Proportional Diierentiated Services: Delay Diierentiation and Packet Scheduling

The proportional diierentiation model provides the network operator with thètuning knobs' for adjusting the per-hop QoS ratios between classes, independent of the class loads. This paper applies the proportional model in the diierentiation of queueing delays, and investigates appropriate packet scheduling mechanisms. The second part of this work 2] considers the corresponding loss rate diierentiation and buuer management problem. Starting from the Proportional Delay Diierentiation (PDD) model, we derive the average queue-ing delay in each class, show the dynamics of the class delays under the PDD constraints, and state the conditions in which the PDD model is feasible. The feasibility model of the model can be determined from the average delays that result with the Strict Priorities scheduler. We then focus on scheduling mechanisms that can implement the PDD model, when it is feasible to do so. The Proportional Average Delay (PAD) scheduler meets the PDD constraints, when they are feasible, but it exhibits a pathological behavior in short timescales. The Waiting Time Priority (WTP) sched-uler 3], on the other hand, approximates the PDD model closely, even in the short timescales of a few packet departures, but only in heavy load conditions. PAD and WTP serve as motivation for the third scheduler, called Hybrid Proportional Delay (HPD). HPD approximates the PDD model closely, when the model is feasible, independent of the class load distribution. Also, HPD provides predictable delay diierentiation even in short timescales.