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Chapter 5Upstream channel capacity and characterisation pIn order to demonstrate the effects of an increased reservation request size, a mixedtraffic configuration was analysed. The mean data rate per active station was set to 41.7kbps, which consisted of 32 kbps Internet and 9.7 kbps Voice over IP traffic. Thesimulations were performed using now both CRAs. In Figures 5.9 to 5.11 we comparethe results for different request limits of 6, 11, 22 and 32 ATM cells, which representthe transmission of packet sizes up to 256, 512, 1024 and 1518 bytes, respectively.Larger values were not considered, since we address the transmission of Internet andVoIP traffic that create packets under 32 ATM cells.Figure 5.9 shows that the access delays can be significantly reduced when the splittingtree algorithm is selected. For both contention resolution algorithms the lowest meanaccess delay (around 50 ms) is yielded with request sizes of 22 and 32 ATM cells,which represent the transmission of 1024 and 1518 byte packets, respectively. It can beappreciated that for these request sizes, the splitting tree algorithm supported up to 8stations more than the number supported by the exponential backoff algorithm. For ashort request size up to 6 ATM cells, this difference was of 10 stations.500Mean Access Delay (ms)450400350300250200150100500ExponentialBackoffSplittingTreeRsv Rsv Msg Msg - - 66 slotsRsv Rsv Msg Msg - - 11slotsRsv Rsv Msg Msg - - 22 22 slotsRsv Rsv Msg Msg - - 32 32 slotsRsv Rsv Msg Msg - - 66 slotsRsv Rsv Msg Msg - - 11slotsRsv Rsv Msg Msg - - 22 22 slots slotsRsv Rsv Msg Msg - - 32 32 slots3 M bps Upstream32 Kbps kbps IP IP &9.7 Kbps kbps VoIPBackoff & Tree A lgorithms20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50umbe r of Active StationsBackoffBackoffTreeTreeFigure 5.9 – Mean access delay vs. No. of active stations for different request sizes.5-19