Distributed queue dual bus (DQDB) has been adopted as the IEEE 802.6 standard for metropolitan area networks (MANs). The access mechanism is based on an implicit distributed queue maintained at each node. DQDB has been shown to achieve maximal aggregate throughput and minimal access delay independent of the size and speed of the network. However, as the size and load of the network increase, DQDB gives rise to throughput unfairness problems due to the long propagation delay. This paper initially presents a survey and assessment of recently proposed remedial DQDB mechanisms. These approaches are categorized as time division multiplexing (TDM)-oriented versus non-TDM-oriented, and exhaustive versus non-exhaustive operations. Regarding TDM as the most promising mechanism, the paper then provides the performance analysis of one TDM-oriented method, called multiple segment control (MSC), proposed by the authors. In the analysis, the network is modelled as a set of identicalM/G/1 queues, each with different services for transmitting the first and remaining segments in a busy period. The average aggregate access delay is then analyzed via an embedded Markov model. The paper shows simulation results to demonstrate the precision of the performance analysis and the performance superiority of MSC over non-TDM-oriented DQDB mechanisms. Furthermore, as will be shown, the network using the TDM-oriented method may incur bandwidth waste should active stations fail. The paper finally proposes a guardian reliability scheme to ensure the robustness of networks.
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