A virtual time CSMA protocols for hard real-time communication

Abstract

We study virtual time CSMA protocols 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. Whenever 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 according to the minimum-laxity-first policy. While both use perfect state information about the nodes and channel, the first is an idealized protocol which obtains this information 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.