Recently, there is an increasing interest in the deployment of femto access points (FAPs), which are short-range low-power home base-stations, over a macro cellular network to improve the total network capacity. Due to the random deployment of FAPs, an analytical understanding of such two-tier networks via stochastic geometry is essential for network operators to fully exploit the potential of femtocells. In this paper, we propose a two-tier network model that captures the geometries of macrocells and femtocells realistically. The success probabilities in spectrum sharing and unsharing for both tiers in the uplink and downlink are derived. Finally, the downlink performance of such two-tier network is discussed numerically.