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Handoff procedure in IEEE 802.11 wireless networks must be accomplished with as little interruption as possible to maintain the required quality of service (QoS). We have developed a fast handoff scheme, called AuthScan, to provide a novel usage of the open system authentication phase to reduce channel scanning latency. AuthScan comprises two steps: firstly, a client caches its handoff history with beacon information. Secondly, when in need of handoff, a client transmits authentication request frames to the selected access points (APs) from the cache instead of broadcasting probe request frames as in active scan to discover the available APs. The next AP is selected by comparing quickly acquired handoff metrics during the handoff procedure. This fast handoff is possible by upgrading only software on the client side. This paper presents the theoretical handoff latency of AuthScan in comparison with other approaches, and shows the effectiveness of our system through implementation and experiments.