Björn Scheuermann

Data aggregation and roadside unit placement for a vanet traffic information system

In this paper we investigate how a VANET-based traffic information system can overcome the two key problems of strictly limited bandwidth and minimal initial deployment. First, we present a domain specific aggregation scheme in order to minimize the required overall bandwidth. Then we propose a genetic algorithm which is able to identify good positions for static roadside units in order to cope with the highly partitioned nature of a VANET in an early deployment stage. A tailored toolchain allows to optimize the placement with respect to an application-centric objective function, based on travel time savings. By means of simulation we assess the performance of the resulting traffic information system and the optimization strategy.

A survey on congestion control for mobile ad hoc networks

Congestion control is a key problem in mobile ad hoc networks. The standard congestion control mechanism of the Transmission Control Protocol (TCP) is not able to handle the special properties of a shared wireless multi-hop channel well. In particular, the frequent changes of the network topology and the shared nature of the wireless channel pose significant challenges. Many approaches have been proposed to overcome these difficulties. In this paper, we give an overview over existing proposals, explain their key ideas and show their interrelations. Copyright

Bitcoin and Beyond: A Technical Survey on Decentralized Digital Currencies

Besides attracting a billion dollar economy, Bitcoin revolutionized the field of digital currencies and influenced many adjacent areas. This also induced significant scientific interest. In this survey, we unroll and structure the manyfold results and research directions. We start by introducing the Bitcoin protocol and its building blocks. From there we continue to explore the design space by discussing existing contributions and results. In the process, we deduce the fundamental structures and insights at the core of the Bitcoin protocol and its applications. As we show and discuss, many key ideas are likewise applicable in various other fields, so that their impact reaches far beyond Bitcoin itself.

Probabilistic aggregation for data dissemination in VANETs

We propose an algorithm for the hierarchical aggregation of observations in dissemination-based, distributed traffic information systems. Instead of carrying specific values (e.g., the number offree parking places in a given area), our aggregates contain a modified Flajolet-Martin sketch as a probabilistic approximation. The main advantage of this approach is that the aggregates are duplicate insensitive. This overcomes two central problems of existing aggregation schemes for VANET applications. First, when multiple aggregates of observations for the same area are available, it is possible to combine them into an aggregate containing all information from the original aggregates. This is fundamentally different from existing approaches where typically one of the aggregates is selected for further use while the rest is discarded. Second, any observation or aggregate can be included into higher level aggregates, regardless if it has already been previously - directly or indirectly - added. As a result of those characteristics the quality of the aggregates is high, while their construction is very flexible. We demonstrate these traits of our approach by a simulation study.

The feasibility of information dissemination in vehicular ad-hoc networks

In this paper we consider information dissemination in vehicular ad-hoc networks (VANETs) in city scenarios. Information dissemination is an important building block of many proposed VANET applications. These applications need a certain dissemination performance to work satisfactorily. This is critical during the rollout of VANETs, when only few cars participate. After analytical considerations, we focus on simulations using a detailed model of a whole city. We assess the dissemination performance depending on the amount of equipped vehicles on the road. For few equipped vehicles, we show that dissemination speed and coverage will not be sufficient. Therefore, we propose to use specialized, but simple and cheap infrastructure, stationary supporting units (SSUs). If a small number of SSUs is installed in a city and connected via some backbone network, the dissemination performance improves dramatically, especially during the VANET rollout phase. Thus, SSUs allow for a faster and earlier rollout of working, dissemination-based VANET applications

Predicting Parking Lot Occupancy in Vehicular Ad Hoc Networks

The search for free parking places is a promising application for vehicular ad hoc networks (VANETs). In order to guide drivers to a free parking place at their destination, it is necessary to estimate the occupancy state of the parking lots within the destination area at time of arrival. In this paper, we present a model to predict parking lot occupancy based on information exchanged among vehicles. In particular, our model takes the age of received parking lot information and the time needed to arrive at a certain parking lot into account and estimates the future parking situation at time of arrival. It is based on queueing theory and uses a continuous-time homogeneous Markov model. We have evaluated the model in a simulation study based on a detailed model of the city of Brunswick, Germany.

Challenge: peers on wheels - a road to new traffic information systems

In the context of vehicular ad-hoc networks (VANETs), a number of highly promising convenience applications have been proposed. These include collecting and distributing information on the traffic situation, distributed monitoring of road and weather conditions, and finding available parking places in a distributed, cooperative manner. Unfortunately, all of these applications face major problems when a VANET is used as a means to distribute the required information. In particular a large number of vehicles needs to be equipped with dedicated VANET technology before these applications can provide a useful service. Even if customers were willing to purchase a system which is not immediately useful, it would still take quite some time until the required density of equipped cars is reached. In contrast, affordable always-on mobile Internet access is already mainstream. Such Internet connectivity could be used to build the proposed applications in a different fashion: by using peer-to-peer communication, essentially creating a peer-to-peer network of cars sharing traffic information. This allows to overcome the limitations of VANETs, while it preserves their key benefits of decentralization and robustness. In this paper, we describe the technical challenges that arise from such an approach, point out relevant research directions, and outline possible starting points for solutions.

Implicit hop-by-hop congestion control in wireless multihop networks

It has been shown that TCP and TCP-like congestion control are highly problematic in wireless multihop networks. In this paper we present a novel hop-by-hop congestion control protocol that has been tailored to the specific properties of the shared medium. In the proposed scheme, backpressure towards the source node is established implicitly, by passively observing the medium. A lightweight error detection and correction mechanism guarantees a fast reaction to changing medium conditions and low overhead. Our approach is equally applicable to TCP- and UDP-like data streams. We demonstrate the performance of our approach by an in-depth simulation study. These findings are underlined by testbed results obtained using an implementation of our protocol on real hardware.

PeerTIS: a peer-to-peer traffic information system

In this paper we propose a traffic information system based on the distribution of knowledge provided by the cars themselves. Prior work in this area attempted to realize this distribution via vehicular ad-hoc networks, i.e., by direct communication between cars. Such an approach faces serious problems due to capacity constraints, high data dissemination latencies, and limited initial deployment of the required technology. In this paper, we present a solution that is not based on ad-hoc networking, but is still fully decentralized. It establishes a peer-to-peer overlay over the Internet, using cellular Internet access. We present a structure for the overlay, a prototype implementation in a simulation environment, and results that underline the feasibility of such a system in a city scenario. We also provide an estimate of expected user benefits when our system is used for dynamic route guidance.

A fundamental scalability criterion for data aggregation in VANETs

The distribution of dynamic information from many sources to many destinations is a key challenge for VANET applications such as cooperative traffic information management or decentralized parking guidance systems. In order for these systems to remain scalable it has been proposed to aggregate the information within the network as it travels from the sources to the destinations. However, so far it has remained unclear by what amount the aggregation scheme needs to reduce the original data in order to be considered scalable. In this paper we prove formally that any suitable aggregation scheme must reduce the bandwidth at which information about an area at distance d is provided to the cars asymptotically faster than 1/d2. Furthermore, we constructively show that this bound is tight: for any arbitrary ε>0, there exists a scalable aggregation scheme that reduces information asymptotically like 1/d(2+ε).