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Default 5GHz channel width

Getting noticed

Default 5GHz channel width

Hi!  

 

I have an environment with MR42s and it has been recommended to change the default 5GHz channel width from 40 to 20.  Everything I can find seems to indicate that 40MHz or greater is the way to go for 5GHz.  I see almost nothing touting the benefits of a 20MHz channel width.

 

Anyone have any insight on this?

 

Thanks!

Doug

5 REPLIES 5
Building a reputation

Re: Default 5GHz channel width

Hi Doug, nothing wrong with using 20, 40 or even 80(if you can). However, keep in mind this all depends on your environment, one major factor being, do you have lots of APs, and/or do you have lots of “other people” APs around you? 

Highlighted
Kind of a big deal

Re: Default 5GHz channel width


@american_niseiwrote:

and it has been recommended to change the default 5GHz channel width from 40 to 20.  Everything I can find seems to indicate that 40MHz or greater is the way to go for 5GHz.  I see almost nothing touting the benefits of a 20MHz channel width

 


Well, as you observe, the default 5 GHz channel width is 20 MHz. You won't see much written about the benefits of using  20 MHz channels because most of what is written is marketing material; but it does exist.

 

  • a 40 MHz wide channel is twice as likely to suffer from interference as a 20 MHz wide channel. The larger channel occupies twice as much of the available spectrum, so twice as likely to suffer from interference.
  • a 40 MHz wide channel reduces the amount of space available for other APs, potentially causing conflicts and congestion.
  • whilst virtually all client devices can use a 40 MHz channel, the same does not apply to 80 MHz and 160 MHz channels.
  • if operating in Band B UNII-2 Extended  (5490 - 5710 MHz), then you are doubling the likelihood that weather radar will cause DFS, TPC and CAC to become active. Once that happens, depending on which channels are being used, an AP can go off-air for up to 60 minutes ensuring that it does not start broadcasting on an occupied radar channel. Basically the Wi-Fi shuts down, and you can't do anything about it, apart from avoiding the weather radar channels,

 

DFS is required in the frequency range 5725-5850 MHz to ensure an appropriate protection to the radiolocation service (including frequency hopping radars), DAA is required in the frequency range 5855-5875 MHz for the protection of ITS, in the frequency range 5725-5875 MHz for the protection of BFWA, and in the frequency range 5795-5815 MHz for the protection of TTT applications.

If you are suffering interference related performance/throughput issues, it is a good idea to see if you can wind back the TX power of the AP radios without the client devices losing connectivity and reducing the likelihood of interference by reducing the channel width. If still suffering from interference , change the channel number to the channel just vacated and see if that makes a difference.

 

It is perfectly normal advice, as far as throughput/connectivity/interference issues are concerned to reduce channel width as the first step of nailing down the problem.

 

Without knowing a great deal more about your situation and environment, I could not comment on further steps to take to solve the issue. I can tell you this, however, turning up the TX volume,and widening the channel width always has unintended consequences.

 

Having said that, at my test site, I run 80 MHz channels because I am testing the efficacy of DFS/TPC/CAC and I am exposed to weather radar, marine, aviation and military use of the 5 GHz portion of the spectrum.

Robin St.Clair | Principal, Caithness Analytics | @uberseehandel
Kind of a big deal

Re: Default 5GHz channel width

In my experience, @Uberseehandel 's first bullet point:

 

  • a 40 MHz wide channel is twice as likely to suffer from interference as a 20 MHz wide channel. The larger channel occupies twice as much of the available spectrum, so twice as likely to suffer from interference.

Is your single biggest issue with wider channels. You have to realize that your physical layer, RF spectrum, acts like a hub with clients operating in half-duplex mode. By narrowing your channels you make their communication more efficient and precise with regards to RF spectrum utilization. 

 

I think this is reinforced with the upcoming features being included in 802.11ax with OFDMA that will allow for channels smaller than 20MHz so that more clients can get onto the network at once, more reliably. 

 

Remember, faster wifi doesn't equal better wifi 😉

Getting noticed

Re: Default 5GHz channel width

Thanks everyone for your responses!  The environment is a high-density, high-utilization co-working location.  There can be over 300 people in the space.  I currently have a mix of MR32, 33, and 42s in the space.  Running everything on auto except the channel width which I had set to 40.

 

I've had both connectivity issues (people getting booted off the AP) and speed issues.  I don't have DFS turned on and am not using any DFS frequencies.

 

 

Kind of a big deal

Re: Default 5GHz channel width


@american_niseiwrote:

 . . The environment is a high-density, high-utilization co-working location.  There can be over 300 people in the space.  Running everything on auto except the channel width which I had set to 40.

 

I've had both connectivity issues (people getting booted off the AP) and speed issues.  I don't have DFS turned on and am not using any DFS frequencies.

 


Hi,

 

It looks like you have some challenges there.

 

Firstly, be aware that the specifics of operating in the 5 GHz band are dependent upon the regulatory area you are in. I know there have been some changes in the US and that in East Asia the legislation has changed, I don't keep up with this stuff globally (I have to eat), unless I need to.

 

Depending on where you are, if you are not using DFS channels (using DFS is not optional, it is mandatory if using a DFS channel).

  • In Europe, the only 20 MHz wide channels that are nor subject to DFS services are:
  • 36, 40, 44 and 48 - all at 20 MHz
  • the equivalent 40 MHz channels are 38, 46
  • 80 MHz channels 42 
  • That is it for most of the world, including the US, if you avoid the DFS channels.

So you only have 2 non overlapping channels available. High density, you are going to have problems.

 

The advice you have been given is dead right.

 

In order to so something about improving your overall service, these are the steps I would take.

 

Preamble - almost everything in the 5 GHz band is counter-intuitive.

 

With only 2@40 Mhz / 4@20 MHz channels to work with, you have some severe limitations.

You need to better understand your environment. Start out by carrying out a site survey. Use Ekahau. If the budget is tight use their free tool, HeatMapper. Do read the very clear documentation, there is plenty of it, all helpful.

 

Generally, bear in mind:

  • better to have more low powered APs  operating at low volumes, than fewer powerful APs operating at max TX volume.
  • avoid having signals leaking into adjacent spaces serviced by other APs
  • you have little control over Wi-Fi signals from neighbours leaking into your space, unless you can find a way to address the issue in a non-confrontational manner.
  • Auto channel and auto TX volume is not as efficient as carrying out a survey and adjusting channels, TX volumes and AP locations explicitly.

 

These are just guidelines. If the problem is serious, and it sounds as if it is, hire a qualified professional to sort it out. If you have shouty Wi-Fi neighbours, don't get into a fight with them. Some of the cheaper brands of AP are not as particular about how high TX volumes are set when their domain is set to certain countries.

 

 

Robin St.Clair | Principal, Caithness Analytics | @uberseehandel
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