OVERVIEW

Wireless radio communication is an essential component of sensor networks and enables sensor nodes to perform significant local coordination, distributed signal processing, and network self-configuration to achieve scalable, robust and long-lived networks.  The quality of the wireless channel depends on multiple factors, such as the environment, the radio frequency, the modulation scheme, and even the RF transceiver hardware in use. 

These networks will be deployed in harsh environments from the communication perspective, with significant multi-path effects.  In addition, the low power radios typically used in sensor networks do not have sufficient frequency diversity to be resilient to multi-path communication. Under these conditions, wireless communication is known to be unpredictable and has been shown to vary drastically with small spatial changes and on different time scales.  Even though most sensor network algorithms are designed to be adaptive to the variations in the communication channel, there are several parameters that need to be adjusted to the operating conditions in order to improve performance.  Furthermore, the real communication channels are very difficult to model for the wide range of target environments and the different type of radios, frequencies, and modulation schemes in use.  Thus, it is difficult to extensively test the algorithms under development in simulations under realistic conditions.  Given the variability of the communication channel, and the difficulty to model it accurately, it is essential to get qualitative and quantitative data that may allow us to better understand the channel characteristics in the target deployment area. 

APPROACHes

SCALE [1] is a measurement tool design to study wireless communication channels with low power radios in new environments.  It facilitates the characterization of the most basic communication metric from the application point of view: packet delivery.  The tool enables the collection of packet delivery statistics using the same specific hardware platform and in the same environment intended for deployment.  The data gathered by SCALE will allow protocol developers and engineers to better estimate the appropriate density, system parameter tuning constants, and expected performance of protocols and algorithms (data capacity, convergence time, latency).

SCALE is highly configurable, including the following parameters: packet probe size, inter-packet period time, and ttransmission power gain, among others. This flexibility permits performing experiments under multiple different varied conditions. More importantly, it allows repeated measurements while constraining all parameters other than the one being varied, allowing us to systematically probe the effects of that particular parameter.  The tool can be run transparently in a centralized way with all the software running in a central PC and connected to the nodes via serial cables, or in a fully distributed way with the software running in different distributed nodes.  SCALE also provides a visualization screen to help viewing the connectivity data in real-time and after each experiment completes.  Using up to 55 nodes, we were able to measure and study the connectivity conditions of two hardware platforms, Mica 1 and 2 motes, in three different environments: an outdoor habitat reserve, an urban outdoor environment on a university campus, and an office building.  

SYSTEMS / EXPERIMENTS

The results of our measurements using SCALE revealed some interesting findings.  By analyzing data from a rich set of links with different distances, directions, antennae elevations from the ground, with or without line of sight --conditions that we expect to find in sensor network deployments-- we found that there is no clear correlation between packet delivery and distance in an area of more than 50% of the total communication range.  In addition, we found that temporal variations of packet delivery are not correlated with distance from the transmitter or transmission power level, but to the mean reception rate of each particular link.  We also found that the percentage of link asymmetries varies from 5% up to 30% in some cases, and there was no obvious correlation between link asymmetries and distance and/or transmission power levels.  By using this tool, we provide significant quantitative evidence that supports the commonly held belief that link asymmetries are due to hardware calibration differences.

ACCOMPLISHMENTS

We have fully implemented SCALE in the EmStar [2] framework.  We have performed several experiments under real conditions using the mica Berkeley motes in multiple environments using different configuration parameters.

FUTURE DIRECTIONS

In the near future, we plan to integrate SCALE with some in situ localization systems under development to eliminate the extent of human intervention needed and make the system more autonomous.  In addition, we plan to extend the low-level radio interface to collect some of the signal-to-noise information available in some of the RF transceivers used in sensor networks.

Another venue of exploration will be the development of propagation models for simulations that better characterize the real conditions found in sensor networks deployments.

PEOPLE

FACULTY

Prof. Deborah Estrin

GRADUATE STUDENTS

Naim Busek
Alberto Cerpa