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Chapter 5Upstream channel capacity and characterisation pThe mean packet delay curve is stable (producing considerably low mean access delaysunder ≈ 33 ms) up to the point of saturation, which was found to be at approximately61% of the cc, lower than the capacity achieved by the system throughput. From thispoint, even a slight increase in offered load may result in system instability, as we canobserve with an offered load approximately of 62% of the cc.Figure 5.2 gives a better insight into the delay characteristics of the system by plottingthe number of frames that experienced delays of a certain value. In this figure at 61% ofthe cc, 75% of the Ethernet packets were transmitted in less than 33 ms and the other25% under 67 ms. At 62% traffic load, the saturation point is evident, and access delayshave been considerably increased, since only 11% of the packets are now transmittedunder 67 ms. Hence, the system cannot maintain maximum throughput whilst providingbounded delay characteristics. Therefore, the maximum system throughput sustained is1.88 Mbps (61% cc). In the next scenario we address the number of stations that can besupported.1.0Load 54%0.9Cumulatitve Probability0.80.70.60.50.40.3Load 61%3 Mbps UpstreamEthernet Frames (1518-bytes)Ba ckoff AlgorithmL oad 62%Load 6 3%0.20.1Load 70%Load 85%0.00 33 67 100 133 167 200 233 267 300 333 367 400 433 467 500Me an Acce ss De lay (ms)Figure 5.2 - Cumulative probability vs. mean access delay.5-11