The fast growing complexity of today's real time embedded systems necessitates new design methods and tools to face the problems of design, analysis, integration and validation of complex systems. We present a system level design method for embedded real-time systems combining the informal strengths of UML with the formal strengths of SDL. We demonstrate our flow by the design example of a telecommunications application from the wireless or access domain, showing the applicability of the flow to control and data-dominated types of systems. Finally we will show how the application results and other end-user needs and requirements influenced the current UML 2.0 proposal with support for real-time and embedded systems.