American Megatrends ManageTrends 2.7, User Manual

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Appendix J : WDS and Deduplication in StorTrends iTX 305
Other factors that exert an effect on throughput are the latency and window sizes employed by the
TCP protocol. The end-to-end acknowledgment model of TCP entails an inverse dependency
between throughput and latency. This dependency, by extension, superimposes an inverse
relationship envelope over the achievable throughput, as can be seen from the figure below.
Figure 2: StorTrends Transfer Profile vs. TCP Transfer Profile
Based on the data above, it is clear that for short distances, where latencies are in the sub-ms range
(for example, in LAN environments), the throughput is dictated entirely by the link bandwidth.
However, the situation is quite the opposite as the latencies increase. Generally speaking, as
bandwidth speed increases, the effect of latency becomes more pronounced. For example, a 1GB
data transfer via satellites using 45mbps bandwidth could take more than 24 hours, as opposed to
just the few minutes it would have taken if the latencies were minimal. Due to the nature of the
TCP protocol, replication over long distances is essentially bottlenecked by latencies, and higher
bandwidth transports do very little to help. The result of this bottleneck effect is a gross under-
utilization of available bandwidth.