Vehicular Sensor Networks (VSNs) are an emerging area of research that combines technologies developed in the domains of Intelligent Transport Systems (ITS) and Wireless Sensor Networks. Data dissemination is an important aspect of these networks. It enables vehicles to share relevant sensor data about accidents, traffic load, or pollution. Several protocols are proposed for Vehicle to Vehicle (V2V) communication, but they are prone to intermittent connectivity. In this chapter, the authors propose a roadside infrastructure to ensure stable connectivity by adding vehicle to infrastructure to the V2V communication. They introduce a data dissemination protocol, Hexagonal Cell-Based Data Dissemination, adapting it for VSNs within a metropolitan area. The virtual architecture of the proposed data dissemination protocol exploits the typical radial configuration of main roads in a city, and uses them as the basis for the communication infrastructure where data and queries are stored. The design of the communication infrastructure in accordance with the road infrastructure distributes the network data in locations that are close or easily reachable by most of the vehicles. The protocol performs a geographical routing and is suitable for highly dynamic networks, supporting a high number of mobile sources and destinations of data. It ensures reliable data delivery and fast response. The authors evaluate the performance of the proposed protocol in terms of data delivery ratio and data delivery delay. The simulation results show that HexDD significantly improves the data packet delivery ratio in VANETs.
TopIntroduction
Traditionally, towns were built in a very specific fashion. In the center would be the church or town hall and a market square, surrounded by one or more circular roads. A number of radial roads would allow visitors to travel from the city gates in the outer wall to the center. Many modern European cities reflect this old city plan in their current street layout. And still the old circular and radial roads are the main traffic arteries in the city. Figure 1 shows the map of the city of Enschede in the Netherlands that clearly illustrates these characteristics. If one had to choose where to build a communication infrastructure in support of vehicular networks in metropolitan areas, these roads would be the prime candidates. As it is, potential support for such networks is scattered over the city in the form of GSM base stations, Wi-Fi hotspots, traffic light and the likes (see Figure 2). The result of this haphazard infrastructure is that some parts of the city have dense communication coverage while other parts have limited coverage or no coverage at all.
Figure 1. OpenStreetMap of Enschede centre overlaid with a hexagonal tessellation of cell size around 70 meters
Figure 2. Virtual infrastructure of the hexagonal tessellation (white cells) overlaid on the existing infrastructure nodes; covered cells are shaded
In the following, we propose a sensing and communication infrastructure, in support of a data dissemination protocol for Vehicular Sensor Networks (VSNs). The infrastructure consists of lampposts that are equipped with small transceivers and sensors, positioned along roads at roughly equal distances, in addition to the existing communication and traffic control infrastructure. Such implementation has many advantages. The lampposts are already in place and no new mechanical constructions to attach the radio nodes to are needed. Electricity is present in every lamppost and is available to power up the transceivers at minimal additional costs. Because existing utilities are used, disruptions during deployment are kept to a minimum.
These roadside wireless sensors form a typical static Wireless Sensor Network (WSN), which provides a full and stable coverage of a city area. This WSN has advantages compared to a vehicular network whose coverage depends on the traffic situation and is usually unevenly distributed over a city. Vehicles together with roadside sensor nodes form a hybrid network that can serve many applications, such as traffic monitoring and control, environmental monitoring, and safety warning. The proposed data dissemination protocol, called Hexagonal cell-based Data Dissemination (HexDD), can be used by these applications.
HexDD protocol was originally created for mission critical WSN applications (Tuysuz-Erman, et al., 2010), and is suitable for highly dynamic networks. The protocol is built upon a virtual hexagonal tessellation of the network area (see Figure 1). The hexagonal tessellation creates a circular structure on the city, starting from the centre, and spreading with hexagonal rings to the end of the city. Three main diagonals crossing at the centre of the city partition its area into equal parts. These three virtual lines together with a hexagonal ring (see Figure 2) form the virtual infrastructure of the HexDD protocol. The layout of this virtual infrastructure is a close approximation of a city street layout, with the main diagonals being the (main) radial roads of the city and the hexagonal ring defined by the inner ring of the city. The HexDD protocol performs routing of data and query messages by exploiting the main roads infrastructure as a communication infrastructure. The protocol provides for routing of messages, queries, or event data, without elaborating on different types or structure of information, neither on data aggregation.
We adapt the protocol for the hybrid WSN and vehicular network, and evaluate its performance in terms of latency and reliability of data delivery. The protocol is compared with a classical non-position-based ad hoc routing protocol, AODV (Perkins, Belding-Royer, & Das, 2003), and a position-based ad hoc routing protocol, GPSR (Karp & Kung, 2000).