Investigating Power Consumption in 802.11 WLANs: Measurement, Visualization, and Improvement
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802.11 based wireless local area network (WLAN) has been increasingly supported by handheld devices. However, supporting WLAN functionality is extremely energy consuming since the connectivity has to be maintained even when the device is in an idle state. The power management specified in the 802.11 standards do not specify detailed techniques to handle the problem caused by power consumption affecting factors (PCAFs), since most of these factors have not been specified in the standards. The objective of this master dissertation is to investigate how different PCAFs affect the power consumption in 802.11 WLANs. As a starting point, several PCAFs are identified and investigated, including the beacon period, the beacon size, the signal strength, the foreground traffic, the background traffic, and the delivery traffic indication map (DTIM). Following the test instructions specified for both an idle mode and a traffic mode, measurement data for these two modes have been collected from the test-bed. According to the experimental study, it has been shown that all the PCAFs have different qualitative and quantitative effects on the power consumption of 802.11 WLANs. For example, the uplink traffic mode consume more energy that the downlink one. The background traffic eliminates the effectiveness of the power management the most among all PCAFs. By making use of the findings from the experimental study combined with the concept of 802.11 theory, analytical expressions for calculating the power consumption under a 802.11 legacy polling scheme are derived. The accuracy of the obtained equations have been validated by the experimental data measured from the test-bed. Moreover, a WLAN Power visualization tool (WPVT) has been implemented by Java, which can display theoretical power consumption curves for both an idle mode and a traffic mode if PCAF configurations are fixed. All theoretical curves are determined according to the derived analytical formula. Different with the traditional way where a test-bed, hardware configurations, multimeter testing, etc., are required, the WPVT only needs simple PCAF configurations. In a sense, the WPVT facilitates the investigation of PCAFs in 802.11 WLANs due to the fact of less time consuming as compared with the traditional experimental study. Finally, a new power saving algorithm has been proposed, which has taken the time drift of the synchronization clock between an access point (AP) and a station (STA) into account. Probability density function (PDF) curves of the power consumption have been presented and simulated for both the Legacy polling scheme and the new mechanism. It turns out that the new mechanism outperforms the Legacy polling scheme with respect to the power consumption. The investigation presented in this dissertation provides first-hand empirical results and guideline for studying PCAFs in the 802.11 WLANs. Both the methodologies and the investigation results can supplement existing prior research work related to power conservation topics. This is the main potential outcome through the thesis, which can also act as the main contribution to knowledge.
Masteroppgave i informasjons- og kommunikasjonsteknologi 2009 – Universitetet i Agder, Grimstad