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Chapter 5Upstream channel capacity and characterisation pthe cc).As a final analysis, Table 5.5 presents a summary of the maximum system performanceand bandwidth characterisation for the six different packet sizes considered. In this tablewe can appreciate that regardless of the packet size, the maximum system utilisation isaround 89-90% of the link capacity, with a maximum deviation betweenthroughput/utilisation of ≈ 55% and ≈ 24% for minimum and maximum Ethernet packetsizes, respectively. There are several factors that may account for these deviations.A major component is due to the relatively large amount of overhead involved at MAC(ATM encapsulation) and PHY (forward error correction and synchronisation) layers.This can be as large as 34% and 23% of the cc when delivering small and large packetsof 64 and 1518 bytes, respectively.Another factor is the amount of bandwidth used for the transmission of successfulreservation requests. For 64-byte packets, this amount of bandwidth (of ≈ 12%)represents a significant portion of cc. However, for 1518-byte packets, this bandwidth(of ≈ 1%) represents a tiny fraction of the link capacity. This difference is because shortmessages require proportionally more reservation requests in order to transmit the sameamount of data than with larger messages. For instance to transmit 32 kbps, a stationwill send 75 requests per second with a 64-byte packet size, in comparison to 2.64requests per second with a 1518-byte packet size.Table 5.5 - Maximum system performance and bandwidth characterisation.BandwidthPacket size (bytes)Characterisation (%) 64 128 256 512 1024 1518Maximum throughput 34.5 51.3 56.2 62.9 63.4 64.6Maximum utilisation 89.1 90.1 90.5 89.8 88.9 88.5Deviation 54.6 38.8 34.3 26.9 25.5 23.9Supported streams 25.0 48.0 54.0 60.0 60.0 63.0Request collided 8.3 4.1 1.3 0.4 0.1 0.1Request successful 11.8 8.5 4.6 2.6 1.3 0.9MAC&PHY overhead 34.5 26.2 28.4 23.9 24.1 23.0Bandwidth unused 10.9 9.9 9.5 10.2 11.1 11.55-35