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Chapter 6Optimisation of CRA using adaptive CSAs pCumulative Probability1.00.90.80.70.60.50.40.30.20.10.03 Mbps Upstream32 Kbps kbps IP &9.7 Kbps kbps VoIPBackoff CRA33 67 100 133 167 200 233 267 300 333 367 400 433 467 500Mean Access Delay (ms)terms of packet delays and number of streams supported, from Figures 6.9 and 6.10, thelowest access delays were gained using the Forced-CSA.For instance, with 45% of the channel capacity (produced by 33 stations), tolerablemean access delays for VoIP streams (under 50 ms) were seen only with Forced-CSA(FSs-2, CSs-3), supporting up to 33 stations. With a slightly increase in offered load(e.g. 46% produced by 34 stations), only the Simple-CSA (CSs-5) and the Forced-CSA(FSs-2, CSs-3) yielded tolerable low delays for the support of IP traffic.Approximately 73% of all data packets were transmitted in less than 200 ms asillustrated in Figure 6.10. The Simple-CSA (CSs-5) produced relatively low packetaccess delays because it sent more CSs than currently needed to resolve collisions,resulting in a reduction on the average CRGC. On the other hand, the Simple-CSA (CSs-5) wasted many CSs, trying to minimise the CRGC and therefore a slight decrease insystem throughput was obtained.Simple-CSA (CSs-3)Forced-CSA (FSs-1,CSs-3)Forced-CSA (FSs-2,CSs-3)Simple-CSA(CSs-5)Fo rced-CSA(FSs-2,CSs-2)Variable-CSA(CSs-2)Figure 6.10 – Cumulative probability vs. No. of active stations.Exponential backoff algorithm with varying CSA applied.Offered load = 46% produced by 34 stations.As a final remark for this section, in general the Forced-CSA outperforms the other twomechanisms (Simple and Variable-CSA). The Forced-CSA not only provided the lowestaccess delays, but also in most of the cases simulated provided the highest system6-16