Using 40 MHz wide channels in a 2.4 GHz wireless network deployment

Question

We have a site (a secondary school) where all WiFi networks are managed by us. The building should in theory have no rogue APs that introduce interference, however, other RF emitters such as microwaves, game console controllers, and Bluetooth devices do exist.

I, being a young and still learning network engineer, wanted to follow all the best practices as detailed in online resources, as well as published books about network deployment strategies. From what I found, it is generally recommended to use non-overlapping 20 MHz wide channels when deploying a network in the 2.4 GHz spectrum. Configuring APs in a manner where each AP broadcasts on a channel that is not used by any other AP nearby. I have also found a lot of resources that say that using 40 MHz wide channels is generally not recommended in the 2.4 GHz band since there are only 2 non-overlapping chunks of the available spectrum and that it would be extremely difficult to divide it in a manner where APs do not interfere with each other.

Unfortunately, our senior network engineer would like to use 20 MHz Ce channels (40 MHz effective). His logic is that we should use as much of the spectrum as possible and that the best way to get a transmitter to stop occupying the spectrum is to allow it to transmit the data at the fastest data rate possible (since a higher max speed is reachable with 40 MHz). He tells me that that I do not understand how 802.11 works and that APs operating in the same chunk of the spectrum can utilize it better if allowed to use the entire spectrum due to collision avoidance algorithms such as CSMA/CA, as opposed to dividing the spectrum into smaller chunks and having each AP use their smaller chunk.

I agree with him that his solution might "better utilize the spectrum", however, I do not see it as bringing a better quality of service to the clients utilizing this network.

Could someone please explain why he is or isn't correct? I cannot find a single online resource that recommends the use of 40 MHz wide channels in the 2.4Ghz spectrum unless you only have 1 or 2 transmitters each using their half of the available spectrum. However, since he is the senior network engineer we have to do things the way he says...

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Additional Information added after Ron Trunk's response

2.4 GHz is not the only band we use. We also have a 5 GHz network, where I have no issue using a 40 MHz wide channel since you can fit a lot more of them without having them overlap.

Answer 1

APs operating in the same chunk of the spectrum can utilize it better if allowed to use the entire spectrum due to collision avoidance algorithms such as CSMA/CA, as opposed to dividing the spectrum into smaller chunks and having each AP use their smaller chunk....Could someone please explain why he is or isn't correct?

A lot depends on your definition of "chunk." As a practical matter, depending where you are, you only get one 40 MHz channels in the 2.4GHz band (US), and it's more susceptible to interference than a 20 MHz one. The number of adjacent APs, number of clients, and types of traffic will all affect performance and may make narrower channels work better than wide ones.

A better question is why use 2.4 GHz at all? Any device capable of 40GHz on 2.4 GHz is almost certainly capable of using the 5GHz band, where there are plenty of channels and less interference. But perhaps there is a justifiable reason that we don't know.

However, since he is the senior network engineer we have to do things the way he says

This is the bottom line, and I doubt that anything that anyone here says will change his mind. You can rest easier that your opinions are just as valid as his.

Answer 2

Looking at the context of your question, it appears the main goal is to reduce interference. That being the case, channel widths are only one part of the consideration. One of the big problems I've seen in many installations is the proximity of access points combined with inadequate configuration. I don't know if you've completed a wifi survey and no idea what hardware you're using but essentially you need to know how close the AP's are to each other and determine how they interact. I see a lot of environments where AP's will be very close but transmitting at full power, causes many issues. You could lower the transmission power of an AP as much as necessary to ensure the least amount of unnecessary signal overlap. If you haven't done this already, definitely will improve user experience if you're having trouble.

Answer 3

40 MHz channels were only introduced in 802.11n (WiFi 4 in the new naming scheme from the WiFi Alliance). 802.11g (WiFi 3) and earlier physical layers, could not use 40 MHz channels. 802.11n, however, operates at both 2.4 GHz and 5 GHz bands. So if your clients support 40 MHz bands (802.11n, 802.11ac, 802.11ax - or WiFi 4, 5, 6, respectively), they should be able to do so at 5 GHz as well.

By the way, you might be happy to know that with 802.11ax, there is much better support for dense deployments, sharing of spectrum, etc., so some of those considerations mentioned by the senior engineer, while having some truth for earlier generations of WiFi, would not be relevant with 802.11ax (WiFi 6).