[show abstract][hide abstract] ABSTRACT: In several sensing applications the parameter being sensed exhibits a high spatial correlation. For example, if the temperature of a region is being monitored, there are distinct hot and cold spots. The area close to the hot spots is usually warmer than average, with a temperature gradient between the hot and cold spots. We exploit this correlation of sensor data to form a forest of logical trees, with the trees collectively spanning all the sensor nodes. The root of a tree corresponds to a sensor reporting the local peak value. The tree nodes represent the value gradient: each node's sensed value is smaller than that of its parent, and greater than that of its children. GrAFS provides a mechanism to maintain some information at the local peaks and the sink. Using this information the sink can answer several queries either directly, or by probing the region of the sensor field that holds the answer. Thus, queries can be answered in a time and/or bandwidth efficient manner. The GrAFS approach to data aggregation can easily adapt to changes in the spatial distribution of sensed values, and also cope with message losses and sensor node failures. Implementation on MICA2 motes and simulation experiments conducted using TinyOS quantify the performance of GrAFS.
International Conference on Parallel Processing, ICPP 2011, Taipei, Taiwan, September 13-16, 2011; 01/2011
[show abstract][hide abstract] ABSTRACT: This article discusses lessons learned from use of our publicly available wireless networking testbed, assert. In  we discussed the design and implementation phase of a testbed based on a set of propositions. By opening the testbed to users not familiar with underlying architecture of assert, we put these propositions to the test. While fidelity of the testbed is a major challenge for developers, users assume the fidelity as a given and look for ease of use comparable with a simulator. We list the main demands by users of our testbed and the way we addressed these demands during an enhancement phase.
[show abstract][hide abstract] ABSTRACT: Application-specific data aggregation can play a significant role in energy-efficient operation of wireless sensor networks. Existing aggregation techniques rely heavily on the routing protocol to build shortest paths to route node measurements to the base station and are limited in the types of supported queries. We propose an aggregation scheme that utilizes the inherent information gradients present in the network. The query is directed to the source of information, resulting in better load sharing in the network. We support a variety of queries ranging from simple maximum, minimum or average of the readings of sensor nodes to more complex quantile queries such as k highest values or k<sup>th</sup> highest value through a generic query algorithm. The query algorithm shifts the computation to the querying agent, thus eliminating any in-network aggregation.
[show abstract][hide abstract] ABSTRACT: Most researchers conduct wireless networking experiments in their laboratory or similar indoor environments. Such environments are veritable RF jungles, especially when we consider the ISM bands. In this paper we examine and test several common explicit and implicit assumptions that researchers tend to make about the wireless environment. Although these assumptions are acknowledged by most researchers, the extent of their impact is often underestimated. We find that because the environment is always in flux, it is almost impossible to reproduce the results of an experiment. Hence, there is a high risk of misinterpreting the data obtained from such experiments. Through this paper we try to caution experimenters against such risky assumptions when they venture into the RF jungle. After a successful proof-of-concept experiment, we advocate the use of wireless networking testbeds that provide experimenters better control over the RF environment by using coaxial cables, programmable attenuators and power dividers/combiners.
Communications, 2009. ICC '09. IEEE International Conference on; 07/2009