Simulation is essential in WSN study. However, the nature of WSN makes it an unexpectedly complex task. The extremely large number of nodes, the need for an environment model, and the cross-layer dependencies of the models are some of the reasons for this complexity. Many of the existing simulation tools do not properly handle these issues. In this paper, OBIWAN, a new simulator for WSN is presented. In its design, critical issues like reusability, scalability and cross-layer dependencies have been addressed. The results on its capabilities and performance are provided through a case study involving several thousand nodes and an environment model