1. INTRODUCTION
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.
2. EVOLUTION
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
(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 VII.
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) [1] , High Tech Automotive system
( Dutch) [2], Car-2-Car Communication Consortium C2C-CC (Europe) [3] , European
Association for Collaborative Automotive research (EUCAR) (Europe ) [4].
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
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 [5]
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.
3. ARCHITECTURE
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Vehicular
network can be deployed by network operators and service providers or through
integration between operators, providers, and a governmental authority. Recent
advances in wireless technologies and the current and advancing trends in ad
hoc network scenarios allow a number of deployment architectures for vehicular
networks, in highway, rural, and city environments. Such architectures should
allow communication among nearby vehicles and between vehicles and nearby fixed
roadside equipment.
e
Figure 1 illustrates
the reference architecture. This reference architecture is proposed within the
C2C-CC, distinguishing it from 3 domains: in-vehicle, ad hoc and infrastructure
domain[6]. The in-vehicle domain refers to a local network inside each vehicle
logically composed of two types of units:
(i)
an on-board unit (OBU) and
(ii)
one or more application unit(s) (AUs).
An OBU is a device in
the vehicle having communication capabilities (wireless and/or wired), while an
AU is a device executing a single or a set of applications while making use of
the OBU’s communication capabilities. Indeed,
an AU can be an integrated part of a vehicle and be permanently connected to an
OBU. It can also be a portable device such as a laptop or PDA that can
dynamically attach to (and detach from) an OBU. The AU and OBU are usually
connected with a wired connection, while wireless connection is also possible
(using, e.g., Bluetooth, WUSB, or UWB). This distinction between AU and OBU is logical,
and they can also reside in a single physical unit.
The ad hoc domain is a
network composed of vehicles equipped with OBUs and road side units (RSUs) that
are stationary along the road. OBUs of different vehicles form a mobile ad hoc
network (MANET), where an OBU is equipped with communication devices, including
at least a short-range wireless communication device dedicated for road safety.
OBUs and RSUs can be seen as nodes of an ad
hoc network, respectively, mobile and static nodes. An RSU can be attached to
an infrastructure network, which in turn can be connected to the Internet. RSUs
can also communicate to each other directly or via multihop, and their primary
role is the improvement of road safety, by executing special applications and
by sending, receiving, or forwarding data in the ad hoc domain.
Two types of
infrastructure domain access exist: RSU and hot spot. RSUs may allow OBUs to
access the infrastructure, and consequently to be connected to the Internet.
OBUs may also communicate with Internet via public, commercial, or private hot
spots (Wi-Fi hot spots). In the absence of RSUs and hot spots, OBUs can utilize
communication capabilities of cellular radio networks (GSM, GPRS, UMTS, WiMax,
and 4G) if they are integrated in the OBU.
4. REQUIREMENTS
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.
5. APPLICATIONS
Vehicular networking
applications can be classified as
1) Active road safety
applications,
2) Traffic efficiency and management
applications and
3) Infotainment applications.
5. 1 ACTIVE ROAD
Active road safety
applications are those that are primarily employed to decrease the probability
of traffic accidents and the loss of life of the occupants of vehicles [7] . A
significant percentage of accidents that occur every year in all parts of the
world are associated with intersection, head, rear-end and lateral vehicle
collisions. Active road safety applications primarily provide information and
assistance to drivers to avoid such collisions with other vehicles. This can be
accomplished by sharing information between vehicles and road side units which
is then used to predict collisions. Such information can represent vehicle
position, intersection position, speed and distance heading. Moreover,
information exchange between the vehicles and the road side units is used to
locate hazardous locations on roads, such as slippery sections or potholes.
Some examples of active road safety applications are given below as
Intersection collision warning: In this use case, the risk of lateral collisions for vehicles
that are approaching road intersections is detected by vehicles or road side
units. This information is signaled to the approaching vehicles in order to
lessen the risk of lateral collisions.
Lane change assistance: The risk of lateral collisions for vehicles that are
accomplishing a lane change with blind spot for trucks is reduced.
Overtaking vehicle warning: Aims to prevent collision between vehicles in an overtake
situation, where one vehicle, say vehicle1 is willing to overtake a vehicle,
say vehicle3, while another vehicle, say vehicle2 is already doing an overtaking
maneuver on vehicle3. Collision between vehicle1 and vehicle2 is prevented when
vehicle2 informs vehicle1 to stop its overtaking procedure.
Head on collision warning: the risk of a head on collision is reduced by sending early
warnings to vehicles that are traveling in opposite directions. This use case
is also denoted as “Do Not Pass Warning”[8].
Rear end collision warning: the risk of rear-end collisions for example due to a slow down
or road curvature (e.g., curves,hills) is reduced. The driver of a vehicle is
informed of a possible risk of rear-end collision in front.
Co-operative forward collision warning: a risk of forward collision accident is detected through the
cooperation between vehicles. Such types of accidents are then avoided by using
either cooperation between vehicles or through driver assistance.
Emergency vehicle warning: an active emergency vehicle, e.g., ambulance, police car,
informs other vehicles in its neighborhood to free an emergency corridor. This
information can be re-broadcasted in the neighborhood by other vehicles and
road side units.
Pre-crash Sensing/Warning: in this use case, it is considered that a crash is unavoidable
and will take place. Vehicles and the available road side units periodically
share information to predict collisions. The exchanged information includes detailed
position data and vehicle size and it can be used to enable an optimized usage
of vehicle equipment to decrease the effect of a crash. Such equipment can be
actuators, air bags, motorized seat belt pre-tensioners and extensible bumpers.
Co-operative merging assistance: vehicles involved in a junction merging maneuver negotiate and
cooperate with each other and with road side units to realize this maneuver and
avoid collisions.
Emergency electronic brake lights: vehicle that has to hard brake informs other vehicles, by using
the cooperation of other vehicles and/or road side units, about this situation.
Wrong way driving warning: a vehicle detecting that it is driving in wrong way, e.g.,
forbidden heading, signals this situation to other vehicles and road side
units.
Stationary vehicle warning: in this use case, any vehicle that is disabled, due to an
accident, breakdown or any other reason, informs other vehicles and road side
units about this situation.
Traffic condition warning: any vehicle that detects some rapid traffic evolution, informs
other vehicles and road side units about this situation.
Signal violation warning: one or more road side units detect a traffic signal violation.
This violation information is broadcasted by the road side unit(s) to all
vehicles in the neighborhood.
Collision risk warning: a road side unit detects a risk of collision between two or more
vehicles that do not have the capability to communicate. This information is
broadcasted by the road side unit towards all vehicles in the neighborhood of this
event.
Hazardous location notification: any vehicle or any road side unit signals to other vehicles about
hazardous locations, such as an obstacle on the road, a construction work or
slippery
road conditions.
Control Loss Warning: if an additional use case is described that is intended to
enable the driver of a vehicle to generate and broadcast a control-loss event
to surrounding vehicles. Upon receiving this information the surrounding vehicles
determine the relevance of the event and provide a warning to the drivers, if
appropriate.
5.2 TRAFFIC EFFICIENCY AND MANAGEMENT APPLICATIONS
Traffic efficiency and management applications focus on
improving the vehicle traffic flow, traffic coordination and traffic assistance
and provide updated local information, maps and in general, messages of
relevance bounded in space and/or time. Speed management and Co-operative
navigation are two
typical groups of this
type of application.
a) Speed management: Speed management applications aim to assist the driver to
manage the speed of his/her vehicle for smooth driving and to avoid unnecessary
stopping. Regulatory/contextual speed limit notification and green light optimal
speed advisory are two examples of this type.
b) Co-operative navigation: This type of applications is used to increase the traffic
efficiency by managing the navigation of vehicles through cooperation among
vehicles and through cooperation between vehicles and road side units. Some
examples of this type are traffic information and recommended itinerary
provisioning, co-operative adaptive cruise control and platooning.
5.3 INFOTAINMENT APPLICATIONS
a)
Co-operative local services: This type of applications focus on
infotainment that can be obtained from locally based services such as point of
interest notification, local electronic
commerce and media
downloading.
b)
Global Internet services: Focus is on data that can be obtained from global
Internet services. Typical examples are Communities services, which include
insurance and financial
services, fleet management
and parking zone management, and ITS station life cycle, which focus on
software and data updates.
6. TECHNICAL CHALLENGES
Vehicular networks’ special
behavior and characteristics create some challenges for vehicular
communication, which can greatly impact the future deployment of these
networks. A number of technical challenges need to services for drivers and
passengers in such networks. Scalability and Interoperability are two important
issues that should be satisfied, and the employed protocols and mechanisms
should be scalable to numerous vehicles and interoperable with different
wireless technologies.
The following
are the challenges of vehicular networking
1.Addressing and Geographical addressing: Some vehicular networking applications
require that the addresses are linked to the physical position of a vehicle or
to a geographic region. Mobility makes tracking and managing of “geo-addresses”
extremely challenging.
2.. Risk analysis and management: Risk analysis and management is used to identify and manage the
assets, threats and potential attacks in vehicular communication. Solutions on
managing such attacks have been proposed, but models of attacker behavior are
still missing.
3.Data centric trust and verification: For many vehicular
applications the trustworthiness of the data is more useful than the
trustworthiness of the nodes that are communicating this data. Data-centric
trust and verification provides the security means to vehicular applications to ensure
that the communicated information can be trusted and that the receiver can
verify the integrity of the received information in order to protect the
vehicular network from the in-transit traffic tampering and impersonation
security threats and attacks[8] . Public key cryptosystems can be used here but
the main challenge is associated with the overhead that is introduced by the
use of the public key cryptosystem[9].
4.Antonimity, privacy and liability: Vehicles receiving information from other vehicles or
other network entities need to be able
to somehow trust the entity that generated this information. At the same time,
privacy of drivers is a basic right that is protected, in many countries, by
laws. Privacy can be provided using anonymous vehicle identities. One of the
main challenges here is the development of a solution that is able to support
the tradeoff between the authentication, privacy and liability, when the
network has to (partially) disclose the communicated information and its origin
to certain governmental authorities.
5.Secure localization: Secure Localization is a Denial of Service (DoS) resilience
mechanism related to the means of protecting the vehicular network against
attackers that are deliberately willing to retrieve the location of vehicles.
6.Forwarding Algorithms: Forwarding of packets
is different than routing, where the goal of routing is to choose the best
possible route to reach destination(s), whereas forwarding is concerned about
how data packets are transferred from one node to another after a route is
chosen.
7.Delay Constraints: Data packets sent by vehicular networking applications usually
have time and location significance. Primary challenge in designing vehicular
communication protocols is to provide good delay performance under the
constraints of vehicular speeds, unreliable connectivity, and fast topological
changes.
8.Prioritization of data packets and
congestion control: Data
packets carrying traffic safety and traffic efficiency information usually have
higher significance and therefore should be forwarded ”faster” than other
packets. Majority of the research activities have focused on how to provide the
highest priority to the emergency type of data packets. When an emergency
occurs, the channel utilization is likely to degrade due to massive broadcast
of emergency messages.
9.Reliability and cross layering between network and transport
layer: Due to the wireless nature of the vehicle to
vehicle communication network, a route may suddenly break. It is therefore
important to provide as much reliable as possible transport service on top of
the inherently unreliable network. Designing cross-layer protocols, which span
between transport and routing layers, can be beneficial in vehicular networks
that support real-time and multimedia applications.
7.
FUTURE WORK
The
main recommendations for future work can be listed out as
Geographical addressing:
The most promising, but also the most complex one is the
geographical addressing family that extends IP routing and IP addressing in
order to cope with GPS addresses. While several solutions associated with this
family have been proposed, more research and standardization activities are
needed for a successful realization.
Data-centric Trust and Verification:
The proactive data-centric trust and verification security
concept has been researched extensively. However, the tamper-resistance
hardware used in a vehicle to detect unnecessary accident warnings, needs to be
further researched. The reactive security concept has been studied in a smaller
scale. More work is needed in the area of context verification, where a vehicle
is able to realize an intrusion detection system by comparing received
information on parameters associated with status and environment
with its own available
information.
Anonymity and privacy:
It
is being extensively investigated. However, an open area is anonymity and
adaptive privacy,where users are allowed to select the privacy that they wish
to have.
Forwarding algorithms:
The main challenge in designing forwarding algorithms for
VANETs is to provide reliable packet ransmission with minimum delay, maximum
throughput, and low communication overhead. Future research must focus on
protocols targeted at heterogeneous systems to handle applications with diverse
QoS requirements. Respecting the requirements of applications while solving the
fundamental communication problems in VANETs is a significant challenge in
designing future forwarding algorithms.
Delay constraints:
The primary challenge in designing protocols is to
provide good delay performance under the constraints of high vehicular speeds,
unreliable connectivity, and fast topological changes. In this section, we
discussed several methods that incorporate delay constraints in various layers.
To provide overall system improvement, future solutions must focus on
cross-layer protocols that strike a balance among conflicting issues from
different layers with an objective of end-to-end delay minimization.
Prioritization of data packets:
The new standards like 802.11e and IEEE 802.11p [10]
provide guidelines for packet prioritization. While there is some research in
adopting these standards, more work needs to be done in effectively lever aging
them. For example, cross-layer protocols that operate in multiple layers to
provide priorities among different flows and different applications.
Furthermore, developing efficient scheduling strategies that enable delay-aware
transmission of packets with different priorities is also a matter of concern
for
future VANET applications.
Reliability
and cross-layering between transport and network layers:
Since many safety-related and other applications require
geocasting or broadcasting, there is a clear need for new approaches that are
not based on traditional transport protocols. It is even more challenging the
case of geocasting protocols since the relay nodes in such methods do not
maintain any state information. Cross-layer design holds a promising future in
realizing effective protocols that address issues related to congestion and
link disruption.
CONCLUSION
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.
.
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