On the Fidelity of 802.11 Packet Traces

Packet traces from 802.11 wireless networks are incomplete both fundamentally, because antennas do not pick up every transmission, and practically, because the hardware and software of collection may be under provisioned. One strategy toward improving the completeness of a trace of wireless network traffic is to deploy several monitors; these are likely to capture (and miss) different packets. Merging these traces into a single, coherent view requires inferring access point (AP) and client behavior; these inferences introduce errors. In this paper, we present methods to evaluate the fidelity of merged and independent wireless network traces. We show that wireless traces contain sufficient information to measure their completeness and clock accuracy. Specifically, packet sequence numbers indicate when packets have been dropped, and AP beacon intervals help determine the accuracy of packet timestamps. We also show that trace completeness and clock accuracy can vary based on load. We apply these metrics to evaluate fidelity in two ways: (1) to visualize the completeness of different 802.11 traces, which we show with several traces available on CRAWDAD and (2) to estimate the uncertainty in the time measurements made by the individual monitors.