# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 2 Related Work  
While some studies have proposed hybrid approaches, limitations remain in addressing the key requirements for practical emergency response applications. Research on emergency message dissemination in vehicular networks has focused on three main approaches: flooding-based protocols, clustering techniques, and position-based methods. Flooding-based protocols aim to quickly propagate alerts, such as an urban multi-hop broadcast protocol (UMBP) proposed in 2015 by [22] to disseminate emergency messages that quickly selects remote neighboring nodes to reduce dissemination delays. However, it does not consider dynamic network conditions which can impact delays and reliability in dense environments. In 2017, [23] introduced a protocol relying on message contents for broadcasting. Some potential issues are that restricting broadcasts based on static position fields may fail to promptly propagate alerts through new or changing network topologies. Reliance only on received messages limits context awareness of local channel load and mem ber positions over time. Performance could degrade under high congestion or disruption without coordination between nodes. Furthermore, in 2022, [24] proposed an optimized Ad-hoc Multi-hop Broadcast protocol, aiming to reduce traffic while shortening times and improving reliability. However, performance may degrade under high congestion without coordination. Reliability and scalability need examination under varying conditions. Clustering techniques organize vehicles to curb flooding. In 2018, [25] proposed a Cluster-based Recursive Broadcast (CRB) approach which forms temporary clusters to limit broadcasts to cluster heads. One issue is that dynamically forming and maintaining up-to-date clusters comes with significant messaging overhead, especially under low-density conditions where membership changes frequently. Relying on a single cluster head (CH) also reduces resilience--if that node becomes disconnected due to mobility, it could disrupt message propagation within the affected cluster. Additionally, selecting the next CH based solely on distance may not always choose the optimum node to maximize coverage throughout the affected vehicular network. However, dynamically maintaining clusters incurs heavy overhead, especially under low densities. In 2021, [26] proposed a connected dominating set-based forwarding (CDSF) approach for traffic management in VANETs which identifies congested routes. However, CDSF neglects non-linear mobility patterns and does not fully address reliability issues or responsiveness required for practical traffic management in complex urban driving scenarios over a long duration. Position-based methods leverage location metadata. In 2017, [23] introduced a protocol relying on message contents for rebroadcasting. However, relying only on static position fields may fail to promptly propagate alerts through changing topologies. Furthermore, [27] proposed the Velocity and Position-based broadcast suppression protocol to minimize storms through source vehicle metadata, avoiding periodic exchanges. Receivers calculate forwarding probability based on distance and speed. However, only considering distance/speed may lead to incorrect assessments over time. Performance could degrade under high loads without coordination. VP-CAST does not fully address reliability, coordination, and responsiveness to varying environments. In 2023, [28] proposed ASPBT for DSRC at intersections which divides areas into partitions based on density. However, details on beacon retransmissions are lacking. Additionally, in 2023, [29] proposed a geographic routing strategy called Trust-Based Geographical Routing (TBGR) to improve the reliability of emergency message delivery in vehicular networks. While TBGR improves performance, limitations include not factoring interference into link quality calculations and only simulating one attack type. While these studies made progress, issues remained. Flooding schemes lack reliability under congestion without coordination.