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My 100% signal strength Wi-Fi network is being interfered or blocked by a low signal Wi-Fi network. When I monitor Wi-Fi using inSSIDer 2.1, as soon as a particular network (with 30% signal strength) comes in, my network becomes slow, pinging my router times out intermittently, etc.

My router is a D-Link 2750U double antenna.

How is it possible that a far away signal is able to interfere with my nearby signal?

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Each WiFi "channel" spans a certain range of frequencies around 2.4 GHz, and it's possible that both networks are configured to use the same channel. There are utilities you can use to determine what network is on what channel, and you can change your own network to a channel that is not being used (or with lower interference) for the best performance.

You can use a tool like inSSIDer (as you listed above), or in some cases, the WiFi card manufacturer may provide a similar utility. There are also a wide variety of alternatives to the tool I listed above.

As for why the low signal network affects your high strength one, the key fact is that "noise is noise". From your perspective, any transmission not from your network is considered random noise, and will corrupt packets. If a network on the same channel has a 30% signal strength (assuming both networks are transmitting packets at the maximum rates), this introduces 30% error into your signal (assuming the 0's and 1's being transmitted are truly random).

Thus, 70% of your packets still arrive fine, but the remaining 30% need to be re-transmitted, affecting the total available bandwidth for your network.

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Those percentage numbers are nonsense. Otherwise, this is correct. – David Schwartz Mar 8 '12 at 20:42
@DavidSchwartz for the more advanced readers, assuming that the interfering signal is true AWGN, and assuming no changes in transmission power, the true error rate follows the Q-function, and is highly dependent on the inter-symbol interference, itself based on the signaling method (NRZ, Manchester, etc...). For a non-corrective transmission algorithm, those numbers are understated - although we know what WiFi does use checksums, greatly reducing the error rate probability. – Breakthrough Mar 9 '12 at 13:07

Did you:

  1. Change your SSID. Does it match theirs? If so you will have issues.
  2. Enable WPA2 encryption.
  3. Change channels. Your software should tell you which channel you are on and what channel they are on. If you use the default 6 and so do they, change to 1 or 11. Basically, get as far away from their channel as possible.
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