Guaranteeing Hard Real Time End-to-End Communications Deadlines

In a distributed hard real time system communication between tasks on different processor must occur in bounded time. The inevitable communication delay (termed the end-to-end delay) is composed from the delay in transmitting a message on the communications media, and also from the delay in delivering the data to the destination task. This paper gives schedulability analysis bounding the media access delay and the delivery delay, and hence allows the end-to-end delay for a message to be bounded. Two access protocols are considered, TDMA and a 802.5-style token ring. Two approaches are also considered for delivery: an ondemand protocol in which each packet is delivered to the host when it arrives, and a periodic server approach in which the host polls for incoming messages. The schedulability analysis covers all combinations of these access and delivery methods. In addition the effect of incoming messages on the destination processor and its workload is addressed.