Opinions on Channel Width in a 1:1 Classroom

[Garrett]

Currently in our district we have an AP per classroom due to the numerous devices connecting.

We are already limiting users on bandwidth. With that in mind what is the ideal Channel Width for a high density area like a classroom?

The argument that is being had is performance. Does 40MHz give better performance?

Or does it not matter since we are limiting devices speeds anyway? 

My understanding has always been 20MHz channels for a high density environment in a must. 

This gives a more stable wireless network and less chance of channel overlapping.

Any thoughts?

[SoCalRacer]

Depends on the devices connecting and your RF environment.

Devices:

If you control the devices connecting then you have better options available. If you don't which is what I suspect, and these are something like BYOD students devices, you have no idea what they will be and backwards compatibility is going to tell me that you should go for 20Mhz.

Environment:

40Mhz will give you higher bandwidth/performance but will overlap other channels so if you have many outside wifi networks visible in your area 20Mhz would be best. 

[Garrett]

So the main devices connecting are actually all owned by us. 

Those being Teachers Windows devices and HP Chromebooks. (Then obviously guest users on guest WiFi)

As far as our RF environment. We are pretty segregated from most neighbors and only pushing out 3 SSID's per AP.

So you are on the side there will be performance increase on 40MHz. But in what way?

The reason I am curios is we are capping devices at 25mbps. 

What are these "performance" increases if its already being capped? Are there other performance features besides from just bandwidth speed that 40MHz is improving?

Thanks!  

[RubenG]

---

@Garrett wrote:

The reason I am curios is we are capping devices at 25mbps. 

What are these "performance" increases if its already being capped? Are there other performance features besides from just bandwidth speed that 40MHz is improving?

No other performance increases would be noticed if you are capping the client and there are a handful of clients, and none are utilizing wireless to the 25mbps cap simultaneously.  

You will see a performance increase if you have 30 clients in a classroom and they are all utilizing the wifi to download a large file at the same time.

No other benefits, just speed. More downsides depending on how you utilize and how your environment is. I would recommend viewing your AP status page and seeing how heavily utilized it is. 

[Garrett]

Just for clarification, 

When you say:

"You will see a performance increase if you have 30 clients in a classroom and they are all utilizing the wifi to download a large file at the same time."

What are you meaning by that?

The throughput is better?

The 40MHz channel width can handle more data at once? comparatively to 20MHz?

For example if you get 30 chromebooks all turning on at once and all connecting to the AP and browsing it, the 40MHz could handle that better than 20MHz?

[RubenG]

Correct. 40 MHz will have a higher throughput.

For example, if we look at speeds for 3 spatial Streams. The fastest theoretical possible speed for a 20 MHz would be: about 300mbps (rounding up)

for 40 MHz: 600mbps

assuming that the devices are only downloading, not broadcasting anything else. They are all streaming a super 8K HD video quality or downloading a very large file at the same time. Then each of the 30 clients would be limited to 1/30th of the speed. The average max speeds you would see would be something like this.

40 MHz ~ 20 mbps

20 MHz ~ 10 mbps

So overall you would be able to get better speeds for each client if you had high density and demand like in the example above.

Keep in mind that this is only an example and does not necessarily correspond to the values you will see in your environment. There are other factors that need to be considered when you think of the actual throughput that won't allow you to get the speeds mentioned. Such as half duplex, broadcasting by the AP, interference, etc.

It isn't necessarily that it can handle it better, but more speed will be available. Realistically it is better to operate on 20 MHz because of channel overlap and reducing the available number of channels will reduce the available open channels for the AP to use when it has to back off due to high utilization. 

I would recommend you check your environment and run a passive survey, see if any AP is over utilizing the Channel. see what kind of speeds you are getting on the dashboard. To see if you require the boost in speed.

[Garrett]

If you do not mind me asking. How are you calculating those numbers. Is that kind of just common knowledge? 

That 20MHz vs 40MHz has half the speed? or??

Also, just to clarify on what you stated earlier, if we are capping speeds per client @ 25mbps on a 20MHz channel width, there is the possibility that speeds could dip under that cap because of traffic. (a lot of traffic?)

Whereas 40MHz would have a less likely-hood of dropping under 25mbps because of the faster speeds it is capable of? 

Not to be repetitive, but I am learning, and for the most part just want to fully understand why 40MHz would be better than 20MHz in a throttled environment.  

Thanks by the way Ruben, I appreciate the time you are taking to respond. 

[GIdenJoe]

The most important fact has not been checked.

Since you're using 1AP per classroom you should survey those areas around that classroom and above and below to see how far that signal actually bleeds even at lowest transmit powers.

Keep in mind that although people use -67 dBm as a celborder definition, the signal can cause signal detect at 4 dB above noise floor.  That means if your noise floor is -92 at your area your AP and connected clients can still be heard at -88 dBm from neighboring AP's causing deferred transmissions.

So when determining your channel plan, see how far the signal goes down to -86 or -88 depending on noise and that is your actual border wherein you cannot reuse that channel.  So if you still  have room for enough channels, then go with 40MHz, but if you don't have enough channels, move down to 20MHz.  Also consider changing your RX-SOP thresholds.  And of course use high mandatory datarates like 24Mbps minimum to avoid overhead.

[Garrett]

The site survey piece is definitely something we have been thinking of doing. 

Just getting some more visibility into the classrooms would be huge for exactly what you are talking about. 

Currently we have our Meraki AP's set to auto on most everything. Obviously RX-SOP from what I understand being one of those changes that you would only want to do with good visibility in your wireless network. Hints the site survey piece. 

Then quick follow up question. Are you under the opinion as well that if per client bandwidth is currently being capped at 25 mbps, that the "performance" difference will be minimal? 

That is one key aspect that I am trying to read up on. Does that limited speed make so the possibilities of what 40MHz can perform does not matter? Is the main "performance" difference between the two just speed? 

[GIdenJoe]

Well, if you're able to use 40 MHz if the channel plan allows for it. Then you will have the advantage of using it because even though you cap the througput of every client to 25 Mbps, they will need less time on the air to actually transmit that 25Mbps of traffic.  Because if 40MHz is possible, the potential datarate per device can go up and you could potentially have more efficient use of the airtime.

However if the channel plan doesn't allow it because you get co-channel interference from a distant AP on the same channel and then you need to fall back to 20 MHz.

This is one of the main reasons to keep your celsizes on the small side and avoid putting AP's in corridors.

[RubenG]

---

@Garrett wrote:

My understanding has always been 20MHz channels for a high density environment in a must. 

This gives a more stable wireless network and less chance of channel overlapping.

Typically it is recommended that in high-density environments you utilize 20 MHz because you will have multiple APs and you don't want any co-channel interference. Reducing to 20 MHz will allow you to utilize a larger number of channels and you won't have APs utilizing the same channel.

You can utilize 40 MHz Channels with 9 APs utilizing every single 40 MHz channel, in an auditorium for example. The problem here is if some sort of interference or a rogue AP is introduced to your environment. If said device utilizes the channel very heavily then your APs ability to service your wireless clients will be degraded and all clients on that AP will experience poor or slow connections.

The above example was for an auditorium, where high density is expected and APs would be within audible distance of each other. Now if you have a more spread out building system, 1 classroom per building/bungalow, you can get away with the APs utilizing 40 MHz. The reason being is that the APs will be at a fairly good distance from each other. So if some sort of interference or rogue AP is introduced that utilizes the entire channel then the AP will be able to move to another Channel with Meraki's AutoChannel. With this, of course, you would have other factors to determine. Like how many audible APs are at a particular location, how strong is it broadcasting, how is the channel overlap, etc. And you would have to run a predictive or have someone run a passive survey.

It would depend on your environment whether you can get away with running 40 MHz. How close are your classrooms, how thick are your walls, any RF interference, how many floors, etc. Which would involve some sort of wireless survey.

---

@Garrett wrote:

The argument that is being had is performance. Does 40MHz give better performance?

Or does it not matter since we are limiting devices speeds anyway? 

---

This will depend on what you are limiting the clients too and how many clients you are anticipating per classroom. Utilizing that you can determine if the 20 MHz channel will provide enough throughput. You also have to consider if every single device will be downloading/uploading at that speed simultaneously. But yes, utilizing 40 MHz will essentially double your speed if your environment permits.