We propose a MAC layer handoff mechanism for IEEE 802.11 WLAN to give benefit to bandwidth-greedy applications at STAs. In order to find an optimal AP, we investigate a new measurement method called TFC (Transient Frame Capture) to estimate the achievable throughput from APs. Since TFC is employed under promiscuous mode, it avoids service degradation through the current associated AP. In addition, the mechanism is a client-only solution which does not require any modification on APs. We develop an analytic model for the throughput estimation and demonstrate it through experimental studies.