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MOST

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MOST (Media Oriented Systems Transport)

Overview

MOST is an automotive multimedia networking standard for in-vehicle audio, video, and data. It uses optical fiber or coaxial cable in a ring topology and supports synchronous (audio/video) and asynchronous (IP) channels at 25/150/1000 Mbps.

1. Theory & Fundamentals

  • Ring topology: data travels one direction around the ring
  • Media: optical POF (plastic optical fiber) or coax (electrical)
  • Speeds: MOST25 (25 Mbps), MOST150 (150 Mbps), MOST1000 (1 Gbps)
  • Channel types: Synchronous (audio/video, guaranteed bandwidth), Asynchronous (IP packets), Control (small messages)
  • Master: Timing Master generates frame clock; all nodes lock to it
  • Addressing: 2-byte node address; function catalog defines services

2. Frame / Packet Structure

MOST25 Frame (48 bytes @ 44.1 kHz × 25 Mbps / 8):
  Preamble | Header(2B) | Data(44B) | CRC(2B)
  Header: Timing Master | Start | Boundary
  Data: Synchronous channels (audio PCM) + Async channel + Control

MOST150 Frame (1536 bits at 150 Mbps):
  Preamble(32b) | Boundary(12b) | Sync channels + Async(1404b) | CRC(88b)

MOST Control Message:
  FBlockID | InstID | FktID | OpType | TelID | Data
  FBlockID: functional block (audio, video, navigation)

3. Protocol Mechanics

  • Network Allocation Message (NAM): nodes allocate synchronous bandwidth
  • Connection labels: channel number assigned to audio connection
  • FBlock: software component; addressed by FBlockID + InstanceID
  • Power: MOST ring maintains master/slave power states
  • Shutdown: graceful ring break protocol

4. Hardware Implementation

  • MOST transceiver: OS8104, OS81118 (Microchip/SMSC)
  • POF: 650 nm red LED transmitter; PIN photodiode receiver
  • Coax: electrical MOST150/1000 with same protocol
  • MCU interface: SPI or I2C to MOST transceiver IC
  • Systems: BMW iDrive, Audi MMI, Mercedes COMAND

5. Register-Level / Configuration

// MOST control message send via OS8104 SPI
void MOST_SendControl(uint8_t fblock, uint8_t inst, uint16_t fkt, uint8_t op, uint8_t *data, uint8_t len) {
    uint8_t msg[16]={fblock,inst,fkt>>8,fkt&0xFF,op,len};
    memcpy(&msg[6],data,len);
    OS8104_SPI_Send(MSG_TX_REG,msg,6+len);
}

6. Driver / Software Development

  • Implement init, TX, RX functions; use interrupts or DMA
  • Handle errors: timeout, CRC mismatch, arbitration loss
  • Use circular/ring buffers for high-throughput RX
  • Add retry logic and watchdog for reliability
  • Separate hardware layer from protocol logic

7. Debugging & Testing

  • MOST Spy software (SMSC/Microchip): ring monitor and frame decode
  • Oscilloscope on optical receiver: verify light signal levels
  • Common issues: ring broken (node powered off breaks ring); timing master fail
  • Logic analyzer on SPI to OS8104 for command verification

8. Real-World Applications

  1. Premium car audio systems (BMW, Audi, Mercedes)
  2. Rear-seat entertainment
  3. Navigation and telematics integration
  4. Instrument cluster video
  5. Surround-sound DSP networks

9. Advanced Topics & Edge Cases

  • MOST replaced by Ethernet AVB in new vehicles
  • Ethernet AVB + SOME/IP now preferred for bandwidth and IP compatibility
  • MOST Network Services (MNS): OS-level API for MOST applications
  • MOST Cooperation: Industry group maintaining standard

10. Standards & Variants

Version Speed Medium Notes
MOST25 25 Mbps POF Legacy
MOST150 150 Mbps POF/Coax Current
MOST1000 1 Gbps Coax High-end

💡 Practical Examples

Example 1

Stream audio: allocate 6 synchronous channels (48kHz 16-bit stereo) from head unit to amplifier

Example 2

Control: send FBlock 0x01 (audio) Volume.Set(50) command to amplifier node

Example 3

Ring diagnostics: query each node for NI state; detect broken ring segment

🧪 Practice Questions

Beginner

  1. What topology does MOST use?
  2. What are the 3 channel types?
  3. What medium does MOST25 use?
  4. What is a FBlock?
  5. What replaces MOST in new vehicles?

    6. Intermediate

  6. Explain MOST synchronous channel allocation.
  7. Implement a MOST control message handler for volume control.
  8. How does MOST handle ring break?
  9. What is MOST1000 and when is it used?
  10. Compare MOST150 vs Ethernet AVB for automotive multimedia.

    11. Advanced

  11. Design a MOST150 car audio network with 8 nodes.
  12. Implement MOST application layer (FBlock) for a DSP processor.
  13. Build a MOST ring monitor with node status display.
  14. Migrate a MOST25 system to Ethernet AVB.
  15. Implement MOST low-power (Standby) mode correctly.

    16. Hands-on Projects

  16. MOST Control: send volume/source commands to simulated amplifier.
  17. Ring Monitor: detect and report broken ring segments.
  18. Audio Stream: allocate sync channels, stream PCM audio.

Checklist

  • [ ] Explain MOST ring topology and channel types
  • [ ] Configure OS8104/OS81118 transceiver via SPI
  • [ ] Send and receive control messages
  • [ ] Allocate synchronous channels for audio
  • [ ] Handle ring break detection
  • [ ] Implement FBlock application layer
  • [ ] Debug with MOST Spy
  • [ ] Understand timing master role
  • [ ] Compare MOST with Ethernet AVB
  • [ ] Migrate design to MOST1000
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