This paper proposes an open hierarchical network architecture for the Intet of Things (IoT), which can provide a unified network topology by using heterogeneous Wireless Sensor Networks (WSNs). With this proposed architecture, our research focuses on the optimal deployment strategy of the nodes on the convergence level. We aim at the maximization of the sub-networks lifetime while minimizing the deployment cost. Meanwhile, a novel metric named as the Ratio of lifetime to Cost (RLC) is proposed to estimate the efficiency of convergence nodes deployment. Simulation results indicate that the proposed deployment algorithm can a-chieve the optimal number of convergence nodes. The proposed deployment strategy is able to a-chieve a balanced tradeoff between the network lifetime and the deployment cost.