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