ALTERNATIVE STRATEGIES FOR IMPROVING THE FAIRNESS IN AND AN ANALYTICAL MODEL OF THE DQDB NETWORK

This paper deals with the distributed queue dual bus (DQDB) (IEEE 802.6) network, and makes two independent contributions. First, the unfairness problem of DQDB is addressed, and several alternative solutions that can improve the network's fairness are proposed. They include 1) the proportional assignment scheme (PR) (under which the bandwidths assigned to the individual stations are in proportion to their offered traffic during heavy load conditions); 2) the (multiple-request) FCFS-message-queue-based DQDB scheme (MD) (under which a station follows the regular DQDB protocol, but it is allowed to make multiple outstanding requests, one for each of its queued segments, in order to create a distributed FCFS message queue rather than a FCFS segment queue); and 3) a combination of MD and PR, denoted by MP. Implementation methods that require simple additional hardware on top of the regular DQDB interface are outlined. Simulation examples are employed to compare the performance of the above schemes and to gain insights into their characteristics. The performance of these schemes are also compared with those of original DQDB and bandwidth balancing DQDB. The second contribution of this paper is the development of an analytical model of the DQDB network. By employing some constrained assumptions for analytical tractability, a Markov chain model for (an earlier version of) the entire DQDB network is formulated, and the corresponding state space explosion problem is highlighted. For reasonably small systems, the analytical model can predict an individual station's throughput and mean segment delay for known (possibly-asymmetric) loading patterns. The model is verified via simulation.