This paper describes a modular and scalable simulation environment, called GloMoSim, to evaluate end-to-end performance of integrated wired and wireless networks. GloMoSim has been implemented on sequential and parallel computers and can be used to simulate networks with a large number of nodes. It is designed as a set of library modules, each of which simulates protocols within a specific layer of the communication stack. Common APIs have been specified between neighboring layers on the protocol stacks. The modularity facilitates the study of the interaction between layers as well as the evaluation and comparison of different layers. The parallel structure of the simulator enables the scaling to large network sites without compromising the accuracy. Two sets of experiments (parallel scaling; TCP and MAC layer interaction) illustrate the features of the GloMoSim platform.