Many promising congestion control protocols use explicit feedback from the network to achieve high performance. These protocols often use congestion signals whose computation requires an estimate of link capacity. Such estimates are not available in networks where capacity varies over time. This paper studies the impact of inaccurate capacity estimates on the performance of congestion control protocols over variable capacity links. As a case study, we focus on 802.11 WLANs. We show that such estimates can lead to either under-utilization or unfairness and network overload. Using a model, we characterize the available capacity of a node in a 802.11 WLAN and then study a method for capacity estimation. Using simulations, we show that the method leads to high utilization and fairness over shared, multi-access networks.