Improving the Profibus medium access control protocol

Abstract

In this paper, we present a critical study of the master stations Profibus Medium Access Control (MAC) protocol (Vasques, 1996). In Profibus network, the master stations use the timed token protocol to communicate between each other. According to our study, in the worst case, if all the master stations have permanently high priority traffic, and one of them has in addition low priority traffic, then, this station will send all its messages and the following stations will send only one high priority message at each token arrival. To resolve this problem, we propose to allocate to each master station a time-capacity H k to transmit its high priority messages, and a time-capacity L k to transmit the rest of its traffic (low priority). In addition, in order to favour high priority messages transmission, if a given master station has a remaining time (HR k or LR k ) from respectively its (H k or L k ), the following station may exploit it to transmit more high priority messages during a token cycle time.