Real-Time Performance of Two Token Ring Protocols

When designing real-time communication protocols, the primary objective is to guarantee the deadlines of synchronous messages while sustaining a high aggregate throughput. In this paper, we study two token ring protocols for their suitability in real-time systems. The priority driven protocol (e.g., IEEE 802.5) allows implementation of a priority based real-time scheduling discipline like the rate monotonic algorithm. The timed token protocol (e.g., FDDI) provides guaranteed bandwidth and bounded access time for synchronous messages. We study these two protocols by deriving their schedulability conditions, i.e., the conditions which determine whether a given message set can be guaranteed. Using these conditions, we evaluate the average performance of these protocols under diierent operating conditions. We observe that neither protocol dominates the other for the entire range of the system parameter space. We conclude that the priority driven protocol performs better for bandwidths in the range of 1-10 Mbit/s while the timed token protocol has a superior performance at higher bandwidths.