8. Priority Based QoS - More

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To see how CoS worked, I performed a couple of IxChariot runs. I plugged two notebooks and a desktop (all running Win XP SP2) into the 728TS, initially setting the CoS for both notebook ports to 0. I then set IxChariot to use the IxChariot throughput.scr script, to send TCP/IP data simultaneously from both notebooks to the desktop for about a minute. During the run, I incremented the priority level of one of the ports (designated "CoS Variable" in the plots) by one every 10 seconds (give or take).

Figure 12: CoS throughput vs. modes - Strict Priority
(click image to enlarge)

Figure 12 shows the results with the QoS Queue control at its default Strict Priority (SP) setting. I didn't expect to see a succession of widening throughput differences, since that's not how priority-based QoS works. But what's interesting is that the two ports actually swap highest throughput honors depending on the CoS value of the "CoS Variable" test pair.

The "CoS 0" pair gets maximum bandwidth in Figure 12 when the "CoS Variable" pair has CoS=1 and 2, then drops down to where it's supposed to be for CoS = 4 - 7. And when CoS=0 and 3, the two traffic streams get an equal shot at bandwidth. The changes seem to correspond with the default CoS to Queue settings (Figure 11), so this behavior appears to be by design?but it's certainly not obvious!

Tip! This documentation for the Dell PowerConnect 27XX series managed switches does a little better job of explaining CoS setup than the Netgear documentation.

Tip! This excerpt from "Practical Service Level Management: Delivering High-Quality Web-Based Services" can help you wrap your head around these QoS technologies.

Figure 13 shows the results of the same experiment, but this time with the QoS Queue control set to WRR (Weighted Round Robin). This technique is supposed to ensure that no priority level dominates the available bandwidth, which seems to be what Figure 13 confirms. The two streams still swap the top bandwidth position depending on the CoS value, but bandwidth sharing appears to be more equitable.

Figure 13: CoS throughput vs. modes - Weighted Round Robin (WRR)
(click image to enlarge)