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Chapter 6Optimisation of CRA using adaptive CSAs pLarger values, (above 4 CSs per signalling frame) cause a waste of bandwidth, since theCSA tends to allocate more contention slots than those needed. This in turn results in areduction in system throughput. Furthermore, this causes increased access delay asstations transmit reservation requests more frequently. Such requests at the headendhave to wait longer for grant since there is a reduction in the bandwidth assigned forreservation access.From Figure 6.3, it is evident that on high traffic offered loads, beyond 50% of the cc,(generated by at least 24 stations), the average number of contention slots needed perrequest ranged from 3 to 3.5 slots, when the minimum number of CSs per signallingframe was below 5 slots.In Figure 6.2 we can appreciate that the maximum system throughput achieved wasapproximately 51% (for CSs-4) with an offered load about 52% of the channel capacityyielded by 25 stations. This difference of approximately 1% between theoretical andsimulation results was attributed to the fact that the CSA assigned on average 3.08contention slots per successful request (see Figure 6.3), instead of the optimum value of2.718. Furthermore, the average Pk size (registered by simulations) was 374 bytes with8.42 slots requested per packet, which made the difference of 1%.Average umber ofContention Slots per Request7.57.06.56.05.55.04.54.03.53.02.5CSs-1CSs-2CSs-3CSs-4CSs-5CSs-6CSs-73 Mbps Upstream64 Kbps kbps IPBackoff CRA18 19 20 21 22 23 24 25 26 27 28 29 30umber of Active StationsFigure 6.3 – Average No. of CSs per reservation request vs. No. of active stations.Exponential backoff algorithm with a Simple-CSA.6-5