## 2 Related Work
### Communication in Vehicular Networks
However, the application of this solution to urban scenarios where vehicles display more random movements is not considered, not to mention the fact that preserving synchronized clocks requires more control packets (overheads) than asynchronous ones. An infrastructure-based solution involving clustered vehicles is proposed in [20]. RSUs are used as gateways in order to maintain the connection throughout the network, while they compute more distant movements and topology changes with the participation of an additional transport control center (TCC). The extra deployment of infrastructure might increase the costs of this infrastructure and the centralized entity. The authors in [21] propose the communication of head clusters with close fixed nodes called sinks, as an additional infrastructure with fog computing, in order to avoid congestion. The purpose is to maintain connectivity throughout the network. Nevertheless, congestion is only avoided at sinks, since beacon packets that maintain the clusters are not avoided. Two mechanisms for the dissemination of emergency messages are proposed in [22]. The first is a known code that reduces the packet size to be communicated among vehicles. The second is an inter-cluster communication protocol based on the A* algorithm [23] for message delivery. The solution is, however, cluster-based, with centralized (heads) dissemination and beacon messages that are larger than the emergency ones, meaning more overheads. Reduced relay nodes in a geo-routing protocol are proposed in [24] for an overall reduced hop number and end-to-end propagation delay time. Frontier vehicles are selected as relays in consecutive areas that might not be permanently connected, i.e., the store-carry-forward approach is also applied. Since the scheme is only V2V and under ideal conditions, the authors do not mention how they deal with line-of sight and density problems. A predictive vehicular location is proposed in [25] by means of a Kalman filter [26]. The main motivation is to avoid the excessive use of beacon messages and predict more distant positions. The vehicle with minimum error acts as the relay for the data geo-route to a destination zone; however, it is unclear if the vehicles execute the algorithm with limited resources or offline and how they obtain the actual position if the Global Positioning System, or GPS, is partially reliable. The authors in [27] present a bio-inspired approach based on Particle Swarm Optimization (PSO) and aim to achieve a high data delivery rate, while the critical response time is reduced. Hello packets determine communication ranges and a Next Hop Vehicle (NHV), computed by the mentioned heuristic, relays data to the next region. Full knowledge of neighbor vehicles is assumed. Ant Colony Optimization (ACO) is used in a bio-inspired geo-routing protocol [28]. Besides RSUs, a central entity sends agents (ants) with the purpose of finding less con gested routes in the physical environment to provide shorter paths and travel times. High pheromone intensity means a high vehicular density to be avoided if a dynamic threshold is exceeded. Note that this solution requires reliable links and extra entities and its scope is to avoid traffic jams, instead of dealing with intermittent communication due to vehicular mobility. A bio-inspired protocol based on spider behavior is presented by [29]. Their aim is to avoid V2I communication by dispatching agents from point to point in a spiderweb-like network and returning with the shortest path to the destination node. However, the reported overheads are higher than those of the compared works and reliable geographic services and connectivity is assumed. Data dissemination under unstable links solution is provided by [30]. Both sparse and dense vehicular scenarios are considered, while alternative routes are computed if the main route presents failures. Despite the recovery mechanism, a greedy algorithm is used to calculate routes, where frontier nodes are used as relays if moving in the same direction.