We propose a process calculus to model distributed wireless networks. The calculus focuses on high-level behaviour, emphasising local broadcast communication and probabilistic behaviour.Our formulation of such systems emphasises their interfaces, through which their behaviour can be observed and tested, although this complicates their contextual analysis. Nevertheless we propose a novel operator with which networks can be decomposed into components. Using this operator we define probabilistic generalisations of the well-known may-testing and must-testing preorders.We define an extensional probabilistic labelled transition system in which actions represent particular interactions networks support via their interfaces. We show that novel variations on probabilistic simulations support compositional reasoning for these networks which are sound with respect to the testing preorders. Finally, and rather surprisingly, we show that these simulations turn out not to be complete.