Muhammad Hamad Alizai

Bursty traffic over bursty links

Accurate estimation of link quality is the key to enable efficient routing in wireless sensor networks. Current link estimators focus mainly on identifying long-term stable links for routing. They leave out a potentially large set of intermediate links offering significant routing progress. Fine-grained analysis of link qualities reveals that such intermediate links are bursty, i.e., stable in the short term. In this paper, we use short-term estimation of wireless links to accurately identify short-term stable periods of transmission on bursty links. Our approach allows a routing protocol to forward packets over bursty links if they offer better routing progress than long-term stable links. We integrate a Short Term Link Estimator and its associated routing strategy with a standard routing protocol for sensor networks. Our evaluation reveals an average of 19% and a maximum of 42% reduction in the overall transmissions when routing over long-range bursty links. Our approach is not tied to any specific routing protocol and integrates seamlessly with existing routing protocols and link estimators.

KleeNet: discovering insidious interaction bugs in wireless sensor networks before deployment

Complex interactions and the distributed nature of wireless sensor networks make automated testing and debugging before deployment a necessity. A main challenge is to detect bugs that occur due to non-deterministic events, such as node reboots or packet duplicates. Often, these events have the potential to drive a sensor network and its applications into corner-case situations, exhibiting bugs that are hard to detect using existing testing and debugging techniques. In this paper, we present KleeNet, a debugging environment that effectively discovers such bugs before deployment. KleeNet executes unmodified sensor network applications on symbolic input and automatically injects non-deterministic failures. As a result, KleeNet generates distributed execution paths at high-coverage, including low-probability corner-case situations. As a case study, we integrated KleeNet into the Contiki OS and show its effectiveness by detecting four insidious bugs in the μIP TCP/IP protocol stack. One of these bugs is critical and lead to refusal of further connections.

Dynamic TinyOS: Modular and Transparent Incremental Code-Updates for Sensor Networks

Long-term deployments of sensor networks in physically inaccessible environments make remote re-programmability of sensor nodes a necessity. Ranging from full image replacement to virtual machines, a variety of mechanisms exist today to deploy new software or to fix bugs in deployed systems. However, TinyOS - the current state of the art sensor node operating system - is still limited to full image replacement as nodes execute a statically-linked system-image generated at compilation time. In this paper we introduce Dynamic TinyOS to enable the dynamic exchange of software components and thus incrementally update the operating system and its applications. The core idea is to preserve the modularity of TinyOS, i.e.~its componentization, which is lost during the normal compilation process, and enable runtime composition of TinyOS components on the sensor node. The proposed solution integrates seamlessly into the system architecture of TinyOS: It does not require any changes to the programming model of TinyOS and existing components can be reused transparently. Our evaluation shows that Dynamic TinyOS incurs a low performance overhead while keeping a smaller - upto one third - memory footprint than other comparable solutions.

Energy Harvesting and Wireless Transfer in Sensor Network Applications: Concepts and Experiences

Advances in micro-electronics and miniaturized mechanical systems are redefining the scope and extent of the energy constraints found in battery-operated wireless sensor networks (WSNs). On one hand, ambient energy harvesting may prolong the systems’ lifetime or possibly enable perpetual operation. On the other hand, wireless energy transfer allows systems to decouple the energy sources from the sensing locations, enabling deployments previously unfeasible. As a result of applying these technologies to WSNs, the assumption of a finite energy budget is replaced with that of potentially infinite, yet intermittent, energy supply, profoundly impacting the design, implementation, and operation of WSNs. This article discusses these aspects by surveying paradigmatic examples of existing solutions in both fields and by reporting on real-world experiences found in the literature. The discussion is instrumental in providing a foundation for selecting the most appropriate energy harvesting or wireless transfer technology based on the application at hand. We conclude by outlining research directions originating from the fundamental change of perspective that energy harvesting and wireless transfer bring about.

BurrowView - seeing the world through the eyes of rats

For a long time, life sciences were restricted to look at animal habitats only post-factum. Pervasive computing puts us in the novel position to gain live views. In this paper we present BurrowView, an application that tracks the movement of rats in their natural habitat and reconstructs the underground tunnel system. To make reliable statements, special consideration has been taken with regard to the information quality. Our system is able to reconstruct paths up to a resolution of 20 cm, the length of a rat without its tail.

Exploiting the Burstiness of Intermediate-Quality Wireless Links

We address the challenge of link estimation and routing over highly dynamic links, thats is, bursty links that rapidly shift between reliable and unreliable periods of transmissions. Based on significant empirical evidence of over 100,000 transmissions over each link in 802.15.4 and 802.11 testbeds, we propose two metrics, expected future transmissions (EFT) and MAC3, for runtime estimation of bursty wireless links. We introduce a bursty link estimator (BLE) that based on these two metrics, accurately estimates bursty links in the network rendering them available for data transmissions. Finally, we present bursty routing extensions (BRE): an adaptive routing strategy that uses BLE for forwarding packets over bursty links if they offer better routing progress than long-term stable links. Our evaluation, comprising experimental data from widely used IEEE 802.15.4-based testbeds, reveals an average of 19% and a maximum of 42% reduction in the number of transmissions when routing over long-range bursty links typically ignored by routing protocols. Additionally, we show that both BLE and BRE are not tied to any specific routing protocol and integrate seamlessly with existing routing protocols and link estimators.

KleeNet: automatic bug hunting in sensor network applications

We present KleeNet, a Klee based bug hunting tool for sensor network applications before deployment. KleeNet automatically tests code for all possible inputs, ensures memory safety, and integrates well into TinyOS based application development life cycle, making it easy for developers to test their applications.

Sensors with lasers: building a WSN power grid

We present here a first prac architecture that allows us to decouple en activities in WSN. Such a separation of us to utilize abundant energy sources d location, allowing unrestricted lifetime energy consumption in WSN. We demons practical decoupling using low-cost and -beaming that powers current WSN platf We design and implement LAMP: a tiered energy supply to both mesh and clust using an energy distribution protocol. W show that, for an additional cost of

Refector: heuristic header error recovery for error-tolerant transmissions

2 support perpetual mesh functionality for nodes in clustered operation.

Efficient online estimation of bursty wireless links

High bit error rates reduce the performance of wireless networks. This is exacerbated by the enforcement of bit-by-bit correct transmissions and the resulting retransmission overhead. Recently, research has focused on more efficient link layer mechanisms and on tolerating payload errors. Header errors, however, still cause todays network and transport protocols to drop the erroneous packets. Instead of retransmitting such packets, we investigate a novel concept (called Refector) of heuristically repairing header bit errors. Refector accepts erroneous packets on end hosts and exploits protocol knowledge and protocol state to assign packets to their correct destination applications. It operates on layers 3 and 4, is independent of the underlying MAC and PHY, and requires no changes to hardware, firmware, and communication behavior. We evaluate the Refector concept via a prototype implementation deployed in an 802.11 network. Our results show that Refector reduces packet loss in the network by more than 25% when compared to payload-error-tolerant protocols such as UDP-Lite.