Synopsis Diffusion for Robust Aggregation in Sensor Networks

Early approaches for computing duplicate-sensitive aggregates in sensor networks (e.g., in TAG) have used a tree topology, in order to conserve energy and to avoid double-counting sensor readings. However, a tree topology is not robust against node and communication failures that are common in sensor networks. Recent research has shown that for several duplicate-sensitive aggregates (e.g., Count, Sum), significantly more accurate and reliable answers can be obtained by combining energy-efficient multipath routing schemes with clever algorithms that avoid doublecounting. In this paper, we establish a formal foundation for this new approach, which we call synopsis diffusion. Synopsis diffusion avoids double-counting through the use of orderand duplicate-insensitive (ODI) synopses that compactly summarize intermediate results during in-network aggregation. We provide a surprisingly simple test that makes it easy to check the correctness of an ODI synopsis and to develop new ODI synopses. We also show that the properties of ODI synopses and synopsis diffusion create implicit acknowledgments of packet delivery. We show that this property can, in turn, enable the system to adapt message routing to dynamic message loss conditions, even in the presence of asymmetric links. Finally, we illustrate using extensive simulations the significant robustness, accuracy, and energy-efficiency improvements of synopsis diffusion over previous approaches.