The need for reduction in highway traffic congestion and crashes has become serious challenges throughout the world. In order to overcome these challenges radars, cameras, sensors and other state-of-art technologies are integrated into vehicle to improve vehicle safety and driver comfort during travel. In addition to safety and traffic efficiency, wireless communication can also be shared by commercial and vehicular infotainment applications to, for instance, improve the occupants driving experience.
Intervehicle communication (IVC) is attracting considerable attention from the research community and the automotive industry, where it is beneficial in providing intelligent transportation system (ITS) as well as drivers and passengers’ assistant services. ITS that aim to streamline the operation of vehicles, manage vehicle traffic, assist drivers with safety and other information, along with provisioning of convenience applications for passengers such as automated toll collection systems, driver assist systems and other information provisioning systems.
In this context, Vehicular Ad hoc NETworks (VANETs) are emerging as a new class of wireless network, spontaneously formed between moving vehicles equipped with wireless interfaces that could have similar or different radio interface technologies, employing short-range to medium-range communication systems. A VANET is a form of mobile ad hoc network, providing communications among nearby vehicles and between vehicles and nearby fixed equipment on the roadside.
Vehicular networks are a novel class of wireless networks that have emerged thanks to advances in wireless technologies and the automotive industry. Vehicular networks are spontaneously formed between moving vehicles equipped with wireless interfaces that could be of homogeneous or heterogeneous technologies. These networks, also known as VANETs, are considered as one of the ad hoc network real-life application enabling communications among nearby vehicles as well as between vehicles and nearby ﬁxed equipment, usually described as roadside equipment.
Vehicles can be either private, belonging to individuals or private companies, or public transportation means (e.g., buses and public service vehicles such as police cars). Fixed equipment can belong to the government or private network operators or service providers.
Vehicular networking serves as one of the most important enabling technologies required to implement a myriad of applications related to vehicles, vehicle traffic, drivers, passengers and pedestrians. Vehicular networks are promising in allowing diverse communication services to drivers and passengers. These networks are attracting considerable attention from the research community as well as the automotive industry.
High interest for these networks is also shown from governmental authorities and standardization organizations and a dedicated short-range communications (DSRC) system has emerged in North America, where 75 MHz of spectrum was approved by the U.S. FCC (Federal Communication Commission) in 2003 for such type of communication that mainly targets vehicular networks. On the other hand, the Car-to-Car Communication Consortium (C2C-CC) has been initiated in Europe by car manufacturers and automotive OEMs (original equipment manufacturers), with the main objective of increasing road traffic safety and efficiency by means of intervehicle communication.
The Vehicle Infrastructure Integration initiative was ﬁrst launched by the U.S. Department of Transportation (USDOT) during the ITS World Congress in 2003. Then the Vehicle Infrastructure Integration Consortium was formed in early 2005 by a group of light-duty vehicle manufacturers to actively engage in the design, testing, and evaluation of a deployable VII system for the United States. USDOT’s VII program is divided into three phases:
(i) Phase I—operational testing and demonstration,
(ii) Phase II—research in the areas of enabling technology, institutional issues, and applications to support deployment, and
(iii) Phase III—technology scanning to determine potential new technology horizons for
Vehicular networks present a highly active ﬁeld of research, development, standardization, and ﬁeld trials. Throughout the world, there are many national and international projects in governments, industry, and academics devoted to such networks. These include the consortia like Vehicle Safety Consortium—VSC (United States)  , High Tech Automotive system ( Dutch) , Car-2-Car Communication Consortium C2C-CC (Europe)  , European Association for Collaborative Automotive research (EUCAR) (Europe ) .
VSC(Vehicle Safety Communications)
Consortium speciﬁed several performance requirements derived from the trafﬁc safety applications. From these requirements, the most signiﬁcant ones are: (1) safety messages should have a maximum latency of 100 ms, (2) a generation frequency of 10 messages per second and (3) they should be able to travel for a minimum range of 150 meters.
C2C-CC (Car 2 Car Communication Consortium)
It is a non-proﬁt organization initiated in the summer of 2002 by the European vehicle manufacturers, which is open for suppliers, research organizations and other partners. C2C-CC cooperates closely with ETSI TC ITS and the ISO/TC 204 on the speciﬁcation of the ITS European and ISO standards.
HTAS (High Tech Automotive Systems)
It is a Dutch organization that drives innovation through cooperation of Industry, Knowledge Centers and Government.
EUCAR (European Association for Collaborative Automotive Research)
It was established in 1994, evolved from the previous Joint Research Committee (JRC) of the European motor vehicle manufacturers. EUCAR supports strategic co -operations in research and development activities in order to progressively achieve the creation of technologies for the optimization of the motor vehicle of the future.
2.1 SPECIAL CHARECTERISTICS
Vehicular networks have special behavior and characteristics, distinguishing them from other types of mobile networks. In comparison to other communication networks, vehicular networks come with unique attractive features as follows 
Unlimited transmission power: Mobile device power issues are usually not a signiﬁcant constraint in vehicular networks as in the case of classical ad hoc or sensor networks, since the node (vehicle) itself can provide continuous power to computing and communication devices.
Higher computational capability: Indeed, operating vehicles can afford signiﬁcant computing, communication, and sensing capabilities.
Predictable mobility: Unlike classic mobile adhoc networks, where it is hard to predict the nodes’ mobility, vehicles tend to have very predictable movements that are (usually) limited to roadways. Roadway information is often available from positioning systems and map based technologies such as GPS. Given the average speed, current speed, and road trajectory, the future position of a vehicle can be predicted.
To bring its potency to fruition, vehicular networks have to cope with some challenging characteristics, which include
Potentially large scale: Unlike most ad hoc networks studied in the literature that usually assume a limited network size, vehicular networks can in principle extend over the entire road network and so include many participants.
High mobility: The environment in which vehicular networks operate is extremely dynamic, and includes extreme conﬁgurations: on highways, relative speeds of up to 300 km/h may occur, while density of nodes may be 1–2 vehicles 1 km on low busy roads. On the other hand, in the city, relative speeds can reach up to 60 km/h and nodes’ density can be very high, especially during rush hour. Partitioned network: Vehicular networks will be frequently partitioned. The dynamic nature of trafﬁc may result in large intervehicle gaps in sparsely populated scenarios, and hence in several isolated clusters of nodes.
Network topology and connectivity: Vehicular network scenarios are very different from classic ad hoc networks. Since vehicles are moving and changing their position constantly, scenarios are very dynamic. Therefore the network topology changes frequently as the links between nodes connect and disconnect very often. Indeed, the degree to which the network is connected is highly dependent on two factors: the range of wireless links and the fraction of participant vehicles, where only a fraction of vehicles on the road could
be equipped with wireless interfaces.
Vehicular network requirements can be grouped into the following classes:
a) Strategic requirements: These requirements are related to:
(1) The level of vehicular network deployment, e.g.,minimum enetration threshold and
(2) Strategies deﬁned by governments and commissions.
b) Economical requirements: These requirements are related to economical factors, such as business value once the minimum penetration value is reached, perceived customer value of the use case, purchase cost and ongoing cost and time needed for the global return of the invested ﬁnancial resources.
c) System capabilities requirements: These requirements are related to the system capabilities, which are:
Radio communication capabilities, such as (1) single hop radio communication range, (2) used radio frequency channels,(3) available bandwidth and bit rate, (4) robustness of the radio communication channel, (5) level of compensation for radio signal propagation difﬁculties by e.g., using road side units.
Network communication capabilities, such as (1) mode of dissemination: unicast, broadcast, multicast, geocast (broadcast only within a speciﬁed area), (2) data aggregation, (3)congestion control, (4) message priority, (5) management means for channel and connectivity realization, (6) support of IPv6 or IPv4 addressing, (7) mobility management associated with changes of point of attachment to the Internet.
Vehicle absolute positioning capabilities, such as (1)Global Navigation Satellite System (GNSS), e.g., Global Positioning System (GPS), (2) Combined positioning capabilities,e.g., combined GNSS with information provided by a local geographical map.
Other vehicle capabilities, such as (1) vehicle interfaces for sensors and radars, (2) vehicle navigation capabilities.
Vehicle communication security capabilities, such as (1)respect of privacy and anonymity, (2) integrity and conﬁdentiality, (3)resistance to external security attacks, (4)authenticity of received data, (5) data and system integrity.
d) System performance requirements: These requirements are related to the system performance, which are: (1) Vehicle communication performance, such as maximum latency time, frequency of updating and resending information, (2) vehicle positioning accuracy, (3) system reliability and dependability, such as radio coverage, bit error rate, black zones (zones without coverage). (4) Performance of security operations, such as performance of signing and verifying messages and certiﬁcates.
e) Organizational requirements: These requirements are related to organizational activities associated with deployment, which are: (1) common and consistent naming repository and address directory for applications and use cases, (2) IPv6 or IPv4 address allocation schemes, (3) suitable organization to ensure interoperability between different Intelligent Transport Systems, (4) suitable organization to ensure the support of security requirements, (5) suitable organization to ensure the global distribution of global names and addresses in vehicles.
Vehicular networking applications can be classiﬁed as
1) Active road safety applications,
2) Traffic efficiency and management applications and
3) Infotainment applications.
Vehicular networking is the enabling technology that will support several applications varying from global Internet services and applications up to active road safety applications. This is a survey that introduced and discussed the possible applications and use cases that could be supported by vehicular networks in the near and long term future. Furthermore, the several requirements, e.g., communication performance requirements, imposed by such applications are emphasized. Moreover, the government and international projects and programs that were and are being conducted in the USA, Dutch and Europe are presented. Finally the recent main research challenges associated with vehicular networking are introduced and possible future works have been discussed.