Editor's Note: Wireless sensor networks lie at the heart of emerging applications in nearly every industry segment. In building these networks, designers contend with issues that encompass real-time communications, efficient high-bandwidth data exchange, multiple network topologies, selection of optimal routing strategies, and more. The book, Building Wireless Sensor Networks, offers detailed treatments on critical requirements and promising solutions in each of these areas and more.
This excerpt focuses on design challenges and methods associated with creating a vehicular ad hoc network (VANET). To share data as vehicles pass on roads or rest in parking areas, a VANET must contend with issues as varied as the physics of signal propagation, the fluid nature of data routing, and the security vulnerabilities associated with participation in an ad hoc network. Because of the changing nature of a VANET, designers need a broad understanding of these issues. In this excerpt from the book, the authors offer an in-depth discussion that defines the nature of VANET challenges and discusses alternatives for their solution.
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Adapted from Building Wireless Sensor Networks , by Smain Femmam, Editor.
Chapter 3. Routing and data diffusion in vehicular ad hoc networks
By Frédéric Drouhin and Sébastien Bindel
Data delivery is a crucial task in vehicular networks since current applications require the cooperation of each and every vehicle. Regarding the interaction between the driver and the vehicle, as defined by the National Highway Traffic Safety Administration [NHT 16], a fully autonomous car performs a given driving task under any variety of conditions. Therefore, all decisions taken by the system become crucial, simply because no actual human hand is on the “steering wheel” of the driving system. Information provided by sensors make it possible to construct a local vision of the vehicle; however, the construction of a global vision of the actual situation at hand requires the exchange of information and data. This task is ensured by data delivery services, with the aim of being efficient and reliable. Security aspects must also be taken into account in order to disseminate, receive and process reliable information and data.
The term vehicular ad hoc network (VANET) usually refers to all wireless vehicular networks. Considered as a subset of mobile ad hoc networks (MANET) by [COR 99], VANETs share common features: a dynamic topology due the mobility of nodes, a limited available bandwidth and limited security since the communication medium is shared by all stations. However, vehicles do not have limited energy capacity, because they have an alternator.
Regarding these features, a data delivery service has to ensure reliable communication despite the mobility of nodes, minimize the bandwidth consumption and secure communication. In this chapter, data delivery service in VANET is investigated through three aspects: (i) how to select a destination, (ii) how to route data to a destination and (iii) how to secure communication. The selection of destination is performed through a transmission method, which can reach one or several destinations. In praxis, a special kind of address is assigned to identify nodes such as the unicast, broadcast, multicast or anycast addresses. Data routing is ensured by routing protocols, which determines the path between two non-adjacent vehicles. The computation of the best path relies on information provided by a metric which assesses the “cost” of each path to reach a desired destination. Communication security provides additional services according to the application requirement. Two kinds of security must be considered: passive attacks and active attacks. Within passive attacks, only monitoring tasks are performed, unlike in active attacks wherein an action is performed by a hacker.
The remainder of this chapter is organized as follows. Section 3.2 describes the context and challenges related to each considered aspect of the data delivery service. In section 3.3, routing protocols related to vehicular networks are detailed. In section 3.4, security aspects are detailed. Section 3.5 closes this chapter and provides outlook.
3.2. Background and challenges
The deployment of routing and security solutions requires compliance with the characteristics and standards of vehicular networks. Regarding the features of VANET, the high dynamic of the topology and the uncertain and random density of vehicles have a significant impact on the connectivity to the network and the delivery delay. Information provided by on-board sensors gives local information, such as the position, useful for commutation. Communications in vehicular networks rely on three architectures. The first one is the Vehicle-to-Vehicle ad hoc network (V2V) where vehicles communicate directly to each other and form a fully distributed network. The second one is the Vehicle-to-Infrastructure network (V2I) wherein vehicles communicate only with the roadside infrastructure via RoadSite Units (RSU) and form a centralized network. The last one is the hybrid architecture combining both the V2V and V2I infrastructures. A vehicle can communicate either in a single hop or multi-hop fashion. The design of routing and security solutions needs to take into account the network architecture, the communication standards defining the protocol stack, and the signal propagation to understand the disturbances generated by the environment.
3.2.1. Communication standard
The dedicated short-range communication (DSRC) system has been specifically used for vehicular communications. It is a short/medium-range technology that operates at the 5.9 GHz band that has been widely standardized. The most investigated standard is certainly the one designed by the Institute of Electrical and Electronics Engineers (IEEE). It includes two standards, the IEEE 802.11p and the Wireless Access in Vehicular Environments (WAVE).
The standard IEEE 802.11p was introduced in 2004 as an amendment of the IEEE 802.11 in order to address vehicular communication. It describes the requirements of the physical and data link layers and is part of the WAVE architecture dedicated to intelligent transport systems (ITS). The physical amendment of 802.11p is similar to the IEEE 802.11a, both work in the range of 5 GHz but have a different bandwidth, 20 MHz for 802.11a and 10 MHz for 802.11p. Table 3.1 lists the remaining differences. Assuming a theoretical communication range up to 1000 m (V2V and V2I), [GAL 06] have shown in praxis that the maximum range in line of sight (LOS) is 880 m and in non-line of sight (NLOS) between 58 m and 230 m.
Table 3.1. IEEE 802.11a and IEEE 802.11p parameters
The aim of WAVE architecture is to give wireless access in a vehicular environment. The standard defined two stacks, one dedicated to the data plan, Figure 3.1, the other dedicated to the management plan, a resource manager and a security service. Regarding the data plane, WAVE includes IEEE 802.11p amendment to define physical and the lower layer of the data link. In WAVE, the channel is split in two, one half dedicated to signalization, called the Control Channel (CCH) and the other dedicated to information transmission over IP, called the Service Channel (SCH). This functionality is detailed in the IEEE 1609.4. The IEEE 1609.3 standard defined network services including Logical Link Control (LLC), IP and transport layers and the management plane layer. The resource manager defined in the IEEE 1609.1 standard runs at the application layer and is destined to manage services provided by the applications. Security services defined in the IEEE 1609.2 define security mechanisms for applications and manage messages to guarantee confidentiality, authenticity, integrity and anonymity.
Figure 3.1. WAVE communication stack: data plane
3.2.2. Signal disturbance
In wireless networks, the environment plays a significant role in network performances because it disturbs the signal propagation. This feature has to be considered in the design of routing protocols in order to be suitable for vehicular networks. An electromagnetic wave is made up of an electric field (E ) and a magnetic field (B ), oscillating at the same frequency and spreading in the same direction, as depicted in Figure 3.2.
Figure 3.2. Illustration of an electromagnetic wave
The distance between two oscillations is called the wavelength and denoted λ (m ). Let c be the speed of light (3.108 m.s−1) and f the frequency (5.9 GHz for VANET), then the wavelength is computed as follows:
Four main effects responsible for the signal disturbance can be distinguished. First, path loss, which represents the attenuation of the signal between the emitter and the receiver. Second, large-scale shadowing describes a fading occurring on a large scale. Third, small-scale fading occurring on a small scale. Fourth, the Doppler effect which is the change of the wavelength between an emitter and a receiver in motion. Figure 3.3 depicts the effect of path loss, shadowing and the multi-path effect versus the distance.
Figure 3.3. Path loss, shadowing and multi-path effects versus the distance
The next installment of this series provides an in-depth discussion of the four main effects responsible for signal disturbance.
Reprinted with permission from Elsevier/ISTE Press, Copyright © 2017
Frédéric Drouhin is an Assistant Professor in the Laboratoire Modélisation Intelligence Processus Systèmes (MIPS) at the Université de Haute Alsace.
Sébastien Bindel is an Associate Professor in the Département Réseaux et Télécommunications at Université de Haute-Alsace.