Timing Properties of the FDDI-M Medium Access Protocol

Abstract

The FDDI Standard speci"es a 100 Mbits/s "bre-optic token ring network that has been implemented and widely installed. The FDDI medium access control (MAC) protocol, however, has the inherent de"ciency that, at most, one half of the bandwidth of a FDDI ring can be used to transmit synchronous messages. In a recent paper, a modi"cation to the FDDI MAC protocol, called FDDI-M, was proposed to overcome this de"ciency. It has been shown using simulation that FDDI-M doubles a ring's ability of supporting synchronous traf"c while at the same time it achieves a higher throughput for asynchronous traf"c 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 (WCAU) 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.

Token Passing Techniques for Hard Real-Time Communication

“…In (Chan et al, 1997), the authors showed that the maximum token rotation time for the Modified Timed Token Protocol (MTTP) is bounded by TTRT. This means that, under MTTP it is possible to select a value for TTRT no greater than the minimum deadline D. A new token rotation time is defined as TTRT n = TTRT -T S , and used instead of TTRT; 2.…”

“…For a complete survey, an interested reader can see (Zhang et al, 2004) for TTP; (Chan et al, 1997) and (Daoxu et al, 1998) for MTTP; (Franchino et al, 2007), and (Franchino et al, 2008a) for BuST. In the following, we compare the schemes performance under the token passing protocols considered in this chapter.…”

“…The results and proofs in (Chen & Zhao, 1992) and (Chan et al, 1997) have been used as basis to derive the properties of BuST shown in the following. For comparison, the same type of results for TPP and MTTP, available in literature, are reported as well.…”

BuST: Budget Sharing Token protocol for hard real-time communication

“…Note that, with FDDI, the maximum time allocated to asynchronous messages is equal to TTRT − τ . Chan et al [4] proved that for FDDI-M, the maximum amount of time between two consecutive token arrivals at a given node is TTRT . From the protocol constraint, the total synchronous bandwidth must be no greater than…”

“…For FDDI [5] and FDDI-M [4], the WCAU for the PA scheme can asymptotically approach 0%. However, by setting TTRT as suggested in Corollary 2, we can get a non-null WCAU also for FDDI-M.…”