The problem of segmentation arises with FlexRay in a form similar to CAN. Theoretically, a FlexRay message but can contain up to 254 bytes of data, almost one will choose the message length but much shorter - at least in the static segment. Therefore Autosar has submitted a proposal for a transport layer for FlexRay, which also serves as the basis for the ISO standard 10681-2. However, both the AUTOSAR standard and the draft ISO standard has experienced a number of modifications so that it would be risky at the moment to say that the transport layer for FlexRay is already absolutely stable.

The message format has been modified from ISOTP for CAN only slightly. For example, in the form of PCI information. However, we wanted to pose with the transport protocol significantly longer messages. For this reason we modified the single-frame on a different identification code in a single-frame extended. Here the field for the data length is selected so that you can pack up to 250 bytes in a message.

Modified message format for AUTOSAR FR TP

Then there is a first and a Consecutive-frame, as we know it CAN and in addition an extended variation of the first frame in which the data length field is chosen so that it can address an overall data length of up to four gigabytes . In CAN, however, it was only four kilobytes.

At the beginning of the transport messages you had to insert a field FlexRay for the addressing information, which is in CAN is not so. There you have these codes on the CAN ID. With FlexRay are now in the first part of the payload of a FlexRay message sent, the target and source address. The meaning of these two Mitsendens addresses for source and receiver control unit is that it will not for any reservation for the FlexRay communication between a sender and a receiver has a separate slot in the scheduling table. This is why: If you want to use a slot more than once, there must be a mechanism to distinguish between who is and who the receiver station. For this reason, this information is mitversendet on the transport protocol layer.

Finally, it gives you extra acknowledge frame that is sent from the receiver to the sender to confirm that the data are received properly. This is necessary because, in contrast to CAN, FlexRay at the embassy in no error feedback between receiver and transmitter are. On the application level must be taken in case of failure of retransmission, while the CAN controller repeated here to send automatically.