Statistical analysis of RTT variability in GPRS and UMTS networks

Abstract: We study the data from a passive traffic measurement representing a 30 hour IP traffic trace from a Finnish operator’s GPRS/UMTS network. Of specific interest is the variability of the Round Trip Times (RTTs) of the TCP flows in the network. The RTTs are analysed at both microand macroscopic level. The microscopic level involves detailed analysis of individual flows and we are able to infer various issues affecting the RTT process of a particular flow, such as changes in the rate of the radio channel and the impact of simultaneous TCP flows at the mobile device. Also, Lomb periodograms are used to detect periodic behavior in the applications. In the macroscopic analysis we search for dominating RTT values first by Energy Function Plots and then by exploring spikes of empirical PDFs of aggrgate RTTs. Comparison of empirical CDFs show significant shift in quantiles after considerable traffic increase which is found to be due to bandwidth sharing at individual mobile devices. Finally, we investigate further the impact of bandwidth sharing at the mobile device and show how the increased amount of simultaneous TCP connections seriously affects the RTTs observed by a given flow, both in GPRS and in UMTS.