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Broadcast Link Quality Measurements in 802.11 Networks

Broadcast probes are typically used in Wireless LAN networks to assess the quality of the radio link and/or to runtime derive metrics to be used by higher layer protocols (e.g., routing in WLAN-based ad hoc or mesh networks). This work raises the concern that proprietary vendor operation may severely affect broadcast-based measurements. More precisely, we present and analyze experimental outdoor link quality measurement results obtained with a device/driver pair (Atheros card + MADWiFi driver) widely exploited in the research community. We show that an unexpected periodic transmission power reduction mechanism at the NIC level, undocumented by the device vendor and apparently hard to be disabled, may dramatically impair the measured link quality performance. Our experimental evidence shows that links which would otherwise experience good quality are subject to a significant frame loss ratio, when tested through a broadcast frame transmission methodology. This frame loss ratio is artificially induced by the NIC card operation on broadcast frames: unicast frames are in fact shown to be exempt from such a problem.