The objective is to compose an overall understanding of performance characteristics of the system, using simple techniques for the  whole system where possible and refining only those portions necessary. The model should be based on a relatively small number of parameters and volumetrics. Note that there is a parallel activity for building and refining detailed component models as required. Modeling and sizing activity is generally iterative. The estimates and models will almost certainly need revising as more detail and/or changed input information becomes available as the development of a system continues. Further refinement of details and changes to requirements, as well as the need to test different solution scenarios, may require changes to the various models. This in turn will require further iterations to adjust, exercise and calibrate the models.