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aChapter 7Performance optimisation for the support of TCIS pThe maximum throughput and upstream channel utilisation versus the packet sizegenerated is shown in Figure 7.21. In terms of throughput (Figure 7.21a), the DOCSISprotocol is superior to the DVB/DAVIC protocol. For the maximum Ethernet packetsize (1518 bytes), the maximum sustainable throughput per DOCSIS cable modem was≈ 1.6 Mbps, which corresponds to about 62% of the upstream channel capacity, incomparison to ≈ 1 Mbps (32% of the cc) achieved by each DVB/DAVIC cable modem.In terms of utilisation (Figure 7.21b), the difference was 19% of the link capacity infavour of the DOCSIS protocol.However, for small packet sizes under 256 bytes, results reveal that bothcommunication protocols are highly inefficient. The maximum throughput does notexceed 25% of the link capacity even though the offered load was a constant 10 Mbpsstream. The worst case scenario is released for the 64-byte packet size, where thethroughput was as low as 2% of the cc (≈ 56 kbps) for DVB/DAVIC cable modems, incomparison to 7% of the nominal channel capacity (≈ 181 kbps) provided by theDOCSIS cable modems.A closer analysis of the difference between throughput and utilisation, simulation resultsshowed that an increased protocol overhead is involved in the DVB/DAVIC protocolthat reduce considerably the protocol efficiency, as examined in the previous scenario.Throuhgput (Mbps)2.01.81.61.41.21.00.80.60.40.20.0a) System Throughput b) UtilisationDOCSIS: 2.5Mbps UpDVB: 3Mbps Up64 128 256 512 768 1024 1280 1518Packet Size (bytes)Utilisation (Mbps)2.01.81.61.41.21.00.80.60.40.20.0DOCSIS: 2.5Mbps UpDVB: 3Mbps Up64 128 256 512 768 1024 1280 1518Packet Size (bytes)Figure 7.21– Maximum channel capacity for DVB/DAVIC and DOCSIS for one node.7-31