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An Efficient and Practical Synchronous Bandwidth Allocation Scheme for Deadline Guarantees of Synchronous Messages in an FDDI Network

This report addresses issues related to guaranteeing synchronous message sets with message deadlines equal to periods, in an FDDI network where the timed token protocol is used. FDDI guarantees a bounded access time and an average bandwidth for synchronous traffic, and therefore the timed token protocol is well suited for real-time applications. However, this guarantee alone, though necessary, is insufficient for guaranteeing the transmission of synchronous messages before their deadlines. To ensure the timely delivery of synchronous messages, the synchronous bandwidth (a key parameter in the control of synchronous message transmission) must be carefully allocated to individual nodes. In this report, we propose and analyse a new local synchronous bandwidth allocation (SBA) scheme. Generally, our local scheme performs better than any of previously-published local SBA schemes due to the fact that it calculates synchronous bandwidths, with locally-available information, under the consideration of fully using all synchronous bandwidths surely available during the period of the message. Our new local scheme also differs significantly from previously reported local SBA schemes by explicitly taking into account the synchronous bandwidth allocation for the synchronous message sets with the minimum message deadline ( D min ) less than twice the Target Token Rotation Time (TTRT), and as a result can be applied to any of synchronous message sets (with D min > TTRT). The feasibility of allocations produced by the proposed local scheme as well as the worst case achievable utilisation of the scheme is also discussed in this report. Since local SBA schemes, unlike global SBA schemes, can greatly simplify the run-time management of network with the advantage of being flexible and suitable for use in dynamic environment, the proposed local scheme can be used to effectively support synchronous traffic in practice.