Improving fairness in optical-burst-switching networks

Abstract

Approaches for dealing with the beat-down unfairness problem have been mainly limited to electronic packet-switching networks. We investigate two methods for alleviating this problem in optical-burst-switching (OBS) networks. When OBS is used with just-in-time or just-enough-time scheduling, the control packet transmitted ahead of the data burst may fail to secure a free channel, causing the data burst to be dropped at some node along its path. The burst dropping probability increases as the number of hops in the lightpath of the burst increases, resulting in unfairness to bursts with long paths. Our first scheme to deal with this problem is based on a simple equation that adjusts the size of the search space for a free wavelength based on the number of hops traveled by the burst. Our second scheme uses proactive discarding to reduce the probability of dropping bursts with large hop count at the expense of an increase in the dropping probability of bursts with small hop count. The scheme adapts the random-early-discard (RED) strategy to the OBS environment and prioritizes the levels of discarding on the basis of the length of the lightpath. The proactive burst discarding is done in the network access station (NAS) of the source node that generated the burst. Thus discarded bursts do not waste any bandwidth resources in the core of the optical network. Both schemes have simple logic, do not use preemption, do not require complex modification to the lightpath setup scheme, and alleviate the beat-down unfairness without negatively affecting the overall throughput of the system. Detailed performance evaluation of the two schemes is presented and analyzed.