Car to roadside communication using IEEE 802.11p technology
Provision of external services to vehicles has - until recently - been limited because of the lack of highspeed
communications between them and service providers. The lack of standardised communications
interfaces between various car manufacturers hasn't helped. The IEEE 1609 family of standards for Wireless
Access in Vehicular Environments (WAVE) addresses these issues.
IN 2001, THE EC established the goal of cutting
the number of traffic fatalities in half by 2010.
To help achieve this objective, a number of
organisations have been carrying out research
and development into car-to-roadside communication
techniques. Among them is Fraunhofer
ESK, the Munich-based institute that
undertakes applied research into networked
information and communication systems. The
institute has been working on networking
vehicle and the communication infrastructures
(Figs. 1 and 2) to provide drivers with a more
complete operational perspective of the
vehicle, which can add to safety.
Early warning detection of road construction
or accidents can also be provided to reduce
road congestion. In addition, linking the
vehicle and its environment enables optimal
use of existing infrastructures.
Ultimately, nationwide, Europe-wide and
global networks are needed to enable communications
between all vehicles and roadside
access points, or other vehicles.
Fig. 1. Intelligent Transport Systems (ITS) include all types of communications in vehicles, between vehicles and between
vehicles and fixed locations. Such systems include the use of information and communication technologies (ICT) for rail,
water and air transport (picture ETSI).
Fig. 2. Exemplifying networking vehicle and the
communication infrastructures to provide drivers with a
more complete operational perspective of the vehicle.
Car-to-roadside communication is based on a
WLAN (IEEE 802.11p) platform developed
especially for vehicles (Fig. 3), while IEEE 1609
is a higher layer standard on which IEEE
802.11p is based.
Fig. 3. Car-to-roadside communication is based on a
WLAN (IEEE 802.11p) platform developed especially for
IEEE 802.11p is an approved amendment to
IEEE 802.11 that adds WAVE. It therefore
defines the enhancements to 802.11 required
to support Intelligent Transportation Systems
(ITS) applications, including data exchange
between high-speed vehicles and between
those vehicles and the roadside infrastructure
in the licensed ITS band of 5.9GHz (5.85 -
WAVE standards define an architecture and a
complementary, standardised set of services and
interfaces that together enable secure vehicleto-
vehicle (V2V) and vehicle-to-infrastructure
(V2I) wireless communications. Transportation
benefits include greater vehicle safety, better
navigation and traffic management, plus
Note that 802.11p will also be the basis for
Dedicated Short Range Communications (DSRC).
Based on the ISO Communications, Airinterface,
Long and Medium range (CALM)
architecture standard, this is a US Department
of Transportation project that addresses vehiclebased
communication networks, especially toll
collection, vehicle safety services, and
commerce transactions via cars.
The car-to-roadside communication that
Fraunhofer ESK has been looking at is based on
this WLAN IEEE 802.11p platform. It is combined
with a satellite-based positioning system to
enable the exchange of vehicle positioning and
sensor data with the vehicle's environment. The
Universal Mobile Telecommunications System
(UMTS) can be used as an alternative or
additional technology. For time critical applications,
however, UMTS can only be considered
conditionally because the transmission of timed
messages with short delays cannot yet be
The system will have to operate at vehicle
speeds of up to 200km/h and support a transmission
range of 1,000m. These requirements
place high demands on the wireless communication
network. Scalable transmission power
will help avoid collisions on the wireless
segment when traffic is highly dense. To enable
time-critical, safety-relevant applications, the
data will be prioritised and partitioned into
Because of the potential high vehicle speeds,
topological changes are continuously generated
that place many demands upon routing functionality.
Many routing algorithms are available,
but so far none has been found to be adequate
on its own. A situation-dependent, hybrid
solution is therefore needed.
IEEE 802.11p MAC protocol for VANETs
Vehicular Ad-hoc Networks (VANETs) have emerged as a key underlying technology in the realisation
of Intelligent Transportation Systems (ITS). Vehicle communications can greatly reduce the incidence
of accidents. One application is the Intelligent Transportation System (ITS) that includes automatic
control services for improving safety, reducing traffic congestion and increasing passenger comfort.
Although VANETs are particular cases of the general mobile ad-hoc networks, they possess some
characteristics that make its nature exclusive - and present challenges that require a set of new
protocols. Vehicle speeds can be up to 150km/h, while the network topology changes repeatedly and
unpredictably. Hence, the time duration for which a link is active between the vehicles is very short,
especially when the vehicles are travelling in opposite directions.
One approach to enlarge the lifetime of links is to increase the transmission power, but increase in a
vehicle's transmission range will result in increasing the probability of data collision and cause
degradation in the overall system throughput. The other solution requires new protocols exhibiting a
very low latency.
The usefulness of the broadcasted messages depends on latency. For example, if a vehicle is
suddenly stopping or suddenly stops, it should broadcast a message to warn other vehicles of the
probable danger. Considering that the driver needs at least three quarters of a second to initiate a
response, the warning message should be delivered with virtually no system latency.
Position of nodes changes quickly and unpredictably in VANETs, so that, building an efficient routing
table or a list of neighbour nodes will tire out the wireless channel and reduce the network efficiency.
Protocols that rely on prior information about location of nodes are likely to have a bad performance.
However, the topologies of a VANET can be an advantage because the vehicles are not expected to
leave the covered road; therefore, the running direction of vehicles is predictable to some level.
Privacy, safety and security fundamentally effect public acceptance of the technology.With VANETs,
each node corresponds and reports a specific person and location.
VANETs involve both vehicle-to-vehicle (V2V) and vehicle-to-infrastructure/roadside (V2I or V2R)
communications that rely on short-to-medium-range communication techniques.
The IEEE-1609 set of standards for Wireless Access in Vehicular Environments (WAVE) specifies an
architecture that includes new standards for vehicle communication aimed at supporting ITS
applications. IEEE-802.11p forms the bottom layers of the WAVE protocol stack and contains MAC
and PHY layers derived from IEEE-802.11a. This makes IEEE-802.11p more suitable for high speed
vehicles. The WAVE protocol forms a LAN to facilitate ITS applications, and this LAN defines a WAVE
Basic Service Set (WBSS) comprising vehicle-borne OBUs plus RSUs.
The IEEE-802.11p MAC protocol is derived from the IEEE-802.11 distributed coordination function
(DCF), and also uses the IEEE802.11e EDCA based quality-of-service (QoS) amendments. 802.11
MAC uses CSMA/CA that is specified in almost all variants of IEEE-802.11 (802.11a, 802.11b,
802.11g and 802.11p). RTS (Request-To-Send) and CTS (Clear-To-Send) mechanisms are used to
resolve hidden and exposed node problems (such as found in vehicle movements).
In IEEE-802.11p, both the MAC (medium access control) and the PHY (physical) layers belonging to
the DSRC/WAVE protocols are enhanced. Except for slight parameter changes to enable high user
mobility, IEEE-802.11p physical layer is identical to IEEE-802.11a. Moreover, the transmission power
may be higher (up to +44dBm) in 802.11p compared to that in 802.11a. IEEE-802.11p MAC layer is
derived from the basic IEEE-802.11 DCF.
The operating frequency for IEEE-802.11p is the 5.85-5.925GHz range in the licensed 5.9GHz ITS
band. For safety applications requiring higher priority, one channel is dedicated to control. Other
channels are service channels that can serve safety and non-safety services.
The function of the MAC is to coordinate the use of the communication medium. MAC layer protocols
decide which node will access the shared medium at any time. As safety critical applications are
designed to alert drivers about immediate danger, therefore requiring tight delay bounds, a MAC
protocol has to take into consideration these strict application requirements.
The focus of a paper1 on which this box is based is a performance analysis of the IEEE-802.11p MAC
protocol. In the paper, the IEEE-802.11p MAC method was evaluated by developing a VANET model
using MATLAB simulations. The scenario included time-critical packets periodically broadcast in V2V
communications. Channel access delay and the probability of channel access were evaluated.
It was found that when the number of vehicles increased from 100 to 200, channel access delay
increased by around 20ms, and the probability of channel access decreased by about 5%. More
vehicles meant a greater chance of collision, as vehicles have less chance to access the channel.
Moreover, increasing the VANET sensing range from 500 to 1000m reduced channel access
probability by 35%.
The work showed that VANET performs well in small ranges because it becomes easy for vehicles to
communicate with each other. Physical interference and the probability of hidden terminals also
decreased, because in small ranges vehicle location is more likely to be known to other vehicles.
Finally, when simulation times were varied between 50 to 80ms, the probability of vehicles accessing
channels increased by 15%.
1. Performance Evaluation of IEEE 802.11p MAC Protocol for VANETs; Shahzad A.Malik,Madad Ali Shah, Shahid A.Khan,M.Jahanzeb, Umar
Farooq and Adnan Khan. Australian Journal of Basic and Applied Sciences, 4(8): 4089-4098, 2010.
On Board Units (OBUs) and the Roadside Units
(RSUs) (Fig. 4) were designed for the demonstrator.
These are based on embedded hardware
and real-time Linux. The RSUs are linked in a
multi-hop, fault-tolerant meshed network, with
the routing protocol being based upon modified
Optimised Link State Routing (OLSR) protocol.
This is an optimisation of the classical link state
algorithm adjusted to the requirements of a
mobile wireless LAN. Multipoint relays (MPRs)
are selected nodes that forward broadcast
messages during flooding. The technique significantly
reduces the message overhead compared
with classical flooding, where every node
retransmits each message when it receives the
first message copy.
Fig. 4. On Board Units (OBUs) and the Roadside Units
(RSUs) were designed for the demonstrator. These are
based on embedded hardware and real-time Linux.
The current network topology is then
displayed in real-time in the form of graphs,
and the RSUs offer a variety of services through
the meshed network. Requested services or
services in the near vicinity of the vehicle can
be displayed on the OBU. The OBUs and RSUs
transmit beacons at regular intervals. To avoid
placing an unnecessary load on the network,
connections are established only on demand.
The different scenarios call for a reliable and
robust communication platform to ensure that
the entire system will be unaffected by local
disruptions and interference. A security concept
must also be developed to exclude attacks and
manipulation. At the forefront are data security,
authentication and data integrity, issues that
can be addressed through encryption, digital
signatures and certificates.
The deployment of a public key infrastructure
is costly and, as a result, cannot be efficiently
implemented on low-cost hardware. Even so,
the system must guarantee the anonymity of
the driver. This can be achieved by using
temporary and revocable pseudonyms.
In contrast to car-to-car communication, carto-
roadside communication delivers advantages
as soon as the first vehicle is equipped with
the technology. No minimum penetration rate
is called for. The benefits lie mainly in extra
safety, better traffic management and good
access to information and entertainment.
Improved traffic flow - Because the vehicle
OBU communicates with the traffic signal, the
driver knows when the next signal change
occurs, allowing him/her to adjust driving
and/or route accordingly. This can reduce traffic
congestion. This advantage is summarised by
Figs. 5 and 6.
Fig. 5. The vehicle OBU communicates with the traffic
signal, so drivers know when the next signal change
occurs, allowing time to adjust driving and route
accordingly. This can reduce traffic congestion.
Fig. 6.This scenario also works where accidents, road
works or other potential blockages are concerned.Again,
driving and route can be changed to suit.
Increased safety - Vehicle sensors register
accidents and their severity. The vehicle
transmits the accident information to the next
RSU and a high-priority emergency call is
disseminated throughout the mesh network.
This warns other drivers in the vicinity of the
accident, or forwards the accident report to
emergency services control centres, such as
police, fire and rescue services.
More convenience - Service stations supply
the RSUs with up-to-date fuel prices and hours
of operation and forward the data to RSUs in
their vicinity. The vehicle can query the data
for all service stations located along the
planned route and display the service station
with the best prices on the OBU.
Fraunhofer ESK offers concepts based upon
fault-tolerant wireless multi-hop networking
of RSUs and the creation of a corresponding
service platform. The organisation is looking
to jointly develop the storage of up-to-date
information in the RSU network (such as service
station data), analysis and display of the
processed data on the OBU (aggregation and
filtering) and global networking (Internet).
To ensure reliable implementation of such
concepts, an end-to-end simulation environment
comprising traffic and network simulators
is required. These will allow simulation of
processes such as the intelligent aggregation
and filtering of the data, or the routing
algorithms in mesh networks with mixed architectures
(stationary and ad hoc) and their
impact on network loads. The information
forwarding process and the driving strategies
can then be analysed to determine their effectiveness.
Such general frameworks are currently
Network continuity, an increasingly important
issue, is also being improved. This applies not
only to the continuity of multimodal navigation
with up-to-date data, but also office-to-car
continuity, in which the workplace shifts from
the office to the car and is then adapted to
the vehicle environment.
Continuity in information and entertainment
('infotainment') systems can additionally be
realised - for example, an MP3 playlist that is
played at home can be continued in the car at
the very point at which it was stopped.
To ensure interoperability between automobile
manufacturers, all of the communication
protocols and security concepts must be standardised.
Various committees, including IEEE,
C2C CC, ETSI and ISO, are working together to
develop such standards. As member of the Car
to Car Communication Consortium (C2CCC)
Fraunhofer ESK is participating in this process.
Other current topics are new network technologies,
software methodology, networks
embedded systems, car-to-environment
networks, electro mobility, wireless communication
and sensor networks as well as mobile
Josef Jiru is research fellow at Fraunhofer ESK