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The exponential increase in the deployment of IEEE 802.11-based wireless LAN (WLAN) technology has transformed it into an essential building block of the networking infrastructure of commercial enterprises. How to effectively manage these WLAN networks and stop malicious intruders are the two main barriers for further spread of this technology. At the same time, the enormous economies of scale behind IEEE 802.11-based WLAN products also result in rapid technological advances and astounding pricing efficiencies. Today, the street price of a low-end 802.11g access point is as low as $50 USD, with an estimated manufacturing cost of less than $25. In this paper, we describe the design, implementation, and evaluation of a scalable wireless LAN traffic monitoring system called wireless network sentry (Wintry), which provides real-time visibility of the usage of an enterprise's wireless links in a way that is completely transparent to the monitored WLAN networks. Moreover, Wintry is able to leverage low-cost programmable WLAN access points and reduce the total hardware cost to the minimum. A key feature of Wintry is an accurate radio channel busy time estimation algorithm that correctly takes into account both back-off delay and corrupted packets in WLAN traffic load computation without requiring any modification to monitored WLAN devices.