The di culty of embedded system co-design is increasing rapidly due to the increasing complexity of individual parts, the variety of parts available and pressure to use multiple processors to meet performance criteria. Validation tools should contain several features in order to keep up with this trend, including the ability to dynamically change detail levels, built in protection for intellectual property, and support for gradual migration of functionality from a simulation environment to the real hardware. In this paper, we present our approach to the problem which includes a geographically distributed co-simulation framework. This framework is a system of nodes such that each can include either portions of the simulator or real hardware. In support of this, the framework includes a mechanism for maintaining consistent versions of virtual time.