This paper discusses a case study, the co-design of an ATM Network Interface Card (NIC). The NIC is aimed to interface applications with the physical network line. It is composed of a stack of four protocol layers: TCP, IP, AAL and ATM. In this study, the initial specification is given in a language called SDL. The architecture exploration is made using Cosmos, a co-design tool for multiprocessor architecture. Several architectures are produced starting from the same initial SDL specification. The performance evaluation of these solutions was made using CNHDL co-simulation. This paper describes the experiment and the lessons learned about the capabilities and the restrictions of Cosmos and SDL. The use of SDL allows for drastic reduction of the model size when compared to CNHDL model. SDL simulation may be 100 times faster than CNHDL simulation. SDL provides powerful capabilities for systemlevel specification, but lacks facilities for the expression of DSP oriented computation.