The road has become a “moving network”, today vehicles are been designed to carry networks, communicate with other vehicles via a communication link or channel. The 2009 Urban Mobility Report, issued by the Texas Transportation Institute, reveals that in 2007, the congestion caused Urban Americans to travel 4.2 billion hours more and to purchase an extra 2.8 million gallons of fuel [1] [2] . This caused a great cost of 187.2 $billion and an increase of 50% and above in the previous decade [1] [2] .

Recently, the attention of institutes and industries on VANET has grown vastly due to the promising features. The communication between vehicles has created a research field that can enhance the security and the efficiency of transportation system, traffic conditions and also non-safety measures like weather information, location etc. [3] -[5] .

According to configuration of network, VANET can be divided into three categories namely: Wireless Wide Area Network (WWAN), Hybrid Wireless Architecture, and Ad Hoc V2V communication. In the WWAN, the access point of the cellular gateway are fixed, this allows the direct communication between the vehicle and the access point. The Hybrid wireless Architecture uses WWAN access points at some points in the network, while the communication between those access points in the Hybrid Wireless Architecture are achieved with the use of Ad Hoc communications. The third category is the Ad Hoc Vehicle-to-Vehicle communication; this doesn’t require any fixed access point for the vehicles to communicate. Vehicles are designed with their own wireless network card and the setting up of an Ad Hoc network can be actualized for each vehicle.

VANET is a subsystem of Mobile Ad Hoc Network (MANET), VANET communicates with the MANET- like technology with the equipment nearby along the road side, and also to communicate between vehicles. Their characteristics are different from that of other networks [3] -[5] . Unavailability of road information can create a possibility of accurately stating the position of the vehicle at that time. The vehicle is the node in VANET and the nodes are limited to a particular type of topology while in motion which is the road topology. The nodes can provide power for data processing and information transmission to sustain the functioning of the node [4] -[6] .

Although VANET possess the characteristics of a wireless network, there’s a unique character that is associated to the mobility and the unreliable channel condition [6] [7] . Besides the safety application that VANET provides for the road users, there’s also the access to multimedia, mobile e-commerce, weather information etc. [6] -[8] . There are some applications that are specifically designed to aid drivers and improve the services of VANET such as: Advance Driver Assistance System (ADASE2), Crash Avoidance Matrices Partnership (CAMP), NOW (Network on Wheels), Fleet Net, etc. these applications were designed and developed with the joint services of different government and some major car manufactures company [8] -[10] .

The paper will discuss about the Vehicular Ad Hoc Networks (VANET) in detail in the section 2 and discussing about main architecture of VANET in the Section 3. The Section 4 discusses about different classifications of VANET applications and section 5 discusses about main characteristics of VANETs. The common VANET units and entities in classification of environment are discussed in Section 6. The VANET communications patterns are classified and explained in 3 main categories as warning broadcast, group communication and beaconing in section 7. The routing features of VANETs which are geocast, broadcast, unicast and multicast is discussed in section 8 in detail. In section 9, security issues discussed which are based on three main categories such as: availability, authenticity, and confidentiality and research is concluded with section 10.

Vehicular Ad Hoc Network (VANET) utilizes cars as a mobile node to create a mobile network [13] . Vehicles act as a mobile node with the corresponding network. The basic aim of VANET is to improve and increase the safety on our roads and road users, comfort of passengers, and also aid the communication between vehicles and roadside equipment. The VANET communication medium is installed on each node (vehicle) [11] . As shown in Figure 1, each vehicle has its own communication wireless network card which allows ease of communication flow between vehicles and roadside units.

Figure 2 Shows the different domains that exist in VANET. The Mobile Domain consists of (Vehicle and Mobile Devices) such as PDA, Smart Phones, and Laptop etc. The Generic Domain consists of (Internet Infrastructure and Private Infrastructure) such as nodes and servers. While the Infrastructure Domain consists of (Road Infrastructure or Units and the Central Infrastructure) such as the RSU that communicate with the vehicle along the road, and the management center that communicates with the internet.

The Mobile Domain communicates with the Infrastructure Domain and the Infrastructure Domain communicates with the Generic Domain and data flows between the different domain to provide effective and efficient use of the road by the road users.

Since the communication is provided in 2 different way in VANET, there are some fixed node that act as a roadside unit or equipment which enables the ease of VANET to serve as a gateway to the internet and also in accessing geographical data [12] -[14] . Each node in the VANET doesn’t only participate in data transmission and receiving, they also act as a wireless router of the network as different nodes communicate via their own

communication range, permitting cars in the region of 100 to 300 meters of each other to join the network, and create a network with a wide range. As cars fall out of the signal range and drop out of the network, other cars can join in, connecting vehicles to one another so that a mobile Internet is created [15] .

Components of VANET are onboard units and roadside units as shown in Figure 3, we can see how communication is transmitted from the roadside unit to the onboard unit in the vehicle, and also a vehicle to vehicle communication. This creates a better share of information between vehicles.

VANET, vehicles act as nodes, unlike MANET that vehicles are set to move on a predefined road. The vehicles must follow traffic signs and signals and their velocity relies on the speed sign [9] . Wireless devices such as; Personal Digital Assistant (PDA), Remote Keyless Entry Device, Mobile Phones, Laptops etc. are supported by VANET inside the vehicle [16] -[20] . Due to the increase of mobile wireless devices, the demand for the vehicle-to-vehicle (V2V), vehicle-to-roadside (VRC), and vehicle-to-infrastructure (V2I) communication will grow rapidly [20] . There are two type of communication infrastructure available by the VANET; first is the wireless ad hoc network, where there’s communication between vehicles without infrastructural support. Secondly, the communication between the vehicle and the road side unit [14] . The IEEE defined standard of establishing a VANET is 802.11 or 802.16 (WIMAX).

Due to the relatively high speed of nodes (vehicles) in the VANET and the clustering of vehicles in a particular location can cause a very large network at that time due to the independency of each node, a communication standard known as the Dedicated Short Range Communication (DSRC) was developed to fix the issue. This communication standard clearly requires the use of Road Side Units (RSUs) that are installed along the road as gateways between the infrastructure and the nodes (vehicles) and also in reverse [21] . The DSRC communicates on a 5.9 GHZ band and uses 802.11 access methods. USA allocated 75 MHZ of spectrum in the 5.9 GHZ, while Europe allocated 30 MHZ of spectrum in the 5.9 GHZ band for DSRC, this is to be utilized by the Intelligent Transportation Systems (ITS) [22] .

As shown in table 1, there are 7 MHz wide channels. Four of which are service channels that is used for safety and non-safety applications, and there is a control channel (CCH) which is used to control the channel. The two reserved channel (172 and 178) respectively are for future safety applications. Channel 172 is reserved for high accessibility and low inactivity of applications while channel 178 is reserved for high power public safety applications.

The OSI model group similar communication functions into one of the seven logical layers [23] -[25] . In VANET, the session and the presentation layers are omitted and a particular layer can be further broken or partitioned into sub layers in the VANET architecture, as illustrated in Table 2 below [25] . The architecture of VANET may change in different regions, and the protocols and interface will also be different. As shown in Table 3, the protocol stacks for the Dedicated Short Range Communication (DSRC) in the US [26] . At various layers, different protocols are designed to be used of which some are still under development [26] .

The approved amendment to the IEEE 802.11 standard which is IEEE 802.11p standard adds a wireless access in VANET vehicular environment (WAVE). This is focused primarily on the physical layer and MAC sub layer of the protocol stack. The IEEE 1609 standard is a higher protocol standard compared to the IEEE 802.11p. The IEEE 1609 standard functions in the middle layers of the protocol stack and it adaptably supports the safety applications in VANET. While the nonsafety applications are supported through a different set of protocols. The Network, Transport layer services for the nonsafety applications in VANET are supported or provided by IPV6, TCP, and UDP [27] -[29] .

There are two categories of applications that is associated with the VANET; safety and user based applications [30] .

As earlier stated VANET is a sub of MANET, but it has its own distinguished characters such as:

High Mobility: Because vehicles move at high speed it is difficult to predict a node position and also it makes protection of nodes privacy hard.

Rapid Changing Network Topology: Due to the random speed of a node (vehicle), node position is difficult to ascertain and its position changes frequently, this causes the network topology to change frequently in VANET.

Unbounded Network Size: VANET network size is not limited to a particular region or locality, it can be implemented for a city or more, or even for countries. VANET is geographically limitless.

Frequent Exchange of Information: Information can be exchanged amongst vehicles and road side units (RSUs) die to the AD Hoc nature of VANET. This makes the information exchange more frequent and updated.

Wireless Communication: The technology that VANET runs on is a wireless technology, therefore nodes are connected and information exchange are done via a wireless communication channel.

Time Critical: Time limits are set on each information packet that is been sent or received, this enables the delivery of information at the right time to avoid unwanted delays and decisions can be made accordingly by the corresponding node with action taken.

Sufficient Energy: The nodes have huge power source, because the vehicles run on their own battery. There’s no limited power supply for the corresponding components to function properly. This cause demanding techniques to be used by VANET, such as RSA, ECDSA etc.

Better Physical Protection: Because VANET nodes are vehicles, it’s more secured physically. This makes VANET nodes to be more difficult to compromise physically and also reduce physical attack on the infrastructure.

In VANET there are different units involved in the deployment. Although majority are nodes (Vehicles), there are other units or entities that keep the basic operations functioning in the network. Due to the large and complex system model, it has been categorized into four sub models namely: Driver and Vehicle Model, Traffic Flow Model, Communication Model, and application Model [37] -[39] .

Driver and Vehicle Model: This shows the behavior of a single vehicle. In this model two factors are considered such as: different driving styles and the vehicle characteristics. Example a violent driver or passenger and a sport car [39] .

Traffic Flow Model: This model depicts the interaction between vehicles, drivers, and the infrastructure to develop a good road network [39] [40] .

Communication Model: This shows the flow of data or information between or among the road users [41] .

Application Model: This points out the usefulness in the behavior and quality of cooperative VANET applications [41] .

Figure 4 illustrated the VANET units and entities that makes up the VANET model, and it is explained in detail section below.

The use of VANET enables the use of several applications from safety to non-safety applications. These applications exchange messages over VANETs and they are used for different proposes. In the VANET they are four different communication pattern identified [44] [45] . Although other communication pattern exists such as (multimedia access, location based services, etc.).

In the past few years, routing in VANET have been researched widely [42] [52] -[54] . However, due to the characteristics of VANET having a high active topology recurrent connectivity, the commonly used routing protocols that were implemented for MANET have been tested and evaluated for use in VANET environment [55] . Depending on the number of sending and receiving nodes involved, the routing in VANET can be classified into three types namely; Geocast/Broadcast, Multicast, and Unicast approaches.

Security is always a challenge for any infrastructure that is been used in communication. Safety in VANET is of high priority because human lives are involved. The security challenges or issues must be put in place during the design of VANET architecture [53] -[55] . In [56] , the author classified attackers into three categories or dimensions; insider versus outsider, malicious versus rational, and active versus passive.

In VANET security issues, the threats are based into three main groups such as; availability, authenticity, and confidentiality. The following 3 subsections expose these issues in details.

VANET is an area of research that holds promising future and for vehicular users. However, it has its own challenges in the security prospect. VANET aims at reducing the accidents on our roads and increasing the flow of information among vehicle and the road users. The unique nature of VANET springs up issues like illegal tracking and jamming of the network. In this paper, we introduced VANET, its architecture, components, communication pattern and issues in its security. In the course of this research, we found out the routing protocols used in VANET that enabled road users to communicate and receive messages appropriately, such as: Geocast/ Broadcast, Multicast, and Unicast protocol. Also VANET communication pattern, entities and characteristics which include: High Mobility, Rapid Changing, Network Topology, Unbounded Network Size, Frequent Exchange of Information, Wireless Communication, Time Critical, Sufficient Energy and better Physical Protection. The characteristics of VANET expose the usability and efficiency in VANET.

With more research done on the security issues of VANET, I believe that VANET will cause a technological change and improvement for the road users. Useful information exchange can prevent future damage and accidents on our road. Future research would be conducted on comparing the various data security mechanisms and their performance metrics.

ArifSari,OnderOnursal,MuratAkkaya, (2015) Review of the Security Issues in Vehicular Ad Hoc Networks (VANET). International Journal of Communications, Network and System Sciences,08,552-566. doi: 10.4236/ijcns.2015.813050