How is the MOST bus optimized for the automotive industry?

The media-oriented systems transport (MOST) bus was initially intended for implementation on optical fiber to support high bit rates, but fiber and copper transport layers are currently defined. MOST provides a solution for automotive peripherals like radios, CD and DVD players, GPS navigation systems, and infotainment electronic control units (ECUs).

This FAQ describes the MOST bus and system specifications, compares it to other automotive bus alternatives, and introduces the automotive audio bus (A2B) developed to support distributed speakers and microphones in modem automobiles.

MOST was optimized for the automotive sector but is also used in non-automotive applications. It’s ideal for those that benefit from a daisy-chain or ring topology and synchronous data communication to transport audio, video, and data signals on plastic optical fiber (POF) (MOST25, MOST150) or electrical conductor (MOST50, MOST150) physical layers.

The motivations for developing MOST include electromagnetic compatibility (EMC) issues and the weight and size of the interconnect systems for multimedia devices in vehicles. For example, consider a basic system with a multimedia player, mobile phone interface, and radio connected to a user interface (four components).

Conventional interconnects require:

Six cables for this system; total length: 12 meters
Three conductors per cable; total conductor length: 36 meters
36 contacts; total weight: 1700 g

Using the MOST serial bus only requires:

Four cables; total length: 9 meters
One conductor per cable; total conductor length: 9 meters
Eight contacts; total weight: 123 g

MOST offers a significant reduction compared to the conventional interconnect architecture. That savings adds up as the number of devices in a multimedia system increases (Figure 1).

**Figure 1:** MOST was developed to address the rapidly increasing complexity of multimedia interconnects in modern vehicles. (Image: IEEE)

The benefits of using MOST for multimedia data transport include:

More manageable EMC concerns
Fewer cables and connectors
Shorter overall cabling length
Lower weight and smaller volume
Higher reliability and lower cost

The MOST serial bus supports the rapid growth of multimedia devices in today’s automobiles (Figure 2). The physical and data link layer and the seven layers of the ISO/OSI-Model for data communication are defined in the MOST specification. The integration of the MOST protocol in multimedia devices is simplified through the use of standardized interfaces.

MOST is a software protocol and hardware architecture. For multimedia device designers, MOST offers a communications protocol and access to device functionality through a standardized application programming interface (API). The MOST Network Services driver software supports communications and includes Basic Layer System Services (Layer 3, 4, 5) and Application Socket Services (Layer 6). The MOST Network Services driver elements support the MOST protocol between the API on Layer 7 and the Network Interface Controller on the physical layer.

**Figure 2:** MOST is a serial bus designed to efficiently and reliability support the increasing number of multimedia devices in today’s automobiles. (Image: IEEE)

MOST networks
A MOST network can manage up to 46 devices in a ring configuration. The plug-and-play capability enables MOST devices to be quickly added to or deleted from the network. Optionally, these networks can be implemented using a virtual star or other topologies. Redundant double-ring topologies are typically used for safety-critical devices and applications.

One device in a MOST network is considered the timing master, which continuously supplies the network with MOST frames. The start of each frame transfer begins with a preamble that other devices (called timing followers) use for synchronization. Continuous synchronization updates for the timing followers are enabled using the synchronous transfer from the timing master.

So, why not use already established automotive buses? This is because they do not all provide an equivalent tradeoff between cost and performance.

BroadR-Reach is used in some automotive Infotainment networks. It offers speeds up to 10 Gbit/s, but is more costly.
Serial buses like LS-CAN (controller area network), LIN (local interconnect network), and OBDII (onboard diagnostics II) are too slow to carry video.
FlexRay is faster than CAN and other serial buses but has been optimized for timing-critical applications, such as drive by wire rather than media.
Ethernet may be a longer-term contender. It’s used in several automotive applications but is more costly than MOST. Ethernet over fiber might be possible, given the restrictions of CAT5 cabling. But Ethernet is asynchronous while MOST is synchronous.

Automotive audio bus
In contrast to the MOST bus, which is optimized for high data rate multimedia devices, the automotive audio bus (A2B) is rated for 50 Mbps. It enables a master (Head Unit) to control up to 10 audio devices on a serial network (Figure 3).

A typical vehicle cabin has distributed speakers, a subwoofer, and several microphones. Microphones are often distributed throughout the cabin and used for various functions, such as tuning the audio system, road noise reduction, and voice controls (such as hands-free calling from all the seats).

**Figure 3:** The A2B bus is rated for 50 Mbps and can control up to 10 audio devices such as speakers and microphones. (Image: Analog Devices)

A2B is a bidirectional, digital bus capable of transporting inter-IC sound (I2S), time-division multiplexed (TDM), pulse density modulation (PDM) data, I2C control information, and clock and power, using a single, two-wire unshielded twisted pair (UTP) cable. It can function up to 15 m between nodes, and the entire daisy chain can be 40 m long.

The clock in an A2B network is synchronous for all nodes and serial audio. The microphone data is received simultaneously by all nodes. The A2B architecture can be implemented in a stand-alone network, within embedded subnetworks, or as a transport bus connected to another longer distance bus.

The A2B has no jitter and a low fixed latency of 50 µs. Low latency is ensured by simultaneous sampling and delivering data for each frame on all system nodes. The A2B has a configurable 44.1 or 48 kHz frame rate and a bandwidth of up to 50 Mbps, so it’s optimized to deliver higher quality audio compared to analog networks.

Summary
MOST was developed to address issues with electromagnetic compatibility (EMC) and decrease the weight and size of interconnect systems for multimedia devices in automobiles. It can be implemented with POF or copper cabling and can support up to 46 devices on a single network.

MOST offers the best price versus performance tradeoffs compared to alternative automotive buses. However, in the long-term, Ethernet will likely displace MOST for multimedia networks. In addition to MOST, the A2B bus has been developed to support speakers and microphones in vehicle cabins.

References