As wireless communication technology advances, the complexity of Vehicular Ad-hoc Network (VANET) simulations increases. This, however, is at odds with the need for increasingly large-scale Intelligent Transportation Systems (ITS) scenarios to satisfy the demands of increasingly Artificial Intelligence (AI) based solutions. This paper aims to demonstrate that a high-performance simplified VANET simulator can be used for fitness evaluation without loss in solution quality. As an example, we implement a Roadside Unit (RSU) deployment approach based on a genetic algorithm. Disolv, a simplified VANET simulator, is used as a fitness evaluation tool. To validate the solution quality, the best solutions of a few select generations are evaluated with a fully-featured ns-3 driven VANET simulation. From the comparison of fitness values, it can be observed that the values from Disolv allow us to predict those obtained via ns-3. Further, the execution time analysis showcases the substantial performance gains of using a more abstract VANET simulator. A 4.6-hour analysis with Disolv contrasts approximately 300 days with ns-3 for the given scenario and settings. Finally, the potential applications and limitations of using a simplified VANET simulator are discussed.

Platooning control – the automatic distance control in a chain of vehicles – has seen more than 2000 publications in the past 30 years, along with a corresponding number of different algorithms. However, comparisons between such controllers have rarely been done. Moreover, a fair comparison requires that all controller parameters are chosen to be optimal, often manually, which is a labor intensive and hard to replicate process. In this article we demonstrate the benefits of a methodology for parameter selection that encompasses: an evaluator employing a common metric, a simulator component, and an optimizer, all integrated into an optimization framework – along with an open-source reference implementation. We also discuss the trade-offs of different optimization algorithms, both from the literature and custom-built, for parameter optimization of platooning controllers.

Data transfer efficiency is a frequent bottleneck of distributed (co-)simulations and X-in-the-loop systems. One of the key reasons, particularly in Agent-Based Simulation (ABS), is related to the low serialization performance caused by non-optimal data layout in memory. To address these challenges, this paper explores the potentials of a Serialization-oriented Data Layout (SoDaLa) approach for ABS building on Data Oriented Design (DOD) principles. In this work we also introduce ABS_M, a model-based abstraction of memory access in ABS. Using this model, we evaluate the impact of SoDaLa. This is done also to promote the adoption of SoDaLa and to ease the assessment of data layout strategies prior to full implementation in an existing codebases. The results indicate that our proposed approach enhances serialization efficiency and highlights the trade-offs between serialization efficiency and simulation performance across different model specifics and hardware conditions.

Vehicular Ad-hoc Network (VANET) simulations are integral to pave way for rapid adoption of Intelligent Transportation Systems (ITS) applications. Popular VANET simulators are built with high-fidelity network and mobility models to enable comprehensive evaluation. However, the computational complexity of high-fidelity models inhibits scalability. Further, the increasing infrastructure density of 5G and beyond increases the node count, thereby imposing further performance constraints. In this paper, we present a VANET simulation methodology called Disolv to enable large-scale analysis. The key decisions towards performance enhancement include simplifying mobility and network models, abstraction of packets to messages, utilization of a discrete-time instead of discrete-event paradigm for message exchanges, incorporation of data streams, and pre-computation of network links. The proposed methodology is implemented to evaluate the benefits through experiments. Results indicate that significant performance gains are observed with statistically irrelevant loss of precision. Further experiments are carried out to highlight the limitations of the Disolv approach. Finally, ITS applications suitable for evaluation with the Disolv approach are discussed.

We propose SAMAC, a Device-to-Device (D2D) communication scheme that can exploit Low Earth Orbit (LEO) satellites as temporarily-available infrastructure with intermediate round-trip-times for supporting D2D medium access of highly mobile ground nodes such as vehicles. We demonstrate how such a scheme can be kept backwards-compatible to, e.g., IEEE 802.11p and we demonstrate analytically and in computer simulations that, compared to unassisted D2D communication or relaying, in situations where a LEO satellite is available it can improve performance in terms of all of throughput, latency bounds, and reliability.

The use of IoT and AI/ML to extract insights for Data-Driven Decision-Making (DDDM) in Intelligent Traffic Systems (ITS) is becoming increasingly popular. While simulation is a cost-effective and safe way to evaluate such approaches, existing simulators are often impractical due to inefficient control interfaces. In this work, we propose a Discrete-Event, Aggregating, and Relational Control Interfaces (DAR-CI) framework for achieving efficient traffic management simulations through a coupled approach. It enables a non-blocking interaction mode based on a discrete-event synchronization architecture. The overhead caused by data exchange is substantially reduced by supporting the direct retrieval of temporal metrics, data batch processing and customized in-situ aggregation. Combined with flexible, extendable, easy-to-understand, and implementation-friendly semantic specifications, we propose DAR-CI to serve as a universal tool for the traffic simulation community, taking the use and control of traffic simulation to a new level. A proof-of-concept study on the simulation of an adaptive traffic light control system demonstrates a 9.53X speedup compared to TraCI, a widely used protocol for controlling traffic simulators.

Vehicular Edge Computing (VEC) has emerged to offer cloud computing services closer to vehicular users by combining vehicles and edge computing nodes into Vehicular Clouds (VCs). In this scenario, an intelligent task scheduler must decide which VC will run which tasks, considering contextual aspects like vehicular mobility and tasks’ requirements. This is important to minimize both processing time and monetary costs. However, such direct optimization can lead to unfairness in resource usage, easily leading to (as we will show) decreased performance. Towards this end, in this work, we propose FARID, a task scheduling mechanism that considers contextual aspects of its decision process and applies a probabilistic selection function on VCs to balance the processing load and increase the fairness in the use of vehicular resources. Compared to state-of-the-art solutions, FARID has a higher level of fairness and can schedule more tasks while minimizing monetary costs and system latency.

Due to their unique mobility, Low Earth Orbit (LEO) satellites can provide network services in areas with little to no existing infrastructure and due to their low orbit they enable communication at low latencies. We thus motivate the use of LEO satellites for supporting multi-hop Vehicle-to-Everything (V2X) communication as an alternative to dense base station deployment and present CLEO, an approach that can opportunistically use LEO satellites for information dissemination among vehicles. Using extensive computer simulations, we show that not only are relaying latencies low enough for such an approach to be feasible, but we can actually improve performance of information dissemination in terms of all of Packet Delivery Ratio (PDR), Channel Busy Ratio (CBR), and end-to-end latency.

Platooning is a promising application in the field of vehicular networks. It has the potential to improve traffic flow, but also road safety. However, unreliable communication has strong negative effects on platoon stability and, thus, safety on roads. To improve reliability, in this work, we propose using multi-hop communication for platooning using the Decode and Forward (DF) Full-Duplex Relaying (FDR) scheme together with beamforming. We use computer simulations to demonstrate that FDR latency has no observable negative effect on string stability or safety even while performing an emergency brake. We further show that this combined approach reaches a constant Packet Delivery Ratio (PDR) of 100 % even in situations with high interference and/or congestion, where traditional approaches fail.

We study the suitability of Unmanned Aerial Vehicles (UAVs) as purely opportunistic airborne virtual network infrastructure to support urban wireless networks, specifically in two Vehicle-to-Everything (V2X) use cases: First, UAVs being used as relays for cooperative awareness applications; second, UAVs being used to coordinate channel access for platooning in urban areas. We do not require that these UAVs alter trajectory nor speed from those of their random, unrelated primary missions, so that these additional tasks can be executed with close-to-zero impact on the execution of their primary missions. Based on extensive computer simulations we show that, within a wide band of acceptable speeds, flight routes (up to a standard deviation of 300 m from the optimum) as well as altitudes, opportunistic relaying of transmissions via UAVs can yield a benefit to system performance that is on the same order of magnitude as that of optimally deployed UAVs. We further show that an opportunistic channel access control can reduce the total number of packet collisions by approx. 86 % compared to a scenario without any UAVs. Moreover, much of the reduction in impact due to suboptimal missions can be recovered simply by moderately increasing the number of UAVs.

Vehicular Edge Computing (VEC) is a promising paradigm that provides cloud computing services closer to vehicular users. In VEC, vehicles and communication infrastructures can form pools with computational resources to meet vehicular services with low-latency constraints. These resource pools are known as Vehicular Cloud (VC). The usage of VC resources requires a task scheduling process. In this case, depending on its complexity, a vehicular service can be divided into different tasks. An efficient task scheduling needs to orchestrate where and for how long such tasks will run, considering the available pools, the mobility of nodes, and the tasks deadline constraints. Thus, this article proposes an efficient VC task scheduler based on an approximation heuristic and resources prediction to select the best VC for each task, called MARINA. MARINA aims to analyze the behavior of vehicles that share their computational resources with the VC and make scheduling decisions based on the mobility (VC availability) of these vehicles. Simulation results under a realistic scenario demonstrate the efficiency of MARINA compared to existing state-of-the-art mechanisms in terms of the number of tasks scheduled, monetary cost, system latency, and Central Processing Unit (CPU) utilization.

Cooperative Driving requires ultra-reliable communications, and it is now clear that no single technology will ever be able to satisfy such stringent requirements, if only because active jamming can kill (almost) any wireless technology. Cooperative driving with multiple communication technologies which complement each other opens new spaces for research and development, but also poses several challenges. The work we present tackles the fallback and recovery mechanisms that the longitudinal controlling system of a platoon of vehicles can implement as a distributed system with multiple communication interfaces. We present a protocol and procedure to correctly compute the safe transition between different controlling algorithms, down to autonomous (or manual) driving when no communication is possible. To empower the study, we also develop a new version ofPlexe, which is an integral part of this contribution as the only Open Source, free simulation tool that enables the study of such systems with a modular approach, and that we deem offers the community the possibility of boosting research in this field. The results we present demonstrate the feasibility of safe fallback, but also highlight that such complex systems require careful design choices, as naïve approaches can lead to instabilities or even collisions, and that such design can only be done with appropriate in-silico experiments.

Vehicular Edge Computing is a promising paradigm that provides cloud computing services closer to vehicular users. Vehicles and communication infrastructure can cooperatively provide vehicular services with low latency constraints through vehicular cloud formation and using these computational resources via task scheduling. An efficient task scheduler must decide which cloud will run the tasks, considering vehicular mobility and task requirements. This is important to minimize processing time and, consequently, monetary cost. However, the literature solutions do not consider these contextual aspects together, degrading the overall system efficiency. This work presents EFESTO, a task scheduling mechanism that considers contextual aspects in its decision process. The results show that, compared to state-of-the-art solutions, EFESTO can schedule more tasks while minimizing monetary cost and system latency.

Due to the increased computational capacity of Connected and Autonomous Vehicles (CAVs) and worries about transferring private information, it is becoming more and more appealing to store data locally and move network computing to the edge. This trend also extends to Machine Learning (ML) where Federated learning (FL) has emerged as an attractive solution for preserving privacy. Today, to evaluate the implemented vehicular FL mechanisms for ML training, researchers often disregard the impact of CAV mobility, network topology dynamics, or communication patterns, all of which have a large impact on the final system performance. To address this, this work presents FLEXE, an Open Source extension to Veins that offers researchers a simulation environment to run FL experiments in realistic scenarios. FLEXE combines the popular Veins framework with the OpenCV library. Using the example of traffic sign recognition, we demonstrate how FLEXE can support investigations of FL techniques in a vehicular environment.

Platooning has repeatedly been shown to have the potential to increase both the efficiency and the safety of future road traffic. Yet, while there is ample research that illustrates its potential in freeway scenarios, the biggest challenges of traffic efficiency and safety lie in urban environments, which pose unique challenges: They are characterized by the presence of tight speed limits, intersections, traffic lights, and buildings. Moreover, work that has targeted such scenarios has, so far, not considered platoon formation strategies - or only considered idealized vehicle dynamics, platooning controllers, or wireless network dynamics. We fill this gap by investigating dynamic platoon formation at urban intersections controlled by regular traffic lights. We propose a fuel-efficient strategy for platoon formation at such intersections and investigate its performance using realistic simulation models of (all of) vehicle dynamics, platooning controllers, and wireless networking. Our investigation reveals the potential of the proposed strategy to simultaneously save both 15 % of fuel and 14 % of travel time.

Road traffic is continuously increasing worldwide and Vulnerable Road Users (VRUs)such as bicyclists are ever more susceptible to injuries from crashes. Vehicular Ad-hoc Network (VANET)technologies in combination with Advanced Driver Assistance Systems (ADAS)are currently being evaluated to enhance road traffic safety. The same technology is now also considered for warning cyclists about imminent crashes. Prototypes for such VANET -enabled bicycles already exist, however, the primary problem here is verification and test due to immediate safety concerns for the participants. Computer simulation is considered a more feasible alternative; however, realistic behavior models of VRUs are not available in the required quality and quantity. We developed a virtual cycling environment to address this issue, allowing to study and record cyclists' behavior. We collected (and make available)traces from a variety of subjects cycling towards an intersection for three different safety critical scenarios. As proof of concept, we then used these traces in our vehicular networking simulation framework to assess the impact of a simple VANET solution on the cyclists' safety. Our results demonstrate the need for such an integrated framework for empirical studies as well as for simulation-based exploration of system configurations.

Recent works have shown that it would be beneficial for nodes in wireless networks with very dynamic topology to maintain a list of 2-hop neighbors, namely, the neighbors of its neighbors. This is important, for example, for routing, clustering, and message dissemination to all the nodes in a given geographic vicinity. In this paper, we propose a scheme that uses Bloom filters for maintaining 2-hop neighborship information. Furthermore, we developed a novel 2-hop broadcast algorithm making use of the specific nature of our Bloom filter encoded neighbor information. We particularly focus on the Vehicular Ad Hoc Networks (VANETs) application scenario. Here, beaconing is a periodic broadcast of awareness messages by each vehicle to its immediate neighbors. A na€ıve approach would be to include all 2-hop neighbors in each beacon message, which, however, would work only for small or sparse scenarios. We show that our approach significantly reduces the length of the beacon messages, thereby keeping channel load and collision probability considerably lower than in the na€ıve scheme. We further demonstrate the application of our Bloom filter based 2-hop neighbor table for developing higher layer protocols and introduce a multi-hop broadcast protocol called Bloom Hopping.

According to a recent UN report, continuing population growth and urbanization are expected to increase the world’s urban population by 2.5 billion people by 2050, with 2.9 billion extra vehicles. This massive growth in both population and number of vehicles, together with urban transformation and a trend toward mega cities, creates greater and more challenges for achieving smart transportation goals in smart cities. Therefore, new and more integrated modes of transportation, and environment friendly solutions are required to accommodate the rising demands of high liveability in smarter cities that offer safe, secure, affordable, reliable and sustainable transportation in old and new markets alike.

We present a complete simulation and experimentation framework for IEEE 802.11p. The core of the framework is a Software Defined Radio (SDR)-based Orthogonal Frequency Division Multiplexing (OFDM) transceiver that we validated extensively by means of simulations, interoperability tests, and, ultimately, by conducting a field test. Being SDR-based, the transceiver offers important benefits: It provides access to all data down to and including the physical layer, allowing for a better understanding of the system. Based on open and programmable hardware and software, the transceiver is completely transparent and all implementation details can be studied and, if needed, modified. Finally, it enables a seamless switch between simulations and experiments and, thus, helps to bridge the gap between theory and practice. Comparing the transceiver's performance with independent results from simulations and experiments, we underline its potential to be used as a tool for further studies of IEEE 802.11p networks both in field operational tests as well as for simulation-based development of novel physical layer solutions. To make the framework accessible to fellow researchers and to allow reproduction of the results, we released it under an Open Source license.

A major building block of Vehicular Ad Hoc Networks (VANETs) is broadcasting: the use of wireless communication for sharing information among vehicles, or between the vehicles and infrastructure. Dozens of broadcast protocols have been developed in recent years, including protocols for 1-hop broadcasting of vehicle status information (beaconing) and for geocasting-based applications. However, most of these protocols were designed for one application and cannot co-exist, nor can one broadcast solution meet the demands of all applications. These observations motivated our effort to develop a holistic network layer for VANETs. We identify the need for making VANET broadcast context-aware, and for supporting four different classes of broadcast protocols, each with its own properties. These classes are not only able to co-exist on the same network layer, but also to complement one another's functionality. Thus, large applications as well as more holistic Transport protocols can be designed by combining two or more broadcast classes. We discuss the specific characteristics of these classes and design candidate protocols for each class.

We present LAN Radio, an Open Source prototyping system to couple simulation and real world for the wireless channel of IEEE 802.11p based communication. Our main focus is the use in a vehicular networking environment, where field testing and real world experimentation is becoming more relevant these days. Large Field Operational Tests (FOTs) are expensive and difficult to handle in terms of reproducibility and controllability -- particularly for application development and integration tests for automotive Electronic Control Units (ECUs). To support such experiments, we developed an approach to integrate a real system to test into a large scale simulation scenario without the need to change the physical and access layer parts of the communication stack of this system. We closely followed the Hardware In The Loop (HIL) simulation concept but also integrated a wireless communication channel into the picture. LAN Radio is building upon the well established and Open Source development platform OpenWRT to provide optimal extensibility.

We present OpenC2X, an Open Source approach to field testing of vehicular networking solutions. Field Operational Test (FOT) and real-world experimentation are becoming more relevant to our research community as well as to industry and regulation. Unfortunately, available commercial solutions make experimental modifications to the protocol stack time consuming or prohibit it completely. To overcome this limitation, we implemented the ETSI ITS-G5 stack on a standard embedded PC hardware and running Linux system. OpenC2X is the first complete Open Source experimentation and prototyping platform. Our system is fully interoperable to commercial solutions, yet easily extensible with new protocols and applications for vehicular networks.

Advances in miniaturization of sensor nodes enable a wide range of novel application scenarios. At the same time, however, this miniaturization drastically reduces the energy available for communication. We focus on wildlife monitoring applications for bats, which set a weight limit of 2 g for the sensor node including the battery. Here, the protocol design is complicated by the need to recharge a capacitor before each communication attempt. For communications with ground stations, wake-up receivers are used that inherently help mitigate synchronization demands and to provide a superframe structure. We study the not obvious choice of transmission slots within these synchronized superframes. Our findings clearly indicate that slotted access outperforms simple random channel access. Well-planned TDMA schedules only bring little gain compared to random slot selection.

The efficient usage of multiple channels allocated for vehicular communications is deemed crucial to support the increasing spectrum demand of current and upcoming Intelligent Transportation Systems (ITS). In this context, an accurate modeling of adjacent channel interference (ACI) is needed to better understand its adverse effects on simultaneous communications on different channels. In this paper, we modeled ACI in the simulation framework Veins by strictly following the standard specifications in what concerns spectral emission masks. The model was validated in a set of experiments using off-the-shelf chipsets and a software defined radio. We then studied ACI effects at the physical and packet levels in multi-channel vehicular networks through extensive simulations. Our results clearly indicate that slightly larger delays and significantly lower reliability can be experienced when ACI is modeled. Insights are also provided about the usability of the devised ACI simulation model in future studies.

An Intersection Assistance System aim to assist road users in avoiding collisions at intersections, either by warning the driver or by triggering automated actions. Such a system can be realized based on passive scanning only (e.g., using LiDAR) or supported by active Inter-Vehicle Communication (IVC). The main reason to use Inter-Vehicle Communication (IVC) is its ability to provide situation awareness even when a possible crash candidate is not yet in visual range. The IVC research community has identified beaconing, i.e., one-hop broadcast, as the primary communication primitive for vehicular safety applications. Recently, adaptive beaconing approaches have been studied and different congestion control mechanisms have been proposed to cope with the diverse demands of vehicular networks. In this paper, we show that current state-of-the-art congestion control mechanisms are not able to support IAS adequately. Specifically, current approaches fail due to their inherent fairness postulation, i.e., they lack fine grained prioritization. We propose a solution that extends congestion control mechanisms by allowing temporary exceptions for vehicles in dangerous situations, that is, situation-based rate adaptation. We show the applicability for two state-of-the-art congestion control mechanisms, namely Transmit Rate Control (TRC) and Dynamic Beaconing (DynB), in two different vehicular environments, rural and downtown.

We study the effect of radio signal shadowing dynamics, caused by vehicles and by buildings, on the performance of beaconing protocols in Inter-Vehicular Communication (IVC). Recent research indicates that beaconing, i.e., one hop message broadcast, shows excellent characteristics and can outperform other communication approaches for both safety and efficiency applications, which require low latency and wide area information dissemination, respectively. To mitigate the broadcast storm problem, adaptive beaconing solutions have been proposed and designed. We show how shadowing dynamics of moving obstacles hurt IVC, reducing the performance of beaconing protocols. To the best of our knowledge, this is one of the first studies on identifying the problem and the underlying challenges and proposing the opportunities presented by such challenges. Shadowing also limits the risk of overloading the wireless channel. We demonstrate how these challenges and opportunities can be taken into account and outline a novel approach to dynamic beaconing. It provides low-latency communication (i.e., very short beaconing intervals), while ensuring not to overload the wireless channel. The presented simulation results substantiate our theoretical considerations.

Many people use smart phones on a daily basis, yet, their energy consumption is pretty high and the battery power lasts typically only for a single day. In the scope of the EnAct project, we investigate potential energy savings on smart phones by offloading computationally expensive tasks into the cloud. Obviously, also the wireless communication for uploading tasks requires energy. For that reason, it is crucial to understand the trade-off between energy consumption for wireless communication and local computation in order to assert that the overall power consumption is decreased. In this paper, we investigate the communications part of that trade-off. We conducted an extensive set of measurement experiments using typical smart phones. This is the first step towards the development of accurate energy models allowing to predict the energy required for offloading a given task. Our measurements include WiFi, 2G, and 3G networks as well as a set of two different devices. According to our findings, WiFi consumes by far the least energy per time unit, yet, this advantage seems to be due to its higher throughput and the implied shorter download time and not due to lower power consumption over time.

We study the ability of Inter-Vehicle Communication (IVC) solutions to handle real-time requirements in safety scenarios using beaconing as a communication primitive. One of the envisioned safety applications is intersection assistance. The objective of such applications is to either warn the driver or even to act autonomously if other approaching vehicles endanger the vehicle. Fairness, combined with aggressive channel access for low-latency safety messages, has been one of the main research line according to which state of the art congestion control mechanisms have been developed. We show that these solutions are not able to sufficiently support intersection assistance applications. Specifically, we show that current approaches fail exactly due to their fairness postulation. We propose a new situation-aware solution to this fairness dilemma by allowing temporary exceptions for vehicles in dangerous situations. We show the applicability for two state of the art congestion control mechanisms, namely ETSI Transmit Rate Control (TRC) and Dynamic Beaconing (DynB). Our investigation also reveals important research objectives for future IVC protocols, namely how much reactivity and situation-awareness is needed in the highly dynamic environment of vehicular networks.

We study the effectiveness of Inter-Vehicle Communication (IVC) in urban and suburban environments at low node densities, with a particular focus on cooperative awareness and traffic safety. The recently standardized DSRC/WAVE protocol suite defines a platform for such applications, which mainly focus on beaconing, i.e., periodic 1-hop-broadcast. In general, such safety relevant transmissions are defined by time criticality. One of the major problems to be solved is how to tackle the very difficult and complex radio signal attenuation due to buildings and other obstacles, especially in cities. Typical concepts address this problem by requiring all vehicles to also act as relays or by using dedicated Roadside Units (RSUs). We show how such systems may be operated more efficiently and how the situation can be further improved by relying on parked vehicles in addition to, or as a replacement for, RSUs. Given the fact that the U.S. DOT is already evaluating whether to make DSRC mandatory for new cars, wide availability of radio equipped cars can be predicted; also the impact in terms of energy consumption is negligible. We performed an extensive set of simulations to evaluate the negative impact of buildings at low node densities and the benefit of our proposal. Our results clearly indicate that situation awareness can be significantly improved. When disseminating safety critical events in a realistic scenario, reasonable numbers of parked cars can increase cooperative awareness by up to 25%, a factor which requires an unreasonably costly number of RSUs. To the best of our knowledge, we are the first to propose the utilization of parked vehicles as relay nodes for safety applications in vehicular networks.

We study the effect of radio signal shadowing dynamics, caused by vehicles and by buildings, on the performance of beaconing protocols in Inter-Vehicular Communication (IVC). Recent research indicates that beaconing, i.e., one hop message broadcast, shows excellent characteristics and can outperform other communication approaches for both safety and efficiency applications, which require low latency and wide area information dissemination, respectively. We show how shadowing dynamics of moving obstacles hurt IVC, reducing the performance of beaconing protocols. At the same time, shadowing also limits the risk of overloading the wireless channel. To the best of our knowledge, this is the first study identifying the problems and resulting possibilities of such dynamic radio shadowing. We demonstrate how these challenges and opportunities can be taken into account and outline a novel approach to dynamic beaconing. It provides low-latency communication (i.e., very short beaconing intervals), while ensuring not to overload the wireless channel. The presented simulation results substantiate our theoretical considerations.

Security and privacy requirements in vehicular networks are typically addressed using a Public Key Infrastructure (PKI) and pools of pseudonymous certificates for each vehicle. Messages are signed with these certificates, so that misbehaving vehicles can be excluded from the network by disseminating Certificate Revocation Lists (CRLs). We present SmartRevoc, a novel revocation architecture, to solve the main challenges of CRL distribution in vehicular networks: CRL size, dissemination speed, and preservation of location privacy. Using two hash chains, we compute CRLs on vehicles, substantially reducing communication overhead. At the same time our design is privacy preserving, preventing the linking of past pseudonyms to revoked ones. We additionally utilize parked vehicles to epidemically disseminate CRLs and show that even few parked vehicles can outperform an unreasonably high number of Roadside Units (RSUs). Our simulation results clearly indicate that SmartRevoc is a major step forward toward more efficient and secure vehicular networks.

Chirp Spread Spectrum (CSS) based localization techniques are becoming more attractive as they provide improved localization accuracy and robustness compared to WiFi or ZigBee based approaches. However, a remaining problem is the necessary update frequency: In existing CSS based localization systems, the positions of the objects are determined one by one via unicast with nearby anchors instead of using broadcasts. We propose a faster distributed localization scheme for CSS based systems. A portion of nodes are considered cluster heads; they determine the locations of un-localized nodes by dynamically increasing the transmission power. Our novel scheme not only fully utilizes the spatial redundancy, which is crucial for speeding up the localization process. By also allowing to establish new anchors in a two-hop range, we can further increase speed without significantly influencing localization error. The performance of the proposed method is demonstrated through simulation.

We just released an Open Source receiver that is able to decode IEEE 802.11a/g/p Orthogonal Frequency Division Multiplexing (OFDM) frames in software. This is the first Software Defined Radio (SDR) based OFDM receiver supporting channel bandwidths up to 20MHz that is not relying on additional FPGA code. Our receiver comprises all layers from the physical up to decoding the MAC packet and extracting the payload of IEEE 802.11a/g/p frames. In our demonstration, visitors can interact live with the receiver while it is decoding frames that are sent over the air. The impact of moving the antennas and changing the settings are displayed live in time and frequency domain. Furthermore, the decoded frames are fed to Wireshark where the WiFi traffic can be further investigated. It is possible to access and visualize the data in every decoding step from the raw samples, the autocorrelation used for frame detection, the subcarriers before and after equalization, up to the decoded MAC packets. The receiver is completely Open Source and represents one step towards experimental research with SDR.

Content access and downloading in vehicular environments is expected to heavily rely on the availability of roadside infrastructure. Although vehicle-to-vehicle communication is foreseen, data will mostly flow through roadside access points, or RSUs (RoadSide Units), which suffer from less connectivity problems. However, at least in the early stages of deployment, the RSU coverage will be spotty, or limited to main avenues in urban areas. In this paper, we try to address such shortcomings by investigating the possibility of exploiting parked vehicles to extend the RSU service coverage. Our approach leverages optimization models aiming at maximizing both the freshness of the content that downloaders retrieve and the efficiency in the utilization of radio resources. Performance evaluation highlights that the use of parked vehicles enhances the benefits of the content downloading process and leads to a significant offload of the RSUs, with respect to the case where only mobile relays are used.

In the scientific community, Inter-Vehicle Communication (IVC) protocols are frequently evaluated using simulation techniques, often using variants of WiFi stacks instead of IEEE 802.11p, which constitutes the basis for the new DSRC/WAVE standard. We discuss the necessity of using accurate WAVE models based on an extensive set of simulation experiments using: an IEEE 802.11b model, an IEEE 802.11b model tweaked to work in the same frequency range and using similar timings like IEEE 802.11p, as well as a fully featured channel hopping WAVE model. Even though, intuitively, the use of the different protocols will lead to a different network behavior, to the best of our knowledge, there has been no qualitative and quantitative evaluation or comparison of both worlds. According to our results, we can conclude that the simple WiFi model may indeed be used, but only for extremely sparse scenarios - this is exactly what has been validated using field tests. In denser traffic scenarios there is a significant deviation of the protocol behavior between WiFi (and its adapted variant) compared to WAVE. Thus, especially in dense scenarios, the application behavior is strongly influenced if simulated with the wrong model - leading to unrealistic results.

Many safety applications in Intelligent Transportation Systems (ITS) require vehicles to be aware of the presence of nearby cars, but - especially in urban and suburban regions - buildings and other obstacles may block radio transmissions. In the literature, multi-hop relaying by neighboring cars has been demonstrated to perform well at disseminating safety broadcasts in the presence of obstacles. At night, in areas with low traffic density, or when the penetration rate of Car-2-X devices is low, however, there are likely to be too few relaying cars available. This again leads to the problem that vehicles which are not in line-of-sight frequently cannot be sensed either. To the best of our knowledge, we are the first to help overcome this problem by utilizing parked cars as relay nodes. We study the effectiveness and the necessity of this approach with the help of extensive simulative studies and real life experiments. We show how, for scenarios with few equipped cars, the utilization of parked cars proves crucial to support safety applications. When disseminating safety critical events in a realistic scenario, parked cars can increase cooperative awareness by over 40% in total.

Recently, many efforts have been made to develop more efficient Inter-Vehicle Communication (IVC) protocols for on-demand route planning according to observed traffic congestion or incidents, as well as for safety applications. Because practical experiments are often not feasible, simulation of network protocol behavior in Vehicular Ad Hoc Network (VANET) scenarios is strongly demanded for evaluating the applicability of developed network protocols. In this work, we discuss the need for bidirectional coupling of network simulation and road traffic microsimulation for evaluating IVC protocols. As the selection of a mobility model influences the outcome of simulations to a great extent, the use of a representative model is necessary for producing meaningful evaluation results. Based on these observations, we developed the hybrid simulation framework Veins (Vehicles in Network Simulation), composed of the network simulator OMNeT++ and the road traffic simulator SUMO. In a proof-of-concept study, we demonstrate its advantages and the need for bidirectionally coupled simulation based on the evaluation of two protocols for incident warning over VANETs. With our developed methodology, we can advance the state-of-the-art in performance evaluation of IVC and provide means to evaluate developed protocols more accurately.

The aviation community is currently working on the standardization of data communication systems for the future air traffic management. In this context, the ICAO and EUROCONTROL are working on the standardization of IP-based aeronautical telecommunications network and future radio access technologies, respectively. With this work, for the first time, we integrate L-DACS 1, which is one candidate for future radio access technologies, with realistic IP-based network layer functionality and analyze the handover delay performance. We first investigate the effect of link layer retransmissions on handover delay performance. We realized that for regions with low signal-to-noise ratio (BER of 1e-3), link layer retransmissions improve the total handover delay (layer 2 and 3) by about 80%. Considering regions with high signal-to-noise ratio (BER of 1e-5), the benefit of link layer retransmissions becomes negligible due to the reduced number of packet losses. During our analysis, we notice frequent transmission of router advertisement messages causing significant overhead on L-DACS 1 and propose two approaches in order to decrease the overhead to an acceptable range. In the last section, we tackle the increase in handover delay due to a limited number of return link resource request opportunities in congested cells.

We investigate the impact of the human driver behavior on Intelligent Transportation Systems (ITS). Motivated by early studies, which investigated the driver behavior and which provided a rough classification of different reactions on presented traffic information, we integrated such models into our simulation framework Veins. We were able to determine a significant change in the performance of a technically optimized system due to suboptimal reactions of the drivers. The main objective of this study is to outline the need for integrated driver models into the development process of Traffic Information System (TIS) solutions. Based on our simulation framework, not only the driver model can be accurately represented but we were also able to show that simple probabilistic models show exactly the same behavior compared to sophisticated empirical measurement based ones.

We discuss the need for taking the environmental impact as a primary metric for evaluating the quality of algorithms for Intelligent Transportation Systems (ITS). In many studies, the travel time, or its minimization, has been used to demonstrate the advantages of Inter-Vehicle Communication (IVC) solutions combined with dynamic rerouting. Such evaluations are frequently based on simulation experiments. Recently, much progress has been achieved in this domain by coupling network simulation with road traffic microsimulation. We now investigate the relationship between different metrics used for the evaluation, in particular the environmental impact represented by gas consumption and emissions versus the travel time, highlighting cases where both metrics are conflicting.

Efficient data management techniques are needed in wireless sensor networks (WSNs) to counteract issues related to limited resources, e.g. energy, memory, bandwidth, as well as limited connectivity. Self-organizing and cooperative algorithms are thought to be the optimal solution to overcome these limitations. On an abstract level, structured peer-to-peer protocols provide O(1) complexity for storing and retrieving data in the network. However, they rely on underlayer routing techniques. In this paper, we present the virtual cord protocol (VCP), a virtual relative position based efficient routing protocol that also provides means for data management, e.g. insert, get, and delete, as known from typical distributed hash table (DHT) services. The key contributions of this protocol are independence of real location information by relying on relative positions of neighboring nodes, short virtual paths because successors and predecessors are in their vicinity, and high scalability because only information about direct neighbors is needed for routing. Furthermore, VCP inherently prevents dead-ends and it is easy to be implemented.

Early approaches for Traffic Information Systems (TISs) primarily focused on centralized systems using unidirectional downlink communication and employing wireless broadcast or similar techniques. In general, these centralized TISs were operated by public radio stations, thus there were almost no security issues related to this approach. The situation changes when new Inter Vehicle Communication (IVC) techniques are investigated. The advantages of improved timeliness and accuracy of available traffic information come with a number of security concerns. This paper reviews the requirements and objectives for secure TISs. We outline possible solutions to face the security concerns and clearly depict open issues. In conclusion, we advocate more secure TISs that benefit from recent IVC technologies in a more secure and privacy protecting way.

Coupling Vehicular Ad Hoc Networks (VANETs) with wired networks such as the Internet via access points creates a difficult mix of highly mobile nodes and a static infrastructure. In order to evaluate the performance of typical ad hoc routing protocols-in particular, we used Dynamic MANET On Demand (DYMO)-in such VANET scenarios, we combined microsimulation of road traffic and event-driven network simulation. Thus, we were able to analyze protocols of the Internet protocol suite in VANET scenarios with highly accurate mobility models. Varying parameters of DYMO for a multitude of traffic and communication scenarios helped point out approaches for improving the overall performance and revealed problems with the deployment. It could be shown that in realistic scenarios, even for medium densities of active nodes and low network load, overload behavior leads to a drastic decrease of the perceived network quality. Cross-layer optimization of transport and routing protocols therefore seems highly advisable.