Athanassios Boulis

Castalia: revealing pitfalls in designing distributed algorithms in WSN

We present Castalia, a simulator for WSN that models many aspects of the WSN system and uses advanced models especially in terms of the channel and radio behaviour. We show the effects of these features in distributed algorithms that work fine with simpler simulators but fail under Castalia. The demo will present the differences, explain the failures and show how to redesign the algorithms to make them work under more realistic conditions.

Challenges in body area networks for healthcare: the MAC

Body area wireless sensor networks (BANs) are a key component to the ubiquitous healthcare revolution and perhaps one of its most challenging elements from a communications standpoint. The unique characteristics of the wireless channel, coupled with the need for extreme energy efficiency in many healthcare applications, require novel solutions in medium access control protocols. We present the main characteristics and challenges associated with BANs from a healthcare perspective, and present some MAC techniques based on studies of the BAN channel that could be used to address these challenges.

From Simulation to Real Deployments in WSN and Back

The paper presents our efforts to validate some high-level aspects of the WSN simulator we have built as well as the operational functionality of our multi-parameter MAC protocol. In order to do so, we resort to real deployments involving TelosB motes. The simulator, named Castalia, boasts the most accurate wireless channel and radio models for WSN found in current literature. These models are capturing some essential experimental findings. This does not guaranty though that the simulator will behave similarly with a real deployment at the high level (i.e., the protocol or application level). We investigate how our multi-parameter MAC protocol behaves in a real deployment so as to take a first step towards validating and possibly tuning Castalia. The investigation starts by determining the connectivity map for the real deployment and then trying to reproduce it in the simulator. We then proceed with the protocol testing and comparing. We report the difficulties faced and our findings from this process.

Performance and scalability evaluation of the Castalia wireless sensor network simulator

Castalia is an open-source simulator for wireless sensor networks and body area networks which is widely used in the academic and research community. This paper presents a basic evaluation study of Castalia, reporting computation time and memory usage for a variety of scenarios/benchmarks. Moreover, key parameters, such as network size, simulation time, fraction of mobile nodes are varied to reveal Castalias scalability potential. We discuss our results and explain counterintuitive findings in simulators performance. The results and their explanation can be used by Castalia users as a guide to determine the limits they can push their simulations, as well as to make parameter choices that tradeoff accuracy for performance. They also provide an indication of Castalias performance capabilities to potential users.

Experiences and Lessons from Implementing a Wireless Sensor Network MAC Protocol in the Castalia Simulator

We describe our experience from the implementation of the T-MAC protocol for wireless sensor networks in the open-source Castalia simulator. Notwithstanding the popularity of the protocol in the research literature in recent years, we find several practical issues that are not addressed in the original protocol description, which lead to a degree of freedom in the protocol design and implementation and have an impact on its resulting performance. These issues include the ability of the underlying physical layer and hardware to efficiently detect the activation events in the protocol, and necessary changes to the collision resolution and clock synchronization procedures in the presence of varying sleep patterns. Our results highlight the need for rigorous detail in protocol descriptions in the research literature and provide important insights into some of the common pitfalls.

Contention vs. polling: a study in body area networks MAC design

Medium Access Control design for Body Area Networks is challenging due to the uniqueness of the wireless channel characteristics (highly variable in time) and the need for ultra low power while maintaining reasonable performance. Using our temporal BAN channel models we study trade-offs created by a mix of two MAC techniques: i) contention-based access, and ii) polling-based access. By using these techniques at different proportions we see how performance and energy consumption vary. The results reveal design trade-offs in the packet delivery vs. latency vs. consumed energy space. Moreover, we show how optimal points for packet delivery vary depending on the traffic. Finally, explaining the results offers important insight into the behaviour of these techniques under BAN conditions as well as more general issues with medium access for BAN. This insight translates to concrete design suggestions to build more efficient MAC protocols.

Reducing transmission losses in body area networks using variable TDMA scheduling

We consider a typical body area network (BAN) setting in which sensor nodes send data to a common hub regularly on a TDMA basis, as defined by the emerging IEEE 802.15.6 BAN standard. We explore variable TDMA scheduling techniques that allow the order of transmissions within each TDMA round to be decided on the fly, rather than be fixed in advance. Our approach is related to opportunistic scheduling used in other multiuser wireless systems, which aims to maximize the system throughput by allocating transmission opportunities to users with the ‘best’ instantaneous channel. However, the energy overheads of opportunistic scheduling, which requires the continuous reporting of channel states to the scheduler, make it incompatible with body area networks where nodes turn their radio off (‘sleep’) until it is their turn to transmit, as an energy-saving measure. Accordingly, we consider variable TDMA scheduling strategies that decide on the transmission order at the start of each TDMA round based on the last known channel state of each sensor. We formulate the optimization problem of maximizing the expected throughput (or, equivalently, minimizing the expected loss rate) based on a Markov model of the wireless channel, and present some simple approximate heuristics that can be computed quickly yet perform well in practice. We evaluate the performance of these strategies and heuristics numerically in a wide range of scenarios, and demonstrate the marked improvement they achieve over a fixed TDMA allocation.

Energy-efficient retransmission strategies under variable TDMA scheduling in body area networks

We consider a body area network (BAN) setting in which sensor nodes send data to a common hub regularly on a TDMA basis, as defined by the emerging IEEE 802.15.6 BAN standard. Our previous work has established the benefits of variable TDMA scheduling in this setting, where the order of communication with nodes in each TDMA round is determined on the fly by the hub, based on the actual outcomes (success or failure) of their recent transmissions. This technique takes into account the very slow fading nature of wireless channels in BANs, and can minimize the expected rate of transmission losses merely by ordering nodes according to their most recent channel state information, without any additional energy overhead. In this paper, we focus on an extended TDMA setting where the length of each TDMA round is greater than the number of nodes, i.e. a number of ‘spare’ slots are reserved for retransmissions of lost packets. We consider delay-constrained data transmissions based on a Markov model of the wireless channels. We evaluate several variable scheduling strategies in terms of the trade-off between the energy consumption due to the additional retransmissions and the reduction of the loss rate. We show how the best strategy depends on the energy budget of the sensor nodes, and demonstrate the considerable reduction in the loss rate (up to 18%) that can be achieved over a naïve approach where static TDMA scheduling is combined with retransmission slots at the end of the round.

Wireless Sensor Network tesbed for structural Health Monitoring of bridges

The Road and Traffic Authority (RTA) of NSW, Australia, partnered with NICTA, are looking into wireless sensor network technologies to monitor the structural health of bridges. The task involves exploring different sensing, networking, and distributed computation approaches for the specific application of Structural Health Monitoring (SHM). The exploration has to happen on-site with real world conditions. We argue that the on-site exploration needs an over-provisioned testbed that will provide the sensing nodes with power and an alternate wired communication channel. Even though the implementation and deployment of such a testbed is more expensive than deploying a single standalone Wireless Sensor Network (WSN), we show that a single standalone WSN cannot operate throughout the life cycle of experimentation.

Variable Scheduling to Mitigate Channel Losses in Energy-Efficient Body Area Networks

We consider a typical body area network (BAN) setting in which sensor nodes send data to a common hub regularly on a TDMA basis, as defined by the emerging IEEE 802.15.6 BAN standard. To reduce transmission losses caused by the highly dynamic nature of the wireless channel around the human body, we explore variable TDMA scheduling techniques that allow the order of transmissions within each TDMA round to be decided on the fly, rather than being fixed in advance. Using a simple Markov model of the wireless links, we devise a number of scheduling algorithms that can be performed by the hub, which aim to maximize the expected number of successful transmissions in a TDMA round, and thereby significantly reduce transmission losses as compared with a static TDMA schedule. Importantly, these algorithms do not require a priori knowledge of the statistical properties of the wireless channels, and the reliability improvement is achieved entirely via shuffling the order of transmissions among devices, and does not involve any additional energy consumption (e.g., retransmissions). We evaluate these algorithms directly on an experimental set of traces obtained from devices strapped to human subjects performing regular daily activities, and confirm that the benefits of the proposed variable scheduling algorithms extend to this practical setup as well.