Vertical handoff is the basic requirement for convergence of different access technologies and has received tremendous attention from the academia and industry. In recent years, many research works have focused on vertical handoff decision algorithms. However, the evaluation scenarios in different papers are quite different. Thus it is difficult to compare different algorithms in a fair way. This paper analyzes the general motion models of mobile nodes and identifies a set of novel performance evaluation models. Equipped with the models, this paper analyzes the performance of two types of decision algorithms: Hysteresis based algorithm and dwelling-timer based algorithm. Based on the above, a novel vertical handoff decision algorithm, self-adaptive vertical handoff algorithm (SAVA) is presented. Simulations show that SAVA can achieve better performance compared with the conventional methods.