Brahim Ayari

Generic Information Transport for Wireless Sensor Networks

A primary functionality of wireless sensor networks (WSNs) is transporting the information acquired by the sensors as per the desired application requirements. The diverse applications supported by WSNs also stipulate a diverse range of reliability requirements for the transport of various information types. The continuous variation of application requirements and dynamic operational perturbations complicates the design of a generic solution for information transport in WSNs. In this paper, we present a new framework for generic information transport (GIT), which considers varied application requirements and evolvable network conditions in WSNs. GIT manages the information and utilizes a probabilistic approach to ensure tunable reliability of information transport. The GIT framework is distributed in nature and performs its operations locally. The simulation results validate the tunability of the GIT framework. In some setups GIT achieves up to 4-5 times reduction in number of transmissions compared to existing approaches.

FT-PPTC: An Efficient and Fault-Tolerant Commit Protocol for Mobile Environments

Transactions are required not only for wired networks but also for the emerging wireless environments where mobile and fixed hosts participate side by side in the execution of the transaction. This heterogenous environment is characterized by constraints in mobile host capabilities, network connectivity and also an increasing number of possible failure modes. Classical atomic commit protocols used in wired networks are therefore not directly suitable for this heterogenous environment. Furthermore, the few commit protocols designed for mobile transactions either consider mobile hosts only as initiators though not as active participants, or show a high resource blocking time. We present the Fault-Tolerant Pre-Phase Transaction Commit (FT-PPTC) protocol for mobile environments. FT-PPTC decouples the commit of mobile participants from that of fixed participants. Consequently, the commit set can be reduced to a set of entities in the fixed network. Thus, the commit can easily be supported by any traditional atomic commit protocol, such as the established 2PC protocol. We integrate fault-tolerance as a key feature of FT-PPTC. Performance evaluations confirm the efficiency, scalability and low resource blocking time of our approach

ParTAC: A Partition-Tolerant Atomic Commit Protocol for MANETs

The support of distributed atomic transactions in Mobile Ad-hoc Networks (MANET) is a key requirement for many mobile application scenarios. Atomicity is a fundamental property that ensures that all nodes decide a consistent outcome. As MANETs are characterized by frequent perturbations due to network partitioning and the fragility of nodes, providing atomicity is challenging. Existing protocols that ensure strict atomicity in MANETs are either bound to specific mobility pattern or based on building blocks such as consensus or group membership, not allowing arbitrary partitions or requiring exact knowledge about the members of a partition. These assumptions limit the deployment of these protocols to very restricted MANET scenarios, and may lead to poor commit rate, high message overhead or blocking related to intolerably long Commit/Abort decision times. In this paper, we present the first Partition-Tolerant Atomic Commit protocol (ParTAC) for MANETs which does not rely on consensus or group partition membership. As a consequence, ParTAC supports a significantly wider range of mobility patterns and partitioning scenarios than existing protocols. To reduce Commit/Abort decision times and prevent the protocol from blocking, ParTAC follows a best-effort strategy by defining a lifetime for every transaction after which the transaction is aborted. Further, we introduce a new coordination strategy based on a flexible preselection of multiple coordinators among the participating nodes. Thus, the failure of a single coordinator can be tolerated in the presence of network partitioning. Moreover, transactions can be aborted by any coordinator based on lifetime expiration. ParTAC is evaluated using simulations to demonstrate the performance of the protocol in terms of commit rate, message efficiency and Commit/Abort decision time.

On the design of perturbation-resilient atomic commit protocols for mobile transactions

Distributed mobile transactions utilize commit protocols to achieve atomicity and consistent decisions. This is challenging, as mobile environments are typically characterized by frequent perturbations such as network disconnections and node failures. On one hand environmental constraints on mobile participants and wireless links may increase the resource blocking time of fixed participants. On the other hand frequent node and link failures complicate the design of atomic commit protocols by increasing both the transaction abort rate and resource blocking time. Hence, the deployment of classical commit protocols (such as two-phase commit) does not reasonably extend to distributed infrastructure-based mobile environments driving the need for perturbation-resilient commit protocols. In this article, we comprehensively consider and classify the perturbations of the wireless infrastructure-based mobile environment according to their impact on the outcome of commit protocols and on the resource blocking times. For each identified perturbation class a commit solution is provided. Consolidating these subsolutions, we develop a family of fault-tolerant atomic commit protocols that are tunable to meet the desired perturbation needs and provide minimized resource blocking times and optimized transaction commit rates. The framework is also evaluated using simulations and an actual testbed deployment.

Delay-Aware Mobile Transactions

In the expanding e-society, mobile embedded systems are increasingly used to support transactions such as for banking, stock or database applications. Such systems entail a range of heterogeneous entities - both the embedded devices and the networks connecting them. While these systems are exposed to frequent and varied perturbations, the support of atomic distributed transactions is still a fundamental requirement to achieve consistent decisions. Guaranteeing atomicity and high performance in traditional fixed wired networks is based on the assumption that faults like node and link failures occur rarely. This assumption is not supported in current and future mobile embedded systems where the heterogeneity and mobility often result in link and node failures as a dominant operational scenario. In order to continue guaranteeing strict atomicity while providing for high efficiency (low resource blocking time and message overhead) and acceptable commit rate, transactional fault-tolerance techniques need to be revisited perhaps at the cost of transaction execution time. In this paper, a comprehensive classification of perturbations and their impact on the design of mobile transactions is provided. In particular we argue for the delay-awareness of mobile transactions to allow for the fault-tolerance mechanisms to ensure resilience to the various and frequent perturbations.

Map-based Design for Autonomic Wireless Sensor Networks

A prominent functionality of a Wireless Sensor Network (WSN) is environmental monitoring. For this purpose theWSN creates a model for the real world by using abstractions to parse the collected data. Being cross-layer and application-oriented, most ofWSN research does not allow for a widely accepted abstraction. A few approaches such as database-oriented and publish/subscribe provide acceptable abstractions by reducing application dependency and hiding communication details. Unfortunately, these approaches ignore the spatial correlation of sensor readings and still address single sensor nodes. In this work we present a novel approach based on a “world model” that exploits the spatial correlation of sensor readings and represents them as a collection of regions called maps. Maps are a natural way for the presentation of the physical world and its physical phenomena over space and time. Our Map-based World Model (MWM) abstracts from low-level communication issues and supports general applications by allowing for efficient event detection, prediction and queries. In addition our MWM unifies the monitoring of physical phenomena with network monitoring which maximizes its generality. From our approach we deduce a general modeling and design methodology for WSNs. Using a case study we highlight the simplicity of the proposed methodology. We provide the necessary tools to use our architecture and to acquire valuable WSN insights in the established OMNeT++ simulator.

GMTC: A Generalized Commit Approach for Hybrid Mobile Environments

Mobile environments increasingly require distributed atomic transactions to support the growing diversity of financial, gaming, social networking and many other applications. The underlying mobile infrastructure is correspondingly evolving with increasingly diverse wired and wireless elements and also with increasing exposure to a variety of operational perturbations at the mobile elements and communication levels. Consequently, the challenge is not only in providing efficient nonblocking mobile commit (as a fundamental basis behind consistent mobile transactions) but to also provide efficient perturbation-resilient atomic commit in the heterogeneous mobile space. The contribution of this paper is in developing a perturbation-resilient mobile commit protocol that efficiently provides for and preserves strict atomicity for transactional applications. The protocol does not necessarily require access to the powerful communication/computation elements of the wired infrastructure during transaction execution. However, in case access to a wired network becomes possible, it then adapts to utilize this to 1) increase the resilience to network perturbations achieving higher commit rates, and 2) reduce the wireless message overhead and the blocking of transaction participants leading to higher transactions throughput. In contrast, existing solutions are often tailored either for 1) infrastructure-based mobile environments, or 2) infrastructure-less ad hoc networks. To our knowledge, there is no existing commit protocol that can adapt across diverse infrastructure communication modes. The proposed perturbation-resilient generalized mobile transaction commit (GMTC) protocol represents the first atomic commit protocol for hybrid mobile environments which 1) takes advantage of accessing infrastructures, by choosing reliable infrastructure nodes for coordination of transactions and for replication of commit data of mobile participants to tolerate network disconnections, and 2) tolerates network partitioning and delivers best-effort resultsâin terms of transaction commit rate, message complexity, and commit/abort decision time (latency)âif the access to wired infrastructure is unavailable. The protocol performance simulations (covering transaction commit rate, message complexity, and commit/abort decision time) demonstrate the effectiveness of the developed protocol in generalized mobile environments.

Data-Based Agreement for Inter-vehicle Coordination

Data-based agreement is increasingly used to implement traceable coordination across mobile entities such as ad-hoc networked (autonomous) vehicles. In our work, we focus on data-based agreement using database transactions where mobile entities agree on a set of coordinated tasks that need to be performed by them in an atomic way. Atomicity means that all transaction participants agree on a set of tasks which will be performed by them or no one of them is performing any task. The data about the agreed tasks and their corresponding stakeholders are kept in local databases as a proof for the obtained agreement. This proof might be needed by users and regularities/authorities involved depending on the application scenario. In this demo, we demonstrate our effort to provide for partition-aware atomic commit protocols for transactional data-based agreement.