This paper presents a novel mobility modeling approach for a two-tier integrated wireless system that accommodates the system complexity represented by the residencetime correlation between different access networks. Additionally, a novel session model is presented as an adapted version of the proposed mobility model. Furthermore, we develop an analytical framework using this session model to obtain several salient performance metrics such as network utilization times and handoff rates. Simulation results demonstrate that the proposed mobility model is substantially more accurate than existing modeling techniques, and that the proposed analytical framework provide tractable performance evaluation based on the new mobility model.