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Industrial Ethernet Book Issue 64 / 29
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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).

IEEE 802.11p


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 vehicles.

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 - 5.925GHz).

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 automated tolling.

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 technology

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 guaranteed.

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 different channels.

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.

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, 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.

A study

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.

Security

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.

The advantages

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 being developed.

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.

Participation

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 expert systems.

Josef Jiru is research fellow at Fraunhofer ESK

www.esk.fraunhofer.de
Source: Industrial Ethernet Book Issue 64 / 29
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