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I understand and appreciate all the caveats around 20MHz/40MHz channel widths in the 2.4GHz spectrum but I've never been clear whether a client device that only supports 20MHz can work with an access point that's using a 40MHz width channel? Apple devices being the obvious example.

I've heard 40MHz been called "bonding" where two channels are bonded together. In this case, would a 20MHz client device just use one of the bonded channels? Or not connect at all.

munrobasher
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2 Answers2

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The basic answer should be "Yes", most routers configured in 20/40 auto mode will service clients on 20MHZ. The question should rather be: What effect will such a connection have for the client or the router?

Depending on the router, some will in this case drop the general channel width to 20Mhz to all the clients, while some other routers may intermittently have transmission errors with such clients. The outcome depends on both router and client. Newer routers and clients should work better, especially if configured to support only 802.11n or 802.11ac and for bandwidth of 5Ghz.

If the router is configured in the mode of "40MHZ only", then a 20MHZ client cannot connect, as it will lose data and spend all its time in re-connection attempts.

harrymc
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  • I believe the 40Mhz only option is only available for the 5Ghz channel width, not for the 2.4Ghz channel width. – LPChip Jun 06 '19 at 09:28
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    @LPChip: 2.4Ghz is bandwidth and not channel width. 40MHZ channel width on it is taking up two-thirds of the spectrum and is very vulnerable to interference. Normally 40MHZ channel width is only recommended for 5Ghz bandwidth. – harrymc Jun 06 '19 at 10:01
  • Yes I know, but I'm not aware there are any wireless devices on the 2.4Ghz bandwidth that will allow a 40Mhz only. To my knowledge they are either 20 only or both, not 40 only. – LPChip Jun 06 '19 at 11:17
  • @LPChip: I don't know whether such a router exists, but there are many weird ones around. – harrymc Jun 06 '19 at 11:18
  • Right... There are always exceptions, but given how much a 40Mhz consumes I doubt a 40 only exists. Maybe in enterprise gear, but given that the specs for 5Ghz are far greater, it is not too weird it exists on that band. – LPChip Jun 06 '19 at 11:33
  • You would be surprised how many office environments have 40MHz channels configured in the 2.4GHz band. – CustomX Jun 07 '19 at 07:39
  • +1 agreed, there are most certainly routers and flavors of 3rd party firmware that can be set to 40mhz channels only in the 2.4ghz band, And just as stated, 20mhz wide channel clients will not connect. – Tim_Stewart Jun 10 '19 at 15:20
  • How do I know whether my device (cellphone, iPad, laptop) support 40MHz or 80MHz channel width? https://superuser.com/questions/1556658/how-do-i-know-whether-my-device-cellphone-ipad-laptop-support-40mhz-or-80mhz – Rick May 31 '20 at 14:11
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That depends on the configuration of the wifi point.

Wifi has different standards: 802.11/a ac b g n. These letters a, ac, b, g and n are its capability. If the wifi point is configured to not support one or more standards while the client only supports one of those, then the client will not be able to connect. If the wifi point is set to support all modes, then the client will connect.

In addition, the speed can be set to 20Mhz or both 20Mhz/40Mhz. It will use 40Mhz if available or fall back to 20Mhz otherwise. There is no 40Mhz only option.

So just because the network offers 40Mhz, doesn't mean it stops support for the 20Mhz also. It all matters what standards have been enabled. Some people configure their wifi points to only support n in order to ensure that a client that has multiple ways of connecting, always chooses the fastest method, sacrificing backwards compatibility. If this is the case for you too, then the client will not be able to connect. But most of the time, especially if the default settings were used, all protocols are enabled, and the client can connect just fine.

LPChip
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  • “Your client will likely only support the b standard.” – How did you arrive at this conclusion? 11g also uses 20 MHz channels. Apple devices also do not use 40 MHz channels on 2.4 GHz, even on 11n. – Daniel B Jun 06 '19 at 08:56
  • @DanielB I guess that conclusion is wrong. My point was more that it doesn't matter if the device only supports 20Mhz, as long as the standard is supported. Then again, I believe the only option is 20Mhz or 20/40Mhz, and there's no 40Mhz only mode. But I'll edit my post to make this more clear. Thanks for the comment. :) – LPChip Jun 06 '19 at 09:15
  • @lpchip, there are router firmwares that have options for 40mhz wide channels only. The clients only supporting 20mhz **will not connect** to an access point configured this way. – Tim_Stewart Jun 10 '19 at 15:16
  • @Tim_Stewart a custom firmware is usually not following the industry standard but provides features added on demand. That doesn't mean they are best practice. So I'm excluding them because most people here won't have to deal with this, or they will have enough knowledge to not even need to ask the question in the first place. – LPChip Jun 10 '19 at 15:43