Because, at least on my system, looking at iwlwifi...
parm: 11n_disable:disable 11n functionality, bitmap: 1: full, 2: disable agg TX, 4: disable agg RX, 8 enable agg TX (uint)
So 8 is only aggregated TX, while 1 would be aggregated TX and RX. (It does, however, also inexplicably default to 0 on my system.)
e: Oh, apparently this is a pathological re-use of an existing module flag to force something on that they disabled due to instability in some cases, see also:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...
So I'm sorry, but my assumption about =1 is not valid, as I assumed that all of the bits (1,2,4,8) were mutating the same thing, not "one bit does something VERY DIFFERENT".
A quick google turns up lots of people agreeing with you, suggesting the intel wireless driver seems to have trouble on 802.11n. Fine. But 8?
modinfo iwlwifi
parm: 11n_disable:disable 11n functionality, bitmap: 1: full, 2: disable agg TX, 4: disable agg RX, 8 enable agg TX (uint)
Side note: 11n_disable=8 as a way to enable an advanced wifi settings is not the most user-friendly option in the world. It looks like this was added as a patch to fix a compatibility issue with certain NICs[1]. Be careful setting it higher than 8 -- as that would set other flags in the bitmask, potentially turning off 802.11n entirely!
I've not heard of AMPDU before -- but it seems to be an 802.11n-specific protocol extension that allows you to send multiple MPDU blocks (i.e. frames) to the router at a time, rather than one at a time. This seems to trade off latency for error correction between a client and the router [2][3]. So by setting this flag, you're able to transmit multiple frames at a time, rather than retrying each frame individually until it goes through on a busy channel. This really does seem to make a difference, one report online suggests tx throughput goes from 20Mbit/s to 80Mbit/s when agg TX is enabled[4].
Apparently this was introduced in 2014 in order to patch a firmware issue on certain intel NICs. There's a whole conversation on the linux-wireless list about it[5], which seems to conclude that they couldn't reproduce the issue[6]:
> I have seen reports from many users and many devices - and unfortunately I have never been able to reproduce - so I can't know if it affects all the devices. All I can see is that I saw reports on very old devices (5150) and with more recent devices (2230).
[1]: https://github.com/torvalds/linux/commit/205e2210daa975d92ac... [2]: https://routerguide.net/optimize-ampdu-aggregation-on-or-off... [3]: http://eprints.eemcs.utwente.nl/21008/01/eunice.pdf [4]: https://ubuntuforums.org/printthread.php?t=2205924&pp=75 [5]: https://www.spinics.net/lists/linux-wireless/msg118549.html [6]: https://www.spinics.net/lists/linux-wireless/msg118554.html
The importance of full stops outside the quotes ;-)
British style, on the other hand, specifies that a period goes outside the quotations.
http://blog.apastyle.org/apastyle/2011/08/punctuating-around...
I happen to disagree with them, but they have been considered authorities on the proper way to write in the English language for many decades.