The second pair shows that there is a correspondence between variability in rate and time of day. In this second pair of graphs, the upper graph is the same as in the first. Data points in the lower graph represent the corresponding standard deviation for rate across transfer duration, normalised against the average rate. That is, we take the standard deviation for rate samples and then divide by the average rate, thus allowing the standard deviations to be compared on the same scale. In practical terms this graph reflects the level of variability in transfer rate.
High values correspond to highly variable transfer rates, and low values to consistent rates.
There is a clear correspondence between rate, variability, and time of day. We examined standard deviation in this way for several different sites, and all showed similar correspondence, but to varying degrees. This shows that in addition to decreasing, transfer rate fluctuates more when the network is under heavy load. That is, data transfer is more 'bursty'. Furthermore, rate 'troughs' are common, even when the network is not under particularly heavy load.