MOST transmission protocol

The MOST bus supports infotainment in motor vehicles. To ensure that the various input and output devices of the MOST bus can communicate with each other without interference, communication is controlled via the MOST transmission protocol.

The MOST transmission protocol consists of blocks made up of frames. All frames contain data for packetization, streaming and control. The streaming data is synchronized to the MOST clock by the timing master and runs continuously over the MOST bus. The transmission of asynchronous data, for example e-mails, is supported by the packetization data. Both, synchronous and asynchronous transmission share the available bandwidth, which can be variably allocated within a frame.

Control data includes arbitration, stream information such as file format, source and destination address, position of data in the frame, backup, acknowledgement and reservation. They may be distributed over several frames.

The MOST data packet is fed into the polymer fiber at a fixed frequency of 44.1 kHz or 48 kHz. Such a frame consists of many individual streaming channels, with MOST25 there are 60 at 8 bits each, over which the continuous bit streams are transmitted. This results in data rates of 352.8 kbit/s at 44.1 kHz and 384 kbit/s at 48 kHz. The MOST bus physically has a ring topology, so that each MOST frame must pass each MOST device. Bitstreaming starts as soon as the data source and data sink have connected to the same streaming channel.