Abstract
In this work, we compare the performance of a passive eavesdropper in 802.11b/n/ac WLAN networks. In particular, we investigate the downlink of 802.11 networks in infrastructure mode (e. g. from an access point to a terminal) using Commercial-Of-The-Shelf (COTS) devices. Recent 802.11n/ac amendments introduced several physical and link layer features, such as MIMO, spatial diversity, and frame aggregation, to increase the throughput and the capacity of the channel. Several information theoretical studies state that some of those 802.11n/ac features (e. g. beamforming) should provide a degradation of performance for a passive eavesdropper. However, the real impact of those features has not yet been analyzed in a practical context and experimentally evaluated. We present a theoretical discussion and a statistical analysis (using path loss models) to estimate the effects of such features on a passive eavesdropper in 802.11n/ac, using 802.11b as a baseline. We use Signal-to-Noise-Ratio (SNR) and Packet-Error-Rate (PER) as our main metrics. We compute lower and upper bounds for the expected SNR difference between 802.11b and 802.11n/ac using high-level wireless channel characteristics. We show that the PER in 802.11n/ac increases up to 98% (compared to 802.11b) at a distance of 20 meters between the sender and the eavesdropper. To obtain a PER of 0.5 in 802.11n/ac, the attacker’s maximal distance is reduced by up to 129.5 m compared to 802.11b. We perform an extensive set of experiments, using COTS devices in an indoor office environment, to verify our theoretical estimations. The experimental results validate our predicted effects and show that every amendment add extra resiliency against passive COTS eavesdropping.