Scheduling time-constrained messages in a process control environment

Process control applications impose strict requirements on the timing behaviour of the system. The timing behaviour critically depends on the way the underlying resources are managed and made available to the application. Ideally, real-time LAN protocols should be able to handle traffic generated by distributed process control applications in such a way that specific timing constraints are always met. The authors present a performance analysis, based on simulation results, of the three layer MiniMAP (with MMS, LLC type 3 and 4 MAC access classes) with an end-to-end communications scheduler on the top of it. They study the performance of the protocol under four different scheduling policies (first come first served, minimum deadline first, priority, value) and using different load patterns. The performance metric for time-constrained traffic is the percentage of messages received within their deadlines. It is shown that when employing an end-to-end scheduler the performance of MiniMAP improves significantly