Why are some channels not shown in 5GHz 80MHz mode in OpenWrt?

Question

I am using OpenWrt. Some channels are not shown for selection under 5GHz 80MHz.

According to this page:
http://www.revolutionwifi.net/revolutionwifi/2013/03/80211ac-channel-planning.html

Channel 42, 58, 106, 122, 138, 155 should be valid for selection. However, on OpenWrt, the channel numbers just skip on the number I have just mentioned (42, 58, etc)

It seems I am misunderstanding something?

Answer 1

The article you linked to is a bit misleading. When you use multiple contiguous 20MHz-wide channels to make a 40-, 80-, or 160MHz-wide channel, you generally don't name the wide channel based on its center frequency. Instead, you just name the "control channel" and the width, and there are rules in the standard that determine exactly which other 20MHz channels are lashed together to make one mega-channel.

So if you say channel 40 at 80MHz, any 20MHz transmissions you need to make will happen on channel 40, any 40MHz transmissions will go across the 40MHz swath that encompasses both channels 36 and 40 (but we don't really say "you transmit on channel 38", even though that's one way to think about it) and any 80MHz transmissions will go across the 80MHz swath that encompasses channels 36-48 (but we don't really say "you transmit on channel 42", even though that's one way to visualize it).

Specifying "channel 42" wouldn't be enough information to know how to properly operate an 80MHz-capable 802.11ac network, because it doesn't tell you where the 40MHz and 20MHz transmissions should go (they absolutely do NOT all get centered on the center frequency of channel 42).