We study the problem of handoffs in heterogeneous (both wired and wireless) networks. We first present a testbed that integrates multiple network technologies (Ethernet LAN, WiFi and GPRS cellular data network) to provide seamless connectivity to mobile hosts. We then propose a model to analyze the performance of vertical handoffs as well as experimental measures to validate the model. Finally, we discuss the advantages of handoff detection and triggering through link layer mechanisms. An implementation of handoffs’ link layer triggering is presented, and its performance is compared with layer-3 handoff triggering.