# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 5 Simulation and Performance Analysis
Figure 6 compares the performance of three different network systems: a standalone VLC system, a hybrid RF-VLC system, and the standard AODV routing protocol [42; 43] using RF only. The standalone VLC system demonstrates a lower packet delivery ratio compared to the hybrid RF-VLC system. This is due to the line-of-sight limitations of VLC, wherein packets can only be transmitted if the transmitter and receiver have a clear visual path. In the hybrid system, initial broadcasting of packets occurs using RF channels rather than VLC. This enables a broader coverage area for initiating transmissions. Then, at the forwarding stage between intermediate nodes, VLC is used on a hop-by-hop basis wherever the visual path is available. Compared to RF-only communication which has an average range of several hundred meters, VLC has a shorter range of approximately 20-30 m. However, in dense traffic scenarios where nodes are located close together, VLC can provide good connectivity  
Figure 5: Probability Density Functions of accident congestion scenarios. between vehicles. The hybrid approach effectively leverages the benefits of both RF and VLC to optimize overall network performance. In summary, by combining initial broadcasting via RF with subsequent hop-by-hop transmissions using VLC, the hybrid RF-VLC system is able to achieve greater packet delivery reliability than a standalone VLC system hampered by line-of-sight constraints. It also outperforms the conventional AODV protocol that uses pure RF transmissions. Figure 7 compares the broadcast transmission delays between the different network approaches. Transmission delay is measured as the time taken for a packet to reach its destination, starting from the point of origination and including the forwarding stages between intermediate nodes. Transmission delay depends directly on the distance a packet must travel--the farther the distance, the longer it will take to transmit. Assuming a constant transmission speed/bitrate, we would expect a direct proportional (linear) increase in delay as distance increases. i.e., doubling the distance doubles the transmission time. Factors like node processing time, propagation delays, etc. are relatively small contributions compared to transmission time over varying distances in this range (50-2000 m). The nature of radio/wireless propagation means signal strength decays predictably with distance traveled in most conditions, following an inverse square law. This translates to transmission delay, functionally increasing in direct proportion to distance. Due to its short transmission range of 20-30 m, standalone VLC systems demonstrate significantly longer delays, because of more hops before finally reaching the destination, compared to the other options. The AODV protocol that uses only RF, performs better than pure VLC since RF provides a longer communication range between nodes (fewer hops). However, the hybrid RF-VLC system achieves the lowest transmission delays. This is performed by initiating RF broadcasts to establish a wider coverage at the first stage. However, it uses high-bandwidth VLC links at later stages. This hybrid approach reduces the overall number of forwarding hops required. This strategy leverages the strengths of both RF and VLC technologies to minimize the end-to-end delay. The combined RF-VLC system delivers packets to their destinations in the least amount of time due to efficiently exploiting the long-range and high-data capabilities of RF and VLC, respectively, across the different transmission stages. This validates the performance benefits of an integrated hybrid network architecture. Figure 6: Packet Delivery Ratio. Figure 8 directly compares the number of congested vehicles over time both with and without applying our protocol. It clearly shows that the curve with congestion management constantly remains below the curve without congestion management, at all time intervals across the entire simulation duration.