When using survey tools 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 radio frequency (RF) channel for 250 milliseconds (sometimes called the scan time). With these settings, you are 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 will not 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 longer value. For example, Tropos mesh nodes generally have default beacon intervals of 250 milliseconds. They do this to reduce overhead traffic (that is, beacons) on the network. In this case, if your survey tool is set to a scan time of 250 milliseconds or less, then there is a possibility that a beacon will not occur during the 250 milliseconds. In most cases, the scan time is adjustable. To ensure the survey tool you are using is measuring all possible signal data, it is a good idea to be certain that the scan time of the survey tool is greater than the beacon interval.
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