Incorporating Continuation-of-Message Information, Slot Reuse, and Fairness in DQDB Networks

Several important modifications to the distributed queue dual bus (DQDB) (IEEE 802.6) protocol for metropolitan area networks (MANs) are proposed. These modifications are aimed at improving not only the throughput and delay performance of the network but also its fairness performance. The first proposal exploits the continuation-of-message (COM) information in a DQDB segment in order to minimize the mean overhead for multi-packet messages. This reduces the net offered load to the system and thus improves its throughput and delay performance. Then, a scheme for combining the COM information along with the spatial reuse of slots is proposed, thereby achieving even higher throughput and lower delay. Simulation models are used to study the performance gains achievable by using the COM and the slot reuse approaches. Although slot reuse has already been proposed before to improve the DQDB protocol’s performance, the unfairness of such a system has been given little attention so far. Moreover, since the unfairness becomes more pronounced when slots are reused, the study of schemes that can achieve fairness in such environments becomes all the more important. This paper also proposes several schemes to achieve different fairness criteria in a DQDB network employing slot reuse. In particular, the following fairness criteria are studied: (a) an individual station’s throughput proportional to the number of stations downstream from itself, (b) equal throughput at all stations, and (c) an individual station’s throughput proportional to its own offered load. Analytical results corresponding to these schemes are developed, and they are verified via simulation. The proposed schemes can be implemented with just one additional counter and one register (on top of the regular DQDB hardware) at each station. hhhhhhhhhhhhhhhh * This work has been supported by the US Air Force Office of Scientific Research (AFOSR) under Grant No. 89-0292.