## 1 Introduction  
Applications in road safety have encouraged study and research in wireless vehicular communications, both in the industry and in the research community. Vehicular _ad hoc_ networks (VANETs) [1] are seen as a special case of mobile _ad hoc_ networks (MANETs), where nodes are vehicles. Nevertheless, VANETs face particular challenges compared to MANETs, such as faster topology changes, a lower link lifetime or a potentially greater number of nodes taking part in the network, among others. In VANETs, vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communications are both possible.  
Deploying VANET testbeds is quite expensive, and it is not a feasible solution in large-scale scenarios, which may require the deployment of hundreds of vehicles. Due to this fact, network simulation is widely used for investigation in this field. Network simulators are useful and powerful tools to test a broad spectrum of proposals before their implementation. Hence, network simulation is becoming the first step in the process of developing new VANET protocols or services. Complex VANET scenarios with several nodes can be easily managed by simulators, and realistic simulation scenarios are critical to obtain reliable results. A realistic simulation environment requires a node mobility model that guarantees an appropriate distribution of vehicles and a channel model that mainly reproduces the effects of interference and attenuation in different scenarios. These channel models must capture the effects of interference and attenuation depending on the scenario. Furthermore, a realistic simulation environment of an urban area should include the effects of shadowing caused by buildings, as part of the channel model.  
In this work, we make a comparison of three channel modeling techniques, each of which includes a different process to reproduce the effects produced by buildings in the VANET simulation of urban scenarios. These techniques model the attenuation caused by buildings in three ways: an empirical computation of the attenuation produced by buildings, the complete absence of a signal due to buildings and a pre-computed attenuation value based on the discretization of the node position in a simulation.  
We make an analysis of the building shadowing effects in a city area according to each one of the aforementioned models, using two real scenarios with different node densities. In brief, this paper offers a thorough study of the different channel modeling techniques applied in simulation to reproduce the effects of the attenuation caused by buildings in VANETs over an urban scenario. This analysis was very useful to find the impact of such obstacles in the overall performance of the simulated network.  
The paper is organized as follows: Section 2 summarizes previous research work about attenuation models and some proposals and evaluations related to VANETs. Next, Section 3 describes the scenario that we used to test the three attenuation models mentioned above and how those models were implemented in a network simulator. Then, Section 4 is entirely devoted to the evaluation of the selected channel models and the results obtained from the tests. Finally, conclusions are drawn in Section 5.