# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 5 Simulation and Performance Analysis  
In our simulation, we used Sumo 0.19 to construct the mobility road area and map. We also considered Omnet++ version 4.6 for the network simulation. The UDP packets are generated and transmitted using IEEE 802.11p standard at the MAC layer for the RF links. For short-range VLC, a physical layer model based on the IEEE802.15.7 standard [41] is implemented using OMNET++. Transmitter parameters (Ex: optical power and transmitter model) are defined on the transmitter side and photo-diode parameters are defined on the receiver side. The parameters of the optical channel are also defined. The transmitter passes the optical information (Ex: bit rate, signal bandwidth, optical power, etc.) to the receiver. For simulation and evaluation, the number and locations of RSUs along road segments are predetermined based on expected traffic densities and ensuring full coverage of their broadcast zones with limited overlap, as described earlier. Table 2 shows the parameters that we used in our simulation. The model assumes a fixed RSU infrastructure is in place along major roads to partition the coverage area into manageable zones. While vehicles dynamically associate with the nearest RSU, the RSU positions themselves remain static for a given simulated road topology. The number of times the method changed between VLC and RF depended on the vehicle locations and movements relative to the RSU zones. Vehicles entered the network randomly and drove between zones following mobility models. On average, there were 3-8 handovers per vehicle as it drove between zones with different assigned radio technologies (VLC or RF). This was primarily due to the RSU optimizing assignments based on real-time density/interference metrics in each zone. Where direct VLC/RF links between zones were obstructed due to sparse traffic or high interference or at zones at opposite ends of the network, multi-hop routes of up to 10 hops were observed to maximize delivery probability. When a VLC link failed due to occlusion or mobility, the RF beam was realigned to the RSU within 50 ms. Figure 4 demonstrates the effectiveness of the management plan in resolving the troubled traffic conditions from the initial accident through widening congestion to the impact of strategies. The figures were intended merely as a demonstrative aid, to qualitatively illustrate the four stages of accident propagation and congestion development over time. The traffic flow on the highway was proceeding smoothly as vehicles traveled down the road. Cars were moving steadily in lanes at the posted speed limit, with adequate spacing between each vehicle. However, an unexpected collision suddenly occurred at a specific location, as marked by the red circle in Figure 3(a). Figure 3(a) provides a visual depiction of the sudden collision that disrupted the traffic stream at this point on the road. It shows an aerial view of the highway. Within the red circle, several vehicles can be seen merging together, indicating they have impacted each other. The collision seemed to come as a surprise, as cars behind the point of impact continued flowing smoothly without any signs of braking or swerving maneuvers. Leading up to the red circle, there did not appear to be any irregular driving behaviors that might have forewarned of an imminent accident. Figure 3(b) depicts the immediate aftermath of the collision and illustrates how traffic conditions began to deteriorate in the region following this accident. The highway stretch has been represented using a more abstract model, with individual vehicles replaced by nodes shown as green circles. These nodes reflect clusters of vehicles gradually forming on the road as congestion emerges. The tight pack of nodes at the site of the original collision suggests vehicles have bottle-necked here after the accident disrupted the flow. The initial accident served as a perturbation that caused traffic to transform from free-flow to an increasingly jammed flow.