In order to cope with the dramatic growth of cellular data traffic, mobile operators should find cost-effective and easily deployable solutions of capacity increase of their networks. One of these solutions is to use WiFi networks to offload the cellular network. One question when using WiFi networks beside cellular is how they are integrated. Several degrees of integration exist, one of them is to connect the WiFi Access Points (APs) directly to the cellular Base Stations (BSs). This is known as very tight coupling. In this paper, we study the performance of such an architecture. We define a model that takes into account the fluctuation of the coverage (shadowing) and compute how much bandwidth can be saved on the cellular network.