[FS#44] Ath9k - TX performance regression on greater coverage settings

[LEDE Bugs]

The following task has a new comment added:

FS#44 - Ath9k - TX performance regression on greater coverage settings
User who did this - Bluse-Blue (Bluse)

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Hi Koen,

Your observation is by design of the standard and not a malfunction, check IEEE 802.11-2007 17.3.8.6 [[http://www.ie.itcr.ac.cr/acotoc/Ingenieria/Lab%20TEM%20II/Antenas/Especificacion%20802%2011-2007.pdf|IEEE 802.11-2007]]:
"Where dot11RegulatoryClassesRequired is true, the value of the slot time 
shall be increased by the value of 3 μs × coverage class."

So the coverage class set by UCI distance will increase the ack timeout and the slottime. A increased slottime will decrease throughput as the channel access timings are relaxed.
Since this commit [[https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=f77f8234409978fefa0422b12a497451173e39b3|add coverag class to slot time]], the slot time is a function of the coverage class. So I guess your other observation with an older barrier braker release was using a mac80211 subsystem without this patch, hence only ack time out was effected by coverage class settings.
You could reveert the patch from your current release and test the performance in your setup. But keep in mind, the idea of increasing the slot time with increased coverage class, hence increased propagation delay, is based on the fact, that traffic typically flows bidirectional and therefore an increased slot time provides highes channel access probability to the ather direction.

Greatings from Berlin
Stoffel & Bluse

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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=44#comment180