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Today someone told me that when a device has connected to an AP, other APs in the room normally cannot see its packets, because:

  1. Adjacent APs commonly work at different frequencies to prevent interference
  2. The wireless network interface controller will not pass the packet that isn't addressed to it
  3. Sometimes the device does send probe packets, but it only happens when the device decides to roam

My questions are:

  • Is what he said correct?
  • If so, how can a WiFi positioning system, be it fingerprinting-based or triangulation-based, be able to detect the locations of connected devices using just a single AP, or do the APs have to work in promiscuous mode for WiFI positioning?
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Wifi clients will continue to perform Active Probing of the entire Wifi channel space even when already associated to an AP - primarily so they are aware of other APs in the same SSID (on different channels) to roam to.

You can prove this every time you associate to a network, open your wireless settings and see a real-time list of other networks you could associate with.

When you talk about "packets" I assume you mean IP, which in this case is true - while the client probes will be sent out on all channels, IP traffic will only be sent to the AP the client is associated to.

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  • Thanks. But it seems that Active Probing is not enforced by IEEE 802.11 specification, so if the signal strength is quite satisfactory, is it possible the client doesn't send probe packets at all? And if the interval between those probe requests is really long, e.g. 1 time/min, does it mean the WiFi positioning systems can't depend on them?
    – Stupident
    Oct 15 '15 at 11:09
  • It is not enforced, but I believe most clients tend to use it anyway to ensure smoother roaming (and also to maintain a list of available networks if the user wishes to switch) Oct 16 '15 at 6:18

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