Showing papers by "Hannes Hartenstein published in 2004"

Stochastic properties of the random waypoint mobility model

Abstract: The random waypoint model is a commonly used mobility model for simulations of wireless communication networks. By giving a formal description of this model in terms of a discrete-time stochastic process, we investigate some of its fundamental stochastic properties with respect to: (a) the transition length and time of a mobile node between two waypoints, (b) the spatial distribution of nodes, (c) the direction angle at the beginning of a movement transition, and (d) the cell change rate if the model is used in a cellular-structured system area. The results of this paper are of practical value for performance analysis of mobile networks and give a deeper understanding of the behavior of this mobility model. Such understanding is necessary to avoid misinterpretation of simulation results. The movement duration and the cell change rate enable us to make a statement about the "degree of mobility" of a certain simulation scenario. Knowledge of the spatial node distribution is essential for all investigations in which the relative location of the mobile nodes is important. Finally, the direction distribution explains in an analytical manner the effect that nodes tend to move back to the middle of the system area.

Broadcast reception rates and effects of priority access in 802.11-based vehicular ad-hoc networks

Abstract: One key usage of VANET is to support vehicle safety applications. This use case is characterized by the prominence of broadcasts in scaled settings. In this context, we try to answer the following questions: i) what is the probability of reception of a broadcast message by another car depending on its distance to the sender, ii) how to give priority access and an improved reception rate for important warnings, e.g., sent out in an emergency situation, and iii) how are the above two results affected by signal strength fluctuations caused by radio channel fading? We quantify via simulation the probability of reception for the two-ray-ground propagation model as well as for the Nakagami distribution in saturated environments. By making use of some IEEE 802.11e EDCA mechanisms for priority access, we do not only quantify how channel access times can be reduced but also demonstrate how improved reception rates can be achieved. Our results show that the mechanisms for priority access are successful under the two-way-ground model. However, with a non-deterministic radio propagation model like Nakagami's distribution the benefit is still obvious but the general level of probability of reception is much smaller compared to two-ray-ground model. The results indicate that -- particularly for safety-critical and sensor network type of applications -- the proper design of repetition or multi-hop retransmission strategies represents an important aspect of future work for robustness and network stability of vehicular ad hoc networks.

Contention-based forwarding for street scenarios

Abstract: In this paper, we propose to apply Contention-Based Forwarding (CBF) to Vehicular Ad Hoc Networks (VANETs). CBF is a greedy position-based forwarding algorithm that does not require proactive transmission of beacon messages. CBF performance is analyzed using realistic movement patterns of vehicles on a highway. We show by means of simulation that CBF as well as traditional position-based routing (PBR) achieve a delivery rate of almost 100 given that connectivity exists. However, CBF has a much lower forwarding overhead than PBR since PBR can achieve high delivery ratios only by implicitly using a trial-and-error next-hop selection strategy. With CBF, a better total throughput can be achieved. We further discuss several optimizations of CBF for its use in VANETs, in particular a new position-encoding scheme that naturally allows for communication paradigms such as `street geocast` and `street flooding`. The discussions show that CBF can be viewed as a concept for convergence of intelligent flooding, geocast, and multi-hop forwarding in the area of inter-vehicle communication.

FleetNet: Bringing Car-to-Car Communication into the Real World

Abstract: Car-to-car communication by means of wireless technology shows a strong potential to enhance both safety and comfort of road users. The necessity to reach cars beyond the transmission range of the own radio creates the challenge to find algorithms to efficiently forward packets. In these so-called vehicular ad hoc networks, position information has been identified as a crucial component that alleviates some of the limitations of existing topology-based protocols. Thus, we see the use of positional information at the core of a car-to-car communication system that provides advanced applications for active safety, distributed floating car data, as well as user communication and information. A major thrust to leverage a car-to-car communication platform was provided by the FleetNet project, partly funded by the German Ministry of Education and Research BMB+F and led by DaimlerChrysler AG. Based on ad hoc networking principles and the availability of position information, the project developed suitable communication concepts. In addition to simulation studies, a prototype communication and application system was implemented and deployed, embedding safety and convenience applications. With car-based real world field trials successfully conducted, first results and experiences look promising.

Performance measurements of a vehicular ad hoc network

Abstract: Within the framework of the FLEETNET project, a real-world ad hoc network testbed was built using cars as mobile nodes. These cars were equipped with computing and communication devices. As ad hoc routing algorithm, position-based forwarding implemented on Linux-based laptops was used, exploiting position information gathered by on-board navigation systems. We report on measurements and experiences with this testbed for static 1-hop and 3-hop scenarios as well as for mobile 3-hop scenarios. Beside the power, loss-rate, throughput and delivery ratio measurements, we present our methodology for conducting experiments with 'real-world' mobile ad hoc networks and discuss problems of beacon-based position-based routing due to radio fluctuations.

Proceedings of the 1st ACM international workshop on Vehicular ad hoc networks

Abstract: Welcome to the Second ACM International Workshop on Vehicular Ad Hoc Networks! Vehicular ad hoc networks show an enormous potential to increase traveler safety and comfort and can serve to improve the efficiency of the transportation infrastructure. The creation of high-performance, highly reliable, highly scalable, and secure VANET technologies, though, presents an extraordinary challenge to the wireless research community: a high degree of communication reliability is needed under unfavorable channel conditions. Clearly, the specificity of vehicular ad hoc networks in terms of mobility behavior and applications scenarios and requirements makes VANET research an exciting and demanding application- and purpose-driven sub-discipline of wireless networking.The opportunities for vehicular ad hoc networks are growing rapidly. In December 2003, the U.S. FCC approved 75 MHz of spectrum for Dedicated Short Range Communications (DSRC), and the resulting DSRC system is expected to be the first wide-scale VANET in North America. In Japan, two DSRC standards have been adopted (the ARIB STD-T75 in 2001, the ARIB STD-T88 in 2004), and Japanese auto manufactures are working with the Ministry of Land, Infrastructure, and Transportation in the third phase of an ambitious Advanced Safety Vehicle project. The European Union is funding, among others, the "Prevent" and "Global System for Telematics" Integrated Projects. The German Ministry of Education and Research has sponsored the "FleetNet" and "Network on Wheels" projects. Throughout the world, there are many national/international projects in government, industry, and academia devoted to VANETs.Our goal for this workshop is to discuss recent advances and to chart the way forward by bringing together diverse and exciting challenges, ideas and proposals of this very active VANET community.

The impact of wireless radio fluctuations on ad hoc network performance

Abstract: The degree of dynamics with respect to changes of the network's topology strongly affect the performance of a wireless mobile ad hoc network. Main reasons for topological changes are (i) node mobility and (ii) wireless radio fluctuations, e.g., due to fading effects. The impact of node mobility on ad hoc network performance has already been studied intensively. However these studies usually do not consider temporal fluctuations of the wireless channel. We quantitatively study the impact of radio fluctuations on the performance of wireless ad hoc networks. Based on field trial measurements of IEEE 802.11b wireless LAN communication in a static line-of-sight scenario, we derive a basic model for non-deterministic, but temporally dependent, received signal power variations. According to the derived model, we quantify the impact of radio fluctuations on topological change rate (TCR) and link stability as relevant ad hoc network performance metrics via simulation. The effects of pure node mobility and the effects of signal strength fluctuations, as well as the combination of node mobility and fluctuations, are studied. We demonstrate that signal strength fluctuations have a significant impact on ad hoc network performance metrics sometimes 'outperforming' the impact of node mobility. Thus, non-deterministic radio fluctuations cannot be ignored when designing robust ad hoc network protocols based on ad hoc network simulation and analysis.

Definitions of Managed Objects for RObust Header Compression (ROHC)

Abstract: This memo defines a portion of the Management Information Base (MIB) for use with network management protocols in the Internet community. In particular, it describes a set of managed objects that allow monitoring of running instances of RObust Header Compression (ROHC). The managed objects defined in this memo are grouped into three MIB modules. The ROHC-MIB module defines managed objects shared by all ROHC profiles, the ROHC-UNCOMPRESSED-MIB module defines managed objects specific to the ROHC uncompressed profile, the ROHC-RTP-MIB module defines managed objects specific to the ROHC RTP (Real-Time Transport Protocol) profile, the ROHC UDP (User Datagram Protocol) profile, the ROHC ESP (Encapsulating Security Payload) profile, and the ROHC LLA (Link Layer Assisted) profile. Quittek, et al. Standards Track [Page 1] RFC 3816 ROHC MIB June 2004

Der Einfluss von Schwankungen der Übertragungsreichweite auf die Leistungsfähigkeit von Ad-Hoc Netzwerken

Abstract: Die Leistungsfahigkeit eines drahtlosen, mobilen Ad-Hoc Netzwerkes wird im wesentlichen durch die Fahigkeit der Protokolle bestimmt, auf Anderungen der Netzwerktopologie effektiv zu reagieren. Die fur diese Netzwerke charakteristische Eigenschaft der Dynamik in der Netztopologie basiert im wesentlichen auf der Mobilitat der Knoten und auf Schwankungen der Funkreichweite. Wahrend der Mobilitatsaspekt bereits umfassend untersucht wurde, werden die Eigenschaften des drahtlosen Mediums in Modellen meist idealisiert mittels konstantem Funkradius evaluiert. Solche Ergebnisse werden den tatsachlichen Begebenheiten, wie Erfahrungen in Feldtests belegen, nicht gerecht. In dieser Arbeit untersuchen wir den Einfluss von Mobilitat, von zeitabhangigen Schwankungen der Ubertragungsreichweite und der Kombination beider Faktoren auf die Netzwerk-Leistungsmerkmale Topologieanderungsrate (Topological Change Rate, TCR) und Linkstabilitat. Die temporaren Fluktuationen des Funkradius werden mittels eines Gauss-Markov Prozesses erster Ordnung modelliert, wahrend die Auswirkungen der Mobilitat basierend auf dem Random Waypoint Mobilitatsmodell untersucht werden. Die Ergebnisse unserer vergleichenden Simulationsstudie zeigen, dass Fluktuationen der Ubertragungsreichweite zusatzlich zur Mobilitat die Leistungsmetriken eines Ad-Hoc Netzwerkes erheblich beeinflussen. Daher empfehlen wir, Fluktuationen der Radiowellenausbreitung bei dem Entwurf von Ad-Hoc Protokollen und Ad-Hoc Modellierung nicht zu vernachlassigen.