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Accurately measuring the available bandwidth information is critical for providing QoS assurance, especially for the bandwidth-limited 802.11-based wireless networks. However, the shared nature of wireless medium and IEEE 802.11 MAC pose great challenges for estimating the bandwidth accurately. This paper tends to tackle this issue. In particular, we first formally define the available bandwidth in IEEE 802.11 network by considering its unique characteristics. We then present our solution, Passive Available Bandwidth Estimation (PABE). In PABE, the effective link capacity is analyzed by considering the random factors in transmission, and the available channel idle time is estimated by passively monitoring the medium based on a new, lower threshold bandwidth obtained during the normal operations of IEEE 802.11. Our approach incurs very low cost to the network without any explicit message overhead. Through extensive simulation, our data validate that our approach consistently achieves much better performance than other existing algorithms in term of estimation accuracy.