Sniffer placement

We used SNR measurements to place our multiple sniffers. One sniffer was placed adjacent to the AP, to be responsible for capturing the From-AP traffic and the traffic of clients near the AP. The other sniffers were placed as close as possible to the wireless clients. Assuming that clients are uniformly distributed over the coverage area, this meant placing the sniffers so that they cover as much of the AP's coverage area as possible. Generally, if we have $n$ sniffers to place, we split the AP coverage area into $n$ equal areas and place the sniffers in the center of mass of these areas.

Figure 1: SNR Contour Map for controlled experiment: SNR Contour lines for 40, 30, 20 and 15 dB were obtained from SNR measurements. We placed the wireless clients at the locations with different signal conditions based on SNR measurement. Sniffers were placed at locations T, U and V.

To determine the AP coverage area, we first used the SNR (obtained from Prism2 header) seen in Beacon frames from the target AP to draw the contour lines (as shown in Figure 1). The AP coverage area was then determined by choosing a particular SNR contour, e.g., the 15-dB contour line.

We can refine this strategy by noting that, in an environment where multiple APs are installed, the coverage area of an AP may be reduced to the Association Area of the AP. The Association Area of an AP is the area at which a client will favor this AP for association compared with other APs in the area. This behavior may be device-specific and may also vary depending on whether a client has roamed to an area or has just powered on their radio. For the purposes of sniffer placement, we assume that clients will associate with the AP with the highest SNR 9 (9).