Early phases of large projects require responsive guidance from simulation. As such, the simulation response time must be short, while still maintaining validity. Typically, to get development times down, only portions of the system are simulated, and the implications, along with empirically derived results for other portions, are woven together to provide a system level result. This weaving requires a framework so that the result are traceable to their constituents, a necessary condition for the decision makers to consider them valid. We will discuss the structure of the framework that we've developed to support broadband telecommunications network engineering.