Timing Properties of the FDDI-M Medium Access Protocol

The FDDI Standard specifies a 100 Mbits/s fibre-optic token ring network that has been implemented and widely installed. The FDDI medium access control (MAC) protocol, however, has the inherent deficiency that, at most, one half of the bandwidth of a FDDI ring can be used to transmit synchronous messages. In a recent paper, a modification to the FDDI MAC protocol, called FDDI-M, was proposed to overcome this deficiency. It has been shown using simulation that FDDI-M doubles a ring's ability of supporting synchronous traffic while at the same time it achieves a higher throughput for asynchronous traffic than standard FDDI. In this paper we present an analytical study of the timing properties of the FDDI-M protocol. The results presented in this paper complement those of the original paper. Using the worst-case achievable utilization (WC AU) as the performance metric, we evaluate the performance of various synchronous bandwidth allocation (SBA) schemes in guaranteeing synchronous message deadlines. It is found that, in comparison with FDDI, the FDDI-M protocol results in a higher WCAU for the normalized proportional SBA scheme. However, for the local SBA schemes studied, the WCAU values remain at zero.