In recent years mobile computing has experienced an explosive growth, mainly due to the integration of wireless networks with the wired Internet. To deploy bandwidth-greedy multimedia applications on such heterogeneous environments, efficient congestion control algorithms are required. When congestion control protocols such as TFRC are used to transmit multimedia flows, they need to differentiate between congestion losses and wireless losses to behave correctly and efficiently. Currently proposed end-to-end loss differentiation mechanisms can not reliably predict the differences between congestion and wireless losses. In this paper, we explore explicit loss differentiation. We have designed a simple window framework to explicitly and accurately differentiate these losses. By deploying agents at the boundaries of wireless links, we show that we can efficiently implement our window-based scheme with low overhead. We also point out current limitations of our scheme.