Routing and data diffusion in vehicular ad hoc networks
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.