The infotainment bus MOST (Media Oriented System Transport) is different in a number of properties significantly from the bus systems CAN, LIN and FlexRay. These are designed for control and process control in vehicles.

MOST - Media Oriented System Transport

The task to transport audio and video signals, can be with the bit rates of CAN, LIN and FlexRay or not even solve very difficult. Therefore, MOST was designed for a much higher bit rate. The first generation was 25 Mbit / s, equipped with an optical physical layer. It was then developed at the request of Japanese manufacturers a version with an electrical physical layer. This variant can cover 50 Mbps. The latest generation works with MOST 150 Mbit / s.

The networking of infotainment units radio, CD / DVD changer, navigation system, telephone and TV in MOST is using a digital transmission. This is used in any event for all European producers, currently an optical transmission medium, a plastic optical fiber.

MOST - Specifications

A leader in the development of the MOST system, the company OASIS, SMSC today, Harman Becker, and a number of automotive manufacturers. These have formed a consortium founded in 1998 and the MOST Cooperation. Layer 1 and Layer 2, however, are protected by a number of patents the company SMSC. This patent award is relatively restrictive, so that today there is practically only communication controller from SMSC MOST.

Physical Layer and bus topology

The optical fiber allows compared to electrical systems, although an interference-transmission, but requires expensive connectors. The bending radius of optical fibers are restricted. What makes the design of the electrical harnesses demanding. The possible temperature range is also limited.

When topology is used in MOST is usually a ring structure is used. The individual control units are connected with a point to point connection to each other. That is, each controller has an optical input and an optical output. The light signals are regenerated at each controller.

MOST - Physical Layer and bus topology

One of the control unit works as a timing master. This control unit generates the bit clock for the optical signal and generates the frames. All other control devices must be synchronized to that bit clock. If the timing master, or any other control device fails in this ring, then does not the ring.

MOST Data Link Layer 25

The data link layer specifications of the MOST different versions differ in some details, especially in the numerical values. The above figures refer to MOST 25 - the 25 Mbps version of MOST.

The MOST system transports data in so-called frames, as illustrated. The frames have a fixed frequency of 44 kHz or 48, which corresponds to the sampling rate of CD or DVD systems. You can see it, coming from the broadcast.

MOST - Frame or block format

Within a frame we have a header with a number of tax information, a trailer and error control information between a data area. The data area of ​​60 bytes of data is divided into a synchronous and an asynchronous part.

Synchronous data are raw data, for example audio samples that are transmitted in a time slot, called a channel. In contrast to FlexRay, the time slots at MOST be changed dynamically. That is, an audio player can reserve a time slot, for example, to transfer data to an amplifier.

The asynchronous data area is used to transmit data packets, as TCP / IP or navigation data, as illustrated. He allowed within a frame transmission between zero and a maximum of 36 bytes. Since a total of 60 bytes of data available must be reduced, this synchronous part.

MOST - Asynchronous data block

Since an asynchronous data packet can contain up to 48 bytes of user data, communications controller automatically segmented asynchronous data packets. These are then transmitted over several frames. Moreover, this asynchronous data contain a header with a destination and a source address and a number of tax and checksum.

Another way of data transmission, but is not generally used for applications, but especially for the self-administration of the MOST ring, are the control data. The control data includes only 2 bytes per frame. However, exist on a higher level of the transmission protocol is a further block format that can contain up to 17 bytes of user data.

MOST - Tax Data

The Control Data block is automatically segmented by the communications controller, ie, spread over several frames. The asynchronous block and the control data block containing an arbitration mechanism, destination and source addresses, and a checksum. However, there are freely available in the parts of the MOST specification no precise information on how arbitration works, what happens in case of error in detail and what safeguards exist at the time of the transfer of data blocks.