The Cross layer concept is a new way to see the quality of service in the network. It consists in adapting the current mechanisms at one level to the underlying levels and the definition of information to share between, not necessarily adjacent, levels and the global optimisation instead of multiple optimisations. Performance optimisation of the whole system is a crucial step in the process of design and validation of new systems. Models of large, complex and dynamic systems can often be reduced to smaller sub-models, for easier analysis, by processes known as decomposition or aggregation methods. Those techniques have to be implemented in a dynamic simulation tool. It has to be dynamic because simulations are driven by dynamic data and entail the ability to incorporate additional data (either archived or collected online) and reversely the simulator will be able to dynamically steer the measurement process.