## 4 Simulations and Results
### Simulation Results
Nevertheless, there are discrepancies in the percentage of packet losses and in the average end-to-end delay (see Rows 2, 5 of Table 4a). Notice that there is a difference in the average number of neighbors for a density of 67 vehicles/km\({}^{2}\). Similar results are obtained from Tarragona scenario (See Table 4b), but in this case, there are only significant differences in the average end-to-end delay and the average number of hops. For both scenarios, the median of the pre-computed attenuation performance results are not so far from the medians in the realistic scenario. The presence of differences between the aforementioned models is a consequence of the discretization process done in the pre-computed attenuation approach. Lastly, total blockage and pre-computed building attenuation models are compared in order to get an idea of the existing differences between these approaches and the realistic channel model. The reader can observe from Table 4a in the case of the Barcelona scenario and Table 4b for Tarragona that there are statistical differences in the average number of hops of a packet when traveling to reach the access point. Furthermore, differences in the average number of nodes between these two models appear only for the two highest densities applied in this work (133 and 166 nodes/km\({}^{2}\)) in the Barcelona scenario. Hence, the results obtained with these two models could be similar, at least in the percentage, to the packet losses and delay. To conclude this section, we summarize the major features and results drawn from our evaluation:  
* We have simulated three building attenuation models used in the literature for VANETs (_i.e._, realistic, total blocking of the signal and the precomputed attenuation model) under two realistic urban scenarios that have different vehicle densities, junctions and buildings. The study considered four different vehicle densities with ten different mobility traces per density. Moreover, different probability distributions for traffic generation were used in each urban scenario. * The results of statistical tests carried out with four performance metrics (percentage of packet losses, end-to-end delay, average number of hops and neighbors) show differences when employing different attenuation models. * A complete attenuation of the signal due to the presence of buildings, and pre-computed attenuation models in our simulations can be considered for both scenarios as pessimistic bounds for all performance metrics. * We did not find that the differences in the performance metrics are affected by the vehicle density employed during the simulation, except in the case of the average number of neighbors. This metric does not differ from the realistic one with intermediate vehicle density scenarios. These are promissory results, because they lead to the idea of using the total signal attenuation model or pre-computed attenuation files when a realistic attenuation model is difficult to use; for instance, for map areas where building information is not available and a complete blockage outside the street has to be assumed. Pre-computed attenuation files would be a good alternative when scenarios involve a medium or high data traffic load, especially in preliminary studies. Such scenarios require that algorithms to calculate the level of attenuation caused by buildings are executed too many times. This can cause important time consumption due to the number of operations, even when such algorithms are efficient.