MOST bus

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The MOST bus (MOST = M edia O riented S ystems T ransport ) is a serial bus system for the transmission of audio and video, voice and data signals via fiber optics (MOST25, MOST50, MOST150) or via electrical conductors (MOST50). From a physical point of view, MOST is always a ring topology .

MOST is used for multimedia applications in the car and is based on synchronous data communication. MOST technology is used in almost all major car brands in the world, such as Audi , BMW , Cadillac , Buick , General Motors , Hyundai , Jaguar , Land Rover , Mercedes-Benz , Opel , Porsche , Toyota , Volkswagen and Volvo .

Basics of communication

According to the specification, MOST does not only contain the definition of the physical layer and the data link layer . MOST covers all seven layers of the ISO-OSI model of data communication. Uniform interfaces simplify the implementation of the MOST protocol in multimedia devices.

For the application developer, MOST is primarily a protocol definition. This offers the user a uniform interface ( API ) to be able to use the device functions.

The communication functionality is provided by the MOST Network Services . Between the MOST network interface controller , which is based on the physical layer , and the API (layer 7), the MOST network services , consisting of the basic layer system services (layers 3, 4, 5) and the application socket ( Layer 6), the MOST protocol.

MOST networks

A MOST network , usually designed as a ring topology , can contain up to 64 MOST devices. Both the integration and the removal of a MOST device do not cause much effort due to the plug-and-play functionality. Star topologies or other topologies can also be set up virtually - double rings are used as a redundancy structure in safety-critical applications . Also hubs or switches are possible, but not yet established in the automobile.

In a MOST network, a MOST device takes on the role of the timing master, which continuously feeds MOST frames into the ring. The preamble sent at the beginning of the transmission of a MOST frame is used by the timing slaves for synchronization. The timing slaves can continuously re-synchronize using the coding on which the synchronous transmission is based.


With MOST25, a bandwidth of around 23 Mbaud is available for the transmission of streaming (synchronous data transmission) and packet data (asynchronous data transmission)  . This is divided into 60 physical channels that can be selected and configured by the user in groups of four bytes. MOST provides numerous services and mechanisms for the allocation (and deallocation) of the physical channels.

MOST25 supports up to 15 uncompressed stereo audio channels in CD quality or up to 15 MPEG-1 channels for audio-video transmission, each of which requires 4 bytes (i.e. 4 physical channels).

At the same time, MOST offers a channel for the transmission of control messages. With a system frequency of 44.1 kHz, a bandwidth of 705.6 kbit / s is available - or in other words: 2670 control messages can be transmitted per second. With the help of the control messages, MOST devices and synchronous or asynchronous data transmission can be configured. The system frequency is based on the CD standard. Of course, useful data can also be transmitted via the control channel. Due to the protocol overhead (with segmented transmission, 11 of 32 bytes can be used for the application) and the restriction that a MOST node can only use a third of the control channel bandwidth, the effective data rate is around 10 kByte / s.


The MOST50 doubles the bandwidth with 1024 bits per frame. The three channels known from MOST25 remain, but the control channel is given a flexible length and the division between synchronous and asynchronous channel can be made more flexible. For MOST50 there is only Intelligent Network Interface Controller (INIC) for electrical transmission via Unshielded Twisted Pair (UTP).


The MOST150 , which was introduced in October 2007, enlarges the frames to 3072 bits, six times the MOST25. In addition to the three known channels, an Ethernet channel with freely adjustable bandwidth and isochronous transmission on the synchronous channel were introduced. This enables the transmission of synchronous data that require a frequency other than that specified by the MOST frame rate. MOST150 thus provides a physical layer for Ethernet in the vehicle. From a higher-level perspective, MOST150 becomes a multiplex network.

MOST cooperation

Through the MOST Cooperation , which was founded in 1998, MOST technology has become the standard for current and future requirements in multimedia networking in vehicles.


Although the MOST Cooperation has been publishing specifications for the MOST bus for several years, these do not include the link layer (e.g. via the arbitration algorithms or CRC polynomials ), as this is not subject to the MOST Cooperation's cross-licensing . So far, apart from the chips from the MOST inventor SMSC Europe GmbH (formerly OASIS Silicon Systems AG ), only solutions from Analog Devices and various FPGA cores are known. Devices from K2L GmbH, Ruetz System Solutions GmbH, SMSC Europe GmbH ( bought by Microchip Technology in May 2012 ), Vector Informatik GmbH and Telemotive AG are established tools on the market. In March 2008, SMSC and Harman / Becker announced that they would disclose the MOST data link layer to other semiconductor manufacturers in return for license fees.

SMSC and MOST are registered trademarks of SMSC. The MOST Cooperation is free to use the brand.


  • Werner Zimmermann and Ralf Schmidgall: Bus systems in vehicle technology - protocols, standards and software architecture. 5th edition, Springer Vieweg, 2014, ISBN 978-3-658-02418-5 .
  • Grzemba: MOST. The multimedia bus system for use in automobiles . Franzis, Poing, 2007, ISBN 978-3772341496
  • Grzemba: MOST. The Automotive Multimedia Network. From MOST25 to MOST150 . Franzis, Poing, 2011, ISBN 978-3-645-65061-8

Individual evidence

  1. Newsletter of the MOST Cooperation from October 2010 ( Memento of the original from April 18, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot /

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