Cellular networks are facing a data explosion posed by the increasing bandwidth demand of current mobile applications, and cellular operators are trying to leverage auxiliary networks and offload mobile data for relieving this challenge. However, traffic offloading without comprehensive controlling may result poor network utilization and undesirable user experience. In this thesis, we design and implement an integrated architecture for intelligent traffic offloading over collaborative WiFi-cellular networks. Motivated by our measurement, we formulate a mathematical model to estimate and evaluate potential offloading throughput based on various wireless context information, like AP signal strength and bandwidth. To efficiently manage traffic and collect information, we use a centralized SDN architecture in our design. The proposed system enables mobile devices to choose the most beneficial AP for offloading. The experimental evaluation of our prototype implementation demonstrates that this architecture can achieve optimal traffic offloading by considering different real factors instead of making naive decisions. This effort not only explores the feasibility of context-based traffic offloading, but also provides guidelines for designing and implementing a centralized SDN platform for wireless networks.