With the commence of the IMT-Advanced (IMT-A) submission and evaluation process, the vague term "4th generation wireless networks" moves towards existing standards, technologies and hardware. A significant part of the evaluation process is based on the system level simulation of reference scenarios.In this paper, we present how the open Wireless Network Simulator (openWNS), developed in the last 5 years at the department of Communication Networks (ComNets) at RWTH Aachen University, can be used to study protocols of wireless networks. Due to the complexity of IMT-A candidate systems, the features of openWNS are described by means of the lightweight WiFiMAC. This module provides the functions of IEEE 802.11, including amendment n.On this basis, the following details of the simulator are explained: (a) the modular simulation framework for protocol stack development, (b) the WiFiMAC module, (c) simulator calibration to ensure the reliability of the results and (d) the simulation of urban-micro scenarios based on the IMT-A evaluation guidelines.