Weak TDMA for the Deterministic Medium Access on the Controller Area Network

This paper proposes weak TDMA (WTDMA) as a new method for slotted medium access on the controller area network (CAN). Different from TDMA, WTDMA does not require guard times for the temporal isolation of different time slots. Instead, a certain degree of overlap of time slots can be tolerated due to the usage of Carrier Sense Multiple Access/Collision Resolution (CSMA/CR) and the non-preemptive message transmission on CAN. WTDMA can be realized in software with a moderate accuracy of the synchronized clocks of different CAN nodes and without any modifications to the CAN standard. The paper provides sufficient conditions for the correct operation of WTDMA and performs a comprehensive experimental evaluation. We show that bus loads above 90% can be achieved, whereby the message latencies are mostly determined by the message transmission times in the order of hundreds of microseconds. In addition, WTDMA is robust to disturbances such as bit errors. These properties are vital for contemporary mission-critical real-time vehicle applications.