MOST Bus

MOST (Media Oriented Systems Transport) is a high-speed multimedia network technology optimized by the automotive industry. It can be used for applications inside or outside the car. The serial MOST bus uses a daisy-chain topology or ring topology and synchronous data communication to transport audio, video, voice and data signals via plastic optical fiber (POF) (MOST25, MOST150) or electrical conductor (MOST50, MOST150) physical layers. MOST (Media Oriented Systems Transport) is a high-speed multimedia network technology optimized by the automotive industry. It can be used for applications inside or outside the car. The serial MOST bus uses a daisy-chain topology or ring topology and synchronous data communication to transport audio, video, voice and data signals via plastic optical fiber (POF) (MOST25, MOST150) or electrical conductor (MOST50, MOST150) physical layers. MOST technology is used in almost every car brand worldwide, including Audi, BMW, General Motors, Honda, Hyundai, Jaguar, Lancia, Land Rover, Mercedes-Benz, Porsche, Toyota, Volkswagen, SAAB, SKODA, SEAT and Volvo. SMSC and MOST are registered trademarks of Standard Microsystems Corporation (“SMSC”), now owned by Microchip Technology. The MOST specification defines the physical and the data link layer as well as all seven layers of the ISO/OSI-Model of data communication. Standardized interfaces simplify the MOST protocol integration in multimedia devices.For the system developer, MOST is primarily a protocol definition. It provides the user with a standardized interface (API) to access device functionality.The communication functionality is provided by driver software known as MOST Network Services. MOST Network Services include Basic Layer System Services (Layer 3, 4, 5) and Application Socket Services (Layer 6). They process the MOST protocol between a MOST Network Interface Controller (NIC), which is based on the physical layer, and the API (Layer 7). A MOST network is able to manage up to 64 MOST devices in a ring configuration. Plug and play functionality allows MOST devices to be easily attached and removed. MOST networks can also be set up in virtual star network or other topologies. Safety critical applications use redundant double ring configurations. Hubs or switches are also possible, but they are not well-established in the automotive sector. In a MOST network, one device is designated the timing master. Its role is to continuously supply the ring with MOST frames. A preamble is sent at the beginning of the frame transfer. The other devices, known as timing followers, use the preamble for synchronization. Encoding based on synchronous transfer, allows constant post-sync for the timing followers. MOST25 provides a bandwidth of approximately 23 megabaud for streaming (synchronous) as well as package (asynchronous) data transfer over an optical physical layer. It is separated into 60 physical channels. The user can select and configure the channels into groups of four bytes each. MOST25 provides many services and methods for the allocation (and deallocation) of physical channels.MOST25 supports up to 15 uncompressed stereo audio channels with CD-quality sound or up to 15 MPEG-1 channels for audio/video transfer, each of which uses four bytes (four physical channels).MOST also provides a channel for transferring control information. The system frequency of 44.1 kHz allows a bandwidth of 705.6 kbit/s, enabling 2670 control messages per second to be transferred. Control messages are used to configure MOST devices and configure synchronous and asynchronous data transfer. The system frequency closely follows the CD standard. Reference data can also be transferred via the control channel.