I'm analyzing probe request packets and I'm seeing that some of them has an undefined values in HT Capabilities section. Why? Is it not required in all probe request packets?
As I see HT Capability section announced only in 802.11n
There are two 802.11n operating modes: Greenfield (HT) and Mixed (Non-HT). Greenfield can only be used where no legacy systems exist. HT systems will not switch between Greenfield and Mixed, they will only use one or the other.
An 802.11n AP using Non-HT mode sends all frames in the old 802.11a/g format so that legacy stations can understand them. That AP must use 20 MHz channels and none of the new HT features.
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PS. Yep it's not full answer. I don't find yet any notes about this behaviors in RFC's or IEEE's