Timing properties of the FDDI-M medium access protocol for a class of synchronous bandwidth allocation schemes

FDDI networks have been widely deployed to support real-time traffic such as voice and video communications. However its medium access control (MAC) protocol can transmit synchronous messages up to at most one half of the total bandwidth of the FDDI network. Shin and Zheng has proposed a modification to the FDDI MAC protocol, called FDDI-M, which can double a ring´s ability in supporting synchronous traffic (see IEEE Trans. Parallel and Distributed Systems, no.6, p.1125-31, 1995). However, it is widely known that the ability of timed token protocols such as FDDI to guarantee synchronous message deadlines is very dependent on the synchronous bandwidth allocation (SBA) schemes used. We first propose a classification of SBA schemes proposed in the literature which facilitates our comparison of FDDI-M with FDDI. Next, we present an analytical study of the timing properties of the FDDI-M protocol, using the worst case achievable utilization (WCAU) as the performance metric, for a class of SBA schemes. Our results show that FDDI-M improves the WCAU values for those SBA schemes which are non-zero to start with, but those whose WCAU values are zero remain unchanged