In this contribution, Frequency Based Substructuring is applied to a well-known benchmark structure with special attention to correct modelling of the interface problem. Various approaches with the Virtual Point Transformation are used to incorporate the effect of interface flexibility that is present in the clamped connections. An extension of the rigid displacement basis with flexible interface displacement modes is proposed and evaluated. Furthermore, a practical approach using a Transmission Simulator is used in order to improve the quality of the interface description.