We present a novel approach to refine a system model specified with perfectly synchronous communication onto a network-on-chip (NoC) best-effort communication service. It is a top-down procedure with three steps, namely, channel refinement, process refinement, and communication mapping. In channel refinement, synchronous channels are replaced with stochastic channels abstracting the best-effort service. In process refinement, processes are refined in terms of interfaces and synchronization properties. Particularly, we use synchronizers to maintain local synchronization of processes and thus achieve synchronization consistency, which is a key requirement while mapping a synchronous model onto an asynchronous architecture. Within communication mapping, the refined processes and channels are mapped to an NoC architecture. Adopting the Nostrum NoC platform as target architecture, we use a digital equalizer as a tutorial example to illustrate the feasibility of our concepts.