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Chapter 5Upstream channel capacity and characterisation pAs a final remark for this analysis, it was seen that the reservation request mechanism ofthe DVB/DAVIC protocol was not optimised, since a poor performance is obtainedwhen short messages are being transmitted. Therefore, it is evident that a performanceoptimisation or the incorporation of new mechanisms should be considered.5.4.7.2 Multiple node scenarioIn this section, we now consider a multiple-node network scenario to study themaximum performance gain that can be obtained by increasing the packet size. Here,isochronous streams of 32 kbps (produced by each station) were analysed and the interarrivalrates were set according to the packet size. The simulations were carried outusing only the splitting tree algorithm because of its increased performance over theexponential backoff algorithm. The packet sizes considered were 64, 128, 256, 512,1024 and 1518 bytes that cover the whole range from the minimum to the maximumEthernet packet size.Figures 20 to 22 present a general performance overview when the packet size isincreased. The delay and throughput results revealed that for small packet sizes, thecommunication protocol is also highly inefficient.System Throughput (%)7065Pk_Size: 64 bytesPk_Size: 128 bytes60 Pk_Size: 256 bytes55Pk_Size: 512 bytesPk_Size: 1024 bytes50 Pk_Size: 1518 bytes45403530253 Mbps Upstream2032 kbps Kbps isochronous Isochrous streamsTree Algorithm1518 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69umber of Active StationsFigure 5.20 – System throughput vs. No. of active stations for increased packet size.5-32