# Fog-Based Two-Phase Event Monitoring and Data Gathering in Vehicular Sensor Networks
## 1 Introduction  
With the development of vehicular and communication technologies, there emerges a new technology called vehicular ad hoc networks (VANETs) that integrates the capabilities of new generation wireless networks to vehicles [1; 2; 3]. The IEEE 802 committee defined wireless communication standard IEEE 802.11p [4], which serves specifically vehicle to infrastructure (V2I) communication. It can realize a fast V2I wireless communication connection in the urban road environment and ensure transportation safety and communication reliability for moving vehicles. The Federal Communications Commission (FCC) has allocated 75 MHz of bandwidth, which operates at 5.9 GHz for short range communications. Vehicles communicate with other vehicles, directly forming vehicle to vehicle communication (V2V), or communicate with fixed equipment next to the road, referred to as road side unit (RSU), forming vehicle to infrastructure communication (V2I). VANETs enable the concept of smart cars and intelligent transportation systems (ITS), in which information and communication technologies are applied in the fields of road transportation, traffic management and mobility management. The major goals ofthese activities are to increase road safety and transportation efficiency, as well as to reduce the impact of transportation on the environment. Example VANET applications include intersection collision avoidance, electronic brake lights, platooning and traffic information systems [5]. One key and challenging issue in VANETs is vehicular sensing and data-gathering [6; 7; 8]. Vehicular nodes are equipped with more and more sensing units, and large amounts of sensing data such as GPS locations, speeds and video clips are generated [9]. These data are shared or uploaded as input for applications aiming at more intelligent transportation, emergency response and reducing pollution and fuel consumption. In other words, cooperative urban sensing is at the heart of the intelligent and green city traffic management. The key components of the platform will be a combination of a pervasive vehicular sensing system, as well as a central control and analysis system, where data-gathering is a fundamental component that bridges the two systems. However, data-gathering and event-monitoring are also challenging issues in VANETs. VANETs differ from other mobile ad hoc networks (MANET) by their own characteristics. The network scale could be large, and the nodes (vehicles) have high computational ability and no power constraints compared to traditional sensor nodes powered by battery. As indicated by a report from Intel [10], the vehicle is evolving into a supercomputer on wheels. Furthermore, nodes are limited to road topology while moving, and the network usually suffers rapid topology and density changes under various road conditions and high moving speed. The communications are usually fragmented and intermittent-connected. With the rapid development of embedded hardware and communication technologies, there emerges a new kind of node called fog nodes [2; 11; 12], which are deployed at the edge of networks and with the support of location, distribution, scalability, density of devices and mobility. The concept of fog computing extends the traditional cloud computing paradigm to the edge of VANETs. For example, Intel's Next Unit of Computing (NUC) is a small-form-factor computer, whose motherboard measures 4 \(\times\) 4 inches and could be integrated with the RSUs. Therefore, one potential solution to efficient sensing and data-gathering in VANETs is through fog nodes. The fog nodes, e.g., RSUs, are able to provide computation, storage and networking services between the vehicular nodes and ITS central servers, which brings about the opportunity for optimizing the data-gathering and event-detection processes in VANETs.