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Chapter 5Upstream channel capacity and characterisation pThe maximum throughput (see Figure 5.20) yielded for minimum Ethernet packet sizewas as low as ≈ 34% of the link capacity, which can only support up to 25 stationsbefore delays became extremely large for the delivery of isochronous streams (as shownin Figure 5.21). Increasing the packet size can significantly improve the performancewith maximum sustainable throughput up to ≈ 64% of the cc. This increase in systemperformance was recorded for packet sizes of 512 bytes. Larger packet sizes have onlymarginal increase in system throughput and a small increase in packet access delay.From Figure 5.21, for offered loads less than 47% of the cc (produced by up to 45stations), access delays for short packet sizes (under 1024 bytes with the exemption ofthe 64-byte packet size) were relatively shorter than for larger packet sizes (e.g. 1024and 1518 bytes). This is mainly due to the larger transmission delay incurred by longpackets. On average, when a station has gained access to the reservation access region,it takes 1 signalling frame of 3 ms to transmit a packet up to 512 bytes and 2 signallingframes to transmit a packet of 1024 or 1518 bytes. Hence, a minimum difference of 3ms between short and large packet sizes can be noticed in Figure 5.21.Mean Access Delay (ms)504540353025201510503 Mbps Upstream32 32 kbps Kbps isochronous Isochronous streamsTree AlgorithmPk_Size: 64 bytesPk_Size: 256 bytesPk_Size: 1024 bytesPk_Size: 128 bytesPk_Size: 512 bytesPk_Size: 1518 bytes18 21 24 27 30 33 36 39 42 45 48 51 54 57 60 63 66 69umber of Active StationsFigure 5.21 – Mean access delay vs. No. of active stations for increased packet size.5-33