Real-Time Optimisation of TTCAN Networks

Controller Area Network is widely used as a communications network in automotive applications, typically motor cars, commercial vehicles, and utility vehicles. CAN is operated either by spontaneous messaging or by time triggered messaging. Time triggered messaging is the preferred option on modern systems as it allows all messages access to the bus at some defined period in time. Using time triggered messaging alone does not allow real-time access to the network, therefore spontaneous messaging is used in conjunction with the time triggered messaging to ensure this.\ud \ud Presently there are two types of schedulers available for the development of TTCAN message sets. They are the stochastic and heuristic scheduler, which are both useful, but they do not provide the capability of ensuring real-time messages arrive within their deadline.\ud \ud Stochastic schedulers generate message sets by a probability distribution and heuristic schedulers develop a message set solution by trial and error. They both define the optimum message set as the one with the least jitter by use of a cost function analysis.\ud Neither of the two methods take into account the effect the schedules may have on spontaneous real-time messaging.\ud \ud Real-time messages have the best opportunity of meeting their deadline, if the\ud TTCAN messages are not sent sequentially, in fact the larger the arbitration window size between TTCAN messages, the more successful will be the real-time performance of the network.\ud \ud Schedulers are essentially designed to load balance system resources and in the case of a TTCAN network, the ideal situation is that all messages are separated by the same size arbitration window. This provides the optimum real-time performance, however with messages of different time periods being broadcast, it should be realised that arbitration windows will nearly always be of different sizes. A statistical message scheduler has been devised and demonstrated to produced an optimum message set for real-time operation on a TTCAN network and hence improve the results produced by stochastic or heuristic techniques