Improving access delay fairness of DQDB MANs under overload condition

The main advantages of Distributed Queue Dual Bus (DQDB) are the simplicity of its medium access control and that it can utilize all of the channel bandwidth independent of data transmission rate and network size. The data transmission rate of DQDB is in the range from DS3 to SONET speeds. Due to the increased ratio of propagation delay to segment transmission time, the information that a station receives about the current state of DQDB network can be out of date in such a high speed environment. This results in the position dependent characteristics of the network, which is the main reason that unfair throughput and access delay appears, especially when network is overloaded to cause unpredictable behavior. Though Bandwidth Balancing Machine (BWBM) is enabled in the IEEE 802.6 DQDB, and the fairness of throughput may be reached, the unfairness of access delay still exists, especially under overloaded condition. In this paper, we propose an efficient modification of REQ Queue Machine (RQM) with an access protection protocol. The modified RQM scheme eliminates longer access delay for downstream stations along the direction of transmission. The utilization of a modified protection scheme prevents the stubborn unfairness for the upstream stations close to the slot generator. Simulations show that our proposed method can reach fairness in terms of throughput and access delay.