The preamble of the MOST25 frame is used for synchronizing the TimingSlaves to the bit stream and for the initial identification of the frame. The slaves use a PLL switch to synchronize to the network. The TimingMaster generates the preamble based on its oscillator frequency. After the bit stream has been carried over all MOST nodes, it arrives at the master with a phase shift caused by the signal propagation delay in each MOST node. The master thus synchronizes itself to the oncoming frame by means of its PLL, recovers all the bits, regenerates the frame and thus compensates the phase shift. [15]
The Streaming Data Channel and the Packet Data Channel share a total of 60 bytes, which are available in a frame. The bandwidths of the two channels can be adapted to their corresponding requirements via the Boundary Descriptor. The boundary between the two areas can be shifted in steps of 4 bytes (a quadlet). The Streaming Channel can thus have a width between 24 and 60 bytes (6 to 15 quadlets) and the Packet Data Channel a width between 0 and 36 bytes (0 to 9
quadlets). The value of the Boundary Descriptor ranges from 6 to 15 quadlets. It is administered by the MOST Network Interface Controller of the TimingMaster. If the value of the Boundary Descriptor is changed by the TimingMaster, all synchronous connections have to be re-established as audio connection in the SIM Access Profile (SAP) . It is used exclusively for exchanging the SIM data between the cell phone and the car. By means of the SIM Access Profile, the telephone integrated in the car can read out the SIM data from the cell phone and use them as long as it is registered via Bluetooth.
The last byte of the frame is used for controlling of the frame.
The parity bit enables the detection of bit errors in the frame.
- Packet data protocol for MOST25 and MOST50
The protocol header consists of the arbitration field (1 byte), in which the token is stored, the target address (2 bytes), the data field length (1 byte) and the source address (2 bytes). The protocol is secured by a CRC sum (4 bytes), which is automatically generated by the MOST Network Interface Controller. There is no automatic retransmission if there is a CRC sum error. This has to be handled by the higher layers. [15]