Towards reliable geographic broadcasting in vehicular networks

In Vehicular ad hoc Networks (VANETs), safety-related messages are broadcasted amongst cars, helping to improve drivers' awareness of the road situation. VANETs’ reliability are highly affected by channel contention. This thesis first addresses the issue of channel use efficiency in geographical broadcasts (geocasts). Constant connectivity changes inside a VANET make the existing routing algorithms unsuitable. This thesis presents a geocast algorithm that uses a metric to estimate the ratio of useful to useless packet received. Simulations showed that this algorithm is more channel-efficient than the farthest-first strategy. It also exposes a parameter, allowing it to adapt to channel load. Second, this thesis presents a method of estimating channel load for providing feedback to moderate the offered load. A theoretical model showing the relationship between channel load and the idle time between transmissions is presented and used to estimate channel contention. Unsaturated stations on the network were shown to have small but observable effects on this relationship. In simulations, channel estimators based on this model show higher accuracy and faster convergence time than by observing packet collisions. These estimators are also less affected by unsaturated stations than by observing packet collisions. Third, this thesis couples the channel estimator to the geocast algorithm, producing a closed-loop load-reactive system that allows geocasts to adapt to instantaneous channel conditions. Simulations showed that this system is not only shown to be more efficient in channel use and be able to adapt to channel contention, but is also able to self-correct suboptimal retransmission decisions. Finally, this thesis demonstrates that all tested network simulators exhibit unexpected behaviours when simulating broadcasts. This thesis describes in depth the error in ns-3, leading to a set of workarounds that allows results from most versions of ns-3 to be interpreted correctly

Query Processor

contribution Title: Implementation of a resource-aware, data clustering algorithm for the Sun SPOT Distribute

Interference-Aware Geocasting for VANET

With the introduction of standards such as DSRC and WAVE, vehicular ad hoc networks (VANET) are being considered as candidates to improve road safety. Safety applications developed on top of VANET enable vehicle and driver improved awareness of the road situation through vehicles sharing their status and knowledge of the road conditions. In order for these systems to operate, they depend on reliable communication. To overcome fading, shadowing and other effects that cause packet loss, such applications commonly use retransmission schemes to improve packet delivery. However, the wireless channels used for these applications are bandwidth constrained, and retransmission can cause interference which in turn will degrade the reliability of the communication channel. Therefore, it is necessary to find a strategy that balances retransmission with channel occupancy for optimal reliability. In this paper, we present a cooperative retransmission algorithm that overcomes shadowing caused by blocking objects such as heavy vehicles in order to combat the resulting unreliability, whilst controlling the interference caused by retransmissions. The proposed algorithm weighs up the additional area a node can cover, with the amount of interference a retransmission by the same node would cause. We evaluate the performance of the algorithm in urban scenarios using the NS-3 simulator, and find that the proposed algorithm is effective in overcoming the negative effects of shadows caused by blocking objects

Estimating contention of IEEE 802.11 broadcasts based on inter-frame idle slots

Recent advances in communication technology has enabled vehicles to communicate with each other autonomously through the use of IEEE 802.11p protocol. Vehicle-to-vehicle communication regularly makes use of the broadcast mode transmissions, which are not often used prior to this application. Broadcast mode transmissions are more prone to frame loss from channel contention than unicasts due to its inability to adapt, and are unable to recover lost frames. This makes them very sensitive to channel congestion. In this paper, we first apply a variant of Bianchi et al.’s Markov model of the Distributed Coordination Function (DCF), to show that the observed inter-frame idle slots can be expressed as a probability distribution conditional on the number of saturated stations. It therefore follows that the probability distribution for the number of saturated stations can be estimated from inter-frame idle slots through Bayes law. Second, we present a novel passive channel congestion estimation technique that observes the inter-frame idle slot counts and uses a naive Bayes classifier to estimate the current channel contention in terms of the number of concurrent saturated stations. This technique is evaluated using computer simulations, and is shown to produce more accurate estimates with faster convergence time than the existing technique of observing collision probability using channel busy status as a proxy

How well do Yao graph and Theta graph support Greedy Forwarding?

Greedy Forwarding algorithm is a widely-used routing algorithm for wireless networks. However, it can fail if the wireless network topologies contain voids, where a packet cannot be moved closer to destination. Since Yao graph and Theta graph are two types of geometric graphs exploited to construct wireless network topologies, this paper firstly studied whether these two types of graphs can contain voids, showing that when the number of cones in a Yao graph or Theta graph is less than six, Yao graph and Theta graph can have voids, and when the number of cones equals or exceeds six, Yao graph and Theta graph are free of voids. Secondly, this paper experimented on how well Greedy Forwarding is supported on Yao graphs and Theta graphs in terms of stretch, i.e., the ratio between the path length found by Greedy Forwarding and the shortest path length in a graph. The experiments also included comparison with the stretch on Delaunay triangulation, another well-known geometric graph exploited in constructing wireless networks. Overall, our experiments revealed several interesting results

Educational interfaces, software, and technology

One of the primary goals of teaching is to prepare learners for life in the real world. In this ever changing world of technologies such as mobile interaction, cloud computing, natural user interfaces, and gestural interfaces like the Nintendo Wii and Microsoft Kinect people have a greater selection of tools for the task at hand. Teachers and students can leverage these tools to improve learning outcomes. Educational interfaces and software are needed to ensure that new technologies serve a clear purpose in the classrooms and homes of the future. Since teachers are always looking for creative ways to engage 21st century learners there needs to be an academic venue for researchers to discuss novel educational tools and their role in improving learning outcomes. This workshop aims at filling this void: combining the pedagogical expertise of the cooperative learning, and learning sciences communities with the technical creativity of the CHI, UIST and interactive surface communities. The objective of this workshop is to become a conference within two years