# An Emergency Message Routing Protocol for Improved Congestion Management in Hybrid RF/VLC VANETs
## 1 Introduction
### Background  
In VANETs, Road Side Units (RSUs) provide a critical roadside infrastructure for Vehicular-to-Everything (V2X) communication. Indeed, RSUs act as mini base stations and gateways, connecting the vehicular network to the rest of the world. They can also provide cloud-based services transferring data between the vehicles, other RSUs, and the cloud back end or operate as fog nodes with their own computing capabilities [10]. Moreover, modern vehicles in intelligent transportation systems (ITSs) rely on various onboard devices and sensors, and they communicate wirelessly. A main component of an ITS is the onboard unit (OBU) that serves as a router, enabling the messages to be exchanged between a vehicle and other entities, while serving as a hub for the sensors and managing various vehicular functionalities. The OBU connects to the nearest RSU, which in turn extends this connectivity to the cloud networks. Together, the OBUs and RSUs act as smart edge/fog devices providing the basic networking needed for the ITS [4]. Besides, VLC can be used between the RSU and the OBU in some cases [11]. In addition, messages in VANETs can be primarily classified into two categories: safety-related critical (SRC) messages and regular informational messages [12]. The SRC messages are related to potentially hazardous events requiring an urgent response, such as collision and accident warnings, vehicle mechanical issues, and other similar safety notifications. Due to their emergency nature, SRC messages demand low latency and high reliability in dissemination. On the other hand, regular informational messages encompass non-urgent data such as general traffic management updates, in-vehicle infotainment, and periodic beacon signals. As they do not pertain to imminent safety risks, informational messages have less stringent requirements for latency and reliability compared to safety-related critical SRC alerts [13]. Therefore, differentiating the handling of messages based on their urgency and purpose using an appropriate protocol is needed [14]. Routing protocols, originally designed for mobile ad-hoc networks (MANETs), have been widely used in VANETs [15; 16]. However, they are not very efficient due to issues like high end-to-end delays, low data transmission rates, and low packet delivery ratios. Also, the extra bandwidth consumption resulting from the frequent route discovery processes and periodic beaconing of MANET routing protocols is a concern in a highly dynamic VANET environment. On another frontier, VLC is a rapidly emerging technology for vehicular networks due to recent advancements in high-intensity light-emitting diodes (LEDs) that are widely used in vehicles and traffic lights for lighting purposes. The LEDs also have the capability to function as VLC transmitters, with their high-speed switching capabilities. VLC offers significantly less mutual interference compared to RF-based systems due to its short-range and line-of-sight (LoS) transmission properties, [17]. The benefits of using VLC for VANET are as follows:  
**High bandwidth:** The VLC can provide much higher data rates (of several Mbps) compared to the one provided by typical RF systems such as DSRC (n more than 1 Mbps). This option is useful when transmitting large emergency alerts or high-definition sensor/map data. **Increased security and privacy:** The VLC signals are highly directional and they focus within a narrow beam. This makes eavesdropping or intercepting VLC communications much more difficult compared to the case of omnidirectional RF. **Less interference:** The VLC uses a non-overlapping visible light spectrum, which is less prone to interference from other vehicles that can occur in the crowded RF spectrum bands. **No regulatory issues:** The VLC spectrum usage does not require licensing and it is globally available, unlike some RF frequencies. **Improved positioning:** The directional nature of the VLC beams helps in determining the sender and receiver positions more precisely than in the case of non-directional RF.