There has been significant interest and progress in the field of vehicular ad hoc networks over the last several years. VANETs comprise vehicle-to-vehicle and vehicle-to-infrastructure communications based on wireless local area network technologies.
vehicular ad hoc networks: Overview
The rapid increase of vehicular traffic and congestion on the highways began hampering the safe and efficient movement of traffic. Consequently, year by year, we see the ascending rate of car accidents and casualties in most of the countries. There has been significant interest and progress in the field of vehicular ad hoc networks over the last several years. The growth of the increased number of vehicles are equipped with wireless transceivers to communicate with other vehicles to form a special class of wireless networks, known as vehicular ad hoc networks or VANETs. Rapid advances in wireless technologies provide opportunities to utilize these technologies in support of advanced vehicle safety applications.
Figure 1 vehicular ad hoc networks
In particular, the new Dedicated Short Range Communication (DSRC) offers the potential to effectively support vehicle-to-vehicle and vehicle-to-roadside safety communications, which has become known as Vehicle Safety Communication (VSC) technologies. DSRC enables a new class of communication applications that will increase the overall safety and efficiency of the transportation system. The integration of communication technology in state-of the art vehicles has begun years ago: Car phones and Internet access based on cellular technologies as well as Bluetooth adapters for the integration of mobile devices are popular examples. However, the direct communication between vehicles using an Ad Hoc network, referred to as inter-vehicle communication (IVC) or vehicle ad hoc networks (VANETs), is a relatively new approach. Compared to a cellular system, IVC has three key advantages: lower latency due to direct communication, broader coverage and having no service fee.
vehicular ad hoc networks: Definition
The networks that interconnect vehicles on road are called Vehicular Ad hoc Networks (VANETs). A mobile ad hoc network (MANET) consists of mobile nodes that connect themselves in as decentralized, self-organizing manner and may also establish multi-hop routes. If mobile nodes are cars, this is called vehicular ad hoc network”. “The main target of research in VANETs is the improvements of vehicle safety by means of inter vehicular communication (IVC)”. A Vehicular Ad Hoc Network or VANET is a technology that uses moving cars as nodes in a network to create a mobile network. The term VANET was originally adopted to reflect the ad hoc nature of these highly dynamic networks. However, because the term ad hoc network was associated widely with unicast routing- related research, there is currently a debate among the pioneers of this field about redefining the acronym VANET to deemphasize ad hoc networking. Because this discussion has not yet reached consensus, we will continue to refer to vehicle-to-vehicle and vehicle-to-roadside communication based on wireless local area networking technology as a VANET. Vehicular ad hoc network, consisting of a network of vehicles, moving at a relatively high speed, that communicate among themselves with different purposes, being the main purpose that of improving security on the road.
VANET has its own unique characteristics when compared with other types of MANETs, the unique characteristics of VANET include:
- Predictable mobility: VANET differs from other types of mobile ad hoc networks in which nodes move in a random way, because vehicles are constrained by road topology and layout and by the requirement to obey road signs and traffic lights and to respond to other moving vehicles leading to predictability in term of their mobility.
- Providing safe driving, improving passenger comfort and enhancing traffic efficiency: VANET provides direct communications among moving vehicles, thus allowing a set of applications, demanding direct communication between nodes to be applied over the network. Such applications can provide drivers travelling in the same direction with warning messages about accidents, or about the need for sudden hard breaking; leading the driver to build a broader picture of the road ahead. Moreover, additional kinds of applications could be applied via this type of network in order to improve passenger comfort and traffic efficiency by disseminating information about weather, traffic flow and point of interest information (gas station, shopping malls and fast food).
- Variable network density: The network density in VANET varies depending on the traffic density, which can be very high in the case of a traffic jam, or very low, as in suburban traffic.
- Rapid changes in network topology: High speeds typify moving vehicles, especially at the highway leading to rapid changes in network topology. Moreover, driver behaviour is affected by the necessity to react to the data received from the network, which causes changes in the network topology. The life time of the link between vehicles is affected by the radio communication range and the direction of the vehicles; thus increasing the radio communication range leads to an increase in the life time of the link. The life time of the link between vehicles moving in opposite directions is very short lived compared with case in which vehicles move in the same direction. The rapid changes in link connectivity cause the effective network diameter to be small, while many paths are disconnected before they can be utilized.
- Large scale network: The network scale could be large in dense urban areas such as the city centre, highways and at the entrance of the big cities.
- High computational ability: Because the nodes in VANET are vehicles, they can be equipped with a sufficient number of sensors and computational resources; such as processors, a large memory capacity, advanced antenna technology and global position system (GPS). These resources increase the computational capacity of the node, which help obtaining reliable wireless communication and acquiring accurate information regarding its current position, speed and direction.
- Frequent disconnected network: The high speed of the vehicles in one way defines the dynamic topology whereas on the other hand necessitates the frequent requirements of the roadside unit lack of which results a frequent disconnections.
- Communication Environment: Once we are having a mobility model, yet we are not done. As the mobility model may have different features depending upon road architecture, highways, or city environments. Communicating in these situations has to be taken care.
- Unlimited Battery Power and Storage: Nodes in VANETs do not suffer power and storage limitation as in sensor networks; therefore optimizing duty cycle is not as relevant as in sensor networks
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 Yun-Wei Lin, Yuh-Shyan Chen and Sing-Ling Lee “Routing Protocols in Vehicular Ad Hoc Networks: A Survey and Future Perspectives” Journal of Information Science and Engineering 26, 913-932 (2010).
 Yousefi, Saleh, Mahmoud Siadat Mousavi, and Mahmood Fathy, “Vehicular ad hoc networks (VANETs): challenges and perspectives”, 6th International Conference on ITS Telecommunications Proceedings, IEEE, 2006.
 Hartenstein, Hannes, and L. P. Laberteaux, “A tutorial survey on vehicular ad hoc networks”, IEEE Communications magazine 46.6 (2008).