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Chapter 5Upstream channel capacity and characterisation pResults presented in Figure 5.16 indicate that by increasing to two the number ofMPEG2 frames (with MCI information) within the 3 ms period (represented by the label‘MCI-2’ in Figure 5.16) a reduction in access delay of approximately of 4.4 and 4.8 mscan be obtained for the exponential backoff algorithm and the splitting tree algorithm,respectively. This reduction can only be obtained with traffic loads under 47% (45active users) and 53% (51 active users) of the cc for the exponential backoff algorithmand splitting tree algorithm, respectively.However, for high traffic loads, lower access delays are produced with one signallingframe in every 3 ms period (represented by ‘MCI-1’). This change in access delay isbecause as the network load becomes higher than the point of saturation, there is anincreased risk of collision of reservation requests with ‘MCI-2’ (according to simulationresults), which degrades slightly the overall system performance as shown in Figure5.16.Mean Access Delay (ms)1201101009080706050403020100Backoff-MCI-1Backoff-MCI-2Tree-MCI-1Tree-MCI-23 Mbps Upstream32 Kbps kbps IPBackoff & Tree Algorithm30 33 36 39 42 45 48 51 54 57 60 63 66umber of Active StationsFigure 5.16 – Mean access delay vs. No. of active stations for different MCI values.5-28