Adaptive grid applications require up-to-date network resource measurements and predictions to help steer their adaptation to meet performance goals. To this end, we are interested in monitoring the available bandwidth of the underlying networks in the most accurate and least obtrusive way. Bandwidth is either measured by actively injecting data probes into the network or by passively monitoring existing traffic, but there is a definite trade-off between the active approach, which is invasive, and the passive approach, which is rendered ineffective during periods of network idleness. We are developing the Wren bandwidth monitoring tool, which uses packet traces of existing application traffic to measure available bandwidth. We demonstrate that the principles supporting active bandwidth tools can be applied to passive traces of the LAN and WAN traffic generated by high-performance grid applications. We use our results to form a preliminary characterization of the application traffic required by available bandwidth techniques to produce effective measurements. Our results indicate that a low overhead, passive monitoring system supplemented with active measurements can be built to obtain a complete picture of the network's performance.