Currently, there are many research studies which focus on analyzing the performance of Wireless Sensor Networks (WSNs) in several fields such as MAC protocols, routing protocols. In this paper we present an analytical model to evaluate the performance of WSNs based on the RSSI in the application layer. This model utilizes the available RSSI in most popular sensor nodes for analysis and considers the impact of several practical parameters such as data rate, routing information or packet acknowledgements. The use of empirical RSSI as the input can help the model to precisely reflect the real performance of deployed sensor networks. In order to carry out the validation of the model, a live WSN is setup to get the real data to input the proposed analytical model and the simulator for comparison. The results from real test-bed, simulation and analysis show that the proposed analytical model is precise enough and can be applied in estimating performance of WSNs.