Internet applications are increasingly being equipped with rich multimedia data. Most e-business development tools are quite good at authoring multimedia contents, however they lack explicit support for estimating the performance of the envisioned multimedia system at design time. W-CoSim (Web-based Hardware and Software Co-Simulator) is an EJB-based simulation tool used for validating the architecture and the performance of online multimedia systems at design time. WCoSim has four components: Modeler, Translator, Engine and Scenario. Users start from Modeler to describe systems architecture by means of a UML(Unified Modeling Language) deployment diagram, and then specify hardware & software performance parameters such as the size of audio and video data, execution delay and network topology. All information specified in the Modeler are sent to the Translator, and then automatically converted to Java code. Engine and Scenario are responsible to run the Java code and produce results in the form of text and graphs.