However - what happens now if I have a large building (like an airport or shopping centre)? I can scatter these receivers on different channels throughout the building. When you move out of range, I'll stop getting your probe requests and it can be safe to assume you have left the area. If I set up a device to purely sit silent and listen for these probe requests, then write the MAC address and timestamp to a file - then I can tell when you pass within range of my access point - for as long as you are in the area. I'm interested in seeing where you are going - not what access points are available. However, what if we turn the tables a little? The power requirements are much lower - after the initial probe request, the adapter only listens for a fraction of time compared to a passive scan (although this varies per manufacturer and driver). This is where things get interesting.Īctive Scanning is the default in just about every device existing. well, lets say I'd win at the lotto every week before that would happen in the wild. The chances of getting two MAC addresses of the same value are. Now - your MAC Address is considered globally unique. In this process, the wifi card transmits a probe request on the currently tuned channel. So we have implementation #2 - mostly referred to as Active Scanning. Sure, you can only run a scan lets say, every 30 seconds - but that still means you have the receiver powered up for 5 seconds out of every minute. We have 12 (or 14 in some areas) channels in the 2.4Ghz band, so a full scan would take 102,400μs x 2 x 12 = 2,457,600μs or about 2.5 seconds.ĭuring this time, the receiver is in a high power usage mode - so battery life is negatively impacted and power usage for a wifi adapter sky-rockets. To have some hope of hearing most Access Points, you'll need to wait for 2 beacon intervals - say 102,400μs x 2 per channel. The beacon (by default) is only transmitted every 100 TU (102,400μs). The scan is completely passive and nothing is transmitted from your wifi adapter. There are two methods of scanning for active WiFi access points.ġ) Turn the receiver on and listen on each channel for the beacon orĢ) Broadcast a "Who is there?" packet on each channel. Wifi is an interesting protocol when you get into the gritty details.
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