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Chapter 4Simulation and analytical modelling pBy analysing these figures, we can verify that the results obtained using the simulationmodel for the DVB/DAVIC protocol are in good agreement with theoretical results. Forexample, for the worst case scenario that is when a packet size of 64 bytes is used. Fromsimulation results (Figure 4.18), the maximum throughput that a single node canachieve resulted in 224 kbps. From analytical results (Table 4.2), this number resulted in228 kbps. This extremely low performance of the DVB/DAVIC protocol (equivalent to≈ 4% of the channel capacity) is due to the two-phase data transmission cycle, whichrequires a request to be issued in the upstream channel and the data to be transmittedafter a grant message is received in the downstream channel.The delay between the time when a request is issued and the time when it is granted isanother parameter, and needs to be analysed and accurately calculated. This delay,referred here as the ‘scheduler-look ahead (D lah )’, has proven to be one of the majordelay elements in the transmission cycle, which dramatically reduces the throughput.When the packet size was changed to maximum size (1518 bytes) the maximumthroughput sustainable per station resulted in ≈ 2630 kbps (equivalent to 43% of thechannel capacity).In general, results for mean access delays (Figure 4.19) revealed that for all packetsanalysed, the maximum deviation between simulation and analytical results was foundMean Access Delay (ms)100908070605040302010064-bytepacketMean Access Delays for Simulation and Theoretical ResultsSimulation ResultsTheoretical Results128-bytepacket256-bytepacket512-bytepacket1024-bytepacket1518-bytepacket0 250 500 750 1000 1250 1500 1750 2000 2250 2500 2750 3000Offered Load (Kbps)Figure 4.19 – Mean access delay: simulation and theoretical resultsfor different packet sizes.4-33