As sensor networks are finding widespread use across many applications, designers increasingly must not only focus on application development, but also on sensor network optimizations. Given the complexities of sensor networks and the difficulty of analyzing the long-term effects of design changes within a deployed system, simulation is often the only feasible option for evaluating such optimizations. The Arizona Transaction-Level Simulator for Sensor Networks (ATLeS-SN) is a transaction-level modeling based sensor network simulation environment emphasizing modular design for modeling various components within sensor nodes and across the sensor network. We provide an overview of our proposed ATLeS-SN simulation framework and highlight the benefits of this framework for a building monitor application and a forest fire detection and propagation tracking application.