Channels not available for 40MHz; 80MHz and 160MHz @ 5GHz

Where where are official channels for 40Mhz; 80Mhz and 160Mhz @ 5Ghz?

Chipset: MediaTek MT7613BE 802.11nac
OpenWRT: 21.02.1 r16325-88151b8303

OpenWRT only shows the channels @ 20Mhz:

5 GHz channels @ 20MHz

channel 36 transmits at frequency 5180 MHz
channel 40 transmits at frequency 5200 MHz
channel 44 transmits at frequency 5220 MHz
channel 48 transmits at frequency 5240 MHz
channel 52 transmits at frequency 5260 MHz
channel 56 transmits at frequency 5280 MHz
channel 60 transmits at frequency 5300 MHz
channel 64 transmits at frequency 5320 MHz
channel 100 transmits at frequency 5500 MHz
channel 104 transmits at frequency 5520 MHz
channel 108 transmits at frequency 5540 MHz
channel 112 transmits at frequency 5560 MHz
channel 116 transmits at frequency 5580 MHz
channel 120 transmits at frequency 5600 MHz
channel 124 transmits at frequency 5620 MHz
channel 128 transmits at frequency 5640 MHz
channel 132 transmits at frequency 5660 MHz
channel 136 transmits at frequency 5680 MHz
channel 140 transmits at frequency 5700 MHz

But never the channels in between:

5 GHz channels 40MHz

channel 38 transmits at frequency 5190 MHz
channel 46 transmits at frequency 5230 MHz
channel 54 transmits at frequency 5270 MHz
channel 62 transmits at frequency 5310 MHz
channel 102 transmits at frequency 5510 MHz
channel 110 transmits at frequency 5550 MHz
channel 118 transmits at frequency 5590 MHz
channel 126 transmits at frequency 5630 MHz
channel 134 transmits at frequency 5670 MHz

5 GHz channels 80MHz

channel 42 transmits at frequency 5210 MHz
channel 58 transmits at frequency 5290 MHz
channel 106 transmits at frequency 5530 MHz
channel 122 transmits at frequency 5610 MHz

5 GHz channels 160Mhz

channel 50 transmits at frequency 5250 MHz
channel 114 transmits at frequency 5570 MHz

This is expected behavior and normal for all 5GHz wifi devices.

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In OpenWRT, it depends on the Country Code you specifiy...

Screenshot 2021-12-07 163858

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The 20 MHz list is always used. It becomes redundant if the width is wider than 20, and the standard wider band that includes the specified channel will be used. For example if you choose any of 36, 40, 44, or 48 with an 80 MHz bandwidth, the on-air result is exactly the same-- 80 MHz centered over channel 42. Actually setting "42" is not valid.

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In the design for 5Ghz, the table above is what you should use. That is not happening.

It has nothing to do with countert codes, only the available ranges matters.

If you look at the list @psherman linked to, you'll see the 20 MHz channels you list (38, 42, 46, ...) are not available by design. This is to prevent overlap which is a real issue with 2,4 GHz channels.

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The wikipedia 5 GHz table is really unclear in how it's structured. The channels are actually structured in the same way as the 6 GHz channels - see the 6 GHz table.

A 40 MHz channel is almost never specified as its center frequency, but as a 20 MHz channel plus a specification of "adjacent" channels. Either 36+adjacent-up or 40+adjacent-down - I forget what the official convention is. At least it seems that wifianalyzer appears to use adjacent-up when displaying things. wifianalyzer will show the center channel in results, but in parentheses show the actual beacon channel. Other systems will only show the beacon channel

Part of this is that for compatibility reasons, beacons are broadcast at 20 MHz bandwidth on known 20 MHz channels. This allows a 20 MHz-only client to join a 40 MHz capable network

For another way of writing it - https://support.huawei.com/enterprise/en/knowledge/EKB1000079063 - specifically this part - "Two neighboring 20 MHz channels are bundled to form a 40 MHz channel. One channel functions as the main channel, and the other as the auxiliary channel. The main channel sends Beacon packets and data packets, and the auxiliary channel sends other packets."

OpenWRT specifies HT40/VHT80/VHT160 by their main channel, not by center frequency, because that's where the beacons get transmitted

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Not sure howto interpreted the following text just found and have to clear my headup with bedtime :smiley:

Found the following document:
802.11ac Technology Introduction (rohde-schwarz.com)

Page 7:
snip
A few examples (from [9]) will help to illustrate how these parameters work to provide
the center frequency and bandwidth: (Since a VHT STA operates in 5GHz band, the
examples will assume a regulatory class that has a channel starting frequency = 5
GHz.)

Example 1:
A channel specified by
CurrentChannelBandwidth = 80 MHz
CurrentChannelCenterFrequencyIndex1 = 42
CurrentPrimary20MHzChannel = 36

is an 80 MHz channel with

Channel center frequency
= 5 GHz + 5 * 42
= 5210 MHz

Primary 20 MHz center freq
=5 GHz + 5 * 36
= 5180 MH
snip

At the 5Ghz section you see the center channels.

Another nice reading:
WiFi 5GHz band and wide channels - Metis.fi

Try this one form here - https://support.metageek.com/hc/en-us/articles/203532644-802-11ac-Channels -

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This table gives indeed a clear view, 80Mhz starts at 42.

Now, when you choose other channels then 42 and choose 36, will OpenWRT make 42 from it, because it is in the range of the 80Mhz?

I don't thoroughly understand the wording of your inquiry - I'll try though...

If your AP or client is transmitting at 20 or 40 MHz, then yes it will use the appropriate channel:

  • for 20 MHz that's termed 36, 40, 44, or 48
  • for 40 MHz that's termed 38 and 46
  • and yes at 80 MHz - termed 42

Perhaps it'll help if you stop thinking of them as individual channels; but rather as a frequency range used for channels 36-48. As you can see, the larger channel bandwidths are divisible by half of the next size.

(The graphic shows that.)

:warning: Maybe it will also help to remember that you have to pick a 20 MHz channel - as the AP will decrease its TX bandwidth if there's congestion, etc. on the band.

Yes. OpenWrt follows the standard for the 80 MHz blocks, such as "channel 42" covering the four 20 MHz channels centered over 36,40,44,48. It will not transmit an 80 MHz signal that is not one of the standard blocks.

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Ok so by this logic the following channels SHOULD work when width is set to 80Mhz

36 / 5180Mhz
52 / 5260Mhz
100 / 5500Mhz
116 / 5580Mhz
132 / 5660Mhz
149 / 5745Mhz

The reason I ask is that ONLY 36 works for me and I am not sure why.

Also would it make sense to have the UI update so that when the band width is selected the correct channels are displayed with the standard format (center channel)

If you haven't set a country, or you are in a country where only the low part of the band is allowed, then 80 MHz will only work on that first block 36-48 or "42".

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One thing to also keep in mind is that there is a 90 second "listening for radar interference" clock that must run and expire when you switch to, or power up on a DFS channel in a DFS regulated country such as the US. This applies to 60, 100, 116, and 132, etc.

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When you say "set a country" are you talking about in general settings under system properties where the timezone is set or somewhere else?


Nevermind i found it, it was set to 'driver default' instead of 'US' I will test and get back to you. Thanks for the assistance.

After changing the location to US i was able to select a less crowded band (116) and all is working. Thanks again for the assistance @mk24 and @Almondo

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But...what you described is not the format (e.g. what if a client only did 20 MHz).

Glad you got it working by setting the correct country.

So the standard way of assigning channels is this. Notice when selecting a 40 or 80 Mhz channel the center channel is chosen. So if i set the dropdown in the UI to 80 only channels 42, 58, 108, 122, 138, 155 should be available for selection.

OR (and this is even more correct) if we use center frequency there will be 0 overlap in channel numbers which is how the bandplan was designed. There is no need to even allow selection of band width. Each channel will correspond to its correct bandwidth.

So:

  • 36 (20Mhz)
  • 38 (40Mhz)
  • 40 (20Mhz)
  • 42 (80Mhz)
  • 44 (20Mhz)
  • 46 (40Mhz)
  • 48 (20Mhz)
  • 50 (160Mhz)
  • and so on

This is the point of using center channel in the bandplan. Each channel number corresponds to its bandwidth and bandwidth never needs to be selected.

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