When using survey tools, such as AirMagnet Survey and Ekahau, to measure signal strength for generating signal coverage maps, be certain to take into account the 802.11 beacon intervals set in the access points or mesh nodes. The default beacon interval is generally 100 milliseconds, and the default setting for most survey tools is to measure signal strength on each RF channel for 250 milliseconds (sometimes called the “scan time”). With these settings, you’re assured of the access points or mesh nodes transmitting a beacon while the survey tool is measuring the signals on a particular channel. In fact, at least two beacons will occur during that time, so the survey tool won’t miss any of the beacons. A possible problem may occur, however, if the beacon interval on the access points or mesh nodes is set to a shorter value. For example, Tropos mesh nodes generally have default beacon intervals of 250 milliseconds. They do this to reduce overhead traffic (i.e., beacons) on the network. In this case, if your survey tool is set to a scan time of 250 milliseconds, then there’s a possibility that a beacon will not occur during the 250 milliseconds. At these setting, the absence of a beacon has very low probability. In fact, I’ve inadvertently used these settings before for surveying a large area, and there were very few (much less than one percent) missed beacons. I could tell this by the fact that the tool measured signal activity for nearly all sample points. I saw very few where it didn’t. Therefore, the signal coverage generated by the survey tool was still very accurate. In most cases, the scan time is adjustable. To ensure the survey tool you’re using is measuring all possible signal data, it’s a good idea, though, to be certain that the scan time of the survey tool is greater than the beacon interval.