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I. on board units and wireless communication



In the recent decade,
Vehicular Ad hoc Networks (VANETs) are becoming one of the most promising
research fields with the development of wireless technology and advancement in
the vehicular industry.  VANETs are considered
to provide safety information and other informative applications to both
passengers and drivers. VANETs are attracting significant attraction of both
industry and academia due to their unique characteristics like predictable
mobility and high dynamic topology. It is derived from the basic ad hoc network
and is a subclass of the Mobile Ad hoc networks (MANETs), which is a
self-configuring, infrastructure less network connecting mobile users
wirelessly. VANET comes under the family of WLAN 802.11 but uses different
specifications 1. The number of intelligent vehicles equipped with on board
units and wireless communication devices are combined to form VANETs. Due to
this, VANET has established itself as the key component of the Intelligent
Transport System (ITS). In this paper, we discuss the architecture of the VANET
and its characteristics. Moreover, the information regarding the routing
mechanism and various routing components is provided along with the different
routing protocols used for the communication in VANETs.

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Vehicular Ad Hoc Networks
(VANETs) are an emerging new technology which integrates ad hoc networks,
cellular technology and wireless LAN. The idea is to achieve intelligent
inter-vehicle communications and improve traffic safety and efficiency. VANETs
provide (a) Intelligent Transport System (ITS) by enabling vehicle-to-vehicle
communications, and (b) ubiquitous connectivity to mobile users while on the
road 2.  That is why, VANETs are also
called Inter-Vehicle Communications (IVC) or Vehicle to Vehicle (V2V)
Communication. VANETs have variety of applications and ITS is one of its major
application which includes applications like co-operative traffic monitoring,
blind crossing, prevention of collisions, traffic flow controls, nearby
information services and computation of real-time detour routes. Furthermore,
VANETs provide internet connectivity to vehicular nodes while on the move 2. The
vehicles in VANETs follow Wi-Fi 802.11p and Wi-Max 802.16 standards for a
reliable and efficient communication between the vehicles while moving 3. For
the users to download music, send emails, or play games at the back-seat, VANETs
provide internet connectivity to the moving vehicular nodes. To exchange
information between vehicles and between vehicles and road side units (RSUs),
the vehicles are equipped with communication technologies.

        VANETs provide two types of
communications in to share the information regarding critical driving
situations. These are Vehicle-to-Vehicle (V2V) Communication and Vehicle-to-RSU
(V2R) communication. In V2V communications, vehicles communicate with each
other directly to exchange the information where as vehicles communicate
directly with fixed RSUs aside the roads in V2R communication. Both V2V and V2R
communications in VANETs use Dedicated Short-Range Communication (DSRC) radio
for communication purposes 3. DSRC is a two-way wireless communication which
permits high data transmission in active safety applications. The Federal Communications
Commission (FCC) has allocated 75MHz spectrum in the 5.9 GHz band to be used by
ITS vehicle safety and mobility applications.  There are two types of information shared in
VANETs such as safety information and non- safety information. The primary
information that tells drivers about dangers to avoid traffic jams and
accidents comes under the category of safety information where as non-safety
information provides value-added services like nearest hospital, nearest petrol
station and so on to enhance the comfort of the drivers and passengers 3.

Figure 1:
Functions performed by each type of communication in VANETs 6


For routing purposes,
VANETs may use WLAN access points and fixed cellular gateways at traffic
intersections as shown in Fig. 2.

Figure 2:
WLAN/Cellular Network 6


This provides the
connectivity to the mobile nodes in the coverage area of these WLAN access
points and fixed cellular gateways. Moreover, to reduce the deployment cost,
all vehicles and road-side wireless devices can form a mobile ad-hoc network as
shown in Fig. 3.

Figure 3: Ad-hoc
Network 6


This enables
Vehicle-to-Vehicle (V2V) Communication where all the vehicles share important
information with each other such as their positions, speed, collision detection
and so on. VANETs can also use a hybrid network (Fig. 4) which is a combination
of WLAN and ad-hoc networks. In this network, the vehicles communicate with
each other using multi-hop links to stay connected to the world.    

Figure 4: Hybrid
Network 6


The architecture of VANET
is shown in Fig. 5. It is composed of mainly three major components such as
Trusted Authority (TA), On Board Units (OBUs) for moving vehicles and fixed
RSUs.  The information regarding
velocity, breaking and so on is gathered by the group of sensors which are
connected to vehicle’s OBU and this information is then transferred to
surrounding vehicles via wireless medium as messages. The TA is connected to
all the RSUs through a wired connection and all RSUs are connected to each
other. Also, the entire VANET system is sustained by the TA. These three
components are explained as follows 3:

Figure 5:
Architecture of VANET 3

AUTHORITY: The registration of the RSUs, vehicle
users and vehicle OBUs is done by TA. Moreover, the verification of the authorisation
of OBU ID of vehicles or user ID’s is the responsibility of TA to avoid
malicious vehicles 5. High computation power and large storage capacities are
the major requirements of TA. In case of malicious behaviour in the VANET
system, TA has an authority to disclose the real ID of OBUs.

RSUs are generally the fixed units attached to the roadside, parking places or
road intersections. It is composed of an antenna, processor and sensors similar
to an OBU and uses IEEE 802.11p radio technology to access the wireless
channel. A DSRC radio communication is required to communicate with various
vehicles within the range of the RSU. A directional antenna is used to transmit
a message to a dedicated location. The information coming from the TA and
vehicle’s OBU is stored in the RSUs due to its storage capabilities and can be
accessed later 3.

is used to exchange information with RSUs and OBUs of different vehicles. It
acts as a transceiver which is mounted on each vehicle in the VANET system. OBU
is composed of command processor, a user interface, read/write storage for
storing and retrieving data and DSRC radio 4. OBU gets the input from sensors
like GPS receiver, speed sensor, Tamper Proof device (TPD), Event Data Recorder
(EDR) present on each vehicle and these sensors are used to collect information
about the vehicle. The sensitive data like private key, group key and Vehicle
ID is stored in the TPD where-as accident or crash related information is
stored in the EDR. The gathered information by the sensors is then exchanged
between the neighbouring vehicles using wireless medium 3.




The vehicles in VANETs are
radio-enabled acting as mobile nodes and routers for other nodes. VANETs are
similar to other ad-hoc networks in terms of self-organisation,
self-management, short radio transmission and low bandwidth. Following are the
characteristics of VANETs which makes them different from other networks 6:

There is sufficient storage and computing power in VANETS
since vehicles act as nodes instead of small hand-held devices. Therefore, large number of data can be
stored and retrieved later in VANETs.

The topology of the VANETs frequently changes because of the high-speed
movement between the vehicles. For example, the wireless transmission range of
each vehicle is 450 m. Therefore, if the distance between the two cars is less
than 450m, the link is established and the link will last for approximately 14
seconds if the two cars are driving in an opposite direction with a speed of 70

can also be changed due to the high-speed movement of the vehicles. The network
between the vehicles can be disconnected if the density of the vehicles is
less. This creates a problem in such a ubiquitous network. To overcome this,
several relay nodes or access points can be pre-deployed along the road to have
the connectivity every time.

For nodes to communicate with each other, they are equipped
with on-board sensors and the information retrieved from these sensors can be
used to form communication links. For example, GPS are used in the vehicles to
provide location information for routing.

GEOGRAPHICAL NATURE OF COMMUNICATION:  The communication in VANETs is geographical
in nature as it provides the addresses of the destinations where the packets
need to be forwarded. It is different from the traditional networks which uses
unicast or multicast communication where end points use ID or group ID for
their identification.



Routing is part of the network layer of the
protocol stack and is the main function of the 3rd layer. The
physical and data link layers affect the performance of the network layer in
many ways. For example, the communication range of the communicating nodes in
the physical layer may be asymmetric due to some issues. As a result, node A
can send a message to node B but in return, node B may not be able to reply to
node A directly.

So, these types of issues should be addressed
to improve the performance of the routing protocols 7.

routing mechanism used in VANETs is called Multi-Hop Routing. In radio
networks, if the network coverage area is larger than the range of single
nodes, the concept of Multi-hop routing (Fig. 6) is used. In this, other nodes
act as relays to send information from source to the destination. It is used in
various applications such as wireless sensor networks, wireless mesh networks,
mobile ad hoc networks, smartphone ad hoc networks and so on.

Figure 6:
Multi-hop routing in VANETs


The concept of Multi-Hop routing in explained
in Fig.6 where the source uses other nodes to send data to the destination. The
other nodes act as hosts or switching units depending upon their requirement in
the network. There may be multiple routes present to send data from the source
to the destination and which is the best route possible, depends upon the
routing components.



The routing components
used in multi-hop routing can be determined by breaking down the routing
protocol into smaller parts. In this way, the behaviour of these components can
be understood as various applications have different requirements 8. To
maintain the performance and control the functional behaviour of the
application, extra components can be added to the routing protocols to support
extra requirements, services and features. The core routing components of
multi-hop routing protocol such as route discovery, route selection, route
representation and data forwarding are explained in the next section 7.

The process of finding a set of routes between the
sender and the receiver is known as route discovery. It is always the first
stage of any wireless routing component followed by route selection, route
representation and data forwarding. The route discovery mechanism can be of
three types namely proactive, reactive and hybrid. In proactive route
discovery, every node uses up-to-date routing information about the whole
network stored in their routing table to find a path between source and the
destination in a network. In this, the routes between the source and the
destination are pre-determined and the routing information is exchanged between
all the nodes after a certain time or upon change in topology of the network.
The node sends a packet to the destination after consulting its routing table,
which has an up-to-date routing information, and does not wait for a route to
be discovered.  The advantage of this
type of mechanism is that it does not induces any type of delay in sending a
packet because route discovery is a priority. The route discovery uses two
types of routing mechanisms namely distance vector proactive routing and link
state proactive routing.

In reactive route discovery, the route is discovered on
demand of the source. A route discovery process is initiated to find a path to
the destination when sender wants to send any packet. A route reply packet is
sent back to the sender when the route request reaches the intermediate node or
the destination. This route reply packet contains the details about the
discovered route. The hybrid route discovery protocol constitutes of both
proactive and reactive techniques to overcome their disadvantages. It decreases
the control overhead linked with proactive route discovery and the delay
induced in reactive route discovery.

This is a second stage of the routing component where
the output of route discovery stage, which has number of potential routes
between the source and the destination, is examined to select the best possible
route. The route selection in proactive protocols is done using route discovery
stage which uses the information stored in the routing tables of each nodes to
select the best possible route. In reactive protocols, the route selection
process is done either by the source, intermediate node or the destination. For
example, in destination based route selection, the destination selects the
first path through which the first request was made from multiple route

them, both route representation and data forwarding are considered as a single
component. The techniques used for route representation and data forwarding are
exact route and route guidance. In exact match, a path that follows the
sequence of intermediate nodes to the destination is clearly specified. The
methods used for exact route representation and forwarding are routing table
and source routing. On the other hand, route guidance technique uses
self-routing mechanism where the path is not fully determined before sending of
the packet. In this, a node selects the next hop on the fly based on
destination and stores the information of its neighbour regarding their
positions 9


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