Abstract. This publication explores the degree of accuracy to which the propagation delay of WLAN packets can be measured using today's commercial, inexpensive equipment. The aim is to determine the distance between two wireless nodes for location sensing applications. We conducted experiments in which we measured the time difference between sending a data packet and receiving the corresponding immediate acknowledgement. We found the propagation delays correlate closely with distance, having only a measurement error of a few meters. Furthermore, they are more precise than received signal strength indications.To overcome the low time resolution of the given hardware timers, various statistical methods are applied, developed and analyzed. For example, we take advantage of drifting clocks to determine propagation delays that are forty times smaller than the clocks' quantization resolution. Our approach also determines the frequency offset between remote and local crystal clocks.
IEEE 802.11b compliant WLAN technology is increasingly used for cordless telephone services. Often, a WLAN phone is moved during a call. In the present paper we explore to what extent slow user motion influences the wireless link quality. We conducted extensive measurements with speech over commercial WLAN equipment using an experimental environment enforcing controlled motion. Our experiments show that -in contrast to the common assumption -an increase of motion speed can result in a better link quality: The packet loss rate and variance, measured after link-layer retransmissions, decreases. In addition, our measurements demonstrate that the modulation type, the maximal number of retransmissions, the experimental setting, and even the quality of power supply are the dominant influences on link quality.
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