Scheduling algorithm for hard real-time communication in demand priority network

The paper addresses the problem of scheduling periodic messages in demand priority network standardized by IEEE 802.12 Committee. As regards the real time property of the demand priority network, unnecessary blocking time due to its round robin based MAC protocol may cause periodic messages to miss their hard deadlines and result in low schedulability of periodic messages. We propose a new message scheduling algorithm to enforce a priority based preemptive message transmission on the frame basis. Before a node transmits a periodic message, it broadcasts the priority of message for all nodes to construct a network wide ready queue in order of priority. A node can transmit a periodic message only when its message is at the head of the ready queue. We have derived sufficient and necessary conditions for both static and dynamic priority assignment in order to determine the schedulability of periodic messages. The simulation study shows that the proposed algorithm significantly improves the guarantee ratio of periodic messages.