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!

Advertisements

What 802.11ax is not

I normally do not publish the “link to” posts, preferring to share on LinkedIn, but Devin Akin well deserves it. Matches my perspective 98%+, especially the point on 802.11ac stillborn MU-MIMO.

Preamble: Aerohive has released the first 802.11ax APs (the official 802.11ax standard spec is not final yet) – so expect the marketing race.

TL DR: The only good thing is OFDMA sub-carrier allocation (think sub-channels) similar to what exists in GSM/LTE/WiMAX, but it would not work w/o client support and forget 2.4GHz.

Anyway, enjoy the article: http://divdyn.com/802-11ax-not/ and let me know your thoughts!

Setting a package as Device Administrator with StageNow, reversing the DevAdmin class name

I had to perform a simple task recently: set up the Battery Swap application on out TC51 as a Device Administrator, so that it can do its battery swapping preparations correctly (for some reason it’s not set up as such by default). MX and StageNow allow this via the DevMgr CSP. But that CSP requires Package Name and Class Name. Let’s find out and do some more package dumping for fun and profit!

(more…)

WLAN Channel Management F1 Style: Part 1 of 3

We keep saying that overlapping channels are not good – they create unnecessary interference, contention, or both (depending on the distance between the APs). It is a general recommendation to stick to non-overlapping channels and actively hunt and eradicate anyone who overlaps. However, are overlapping channels always bad?

WideChannel6
A typical example of a channel that overlaps with every other!

In this 3-part series I’ll be talking about some channel tuning techniques that may allow cramming more channels in the RF plan, have them overlap – and still get overall better network performance. A valuable LinkedIn member Eduard Garcia-Villegas (who is doing some interesting research in WLANs) had recently provided me with some interesting papers that offer theoretical foundations and practical proof behind this. What does it have to do with F1? Let’s find out!

(more…)

[Recommended] Wireless LAN Professionals Podcast

Just finished listening to http://www.wlanpros.com/wlw052-7-rules-accurate-site-surveys/ by Keith Parsons.

Highly recommended – a very good summarisation of site survey lore:

  • What do do on a site survey
  • What not to do on a site survey
  • How can others cheat on a site survey (= do not do it)
  • Active vs Passive survey
  • Spectrum analyzers
  • And more

P.S. Check out the web page for PDF white papers that summarize the podcast!