WiFi insights – Why you should avoid using channels other than 1, 6, 11 and 14 like the plague

In short: Only use channels 1, 6, 11 (and 14 if you live in Japan) for your 2.4 GHz wireless LAN and simply ignore the other ones. The world will be a better place with no effort at all :-)

Great! Now you have got a WiFi of your own. As you are a prudent guy you’ll carefully choose a free channel so as not to interfere with your neighbors’ WiFis. Scanning for available networks reveals that channels 1, 6, 9 and 11 are already being used, which leaves channel 3 as the way to go.  You are quite confident that everything should be in order now. The truth however is rather disconcerting and may come to you as a shock.

What you have really done is, you have set up a new jammer that will halve the throughput of at least 3 networks – including yours. How can that be true?

Only 3 out of 11  channels do not interfere: 1, 6 and 11

This is due to the channel allocation scheme that is used by 802.11 b/g standards and the behaviour of WiFi equipment: While channels are equally spaced 5 MHz apart, the bandwith consumed by a single transmission path is roughly 20 MHz, effectively occupying 5 channel slots: The center channel it is set to, 2 adjacent channels towards lower frequencies and 2 channels towards higher frequencies as well.

Thus a station set to use channel 1 will occupy channels 1-2-3. The next non-overlapping channel will be 6 occupying channel slots 4-5-6-7-8. The only channel left will be channel 11 occupying channel slots 9-10-11.

Overlapping channels substantially decrease throughput 

Assuming urban surroundings and just 3 useable non-overlapping channels, wireless networks are bound to interfere with each other. If interference cannot be avoided, why not try to minimize interference by “filling the gaps”?  Two stations using the same channel should most certainly experience much more interference than if they were using different ones, even if they were just one slot apart. Suprisingly enough, this is not true.

Wireless stations are by no means dumb passive devices. Before initiating a transmission they will listen for other ongoing transmissions and abide from starting their own transmission when they find that the channel is currently in use, thus minimizing interference (DCF, CSMA) . Sadly enough, this is an important factor that is often overlooked or set aside, even in academic documents. 

Now when you offset two stations by less then 5 channels you’ll impede their capability to properly detect ongoing transmissions – practically blindfolding them and turning them into anti-social criminals.

Furthermore, by virtue of its bandwidth, each transmission that does not adhere to the 1-6-11 rule will use up 2 instead of 1 out of the proper channel bands described. Thus stations set to channels other than 1, 6 or 11 will unknowingly and unneccessarily interfere and disrupt both each other and other adjacent channels, in turn causing frequent re-transmission which will bring throughput down to a crawl.  

What about channels 12, 13 and 14

Within Europe and Japan, there are two additional channels available: 12 and 13. But they are of little use, as they both overlap with channel 11. Still worse, devices may refuse to connect to those channels, unless they have been set up properly, e.g. a Nokia E6x will no longer connect to channels 12 and 13 when you remove your SIMM. 

Channel 14 is a very special case. It is used in Japan only and despite its number it is spaced 12 MHz apart from channel 13, so it might have been named channel 15.4. Transmissions on channel 14 will interfere only minimally with those on channel 11. Therefore, in Japan there are 4 channels that should be used in order to minimize interference: 1, 6, 11 and 14.

Links:

  1. Channel Deployment Issues for 2.4-GHz 802.11 WLANs (Cisco), as PDF
  2. 802.11b WiFi Channels (Moonblink)
  3. DCF (Wikipedia) 
  4. DCF (WiFi Planet)
  5. CSMA (Wikipadia)
  6. 802.11 (Wikipedia)

Comming next: Why SuperG and other 108 MBit transmissions often cause more damage than benefit. Why a 3 dB Antenna boosts more than increasing Transmit Power by 12 dB.

About Reiner

Born 1954 in Ratisbon (Bavaria, Germany), in 1976 punched cards at Berlin Technical University, caught hacking one of its mainframes by Horst Zuse (son of Konrad Zuse), started studying computer science and soon was offered a job whithin their computer department doing systems programming for IBM VM/370. While studying, jobbed around Germany at various places doing all sorts of things, then returned to Berlin to work at SRZ (computer aided typesetting). Never finished my master degree, but chose to take up self-employed work (which didn't turn me rich nor famous). Now working for a mid-sized software company within a very promising department as head of server software development.
This entry was posted in at Home, at other Locations, Computers, English. Bookmark the permalink.

6 Responses to WiFi insights – Why you should avoid using channels other than 1, 6, 11 and 14 like the plague

  1. Pingback: www.ngtech.gr » WiFi Setup- Why channels 1-6-11 are preferred

  2. Wally says:

    How do I know what channel I’m using with a Belkin N wireless Model F5D8233-4?

  3. Reiner says:

    By default, the Belkin uses channel 6 (as most routers do). The F5D8233-4 is a draft-N device, that supports faster than 54 MBit transmissions, but at the expense of occupying more channels (about all the available ones). If your PC uses and needs higher data rates within your LAN (you won’t normally need them for internet access, 54 MBit will not slow down downloads of up to 16 MBit), you should stick with these defaults (main channel=6, extension channel=2). Otherwise you might consider disabling 802.11n.

    You can look up the current setting by logging in to your router (i.e. http://192.168.2.1) and selecting Channel and SSID from its configuration menu.

    The Belkin user manual can be downloaded from http://www.belkin.com/support/article/?lid=en&pid=f5d8233-4&aid=8340&scid=0&fid=3558&fn=f5d8233-4_v1_manual.pdf

  4. Gary Masson says:

    Hey Reiner, thank you for this great post. I’ve been studying Wireless technology these last few weeks and your explanation as to why to only use 1, 6 and 11 is exactly what I was looking for. Putting it in the context of a Jammer is excellent. Thank you sir!

  5. SLeo says:

    I’ve worked in tech support for years. While I’ve known that each channel is a few megahertz wide and very similar channels interfere, my experience has been different. I have often resolved my customer’s wireless issues by using channels 3,4, 8 and 9. While I appreciate the theory, I’ve not seen this work in practice.

  6. Chrisanthi Likousi says:

    Very useful article, thanks for all this information!

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