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IEEE 802.11p MAC protocol for vehicle ad-hoc networks – 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
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 affect public acceptance of the technology.
With VANETs, each node corresponds and reports a specific person and location.
Implementation
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; this LAN defines a WAVE Basic
Service Set (WBSS) comprising vehicle on-board units plus roadside units.
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, the 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.
A study
The focus of a paper 1 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.
Transportation
Fig. 2. 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 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.
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.
Security
The different scenarios call for a reliable and robust communication
platform to ensure that the entire system will be unaffected by local
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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.
The advantages
In contrast to car-to-car communication, car-to-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.
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
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