Abstract

Mobile devices (e.g., laptops, PDAs, cell phones) are increasingly relied on but are used in contexts that put them at risk of physical damage, loss or theft. This paper discusses the dependability evaluation of a cooperative backup service for mobile devices. Participating devices leverage encounters with other devices to temporarily replicate critical data. Permanent backups are created when the participating devices are able to access the fixed infrastructure. Several data replication and scattering strategies are presented,including the use of erasure codes. A methodology to model and evaluate them using Petri nets and Markov chains is described. We demonstrate that our cooperative backup service decreases the probability of data loss by a factor up to the ad hoc to Internet connectivity ratio.

No abstract

International audienceVehicular ad-hoc networks have received increasing interest in the last years for their potential to support a variety of services and applications in order to improve driving safety or traffic efficiency and to provide information and entertainment to the users. This paper focuses on the analysis of some connectivity characteristics in dynamic vehicular communication scenarios, that are important for the design and the performance and dependability assessment of such applications. In particular, we focus on the process describing the occurrence of encounters between cars in single and multi-hop scenarios. Using analytical proofs and simulation experiments, it is shown that under some key assumptions on the movements and the placement of the cars, this process can be approximated as a stationary or a nonhomogeneous Poisson process. Results are also provided concerning the duration of connections in the ad-hoc domain

Abstract. In order to increase availability and reliability of stateful applications, redundancy as provided by replication in cluster solutions is a wellknown and frequently utilized approach. For mobile services in dynamic ad-hoc networks, such replication mechanisms have to be adapted to deal with the frequently higher communication delays and with the intermittent connectivity. Dynamic clustering strategies in which the replica set is adjusted to the current network state can help to handle the network dynamicity. The paper develops a stochastic Petri net model (and its corresponding Markov chain representation) to analyze the resulting availability and replica consistency in such dynamic clusters. The numerical results are interpreted in the context of a vehicular (c2c) communication use-case and can be used to determine optimized cluster configuration parameters. IntroductionMany of the future networking scenarios consist both of wireless multi-hop parts and infrastructure based network components. For new application types and future service platforms, server-based applications access is not only offered by the infrastructure network part, but also by the potentially mobile nodes in the ad-hoc domain. An example for such service provisioning scenarios is the vehicular communication setting [2], in which cars will be able to communicate with each other. This communication will be used for safety critical applications that require high availability. For applications used in automotive traffic this is especially true since application failure could affect driver behavior or directly affect the state of the car, e.g., spurious application of brakes. Traditional solutions for high-availability rely on redundancy offered by cluster implementations, in which Middleware services [1] support the timely replication and fail-over in case of crash failures of individual *