  This paper discusses the latest generation of the MONARC (MOdels of Networked
Analysis at Regional Centers) simulation framework, as a design and modeling
tool for large scale distributed systems applied to HEP experiments. The
simulation of Grid architectures has a vital importance in the future
deployment of Grid systems for providing the users an appropriate feed-back. We
present here an example of simulating complex data processing systems and the
way the framework is used to optimize the overall Grid architecture and/or the
policies that govern the Grid's use.
