Abstract: Due to the increasing complexity of the embedded systems, the gap between a product idea and a working system expands rapidly. To manage the complexity and shorten the design cycles, this article puts forward a method of system-level design, which focuses on the development of executable models which allow qualitative and quantitative properties to be analyzed before the system is actually realized in terms of hardware and software components. System-level design methods should be based on well-defined modeling languages to support creating executable models that properly represent the system to be designed. The parallel objectriented specification language (POOSL) is a system-level modeling language for complex embedded systems. It has been demonstrated to be very effective for modeling systems such as internet router and network processor. A system-level model for SWP is built by using POOSL. Through the simulation of the model, results of the quantitative performance analysis on the influence of the parameters such as the window size, the packet size and the timeout period are given. Furthermore, some key parameters can be determined during the earliest phases of the design process. The system-level method can assess the schemes in the earliest phases of system design.