Microsimulation is increasingly being used for different types of traffic studies and particularly for assessment of ITS applications. This paper explains how a large urban network, the case of Lausanne, has been calibrated with the microsimulator AIMSUN, in order to find results similar to real counting. The different parameters which can be tuned for achieving this goal are described. Vehicles behaviours, network modelling and traffic assignment are tackled. In the former, dynamic traffic assignment and route choice are particularly analysed.