A Virtual Time CSMA Protocol for Hard Real Time Communication 1

We study a virtual time CSMA protocol for hard real time communication systems where messages have explicit deadlines. In this protocol, each node maintains two clocks: a real time clock and a virtual time clock. When-ever a node finds the channel to be idle, it resets its virtual clock to be equal to the real 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 the latest time to send the message. This protocol implements the minimum-laxity-first transmission policy. We compare the performance of our protocol with two baseline protocols both of which transmit messages accord-ing to the minimum-laxity-first policy. While both use per-fect state information about the nodes and channel, the first is an idealized protocol which obtains this informa-tion without paying any cost and the second one pays a reasonable price for it. The simulation study shows that in most cases, our protocol performs close to the first one and better than the second one.