Real-time schedulability 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. The authors compare two token ring protocols for their suitability in hard-real-time systems. A priority driven protocol (e.g., IEEE 802.5) allows implementation of a priority based real-time scheduling discipline like the rate monotonic algorithm. A timed token protocol (e.g., FDDI) provides guaranteed bandwidth and bounded access time for synchronous messages. These two protocols are studied by deriving their schedulability criteria, i.e., the conditions which determine whether a given message set can be guaranteed. Using these criteria, the average performance of these protocols is evaluated under different operating conditions. It is observed that neither protocol dominates the other for the entire range of system parameter space. The conclusion is that the priority driven protocol performs better at low bandwidths (1-10 Mb/s) while the timed token protocol has a superior performance at higher bandwidths