Overhearing the Wireless Interface for 802.11-based Positioning Systems

Abstract

Not only the proliferation of 802.11, but also the capability to determine the position of mobile devices make 802.11 highly appealing for many application areas. Typically, a mobile device that wants to know its position regularly performs active or passive scans to obtain the signal strength measurements of neighboring access points. Active and passive scanning are survey techniques originally intended to be performed once in a while to learn about the presence and signal reception quality of access points within communication range. Based on this survey the best suitable access point is selected as the gateway to the wired network. However, so far, no investigations are known to have been launched into how regular scanning affects concurrent data transmissions from an end-user point of view. In this paper, we explore how common data communication is affected while actively or passively scanning at the same time. We found that with an active scanning interval of less than 2 seconds the network conditions such as throughput and round trip delay are insufficient for interactive applications. The same is true for passive scanning if a scanning interval of less than 7 seconds is chosen. Furthermore, we present a novel scan scheme called Monitor Sniffing to reduce client service disruptions. Monitor Sniffing exploits the fact that 802.11 operates on overlapping channels by overhearing the wireless interface. We have implemented our Monitor Sniffing algorithm using commodity 802.11g hardware, and we demonstrate that it is faster than active and passive scanning and does not disturb concurrent data communication. Finally, our approach only requires software modifications on the client side, making the adoption process quite easy