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Chapter 5Upstream channel capacity and characterisation pFigure 5.5 shows that the lowest access delays can be obtained when stations onlytransmit messages comprised of 1 ATM cell in the contention access region. Low delaysunder 25 ms are produced with an offered load of 35% of the link capacity (created by24 stations), which is more suitable for the support of both Internet applications andVoIP streams. The other values (2, 3 and 5 ATM cells) also offer good results, yieldingmean access delays under 50 ms with the same traffic load. It can be appreciated that ifnone of the messages are allowed to be transmitted using contention access, higherdelays are derived, decreasing the number of supported streams (to 22 stations).100Mean Access Delay (ms)9080706050403020Contention Msg Length -- 0 ATM CellContention Msg Length -- 1 ATM CellContention Msg Length -- 2 ATM CellsContention Msg Length -- 3 ATM CellsContention Msg Length -- 5 ATM CellsContention Msg Length -- 6 ATM Cells3 Mbps Upstre am32 Kbps kbps IP &12.4 Kbps kbps VoIPBackoff A lgorithm10014 16 18 20 22 24 26umber of Active StationsFigure 5.5 – Mean access delay vs. No. of active stationsfor different contention messages.5-15