Felix Schmidt-Eisenlohr

Overhaul of ieee 802.11 modeling and simulation in ns-2

NS-2, with its IEEE 802.11 support, is a widely utilized simulation tool for wireless communications researchers. However, the current NS-2 distribution code has some significant shortcomings both in the overall architecture and the modeling details of the IEEE 802.11 MAC and PHY modules. This paper presents a completely revised architecture and design for these two modules. The resulting PHY is a full featured generic module able to support any single channel frame-based communications (i.e. it is also able to support non-IEEE 802.11 based MAC). The key features include cumulative SINR computation, preamble and PLCP header processing and capture, and frame body capture. The MAC accurately models the basic IEEE 802.11 CSMA/CA mechanism, as required for credible simulation studies. The newly designed MAC models transmission and reception coordination, backoff management and channel state monitoring in a structured and modular manner. In turn, the contributions of this paper make extending the MAC for protocol researches much easier and provide for a significantly higher level of simulation accuracy.

Effects of a realistic channel model on packet forwarding in vehicular ad hoc networks

The discrepancy between real-world radio channel behavior and its standard modeling in simulations (unit disk graph) is a major reason for protocols to perform differently - often worse - than predicted when deployed in a real-world setup. As researchers having to deal with real ad hoc networks are aware of, assuming a fixed border for a nodes communication range might not only lead to inaccurate results but also to a wrong judgment on the comparison between different protocols. We have set up a simulation study to investigate the effects of realistic channel characteristics on packet forwarding strategies for vehicular ad hoc networks. The contributions of this paper are threefold: i) we provide a performance evaluation of various routing/forwarding strategies under the realistic non-deterministic Nakagami radio propagation model and compare the results with the ones obtained using the standard two-ray-ground model. Validated German highway movement patterns are used to model node mobility. ii) We demonstrate that realistic channel conditions present an opportunity and not only a drawback for some forwarding strategies. More specifically, we show that for contention-based forwarding (CBF) techniques, realistic channel characteristics provide a positive impact in terms of an increased average hop distance. iii) We provide an analytical derivation of the expected hop distance for CBF that provides a basis to optimally adjust CBF parameters

Simulation platform for inter-vehicle communications and analysis of periodic information exchange

Due to the large amount of nodes that could comprise a vehicular network, simulation is a valuable tool used for the design and analysis of inter-vehicle communication protocols. In this paper we first describe all the components of a simulation framework that has ns-2 as core building block. Our main contributions are an accurate implementation of the MAC and PHY modules adjusted to vehicular environments together with a well defined set of metrics to evaluate communication protocols for vehicular networks. Second, we make use of the simulation framework to perform a detailed analysis of periodic exchange of broadcast messages when adjusting transmission power and packet generation rate for different propagation models

Introducing probabilistic radio propagation models in OMNeT++ mobility framework and cross validation check with NS-2

When performing wireless network simulations, the lack of precise channel modeling in simulator frameworks becomes a serious problem. Often deterministic models are used for packet propagation, which describe real conditions insufficiently. To close this gap we extended the OMNeT++ Mobility Framework to support probabilistic propagation models. We provide an implementation for the Log-Normal-Shadowing, Nakagami, Rayleigh and Rice wave propagation models and set up a framework that allows easy integration of additional models in future. Due to the characteristics of probabilistic radio models a fixed maximum packet propagation range encounters the problem of inaccurate simulation results as relevant events may be suppressed. On the other hand, unlimited packet propagation, which guarantees for correct simulation runs, causes unnecessary simulation overhead. In this work we present an approach to limit the event delivery to the area where the probability that the event is relevant to the simulation exceeds an adjustable threshold. In order to validate our extensions we successfully performed a detailed crosscheck with the network simulator NS-2 and run a performance evaluation and comparison.

A change in perspective: information-centric modeling of inter-vehicle communication

Inter-vehicle communication with the objective to increase safety and efficiency of our transportation network has been studied extensively in the last few years. However, we are still struggling in answering the fundamental question of to which extent can inter-vehicle communication improve our wide-area transportation network that consists of more than several cities?. In this position paper we discuss from a methodological point of view and by analyzing recent trends how the VANET community could move forward with respect to evaluation methodologies. Based on our observations, we argue that the vehicular networking community might need to extend its perspective -- from bits and packets towards information-centric models -- when assessing efficiency gains in large scale scenarios, thereby providing a multi-scale approach as it is done in various disciplines. We further believe that the community should join forces to convert existing results as well as current and upcoming measurements to provide information-centric models of inter-vehicular communication. We finally propose a modeling approach that could serve as a foundation for future joint efforts and which might take us one step closer to the answer of the fundamental question stated above.

Accurate knowledge of radio channel and network conditions — When does it matter?

One of the main motivations of wireless communication between vehicles is to establish a mutual awareness among vehicles. Such awareness is achieved through the periodic exchange of broadcast messages with information about the current position, heading or speed of a vehicle. The probability that neighboring vehicles receive such messages successfully depends on the current radio channel and network conditions. However, those conditions are not constant but varying due to the wide range of propagation environments, the number of vehicles in the surroundings and the transmission behavior of each vehicle. In order to cope with the current conditions and to achieve a desired communication reliability, communication protocols will have to be dynamically adjusted, e.g. by adjusting transmit power and transmit rate. The questions are therefore: how accurate do vehicles need to know the conditions and in which situations do they need to know them? This paper gives clarifications to those questions by means of a simulation-based sensitivity analysis, and in addition, surveys different methods of obtaining the required knowledge about radio channel and network conditions.

Simulation-based capacity estimates for local broadcast transmissions

The periodic transmission of status updates by all vehicles in a vehicular network represents a service primitive that forms the basis for a lot of envisioned applications, in particular safety related ones. Due to the limited resources that a wireless communication system like IEEE 802.11p is capable to provide, the question raises how much data each node may provide to the system such that the information can still be delivered with the quality of service required by the applications. In this work, local broadcasts capacity is introduced together with straight-forward upper and lower bounds, and estimated by extensive detailed simulations. We show that the ratio of simulation-based capacity estimates and the upper bound is similar for a wide range of system configurations and that the communication system may only be used up to 22% of its upper capacity bound such that service requirements can still be fulfilled.

Simulative Analysis of Vehicle-to-X Communication considering Traffic Safety and Efficiency

Over the past several years, there has been significant interest and progress in using wireless communication technologies for vehicular environments in order to increase traffic safety and efficiency. Due to the fact that these systems are still under development and large-scale tests based on real hardware are difficult to manage, simulations are a widely-used and cost-efficient method to explore such scenarios. Furthermore, simulations provide a possibility to look at specific aspects individually and to identify major influencing effects out of a wide range of configurations. In this context, we use the HP XC4000 for an extensive and detailed sensitivity analysis in order to evaluate the robustness and performance of communication protocols as well as to capture the complex characteristics of such systems in terms of an empirical model.

Visualizing and Understanding Spatio-Temporal Correlations of Data Dissemination in Vehicular Environments

The evaluation of data-dissemination protocols in network simulation is commonly based on statistical averaging. This macroscopic approach does not take into account spatio-temporal effects that may influence protocol behavior on microscopic level. In this paper, we introduce a visualization approach based on ns-2 trace files to graphically reproduce the dissemination process step by step. Our visualizer currently supports the data-dissemination protocol EMDV and can be extended to support other protocols as well.

Vehicle-to-Vehicle Communications: Reception and Interference of Safety-Critical Messages (Fahrzeug-zu-Fahrzeug-Kommunikation: Empfang und Interferenz sicherheitskritischer Nachrichten)

Summary A robust exchange of messages between vehicles via radio communication represents a key problem of vehicular ad hoc network (VANET) research. Environmental influences and the multitude of communicating nodes result in challenging communication conditions that have to be taken into account when assessing the potential benefit of VANETs for traffic safety and efficiency applications. In this paper we discuss an appropriate modeling of three influencing factors on the communication behavior to establish a basis for credible simulation results. In detail, we analyze the effects of fast-fading, capturing, and cumulative noise on vehicular communications and illustrate their considerable influence on a simulations outcome. As a second contribution, we provide an empirical model for the probability of packet reception that is based on the proposed simulation methodology. The model thus allows to determine credible simulation results without being dependent on the complexity of detailed simulations. The saved computational effort facilitates the assessment of VANETs in large-scale scenarios and the consideration of communication specifics in the design process. Zusammenfassung Der robuste Austausch von Nachrichten zwischen Fahrzeugen mittels Funkkommunikation stellt ein wesentliches Problem der Forschung über Fahrzeug-Ad-Hoc-Netzwerke (VANET) dar. Umwelteinflüsse und die große Anzahl miteinander kommunizierender Fahrzeuge verursachen schwierige Kommunikationsbedingungen, die es bei der Abschätzung des möglichen Nutzens von VANETs auf die Verkehrssicherheit und -effizienz zu berücksichtigen gilt. Die vorliegende Arbeit diskutiert eine geeignete Modellierung von Einflussfaktoren auf das Kommunikationsverhalten und schafft damit eine Basis für glaubwürdige Simulationsergebnisse. Wir untersuchen den Einfluss von “Fast Fading” und die Bedeutung von “Packet-Capturing” und diskutieren die Auswirkung von kumuliertem Rauschen auf dem Kommunikationskanal. Für alle Effekte demonstrieren wir ihren entscheidenden Einfluss auf die Ergebnisse einer Simulation. Aufbauend auf dieser detaillierten Modellierung stellen wir ein empirisches Modell vor, das die Empfangswahrscheinlichkeit von Paketen in Abhängigkeit von der Entfernung und der Verkehrsdichte bestimmt. Dieses Modell ermöglicht das Erzielen akkurater Simulationsergebnisse bei reduzierter Komplexität des Simulationsvorgangs. Der eingesparte Rechenaufwand vereinfacht die Simulation von sehr großen Fahrzeugnetzen und die Berücksichtigung von Kommunikationsspezifika in der Anwendungsentwicklung.