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 for IoT – a boon or a bane?

Internet of things is hot now. But what technology should be used to interconnect those devices? Over the years there were multiple contenders, but Wi-Fi was always a bit aside, since it was always perceived as not too energy-efficient. Until recently…

Here’s a picture from a chip maker Rockchip, claiming (claiming!) that their Wi-Fi chip is the most efficient, overperforming even the specially designed ZigBee and BLE chips.

RockchipWiFi

While this seems as a reason to rejoice, promising greater speeds and technology convergence, I’d like to point one thing: 802.11b. Not n, not g, but b. Remember the protection modes, airtime fairness and other nightmares? They might come back in legions. And stay for what seems forever, as Rockchip claims “powering an IoT device for up to 35 years with a single AAA battery”.

What do you think? Is energy-efficient 802.11b chip a boon or a bane for modern WLANs?

P.S. News via SmallNetBuilder – excellent website, whose author is doing great job finding relevant news and testing devices. http://www.smallnetbuilder.com/wireless/wireless-news/32743-rockchip-unveils-iot-wi-fi-soc

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: ‘Too good’ signal is 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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Running a WLAN controller in VMWare Player

Motorola WiNG5.6 firmware images were finally posted last week, and along them, the VX9000 cloud controller DEMO image. I am very excited about it, as it allows me to run a VM version of Motorola’s top controller (or cluster, or hierarchy for that sake) right on my laptop! No need to say about potential for labs and demos this has.

In this blog I’ll review how I installed VX9000 image onto VMWare Player. Officially, VX9000 supports EXSi, Xen, Hyper-V or Amazon EC2, but for small setups something like VMWare Player or VirtualBox also works.

VX9000-GUIlogin

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