, predictable timing behavior for synchronous/isochronous streams, and robust frame handling across MOST nodes.
Related standards
- Other parts of the ISO 21806 MOST series (e.g., physical layer and network/transport layer specifications) complement Part 6 and should be referenced for full system design and conformance.
Keywords: ISO 21806-6:2020, MOST data link layer, Media Oriented Systems Transport, automotive network, TimingMaster, TimingSlave, arbitration, CRC, flow control, isochronous, synchronous.
