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I understand that 2.4GHz Wi-Fi channels overlap, and that the most popular non-overlapping set of channels in the US is 1, 6, and 11. Generally, my signal strength on channels 1, 6, and 11 are much stronger than my neighbors' on the same channel. However, these channels usually have 4 or 5 APs already using them. In this scenario, is it better to use 3, 4, 8, or 9? Or is it better to use the crowded channels 1, 6, and 11?

As a secondary question, does it even matter that my signal strength is much higher than theirs?

Related:

Why use wifi channels other than 1, 6 or 11?

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"the only non-overlapping channels ... are 1, 6, and 11." - You misunderstand the concept of "non-overlapping channels". There are groups of non-overlapping channels. 1, 6 and 11 is only one of those groups. If your neighbor is using channel 5, then if you try to use ch 1 or 6, then you will overlap with his signal. –  sawdust Jun 29 '12 at 21:24
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@sawdust I understand that just fine. Let me amend that statement then - the largest number of non-overlapping channels are the 3 channels 1, 6, and 11. I'm saying that since EVERYBODY uses 1, 6, or 11, should I still use 1, 6, or 11? Or is it better to use 3, 4, 8, or 9 which are in between these popular channels. –  Luke Jun 29 '12 at 21:40
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Probably the best way to find out is to ignore all this conflicting advice, setup iperf on two machines, and test the throughput of each channel. –  endolith Dec 14 '12 at 17:18
    
A lot of people mistake IEEE 802.11 signals for solid cars on a multi-lane highway. They frown upon people driving over the lines, occupying more than one lane. However, Wifi signals are rather like colored plumes of smoke. Along the open lanes, the color plumes are allowed to intermingle. As long as I can still tell the color of my plume of smoke at the end of the road, all is fine. The partial overlap of differently colored plumes is then like a gray mist of noise to my signal. This is the principle of spread spectrum communication. More info here. –  Serge Stroobandt Jul 6 at 15:08

7 Answers 7

up vote 23 down vote accepted

Cisco tested this. The result is that if you use an overlapping channel (anything other than 1,6,11), you get terrible performance and you make everybody else's performance worse. The problem is that any time an AP on the overlapping channel broadcasts, you get stepped on. And because the channels overlap rather than coincide, other network's transmissions are seen as noise, not signal, and don't trigger the bandwidth sharing built into the design.

Non-overlapping channels (1,6,11) work better than overlapping channels. With overlapping channels, you step on each other and can't do anything about it. With non-overlapping channels, you see each other and share the bandwidth.

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Thanks, any link to the results of the test from Cisco? That would make this the definitive answer. –  Luke Jun 30 '12 at 4:00
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Their Channel Deployment Issues paper contains the results of one of their simple tests. –  David Schwartz Jun 30 '12 at 5:00
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David - when that document was written that was probably good advice - you could build a network then and there would not be another wireless network in sight. 11g was fairly new - 11n was on the drawing board - adaptive power and frequency selection, and smart antennas were not available. In today's congested urban environment I don't think you benefit by sticking to 1,6 & 11. If you do someone else's smarter equipment will take advantage. Probably the best advice is to get the latest 11n equipment and let it take care of things for you - or just use all 5GHz eqpt. –  BJ292 Jul 1 '12 at 21:37
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That paper is for setting up multiple APs near each other, not for setting up your single AP to avoid colliding with the transmissions of your relatively quiet neighbors. –  endolith Oct 13 '12 at 1:33
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-1 With my previous router (a crappy WRT54GC) i got terrible performance when using a channel anyone else was using, overlapping channels got much better performance in my experience –  kinokijuf Jul 6 at 11:51

Stick to 1,6, or 11!

Even if other channels seem less crowded, remember that because channels overlap you still have to deal with interference from those busier channels as well. Your "clearer" channels will still have interference originating from the busy channels, so there is little to gain. What happens when you put your system in between two of the "standard" channels is that now you get interference from both of them. So, if you were to use, say, channel 3, you might now get interference from radios on both channel 1 and radios on channel 6 (and everything in between). More than that, you will yourself now cause interference with people using both of those channels. Whenever that happens, those other users will have to re-transmit their message, making the wireless signal in your area even busier.

There are a few studies indicating that, under the right circumstances, it may be possible to get more throughput using a four-channel scheme (such as 1,4,7,11, 1,4,8,11, or 1,5,8,11). However, for this work everyone in your area would have to agree on it. Until you can get everyone cooperating on that scheme, you will get best results by using the least busy of 1,6, or 11. Even then, this was only shown to help for certain kinds of loads and densities.

Finally, be careful when deciding which of 1,6, or 11 is least busy. Tools like InSSIDer will not help you here. They will only show you which neighbors have the strongest signal available on which channels, based on beacons from the access points/routers. They will not tell you how much those neighbors are using the signal. If you have someone next door with a strong access point on channel six, but they hardly ever use it, and other neighbors down the way with weak access points on channels one and eleven, but they use them to work from home and are on them all the time, you may be better off using channel six, even though it might look "bigger" in a tool like InSSIDer.

So how can you know which channel is least busy? This article on the serverfault blog may help:

http://blog.serverfault.com/2012/01/05/a-studied-approach-at-wifi-part-2/

It's the 2nd part of two part series, but the first part is less important to this discussion. The main thing is they recommend a tool called Vistumbler that will allow you see not just signal strength, but also actual traffic. It's takes a bit of doing, but you can use this to really know, not just guess, what channel is going to be least busy in your area.

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In fact, you're better off picking a channel with stronger signals rather than weaker ones. That way, you'll hear each other reliably and share the bandwidth, rather than interfere with each other and both need to retransmit all the time making things even worse. –  David Schwartz Jul 1 '12 at 11:04
    
Wrong! By sticking to the 1-6-11 scheme, you are voluntarily exposing yourself to the RTS/CTS/ACK of alien devices. This forcedly silences your devices on many occasions. It will effectively lower your bandwidth even when capacity in adjacent channels is still available. This problem is known as the exposed node problem. In a corporate setting this problem can be solved by synchronising the nodes. In the wild, this is not readily achievable. More here –  Serge Stroobandt Jul 6 at 14:59

The proof of the pudding is in the eating!

1-6-11 is often worse in moderately congested areas

The 1-6-11 recommendation contained in Cisco's whitepaper about IEEE 802.11 deployment in the corporate environment certainly does not apply to all circumstances! For example, in moderately congested neighbourhoods, one stands a very good chance to benefit from not sticking to this proposed scheme. So, don't be a monkey and consider this:

  • First, note that the signal of a device on a partially overlapping channel is merely noise to the device on the overlapped channel. This is entirely intentional by design. The technique is called spread spectrum.
  • However, the situation usually gets worse when one voluntary abides to the 1-6-11 non-overlapping channel scheme. Doing so will expose your devices to the IEEE 802.11 RTS/CTS/ACK (Request to Send / Clear to Send / Acknowledge) of alien devices, effectively silencing your devices and hence forcedly lowering your bandwidth. This problem is known as the exposed node problem. In a corporate setting this problem can be solved by synchronising the nodes. In the wild, this is not readily achievable.
  • In the end, Shannon's theorem is what dictates the maximum achievable information transfer rate of a channel in function of the noise level on that channel.
  • Your antenna might provide more gain on certain channels and/or in certain directions, both greatly affecting your signal-to-noise ratio.

Hence, I call for actually measuring one's own signal-to-noise level. On a busy time of the day, try a number of seemingly quiet channels in between the most busy channels and far away from the strongest alien signals.

On a GNU/Linux system you can list all access points seen by your WLAN device as follows:

sudo iwlist wlan0 scan

Your own network will also be listed with a Quality value, roughly proportional to the signal-to-noise ratio. Try to maximise this value by changing channels and/or improving your base station antenna gain in your direction (e.g. by using a sector antenna at the edge of your home). Note that antennas often provide a little bit less gain at the band edges (channels 1 & 13/14). Maximum Quality is what you are looking for. The Quality value takes into account noise from overlapping channels.

Channel:3
Frequency:2.422 GHz (Channel 3)
Quality=70/70  Signal level=-40 dBm

If 2.4GHz is too crowded, you may consider falling back to RTS/CTS/ACK channel sharing in the 1-6-11 scheme. Still better; do yourself a favour and upgrade your devices to 5GHz. Much more bandwidth is available on 5GHz and overlapping does not exist.

The important lesson here is: bandwidth is a finite resource. It is especially scarce on the lower frequency bands (2.4GHz). Like with any scarce resource in life, there are only a limited number of possible approaches, listed here using metaphors:

  • The 1-6-11 non-overlapping channel scheme would be the equivalent of a state-sanctioned communist planned economy (i.e. all too often like internal corporate culture).
  • Signal-to-noise optimisation is blatant libertarianism and probably more efficient.
  • And migrating to 5GHz should than be something like... colonising Mars.
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Referring to the statement that "the signal of a device on a partially overlapping channel is merely noise", the use of OFDM uses the available channel width much more efficiently and equally. So this would only be true if you want to go back to 802.11b, which I for one do not. –  YLearn Jul 5 at 16:57
    
@YLearn Interesting point. However, if I am not mistaken, this statement equally applies to single carrier spread spectrum modulation; not only multiple carrier techniques. –  Serge Stroobandt Jul 5 at 21:11
    
If you use a spectrum anayzer, you will see that 802.11b traffic looks like a sine wave and the outside edges of the wave can drop below the noise floor allowing channels to overlap less. When you get to OFDM, the signal pattern is a more of a "plateau" and there is very little of the signal that drops below the noise floor across the 20MHz used so there is no chance for overlapping channels to not interfere with each other, no matter how much noise in the environment. –  YLearn Jul 6 at 3:57
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+1 With my previous router (a crappy WRT54GC) i got terrible performance when using a channel anyone else was using, overlapping channels got much better performance in my experience –  kinokijuf Jul 6 at 11:52
    
@YLearn It is not a story about all or nothing. When channels overlap, this will look like an overall increase of the noise floor on your spectrum analyser. Evidently, the head room between your plateau and the noise floor will be lower. Nevertheless, communication will remain possible, albeit at a lower bit rate according to Shannon's theorem. –  Serge Stroobandt Jul 6 at 12:53

In big corporate networks it's common practice to use channels 1,6 & 11 because it is fairly straightforward (at least on a diagram) to design non-overlapping cells of coverage. As a home user you don't have the same constraints so it makes sense to experiment and look for the best channel. inSSIDer is free and quite popular for checking what's going on in your neighborhood. Collisions will only occur if the interfering signal is strong enough to interfere with the wanted signal. So if your laptop was right next to your AP pretty much nothing is going to interfere. That's generally not the case so it's normally a case of trial and error (and monitoring) to determine the best channel. Plus if you have a few people in the same area all constantly checking for the best channel it can get a bit messy.

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Using the tool to find the answer may be acceptable. However, I'm really curious what the tool is doing to determine the answer. Furthermore, I'm curious if the tool is actually choosing the best answer or if it's just the answer that works in most cases. e.g. Does it take signal strength into consideration? Does it track actual lost packets? Is it able to detect and account for non 802.11 devices in the 2.4GHz range? –  Luke Jun 29 '12 at 23:08
    
Basically the tool is only showing you usage of channels by SSID and graphing that by received signal strength. It is useful to see how your network compares with others and to identify the least used channel space. But it is only a snapshot in time so things may change pretty quickly as users come and go. If you run it from a PC right next to your AP then things will look pretty good from your perspective, so run it from various locations in your home. The dBm scale is logarithmic so 10dBm margin over your neighbors is x 10 signal strength, 20dBm is x 100 and so on. –  BJ292 Jun 29 '12 at 23:41
    
I see, yes I just tried the tool. I thought it could/would make recommendations of what channels to use. It just shows a graph as you suggested. Not unlike WifiAnalyzer for my Android phone (though my android doesn't do 5GHz). I've seen similar tools that will also make recommendations. I can't think of what they were at the moment. –  Luke Jun 30 '12 at 2:09
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This is bad advice. InSSIDer can be misleading here (see my answer) and choosing an "off" channel in a busy 1,6,11 airspace will lead to less throughput for everyone, yourself included. –  Joel Coehoorn Jun 30 '12 at 2:53
    
@Joel I wouldn't say it's bad advice, but it's certainly only part of the picture. Your point is well taken. inSSIDer did help me layout my 5GHz network - a space that my neighbors aren't in just yet (at least not with 802.11). –  Luke Jun 30 '12 at 4:13

In practice there doesn't seem to be too much difference, but if a channel is way overcrowded (e.g. with more than 4 APs using it), you might want to consider switching to a different channel to reduce the likelihood of the signals getting mixed up or otherwise \interfered with by other signals. It also sort of depends on your signal strength. If your signal is really strong, it doesn't matter.

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Well, I am a Ham Radio operator. I have done extensive testing. On my Actiontec or ZyXcel, channel 1 is abysmal! Channel 11 is a close second to the death of channel 1. ACTUAL power readings put 3 and 4 as the strongest signal output and throughput. Channels 6 and 9 are the standard preset. so actually avoid 1,6,9,11. I am a DSL tech also. I have walked people through changing channels from 9-10-11 to 3 or 4. They are amazed at the doubling of the wifi signal on all devices across the board. Leave the signal strength analysis to the experts kids. (it has to do with the relationship of the stage 1 pre-amp I F and the final stage RF, and just never mind) ;o)

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+1 for being a ham and coming out for it. Ham radio operators have a lot of experience running pile-ups. They know the value of experimenting and owning good a antenna. –  Serge Stroobandt Jul 3 at 23:51
    
Unfortunately signal strength is not the key factor in the operation of 802.11 traffic, so this proves nothing. Nor does Ham translate to 802.11 and the Hams I know who are also IT all know this. –  YLearn Jul 5 at 16:40

Some great answers here, but others that just don't understand the technology.

Let me answer with a fictitious, non-technical example. Let's imagine a world where the "highways" are 11 "lanes" wide and the vehicles are 5 lanes wide. Driving part way on the "shoulder" is allowed.

If there is a vehicle going slowly centered on lane 3, this would cause congestion for vehicles centered on lanes 1 and 6. Conversely, if there was a fast vehicle centered on lane 3, it would be impeded by a slow vehicle centered on either lanes 1 or 6.

The best way for traffic to flow effectively and efficiently is if the vehicles are all centered on lanes 1, 6 and 11.

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This is exactly what most people are getting wrong here! IEEE 802.11 signals are not like solid objects. Wifi signals are rather like coloured plumes of smoke. Along the open lanes, the colour plumes are allowed to intermingle. As long as I can still tell the colour of my plume of smoke at the end of the road, all is fine. The partial overlap of differently coloured plumes is then like a grey mist of noise to my signal. This is the principle of spread spectrum communication. –  Serge Stroobandt Jul 6 at 12:40
    
Sorry, I have to disagree. While I agree they are not like solid objects, they are much more so than the plumes of smoke that simply pass through each other with no impact you present. There are significant (and often serious) performance issues when your "plumes of smoke" interact that affect everyone in the environment. –  YLearn Jul 7 at 18:29

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