Ramona Marfievici

How Environmental Factors Impact Outdoor Wireless Sensor Networks: A Case Study

How do the characteristics of the surrounding environment affect the ability of the nodes of a wireless sensor network (WSN) to communicate? Partial answers to this question can be found in the literature, but always with a focus on the short-term, small-scale behavior of individual links, as this directly informs the design of WSN protocols. In this paper, we are instead concerned with the large scale behavior of the overall network, observed over a longer time scale, as our primary interest is to support the deployment of WSNs by characterizing the impact of the target environment. Motivated by a real-world wildlife monitoring application, we report about experimental campaigns in three outdoor environments characterized by varying degrees of vegetation. Experiments are repeated in summer and winter, to account for seasonal variations, and span multiple days, allowing us to assess variations induced by the succession of day and night. Our experiments focus primarily on characterizing the impact of the environment on the physical layer, but we also investigate how this is mirrored at higher layers. We analyze the experimental data along multiple dimensions, yielding quantitative answers to the aforementioned question, and eliciting trends and findings previously not reported in the literature. We argue that this type of study may inspire new methods to better estimate the performance of a WSN in its target deployment environment.

Tales from the C130 Horror Room: A Wireless Sensor Network Story in a Data Center

An important aspect of the management and control of modern data centers is cooling and energy optimization. Airflow and temperature measurements are key components for modeling and predicting environmental changes and cooling demands. For this, a wireless sensor network (WSN) can facilitate the sensor deployment and data collection in a changing environment. However, the challenging characteristics of these scenarios, e.g., temperature fluctuations, noise, and large amounts of metal surfaces and wiring, make it difficult to predict network behavior and therefore network planning and deployment. In this paper we report a 17-month long deployment of 30 wireless sensor nodes in a small data center room, where temperature, humidity and airflow were collected, along with RSSI, LQI, and battery voltage. After an initial unreliable period, a connectivity assessment performed on the network revealed a high noise floor in some of the nodes, which together with a default low CCA threshold triggered no packet transmissions, yielding a low PDR for those nodes. Increasing the CCA setting and relocating the sink allowed the network to achieve a reliability of 99.2% for the last eight months of the deployment, therefore complying with the project requirements. This highlights the necessity of using proper tools and dependable protocols, and defining design methodologies for managing and deploying WSNs in real-world environments.

Modeling WiFi Traffic for White Space Prediction in Wireless Sensor Networks

Cross Technology Interference (CTI) is a prevalent phenomenon in the 2.4 GHz unlicensed spectrum causing packet losses and increased channel contention. In particular, WiFi interference is a severe problem for low-power wireless networks causing a significant degradation of the overall performance. We propose here a proactive approach based on WiFi interference modeling for accurately predicting transmission opportunities for low-power wireless networks. We leverage statistical analysis of real-world WiFi traces to learn aggregated traffic characteristics in terms of Inter-Arrival Time (IAT) that, once captured into a specific 2nd order Markov Modulated Poisson Process (MMPP(2)) model, enable accurate estimation of interference. We further use a hidden Markov model (HMM) for channeloccupancy prediction. We evaluated the performance of: i) the MMPP(2) traffic model w. r. t. real-world traces and an existing Pareto model for accurately characterizing the WiFi traffic and, ii) compared the HMM based white space prediction to random channel access. We report encouraging results for using interference modeling for white space prediction.

Into the SMOG: The Stepping Stone to Centralized WSN Control

Previous research has shown that centralized network control in Wireless Sensor Networks (WSNs) can lead to improved network lifetime, benefit reliability, help to diagnose and localize network failures, assist network recovery, and lead to optimal routing and transmission scheduling. A stepping stone to centralized network control is to build and maintain a complete network topology model that scales and reacts to the network dynamics that occur in low-power wireless networks. We propose SMOG as a mechanism to build and maintain a centralized full network topology model using probabilistic data structures. Extensive analysis of the proposed approach in both simulation and two testbeds shows that SMOG can build a complete model of a WSN of over 100 nodes with 98% accuracy in less than four minutes. Our approach also offers fast recovery from heavy network interference, recovering model accuracy to 98% in less than two and a half minutes.

Towards Detecting WiFi Aggregated Interference for Wireless Sensors Based on Traffic Modelling

We present a technique to identify transmission timing for IEEE 802.15.4 based Wireless Sensor Networks (WSNs) in the presence of WiFi interference. Our technique is based on modeling WiFi traffic with a Modulated Markov Poisson Process (MMPP) model in order to enable us to predict when WiFi transmissions take place and avoid them. We have evaluated the accuracy of our model in a small test-bed. Results are promising and suggest that our approach can increase the reliability of IEEE802.15.4 transmissions.