Carlo Alberto Boano

Radio link quality estimation in wireless sensor networks: A survey

Radio link quality estimation in Wireless Sensor Networks (WSNs) has a fundamental impact on the network performance and also affects the design of higher-layer protocols. Therefore, for about a decade, it has been attracting a vast array of research works. Reported works on link quality estimation are typically based on different assumptions, consider different scenarios, and provide radically different (and sometimes contradictory) results. This article provides a comprehensive survey on related literature, covering the characteristics of low-power links, the fundamental concepts of link quality estimation in WSNs, a taxonomy of existing link quality estimators, and their performance analysis. To the best of our knowledge, this is the first survey tackling in detail link quality estimation in WSNs. We believe our efforts will serve as a reference to orient researchers and system designers in this area.

The Impact of Temperature on Outdoor Industrial Sensornet Applications

Wireless sensor networks are being considered for use in industrial process and control environments. Unlike traditional deployment scenarios for sensor networks, in which energy preservation is the main design principle, industrial environments stress worker safety and uninterrupted production. To fulfill these requirements, sensor networks must be able to provide performance guarantees for radio communication. In this paper, we consider as a case study the deployment of a sensornet in an oil refinery in Portugal, where sensor nodes are deployed outdoors and might experience high temperature fluctuations. We investigate how the variations of ambient temperature influence data delivery performance and link quality in low-power radio communications. We also study the impact that specific implementation requirements, such as the ATEX fire-safety regulations, can have on the design of the overall network. Our experiments show that temperature directly affects the communication between sensor nodes, and that significantly less transmission power is required at low temperatures. We further illustrate that it is possible to save up to 16% energy during nights and cold periods of the year, while still ensuring reliable communication among sensor nodes. In view of these experimental results, we elaborate on how the temperature influences both the design and the deployment of wireless sensor networks in industrial environments.

Making sensornet MAC protocols robust against interference

Radio interference may lead to packet losses, thus negatively affecting the performance of sensornet applications. In this paper, we experimentally assess the impact of external interference on state-of-the-art sensornet MAC protocols. Our experiments illustrate that specific features of existing protocols, e.g., hand-shaking schemes preceding the actual data transmission, play a critical role in this setting. We leverage these results by identifying mechanisms to improve the robustness of existing MAC protocols under interference. These mechanisms include the use of multiple hand-shaking attempts coupled with packet trains and suitable congestion backoff schemes to better tolerate interference. We embed these mechanisms within an existing X-MAC implementation and show that they considerably improve the packet delivery rate while keeping the power consumption at a moderate level.

Low-Power Radio Communication in Industrial Outdoor Deployments: The Impact of Weather Conditions and ATEX-Compliance

Industry recognizes wireless sensor networks as one of the next major technical and economical shifts in automation and control systems. Industrial applications need performance assurances of wireless communication, which is hard to attain given the variability of the environment where such applications are deployed. In this paper, we quantify the impact of environmental conditions, namely temperature, fog, and rain on wireless communication links. We further investigate the influence of industrial casing on communication, evaluating an ATEX enclosure intended for use in potentially explosive atmospheres. Knowledge of the potential impact of environmental conditions and special casings provides an important input to robust communication design during deployment planning.

TempLab: a testbed infrastructure to study the impact of temperature on wireless sensor networks

Temperature has a strong impact on the operations of all electrical and electronic components. In wireless sensor nodes, temperature variations can lead to loss of synchronization, degradation of the link quality, or early battery depletion, and can therefore affect key network metrics such as throughput, delay, and lifetime. Considering that most outdoor deployments are exposed to strong temperature variations across time and space, a deep understanding of how temperature affects network protocols is fundamental to comprehend flaws in their design and to improve their performance. Existing testbed infrastructures, however, do not allow to systematically study the impact of temperature on wireless sensor networks. In this paper we present TempLab, an extension for wireless sensor network testbeds that allows to control the on-board temperature of sensor nodes and to study the effects of temperature variations on the network performance in a precise and repeatable fashion. TempLab can accurately reproduce traces recorded in outdoor environments with fine granularity, while minimizing the hardware costs and configuration overhead. We use TempLab to analyse the detrimental effects of temperature variations (i) on processing performance, (ii) on a tree routing protocol, and (iii) on CSMA-based MAC protocols, deriving insights that would have not been revealed using existing testbed installations.

Accurate Temperature Measurements for Medical Research Using Body Sensor Networks

Medical measurements and clinical trials are often carried out in controlled lab settings -- severely limiting the realism and duration of such studies. Our goal is henceforth to design a body sensor network for unobtrusive and highly accurate profiling of body parameters over weeks in realistic environments. One example application is monitoring the impact of sleep deprivation on periodic processes in the human body known as circadian rhythms, which requires highly accurate profiling of skin temperature across the human body over weeks with real-time feedback to a remote medic. We analyze the requirements on a body sensor network for such applications and highlight the need for self-organizing behavior such as adaptive sampling to ensure energy efficiency and thus longevity, adaptive communication strategies, self-testing, automatic compensation for environmental conditions, or automatic recording of a diary of activities. As a first step towards this goal, we design and build a prototype of such a non-invasive wearable wireless monitoring system for accurate body temperature measurements and real-time feedback to the medic. Through the design, parameterization, and calibration of an active measurement subsystem, we obtain an accuracy of 0.02°C over the typical body temperature range of 16-42°C. We report results from two preliminary trials regarding the impact of circadian rhythms and mental activity on skin temperature, indicating that our tool could indeed become a valuable asset for medical research.

Controllable radio interference for experimental and testing purposes in Wireless Sensor Networks

We address the problem of generating customized, controlled interference for experimental and testing purposes in wireless sensor networks. The known coexistence problems between electronic devices sharing the same ISM radio band drive the design of new solutions to mitigate interference. The validation of these techniques and the assessment of protocols under external interference require the creation of reproducible and well-controlled interference patterns on real nodes, a nontrivial and time-consuming task. In this paper, we study methods to generate a precisely adjustable level of interference on a specific channel, with lowcost equipment and rapid calibration. We focus our work on the platforms carrying the CC2420 radio chip. We show that, by setting the CC2420 in special mode, we can easily generate repeatable and precise patterns of interference. We show how this method is extremely useful for researchers to quickly investigate the behaviour of sensor network protocols and applications under different patterns of interference. We further evaluate the performance of our proposed method.

A Decade of Wireless Sensing Applications: Survey and Taxonomy

The popularity of low-power wireless sensors increased significantly in the last decade, triggering a golden era for wireless sensor network research and development. During the early years of the twenty-first century, wireless sensor network applications have evolved from small demonstrations with a lifetime of only a few hours to complete systems made up of hundreds of tiny wireless nodes deployed in a wide variety of settings, ranging from harsh and remote environments to residential buildings and clinical units. This survey gives an overview of the most relevant applications of wireless sensor network applications deployed during the last ten years, and classifies them using a novel taxonomy that aims to help identifying relevant programming constructs and run-time services. With more than 60 applications reviewed, ranging from military and civilian surveillance to tracking systems, from environmental and structural monitoring to home and building automation, from agriculture and industrial settings to health care, this survey will serve as a reference to guide researchers and system designers.

Quantifying the channel quality for interference-aware wireless sensor networks

Reliability of communications is key to expand application domains for sensor networks. Since Wireless Sensor Networks (WSN) operate in the license-free Industrial Scientific and Medical (ISM) bands and hence share the spectrum with other wireless technologies, addressing interference is an important challenge. In order to minimize its effect, nodes can dynamically adapt radio resources provided information about current spectrum usage is available. We present a new channel quality metric, based on availability of the channel over time, which meaningfully quantifies spectrum usage. We discuss the optimum scanning time for capturing the channel condition while maintaining energy-efficiency. Using data collected from a number of Wi-Fi networks operating in a library building, we show that our metric has strong correlation with the Packet Reception Rate (PRR). This suggests that quantifying interference in the channel can help in adapting resources for better reliability. We present a discussion of the usage of our metric for various resource allocation and adaptation strategies.

Link quality ranking: Getting the best out of unreliable links

Link quality estimation has been an active area of research within the wireless sensor network community. It is now well known that the estimation of reliable links requires few sample packets — less than 10, while the estimation of unreliable links require many more — above 50. In scenarios where unreliable links are ubiquitous, and a rapid transfer of data is needed, traditional estimation techniques are not a viable option. In such scenarios, it is instead sufficient to identify the best link available at any given time. Within this context, we propose Link Quality Ranking (LQR), a mechanism that identifies the best link available when only unreliable links are present. Our testbed results indicate that with one sample packet, the delivery rate of LQR — with respect to the best link available — is above 93%. With 10 sample packets, the performance is above 96%.