Information aggregation is used to merge correlated data items from different nodes before redistributing them. Thus, using aggregation the number of transmissions and the communication overhead can be reduced significantly. Especially for applications which require periodic dissemination of information into a large region, aggregation is a prerequisite. Consider for example a traffic information system where each vehicle periodically disseminates information about road conditions. The aim is to provide drivers with accurate information on traffic conditions for a large road section, so drivers can be informed in time and can take alternate routes in case of traffic congestion, for example. Many aggregation approaches for VANETs use a fixed or structured segmentation of roads for these applications. Based on such segments the aggregation function is applied to fuse data. We argue that such fixed bounds contradict the real situation on roads. Thus either the segments are too large, resulting in loss of accuracy or the segments are too small, resulting in high communication load. Therefore, we propose in this work a system for completely structurefree aggregation. We employ fuzzy reasoning to allow for aggregation decisions to be based on a flexible and extensible set of criteria. These criteria can be application specific and enable a dynamic fragmentation of the road according to the applications' requirements.
Mechanisms for information dissemination are essential for many applications in vehicular ad-hoc networks (VANETs). Currently, simple flooding is often supposed to broadcast information within a geographic region, as for example in many geocast protocols. However, simple flooding has several drawbacks: every node rebroadcasting a message leads to redundancy, contention, and collision, to which is referred as the broadcast storm problem. Hence, in order to fulfill the applications' requirements, more efficient solutions are necessary.In this work we introduce three extensions to an existing probabilistic broadcast protocol to be capable to achieve high reception rates with a wide range of vehicle densities, at the same time lowering the communication complexity. The efficiency of the protocol is shown by simulations in an intersection and highway scenario and its superiority over flooding and the basic protocol is presented.
Efficient information dissemination is one of the challenging tasks in most ad-hoc network application domains, be it in wireless sensor networks, mobile ad-hoc networks, or vehicular ad-hoc networks. It is obvious, that flooding has a high communication overhead, leading to channel congestion and packet collisions. Therefore, more efficient dissemination mechanisms were investigated and proposed by the research community.One class of such algorithms is gossiping, where each node forwards a message with a certain probability. The main challenge in gossiping is the proper determination of the forwarding probability, i.e., on the one hand this value has to be chosen high enough to assure a high delivery ratio, on the other hand it has to be as low as possible to reduce communication overhead. At the same time this probability has to be adapted to dynamic network conditions, like changing node density.In this work an advanced scheme for adaptive gossiping is proposed, capable to adjust the dissemination probability dynamically and in a distributed manner. The gossiping probability is determined at a node for every message that has to be forwarded based on two hop neighborhood information. That way, the proposed protocol is capable of working in arbitrary network topologies and densities, enabling more efficient data dissemination compared to existing approaches.
IntroductionVehicular ad-hoc networks (VANETs) enable promising new possibilities to enhance traffic safety and efficiency. The vision of VANETs is that vehicles communicate spontaneously, in an ad-hoc manner over a wireless medium. Based on this inter-vehicle communication (IVC), vehicles exchange important information, e.g., about road conditions and hazardous situations. Moreover, such information can be propagated via multiple hops, thus making the dissemination of important information possible over longer distances. This is the key advantage of this kind of safety applications compared to conventional safety systems. Whereas conventional safety systems only rely on information sensed in the direct neighborhood by onboard sensors of a vehicle, active safety applications based on IVC can utilize information generated by nodes multiple hops away. Moreover, such information can be enriched on the way with information sensed by relaying cars. This greatly enhances the potential of VANET applications. The advantage is twofold:• Having information about distant hazardous situations like an accident ahead or icy road, the driver can be warned in-time, thus being able to completely avoid the dangerous situation.• Aggregating information from multiple cars enables retaining information on a higher semantic level. This way, applications like cooperative traffic jam warning and cooperative parking place detection can be realized.The enabling technology for such applications is the wireless ad-hoc communication between vehicles. Especially the dissemination of messages in a specific geographic region represents a fundamental service in VANETs to which we refer to as geographic broadcast (GeoCast). This communication paradigm is used by many applications to enhance traffic safety and efficiency but it can also serve as a basic mechanism for other routing protocols. Because of its relevance in the domain of vehicular networks, it is of key importance that the communication protocol enables efficient message dissemination. The realization of a robust and efficient broadcast mechanism is a challenging task due to the wide range of applications envisioned to build upon this communication technology, the rigorous requirements of safety applications, and the special network characteristics of vehicular networks. Therefore, the main focus of this chapter is the efficient broadcast of information for VANET applications. We want to give a broad and in-depth review of recent research in this topic and present simulation results of efficient dissemination protocols designed for such applications. This chapter is organized as follows: In Section 2 we discuss briefly different types of VANET applications, followed by an overview of different communication mechanisms Efficient Information Dissemination in VANETs 3www.intechopen.com
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