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.
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.
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.
