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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Wi-Fi Riddles: Invisible 40MHz

A while ago I stayed in a small hotel that, had virtually all the problems of ‘small hotel wi-fi’ (including unresponsive one-device-per-room hotspot and lack of support outside of business hours. How many people stay at the hotel inside the business hours?). This is very typical of a WLAN installation made by a small local jack-of-all-trades shop. Unfortunately, I see this issue way too often, especially in SMB deployments. I hope this article will help some of the SMB integrators and their clients.

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Wi-Fi Riddles: Strong signal = bad signal?

TooMuchPowerWhile I was working on the next part of the “Unobvious and overlooked Wi-Fi” (which is about channels), I got an interesting knowledge nugget from our engineering. We all know that there is a lower limit to receiver sensitivity, we all know that there must be some upper limit, after which the Rx signal is so powerful, it simply oversaturates the radio. But that is it? Now I know it, even though I did not ask for it explicitly – I merely happened to run into a situation where it matters.. Read on…

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Wi-Fi Riddles: Faster speeds require weaker signals?

A discussion of my previous post in the 802.11 Wireless professionals group lead to a discovery of a new bit of overlooked knowledge! Many thanks to Eduard Garcia-Villegas and Mike Rex, who turned my attention to this one.

Ever wondered why faster rates have smaller effective distance? It was always attributed to signal fading – higher rates require more complex modulation, which, in turn, requires higher SNR (Signal-to-Noise Ratio) value. So at greater distances, with lower SNR, one has to use lower rates. Simple.

Turns out, in addition to that the transmitters may as well lower their power when transmitting at higher rates! This is done because at higher power values transmitters are more likely to introduce errors in the signal they create due to non-linearity in their behaviour (if you want more details – just go to the discussion). Here’s an example from a datasheet that Eduardo provided (the numbers are Tx Power, dBm):

Tx Power: HT20, 2.4GHzTx Power: HT20, 5GHz

Note how the power falls as MCS number increases. Also note how this is different in 5GHz depending on channel number.

Mike Rex added, that the users of alternative consumer WLAN router firmwares (such as DD-WRT and Tomato) sometimes suffer from strange performance and link reliability issues, as these firmwares allow varying radio power levels (and other power settings) beyond what manufacturer has intended, resulting in poor Tx signal quality. This mainly is just a result of the shortcomings and compromises in hardware design (even if the APs are built on the same chipset), but certainly not something a non RF engineer would suspect first! 😉

Well, I learned something – this blog already pays off. 🙂 Many thanks to Eduard, Mike and all others, who tool part in the discussion. This bit has been rolled into the original post to keep all things together.