In this paper, we propose a comparative study between results generated by a real 802.11 testbed in different outdoor and indoor environments and 3 usual network simulators (NS2, QualNet and OPNET). The goal of this study is to evaluate the relevance of the low layers recently implemented in these simulators. The motivation of this paper is to provide a guide to researchers to choose and parameterize a simulator according to a selected context. The study shows that the simulation results can be rather close to the experimental results. However, they are very dependent on the tuning of the physical layer parameters and the selected propagation models.