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Vehicular Ad Hoc Networks (VANETs):
Challenges and Perspectives
Abstract- Vehicular Ad hoc Network (VANET), a subclass of
mobile Ad Hoc networks (MANETs), is a promising approach for
future intelligent transportation system (ITS). These networks
have no fixed infrastructure and instead rely on the vehicles
themselves to provide network functionality. However, due to
mobility constraints, driver behavior, and high mobility, VANETs
exhibit characteristics that are dramatically different from many
generic MANETs. This article provides a comprehensive study of
challenges in these networks, which we concentrate on the
problems and proposed solutions. Then we outline current state of
the research and future perspectives. With this article, readers
can have a more thorough understanding of vehicle ad hoc
networking and the research trends in this area.
I.
INTRODUCTION
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.
Recently, the promises of wireless communications to
support vehicular safety applications have led to several
research projects around world: the Vehicle Safety
Communications Consortium [1] developing the DSRC
technology [2] (USA), the Internet ITS Consortium [3] (Japan),
the PReVENT project [4] (Europe) or the ‘Network on
Wheels’ project (Germany) [5] are some samples.
To cater to the emerging wireless communication needs with
regard to vehicles, in July 2003 ASTM and IEEE adopted the
Dedicated Short Range Communication (DSRC) standard
(ASTM E 2213-03) [6]. The aim of this standard is to provide
wireless communications capabilities for transportation
applications within a 1000 m range at typical highway speeds.
It provides seven 10 MHz channels at the 5.9 GHz licensed
band for ITS applications, with different channels designated
for different applications, including one specifically reserved
for vehicle-to-vehicle communications.
The specific properties of VANETs allow the development
of attractive new services. Some currently discussed examples
in the two most relevant areas safety and comfort are as
follows [7].
1) Comfort Applications: This type of application improves
passenger comfort and traffic efficiency and/or optimizes the
route to a destination. Examples for this category are: traffic-
information system, weather information, gas station or
restaurant location and price information, and interactive
communication such as Internet access or music download.
2) Safety Applications: Applications of this category
increase the safety of passengers by exchanging safety relevant
information via IVC. The information is either presented to the
driver or used to activate an actuator of an active safety system.
Example applications of this class are: emergency warning
system, lane-changing assistant, intersection coordination,
traffic sign/signal violation warning, and road-condition
warning. Applications of this class usually demand direct
vehicle-to-vehicle communication due to the stringent delay
requirements.
Although much effort is needed until these applications
come to reality, dissemination of various messages is the most
important challenge. In this paper we focus on networking
problems which should be addressed for message exchanging
between vehicles in VANETs.
Since VANETs are new topic of interest in scientific and
industry community, we strongly believe a comprehensive
survey study about the topic is needed. In the previous work
[8] the authors had a review of works in various protocol stack
layers. However we will concentrate on the mechanisms
instead of protocol stack layers, and then describe each
mechanism which can be implemented in different layers.
In this work we first classify the challenges as shown in
fig. 1, and then describe networking strategies which should be
considered.
The rest of this paper is organized as follows: We clarify
distinctive networking properties of VANETs in section 3. In
section 4 the literature about safety applications has been
reviewed and in section 5 we will bring to debate previous
works about comfort applications. In the section 6 we briefly
introduce efforts going on for simulation of VANETs and
mobility modeling. Finally in sections 7, 8 we conclude our
survey and outline some open problems for future works.
VANET's
Networking
Safety Messages
Service
differentiation
Carry and
forward
Comfort
Messages
Traditional
MANET
Admission Opportunistic
Control Routing
Power Trajectory
assignment forwarding
Clustering Geographical
routing
Efficient
broadcasting
c) The IVC network has small effective network diameter.
Rapid changes in link's connectivity cause many paths to
disconnect before they can be utilized. In [14] authors studied
the effective network diameter in a typical VANETs. This
characteristic is important for mostly comfort application as
they need to establish unicast and multicast routes (e.g., to the
internet gateway).
d) No significant power constraints, unlike sensor and other
types of mobile networks where limited battery life is a major
concern.
e) Potentially large-scale: In a city center or highways at the
entrance of big cities the network could be quite large scale.
f) Variable Network density: the network's density depends
on vehicular density which is highly variable. In traffic jam
situations the network can be categorized in very dense
networks whilst in suburban traffics it could be a sparse
network.
g) The topology of the network could be affected by driver's
behavior due to his/her reaction to the messages. In other
words the content of messages can change network's topology.
Figure1. Networking challenges in VANETs
II.
NETWORKING PROPERTIES OF VANETS
VANETs are an instantiation of a mobile Ad Hoc networks
(MANETs). MANETs have no fixed infrastructure and instead
rely on ordinary nodes to perform routing of messages and
network management functions. However, Vehicle Ad Hoc
networks behave in fundamentally different ways than the
models that predominated MANET research. Driver behavior,
constraints on mobility, and high speeds create unique
Characteristics in IVC networks. These characteristics have
important implications for design decisions in these networks.
The major differences are as follows.
a) Rapid changes in the VANETs topology are difficult to
manage. Due to high relative speed between cars network's
topology changes very fast. In [9], [10] authors tried to find the
approximation of link's lifetime and [11] tried to find trajectory
duration for a typical highway scenario through simulation.
Although their results could be useful, they are valuable just
for considered scenarios.
b) The IVC network is subject to frequent fragmentation,
even at a high rate of IVC deployment. Although the
connectivity characteristic of MANETs has been studied
broadly, there is few research which tries to tackle this
problem. It is mostly because VANET's connectivity depends
on the scenario. In [12][13] authors tries to captures some
relationships between the model of vehicular mobility and
connectivity of the networks, but since the results are from
simulation they are specific-purpose. Of course being
connective for VANETs is not important for emergency safety
messages since while the network is not connected there is no
problem in safety point of view.
III. SAFETY APPLICATIONS
Examples of vehicle-to-vehicle safety communication may
include collision warning, road obstacle warning, cooperative
driving, intersection collision warning, and lane change
assistance [15].
There are two types of safety messages circulate in the
control channel (e.g., of DSRC) and can be classified
depending on how they are generated: event driven and
periodic. The first ones are the result of the detection of an
unsafe situation, (e.g., a car crash, the proximity of vehicles at
high speed, etc). Periodic messages instead can be seen as
preventive messages in terms of safety, and their information
can also be used by other (non-safety) applications (e.g., traffic
monitoring) or protocols (e.g., routing). Periodic message
exchange (also called beaconing) is needed to make vehicles
aware of their environment. Thus, they will be able to avoid
emergency or unsafe situations even before they appear.
Therefore beacon messages essentially contain the state of the
sending vehicle, i.e., position, direction, speed, etc., and also
aggregated data regarding the state of their neighbors.
It is reasonable to assume that these periodic messages will
be sent in a broadcast fashion since the messages’ content can
be beneficial for all vehicles around. In the following we come
to debate the previous related works attempting to providing
safety applications.
MAC Layer Issues: As mentioned before, event driven
messages should have higher priority than periodic and
comfort messages. Thus some mechanisms for service
differentiation and admission control are needed. In the other
words, we could define three levels of priority: event driven
safety messages, beaconing safety messages and comfort
messages, in decreasing order.