Do non-overlapping channels overlap?

We all know the “non-overlapping” channels 1/6/11 in 2.4GHz (5GHz matter is similar). Do they really not overlap? I keep bumping into this in conversations, and would like to create a point of reference (with pictures) instead of having to repeat same old over and over.

BW- 2m away from AP
Your typical “non-overlapping” 1/6/11 setup

Since we a dealing with broadband technology, the signal is in reality not 100% contained within the allocated 20Mhz band – we only see the tip of the iceberg. Here’s the official 802.11 20-Mhz OFDM channel spectral mask. Note that the “20Mz” channel actually goes up to 30Mhz in every direction (60Mhz total width), albeit up to -45dB weaker, than the central 20Mhz flat part.

Wi-Fi Spectral Mask - Single Channel
802.11 OFDM transmit spectral mask. Power levels are relative to the signal strength in the center.

Now, let’s combine the masks for all the “non-overlapping” together and enjoy the view.

Wi-Fi Spectral Mask - 1-6-11
Spectral masks combined together in 2.4GHz space. Can someone draw me picture with three icebergs please?

Of course, if the APs are spaced far enough, the effect of side bands will be negligible: if I already hear the AP’s central frequency at -87dBm, hearing the sidebands at another 20-26dB lower will do well below the sensitivity threshold. However, if this is not adhered to, here’s a spectrum analyzer capture of channels 1 and 11. Can you see the AP in channel one? What chances are for it to be heard?

Spectrum - 24GHz Ch1 Ch11 overlap
“Non-overlapping” channels 1 and 11.

Summary:

  • Even non-overlapping channels overlap
  • Maintain separation. Either calculate using tools or use 3-5m as a rule of thumb (better use tools!)
  • Stacking APs on top of each other to provide triple density seems a good idea but only works if you are Xirrus, but even they stopped doing it, as far as I know.
  • 2.4GHz is dead, move all enterprise networks to 5.

Hope this clarifies the matter enough. If this useful enough to use as a point of reference when explaining the matters to others? Let me know your thoughts!

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