We first evaluate the relationship between the percentage of gateway vehicles and the packet delivery ratio for a scenario of 4km highway with four lanes and double directions. The results are shown in Fig. 9. We can see that when more vehicles play the role as mobile gateways, the packet delivery ratio increases significantly. Even in low density scenario (20 vehicles/km), the MGRP can achieve 80% packet delivery ratio if the percentage of the gateway vehicles is over 60%. When the percentage of gateway vehicles is 10%, the packet delivery ratio is down to 38%. Besides, the MGRP needs at least 70% gateway vehicles to reach the 100% delivery ratio in the low density scenario. On the other hand, the result shows that it can perform better in middle (30 vehicles/km) and high density scenarios (40 vehicles/km). In these cases, the MGRP needs just 30% of gateway vehicles to achieve nearly 100% delivery ratio.
That is, if there are ten vehicles in the highway scenario, using our protocol just needs three vehicles to equip the OBU devices. Therefore, it does not need too much cost in this network architecture.
Figure 9: The percentage of gateway vehicles vs. packet delivery ratio
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Now, we compare the performance of MGRP and GPSR Fig. 10 shows the packet delivery ratio versa the maximum speed. We can see that although the packet delivery ratios of both protocols decrease as long as the velocity of vehicles rises, our protocol still perform better, because the MGRP utilizes the 3G network and gateway controller to assist packet forwarding so that link disconnect due to high mobility can be greatly avoided.
Figure 10: Packet delivery ratio vs. maximum node speed
Fig. 11 shows the average hop count to the maximum speed. The results reveal that the average hop count increases when the velocity of vehicles rises regardless of MGRP or GPSR. The MGRP can keep count within 5 while the maximum hop count in GPSR is 8. It is because of the fact that we use 3G network to reduce the transmission hops. In addition, we limit the hop counts when the source vehicle intends to find a route to the gateway vehicle or destination vehicle.
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Packet Delivery Ratio (%)
Maximum speed (km/hr)
MGRP GPSR
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Figure 11: Average hop count vs. maximum node speed
Fig. 12 shows the routing overhead versa the maximum speed. The results show that the packet overhead increases when the velocity of vehicles rises regardless of MGRP or GPSR. The packet overhead in MGRP is more than GPSR because we need to maintain the routing table. However, by establishing the routing table we can avoid the local maximum problem in GPSR. Furthermore, our method can decrease the total hop count to the destination node.
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Figure 12: Routing overhead vs. maximum node speed
Figure 13: Packet delivery ratio and overhead vs. hop count
Next, we compare the performance with our routing protocol MGRP and the routing method which utilizes the fixed RSUs to replace the mobile gateway vehicles.
The RSUs are placed on intersections of the streets in Fig. 8. The number of intersections in the map is 51, that is, there are 51 RSUs setting in this example. And there are 150 vehicles equipped IEEE 802.11 interface. The packet delivery ratio,
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routing overhead and average hop count are compared between MGRP and the routing protocol with fixed RSU.
Figure 14: Packet delivery ratio vs. maximum node speed with fixed RSU and mobile gateway vehicle
Fig. 14 shows the packet delivery ratio versa the maximum speed. The packet delivery ratios of both protocols decrease as the speed of vehicles increases. Our protocol MGRP is better than the routing protocol with fixed RSU in terms of packet delivery ratio. Because the gateway vehicles in MGRP can move with the same direction with the other vehicles, it may increases the link lifetime.
Fig. 15 shows the routing overhead versa the maximum speed with fixed RSU and mobile gateway vehicle. The results show that the packet overhead increases when the velocity of vehicles rises regardless of MGRP or using fixed RSU. Our protocol MGRP is better than the routing protocol with fixed RSU in terms of routing overhead because mobile gateway may turn to other direction unexpected in some situations. It may cause the control messages increasing when finding another route path.
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Packet Delivery Ratio (%)
Maximum speed (km/hr)
MGRP FixRSU
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Figure 15: Routing overhead vs. maximum node speed with fixed RSU and mobile gateway vehicle
Figure 16: Average hop count vs. maximum node speed with fixed RSU and mobile gateway vehicle
Fig. 16 shows the average hop count to the maximum speed with fixed RSU and mobile gateway vehicle. The results reveal that the average hop count increases when the velocity of vehicles rises regardless of MGRP or the method of using fixed RSU.
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The result shows the hop count is similar regardless MGRP or the method of using fixed RSU. The reason is that both of those methods use 3G devices forwarding packets, so it can decrease the total hop counts.
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Chapter 5 Conclusions
In this paper, the position-based routing protocol, called mobile gateways routing protocol, (MGRP) has been proposed for vehicular ad hoc networks in city environment. We utilize a part of vehicles as mobile gateway vehicles equipped with the OBU which can forward the data packets through interfaces of 3G or IEEE 802.11.
Other vehicles without 3G interface can forward the packets through wireless network to mobile gateway vehicles, then using 3G interface to forward packets to the gateway controller. Finally, the gateway controller will forward the packets via mobile gateway vehicles nearby the destination vehicle. We design the routing protocol suitable for this hybrid network architecture and it decreases the total hop counts and the probability of links disconnection obviously. The simulation results show that MGRP performs better than the traditional position-based routing protocol GPSR.
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