## 6 Performance Evaluation
### Results Analysis  
The performance of the four systems in terms of RRTT in different traffic densities is presented in Figure 5. We can observe that DIFTOS has the shortest RRTT, especially when the traffic density increases, that is due to the centralized nature of SAINT and CD (partially centralized in case of DIVERT). In CD and SAINT, the average RRTT time increases exponentially with the increase of TD, which is because the centralized server gets busy when more RReq arrive simultaneously. Which is not the case with DIFTOS, because some of the RReq are treated by lower level servers and do not need to be forwarded to the high-level server. Thus, DIFTOS sustains its stability in terms of RRTT, even when the TD is very high (see Figure 5). DIVERT also maintains a relatively short RRTT compared to SAINT and CD, but longer RRTT compared to DIFTOS. That is because the vehicular servers are not organized as a hierarchy like DIFTOS, therefore local RReq need to travel long distances. Figure 6 presents the performance of the four systems in terms of CC in different traffic densities. CD has the least CC among the four systems, that is because CD does not maintain the reservation matrix as the other three systems, it only calculates the optimal path for each vehicle regardless of the traffic conditions. SAINT and DIVERT have similar CC in different densities, and their CC is higher than CD's; this is because they maintain a road reservation matrix, which needs regular updates that require more computation. From this shallow point of view, one could argue that CD, SAINT and DIVERT perform better than DIFTOS in terms of CC, but the calculation in these systems is done in the centralized server (partial at vehicles in DIVERT), while in DIFTOS the computation is distributed among the VVS; thus, the values of CC shown in Figure 6 are the sum of all VVS in case of DIFTOS. Figure 5: Traffic density vs request round trip time. The effectiveness of DIFTOS in terms of travel time is obvious in Figure 7, where DIFTOS achieves the shortest average travel time. DIFTOS could maintain a reasonable TT, even under high traffic density. That is due to the re-routing strategy that allows it to determine the expected congested roads, and re-route vehicles away from these roads before they arrive. DIVERT also achieved a relatively short average TT for the same reason mentioned above. SAINT's average TT is considerably longer than DIFTOS and DIVERT's. However, CD had the longest average TT; this is because it does not consider the road reservations when computing the optimal paths. Figure 6: Traffic density vs. computational cost. Figure 7: Traffic density vs. travel time. The traffic related communication overhead of the four systems is presented in Figure 8. CD had the least amount TM, which is due to its static nature; once the optimal path is determined, the vehicles do not send any update to the server. SAINT, DIVERT and DIFTOS send much more TM compared to CD, which is because they all maintain the road reservation matrix, thus the vehicles in these systems need to send a route update about their position to the server to update the road reservation matrix. DIFTOS's communication overhead is slightly higher than SAINT and DIVERT's, which is because the servers in DIFTOS are designed in a hierarchical manner, and the low level servers need to update their higher servers regularly. The effect of accidents on the computational cost and average travel time is presented in Figures 9 and 10. The number of accidents is increased each time to evaluate the robustness of the four systems against traffic accidents, and their ability to mitigate the effects of such accidents. CD had the best resistance against accidents in term of CC. The reason for this is that in case of congestion due to an accident, the vehicles follow the same congested paths, and do not request the server for a new path, thus no additional computation is required. However, the CD's average TT is the longest among the four systems, due to the aforementioned reason.