Virtual Time CSMA Protocols for Hard Real-Time Communication

We study virtual time CSMA protocols for hard real time communication systems, i, e., systems where messages have explicit deadlines. In this class of CSMA protocols, each node maintains two clocks; a real time clock and a virtual time clock. Whenever a node finds the channel to be idle, it resets its virtual clock. The virtual clock then runs at a higher rate than the real clock. A node transmits a waiting message when the time on the virtual clock is equal to some parameter of the message. Using different message parameters in conjunction with the virtual clock, different transmission policies can be implemented. In particular, use of message arrival time, message length, message laxity, and message deadline implements FCFS, Minimum-Length-First, Minimum-Laxity-First, and Minimum-Deadline-First transmission policies, respectively.