In this paper, QoS characteristics such as TCP throughput is investigated for densely deployed mobile wireless LANs (WLANs). Factors affecting throughput characteristics are discussed and evaluated by using real machines such as smartphones and portable APs. In IEEE 802.11 WLANs, a rate adaptation mechanism controls the transmission rate and one of the dominant factors for QoS. In order to understand the behavior of the rate adaptation control, 1 to 18 sets mobile WLANs are examined under different parameters. Since a behavior of the rate adaptation control is a vender specific one and it strongly depends on interference, signal strength and etc., the real terminals such as smartphones were used in the experiments. Performance anomaly drastically reduces the throughput not only in the WLAN which has a terminal with low transmission rate but also in the neighboring WLANs that share the same channel. In order to avoid unnecessary transmission rate degradation by the rate adaptation control, Context Aware multi Rate Control (CARC) is proposed and evaluated. In CARC, Turning the rate adaptation control on/off is controlled according to a context, for example, signal strength. The evaluation results show that the CARC can be cost-effectively implemented and improves the throughput performance of whole WLANs by 3.5 times than that without the application of CARC.