Developing a functional prototype of a system-on-chip provides a unifying vehicle for model validation and system refinement. Keeping the prototype executable across several abstraction levels, clock domains and design tools is a key requirement to effective prototyping. This paper presents how model-level transactors address design heterogeneity by unifying event-based and cycle-based worlds from specification to implementation. Transactors are used to build a functional prototype of a software-radio component. An executable UML model is bridged to a hardware abstraction of a radio stream developed with Simulink to implement a realistic and working prototype. Model validation and performance measurements are realized through prototype execution and real-time monitoring.