Dot11 Guru

802.11, Troubleshooting

Why can’t I capture 4x20MHz channels if I can hear 1x80MHz channel?

It seems like people have been able to make this work recently.  I haven’t tested it myself personally, so cannot comment on how or how well it works.  Stay tuned for when I find time to test!

It was a question asked in the WiFi Slack group recently.  The discussion hinted in a direction that would eventually answer why, but not without research!

PHY Header

The PHY header consists of various fields.  The field we’re interested in is the “TX TFC” Field.  This is the Training field, and it’s used by the NIC to measure the width of the transmit spectral mask.

Transmit Spectral Mask

A Transmit Spectral Mask is the power contained within a set bandwidth of a radio frequency.  The set bandwidth is actually the channel width, and the spectral mask shape is formed using set parameters (see below).

Piecing it all together

The NIC detects energy on a center frequency,

It uses the Training field in the preamble to measure the channel width of the communication

It then sets to that width to receive and decode the frame.

Therefore, if a 20MHz spectral mask is detected, then the NIC will set itself to 20MHz.  It would not be possible to receive frames on any other channel during this transmission.


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