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Chapter 5Upstream channel capacity and characterisation pIn terms of utilisation, the exponential backoff algorithm utilised approximately 86% ofthe link capacity in comparison with ≈ 90% utilised by the splitting tree algorithm. Inthis analysis, which considered the maximum Ethernet packet size, the increase insystem performance (throughput and utilisation) resulted in approximately of 4% of thecc when the splitting tree algorithm is selected. A bigger increase in systemperformance can be obtained when short packet data transmissions are used. In thefollowing analysis we change the traffic pattern to appreciate this difference better.Results for access delays, Figure 5.4 indicates that saturation points are presentedbeyond 140 stations (with mean packet delay of 39 ms) for the exponential backoffalgorithm and 160 stations (with mean packet delay of 28 ms) for the and splitting treealgorithm. Past these points, a sharp delay increase is recorded, which grows higherthan the mean inter-arrival rate (of 1s) and as a consequence mean access delaysbecome asymptotically unbounded.Mean Access Delay (ms)10009008007006005004003002001003 M bps Ups treamEthernet frames (1518-bytes )12.14 Kbps kbps per stationBackoff & Tree algorithmBackoff AlgorithmSplitting Tree Algorithm020 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340umbe r of Active StationsFigure 5.4 – Mean access delay vs. No. of active stations, exponential backoff andsplitting tree algorithm.5-13