In this paper, we describe the experiment of a CoDesign process to develop a communication system which needs to correctly mix hardware and software parts in order to satisfy the various required performances. The system design process is based on the MCSE methodology and we show its usefulness for CoDesign. CoDesign is shown as an enhancement of the implementation specification step of MCSE. System partitioning is the result of an interactive procedure based on performance and cost evaluations. The complete description of the implementation is obtained by transformations of the functional description: C or C++ for the software, VHDL for the hardware. The links between hardware and software are also synthesized. Such a procedure and associated tools aim at obtaining system prototypes in an efficient and incremental manner. The described example illustrates the benefit of the proposed method, the significance of the functional level and the specific part of a whole system for which CoDesign is appropriate.