MOST Cooperation spec
MOST
Media Oriented Systems Transport — legacy infotainment ring.
Bitrate
25 / 50 / 150 Mbit/s
Topology
Ring (optical or electrical)
Introduced
2001
Overview
MOST is an optical ring network historically used for premium infotainment. Largely replaced by Automotive Ethernet, but still present in many vehicles in service today.
Frame format
| Field | Bits | Description |
|---|---|---|
| Preamble | 8 | Sync |
| Boundary | 8 | Stream/packet boundary descriptor |
| Sync data | ≤480 | Isochronous audio/video data |
| Async data | ≤1014 | Packet data channel |
| Control | 32 | Control channel |
Byte structure — request & response
MOST — Media Oriented Systems Transport
Frame template
[Preamble 1B][Boundary 1B][Sync data ≤60B][Async data ≤127B][Control 4B]Request
| Off | Size | Field | Value | Description |
|---|---|---|---|---|
| 0 | 1 B | Preamble | 0x00 | Frame sync. |
| 1 | 1 B | Boundary descriptor | 0x40 | Defines split between sync and async area. |
| 2–61 | ≤60 B | Sync data | PCM stereo samples | Isochronous audio/video stream. |
| 62… | ≤127 B | Async data | Function block control msg | Packet-switched control message (allocate/release/control). |
| tail | 4 B | Control channel | Function ID + OpType + Data | 4-byte control slot per frame. |
Response
| Off | Size | Field | Value | Description |
|---|---|---|---|---|
| tail | 4 B | Control channel | ACK + Function ID + Result | Network master/slave acknowledges allocation/control. |
| 62… | var | Async data | Status block | Optional async reply with full result. |
MOST cycles repeat at 44.1 or 48 kHz on a ring; a single 'frame' carries audio, video and control simultaneously.
Use cases
- · Audio streaming
- · Video distribution
- · Head-unit ↔ amplifier
Pros
- High isochronous bandwidth
- Optical = EMC immune
Cons
- Ring failure risk
- Phased out
- Proprietary tooling
Request / Response examples
Stream allocation
Request
Allocate4syncbytesforstereoPCM
Response
Allocationtableupdated,channel0x1Aassigned
Network master reserves bandwidth on the ring for an audio stream.