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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 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, 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.
Principles of communication
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.
Some limitations restrict MOST25’s effective data transfer rate to about 10 kB/s. Because of the protocol overhead, the application can use only 11 of 32 bytes at segmented transfer and a MOST node can only use one third of the control channel bandwidth at any time.
MOST50 doubles the bandwidth of a MOST25 system and increases the frame length to 1024 bits. The three established channels (control message channel, streaming data channel, packet data channel) of MOST25 remain the same, but the length of the control channel and the sectioning between the synchronous and asynchronous channels are flexible. Although MOST50 is specified to support both optical and electrical physical layers, the available MOST50 Intelligent Network Interface Controllers (INICs) only support electrical data transfer via a three copper conductor configuration; consisting of a Unshielded Twisted Pair (UTP) set and a single additional control line. The additional control line is connected to each MOST50 network device in a parallel "single shared bus" configuration. Each MOST50 device would contain five copper wire connections in this configuration. Control line (for signals sent from the master) and two UTP sets (each containing D+ D-). One set is used for Data Input (out putted from the preceding device on the network ring) while the other is used for Data Output to the next device on the ring. As with its fibre counterparts, closing or completing the ring (termination at the originating device) is required for any and all network operation.
MOST150 was introduced in October 2007 and provides a physical layer to implement Ethernet in automobiles. It increases the frame length up to 3072 bits, which is about 6 times the bandwidth of MOST25. It also integrates an Ethernet channel with adjustable bandwidth in addition to the three established channels (control message channel, streaming data channel, packet data channel) of the other grades of MOST. MOST150 also permits isochronous transfer on the synchronous channel. Although the transfer of synchronous data requires a frequency other than the one specified by the MOST frame rate, it is also possible with MOST150.
MOST150’s advanced functions and enhanced bandwidth will enable a multiplex network infrastructure capable of transmitting all forms of infotainment data, including video, throughout an automobile.
The optical transmission layer has been widely used in automotive applications for a number of years. It uses plastic optical fibers (POF) with a core diameter of 1 mm as transmission medium, in combination with light emitting diodes (LEDs) in the red wavelength range as transmitters. MOST25 only uses an optical Physical Layer. MOST50 and MOST150 support both optical and electrical Physical Layers.
Main advantages of POF:
- high data rate transmission
- lighter and more flexible compared to shielded electric data lines
- meets strict EMC requirements
- does not cause any interference radiation
- insensitive to electromagnetic interference irradiation
The MOST Cooperation, a partnership of carmakers, setmakers, system architects, and key component suppliers, was founded in 1998. Their objective was to define and adopt a common multimedia network protocol and application object model. As a result of their efforts, MOST technology has emerged as a global standard for implementing current and future requirements for multimedia networking in automobiles.
The MOST Cooperation has published specifications for the MOST Bus for a number of years. However, these specifications do not include details on the Data Link Layer. In March 2008, SMSC (formerly OASIS SiliconSystems) - inventor of the first MOST NIC - and Harman/Becker announced that they would open and license their proprietary Data Link Layer intellectual property to other semiconductor companies on a royalty-bearing basis. At this time MOST chip solutions are available from SMSC, Analog Devices and some FPGA core companies. Development tools are offered by K2L, Ruetz System Solutions, SMSC, Vector Informatik GmbH and Telemotive AG.
- IEEE 1355 has a slice (combination of network medium and speed) TS-FO-02, for polymer optical fiber operating at 200 megabits/second. The specification is faster than MOST, well tested, and open. However, it lacks industry advocates.
- Ethernet is more standard, higher-speed, equally noise immune, being differential and isolated by transformers. However Cat 5 cable may be too expensive for automotive applications. Also, standard cat-5 plugs do not resist vibration. The thin layers of gold rapidly rub off, and corrosion then causes failure. Ruggedized "standard" connectors exist, which hold the connectors steady, but are more expensive.
- CAN (Controller Area Network), LIN (Local Interconnect Network) and other automotive OBD standards are not suitable because they are too slow to carry video.
- FlexRay, also an automotive bus standard, though faster than CAN, is intended for timing critical applications such as drive by wire rather than media.
- Grzemba, Andreas (2007). MOST: Das Multimedia-Bussystem Für Den Einsatz Im Automobil (in German). Poing: Franzis. ISBN 978-3-7723-4149-6.
- Grzemba, Andreas (2011). MOST: The Automotive Multimedia Network; from Most25 to Most150. Poing: Franzis. ISBN 978-3-645-65061-8.
- Zimmermann, Werner; Schmidgall, Ralf (2008). Bussysteme in der Fahrzeugtechnik: Protokolle und Standards; Mit 96 Tabellen (in German) (3rd ed.). Wiesbaden: Vieweg + Teubner. ISBN 978-3-8348-0447-1.