A Survey of Abnormal Traffic Information Detection and Transmission Mechanisms in VSNs <s> What Are Vehicular Sensor Networks? <s> Advances in wireless communications are facilitating the development of inter-vehicle communication systems that will benefit mobility and safety objectives. Recently, these systems, referred as vehicular ad hoc networks (VANETs), are gaining significant prominence from both government agencies and private organizations. VANETs are characterized by high vehicle mobility, unexpected driver behavior and variable traffic environment which bring forth challenges to maintain good connectivity. This study considers VANETs as a nominal system with disturbance. Under the nominal system, the traffic space headway is assumed to follow an approved traffic flow distributions, such as exponential distribution. Disturbance is then used to capture a set of uncertain traffic flow events caused by driver behavior and changes in traffic flow. In addition, robustness factor is incorporated to present the impact of probabilistic disturbance events that disrupt the node connectivity. Under constant disturbance conditions, the lower bound of reachable neighbors for each vehicle to maintain a high connectivity is analytically derived. Furthermore, we obtain the relationship between the number of nodes in a VANET and the reachable neighbors under which the network is asymptotically connected. Finally, in variable disturbance situations, the interaction between robustness factor and macroscopic traffic parameters are investigated based on the simulation data. The validation results demonstrate that the proposed analytical characterization can approximate VANET connectivity very well. Our results facilitate the understanding of VANET connectivity on a freeway segment under different traffic conditions. <s> BIB001 </s> A Survey of Abnormal Traffic Information Detection and Transmission Mechanisms in VSNs <s> What Are Vehicular Sensor Networks? <s> We consider vehicular sensor networks (VSNs) consisting of a large number of sensor nodes deployed on vehicles to facilitate vehicular applications. We try to apply such VSNs to preventing rear-end collisions that are common accidents due to sharp stops. An infrastructure-less framework is proposed, which only relies on vehicles' onboard sensors to prevent such accidents. The proposed framework consists of a distributed warning protocol and a location-based backoff scheme. Vehicle-to-vehicle communications is used to form warning groups, where a warning group is a set of vehicles that drive along the same lane and every pair of adjacent cars is within a certain distance. Only single-hop transmissions are needed to join and leave a group, thus keeping the group maintenance overhead low. When a sudden brake event is detected in a warning group, the location-based backoff scheme can quickly propagate warning messages among its group members. Simulation results show that the proposed approach outperforms existing schemes. <s> BIB002
1.1.1. Origins and Definition. Recently, with the rapid development of various information technologies such as computer, communication, and microelectronics, a large amount of systems for different applications need to be gradually emerged. And every new emerging application system is an integrated body with multiple complex technologies. For instance, under the support of microcomputer and embedded operating systems, the functions of smart onboard devices become richer. They continuously improve the travel quality and driving efficiency from all aspects, so the number of vehicles equipped with smart onboard devices increases greatly. For this reason, as a transport tool, vehicle begins to play a new and important role. Intermittent Connectivity. Due to the high mobility of vehicle nodes and the interference of wireless communication obstacles like city buildings, trees, and so forth, the network topology of VSNs is highly dynamic and the link duration time is quite short. Thus, the network connectivity is worse. Furthermore, there is no fixed end-to-end path between vehicle nodes. Predicable Movement. Since vehicles are limited to move on road, constrained by the road topology and the front vehicles, the movements of vehicle nodes are predicable. Enhanced Transmission and Processing Ability. For the lowcost and low-power WSN nodes, their energy, storage space, transmission range, and processing speed are limited. But VSN nodes have no strict hardware conditions limitation, so their data transmission range and processing performances are enhanced. Node Selfishness. The number of vehicles moving on city road is large, for some reasons like privacy protection or energy saving need, part of vehicle nodes are unwilling to participate in data forwarding. So, the vehicle nodes in VSNs have selfishness. Network Openness. Because vehicle nodes are not always "online", large number of nodes would frequently join or leave the network. When vehicles are parked on the street, some vehicle nodes will be "offline" temporarily; meanwhile, other new vehicle nodes will be "online". Thus, openness is the basic feature of VSNs. Unbalanced Network Topology. Influenced by the road topology and traffic condition, for the same road segments at different time or the different road segments at the same time, the distribution of vehicle nodes is uneven and changes greatly. Thus, the network density switches in sparse and dense states frequently. In addition, the regional network partition phenomenon may produce "information island" effect BIB001 in a time period. 1.1.3. Application and Projects. VSN has extremely important research position and broad application prospects in ITS. After induction and summary, most applications focus on the following aspects. Ensure the Driving Safety BIB002 . Vehicle nodes can sense much dangerous information on road, such as surface gathered water, icy pavement, visual obstacles under weather of dense fog, large falling objects on highway, emergency braking, and car breakdown. With the cooperative intervehicle communication technology in VSNs, the above dangerous information can be broadcasted to the vehicles behind to avoid car collision and ensure the driving safety. Reduce the Driving Time. According to the real-time traffic information gathered from mobile vehicle nodes, the future Table 1 : Projects and application.