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Chapter 5Upstream channel capacity and characterisation p5.4.7.1 Single node scenarioThis analysis addresses the issue of maximum throughput per station that thecommunication protocol can achieve. For this particular case, a single node scenarionetwork is considered and simulations were run for different Ethernet packet sizes (from64 to 1518 bytes). Tests were performed with the exponential backoff algorithm.Results for the splitting tree algorithm reported the same system performance, becausewith only one station collisions are avoided. In order to obtain the maximum achievablethroughput, the station was set to produce the maximum number of packets that it couldreceive from an Ethernet network, which was a 10 Mbps constant stream with variablenumber of packets per second depending on the packet size.The system throughput and upstream channel utilisation versus the packet sizegenerated is shown in Figure 5.18. The results revealed that the communicationprotocol is highly inefficient. The maximum throughput does not exceed 32% (=994kbps) of the link capacity using maximum Ethernet packet size, even though the offeredload was a constant 10 Mbps stream.In this analysis it was found that the scheduler-look ahead is the major delay element inthe transmission cycle, which dramatically reduces the throughput.Throughput & Channel Utilisation (%)504540353025201510503 Mbps Upstream10 Mbps Ethernet TrafficBackoff AlgorithmUtilisationThroughput64 128 256 512 768 1024 1280 1518Packet Size (bytes)Figure 5.18 – System throughput/utilisation vs. packet size.5-30