A PRIORITY BASED MAC PROTOCOL FOR DETERMINISTIC DATA COMMUNICATION OVER ETHERNET

Abstract

In order to realize real-time communication over Ethernet one must be able to bound the medium access time within an acceptable limit. In this dissertation a modified collision resolution algorithm is proposed for achieving deterministic communication over Ethernet. The modified collision resolution algorithm is based on the distributed assignment of special TAG numbers to the stations transmitting real-time traffic. These TAG numbers determine a Round Robin order of transmissions among the real-time stations. They also help in resolving collisions among real time stations by setting the length of the Jam signal, transmitted in case of a collision, to be a function of the TAG number. During a collision, the station with the highest TAG number, and so with the longest Jam, is persisting with its Jam transmission for the longest until all the other stations defer. Thus, the collision terminates and the longest persisting station can transmit its packet successfully. The modified collision resolution algorithm enables stations implementing the IEEE 802.3 MAC standard to transmit on the same network with stations implementing the new algorithm. After introducing the algorithm, we compute an upper bound on the access delay that the protocol guarantees and prove the correctness of the distributed TAG assignment procedure. Finally, we simulate the protocol in a network consisting of real-time stations, which implement the new algorithm together with non real-time stations, which implement the standard IEEE 802.3 MAC. We show that the access delays of the stations transmitting real-time traffic are indeed bounded as predicted.