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Chapter 5Upstream channel capacity and characterisation pFigure 5.6 indicates that a maximum system throughput of ≈ (1.14 Mbps) 37% of the cccan be achieved also with the value of 1 ATM cell, in comparison to ≈ (1 Mbps) 32% ofthe cc yielded when all packets are transmitted using only reservation access. Byallowing a station to transmit short packets in the contention-based region, not only afaster data transmission can be obtained (as appreciated in Figure 5.5) but also a highersystem throughput can be achieved. This is because stations do not need to waste anextra slot to send a reservation request and then wait for its acknowledgement, thusimproving the system performance. This increase in system performance can only begained in small network sizes (under ≈ 50 stations). It has also been noticed that onheavy traffic loads there is a drawback. As the ‘Maximum Contention Access MessageLength’ was increased, more data messages were transmitted using contention access.This in turn increased the collision risk with the transmissions of reservation requestssent by other stations and as a consequence system throughput is reduced. The effect isbetter appreciated in the next two figures.System Throughput (Kbps)12001140108010209609008407807206606003 Mbps Upstream32 Kbps kbps IP &12.4 Kbps kbps VoIPBackoff AlgorithmContention 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 Cells14 16 18 20 22 24 26 28 30 32umber of Active StationsFigure 5.6 – System throughput vs. No. of active stationsfor different contention messages.5-16