with 802.11ac, the difference between uplink and downlink is that the AP can
transmit to multiple users at the same time (multiple signals spacially
multiplexed), but the users transmit back one at a time.
David Lang
On Wed, 1 Dec 2021, David P. Reed wrote:
What's the difference between uplink and downlink? In DOCSIS the rate
asymmetry was the issue. But in WiFi, the air interface is completely symmetric
(802.11ax, though, maybe not because of centrally polling).
In any CSMA link (WiFi), there is no "up" or "down". There is only sender and
receiver, and each station and the AP are always doing both.
The problem with shared media links is that the "waiting queue" is distributed,
so to manage queue depth, ALL of the potential senders must respond aggressively to
excess packets.
This is why a lot (maybe all) of the silicon vendors are making really bad
choices w.r.t. bufferbloat by adding buffering in the transmitter chip itself,
and not discarding or marking when queues build up. It's the same thing that
constantly leads hardware guys to think that more memory for buffers improves
throughput, and only advertising throughput.
To say it again: More memory *doesn't* improve throughput when the queue depths exceed one packet on average,
and it degrades "goodput" at higher levels by causing the ultimate sender to "give up"
due to long latency. (at the extreme, users will just click again on a slow URL, causing all the throughput
to be "badput", because they force the system to transmit it again, while leaving packets clogging
the queues.
So, if you want good performance on a shared radio medium, you need to squish each flow's
queue depth down from sender to receiver to "average < 1 in queue", and also
drop packets when there are too many simultaneous flows competing for airtime. And if your
source process can't schedule itself frequently enough, don't expect the network to replace
buffering at the TCP source and destination - it is not intended to be a storage system.
On Tuesday, November 30, 2021 7:13pm, "Dave Taht" <dave.t...@gmail.com> said:
Money quote: "Figure 2a is a good argument to focus latency
research work on downlink bufferbloat."
It peaked at 1.6s in their test:
https://hal.archives-ouvertes.fr/hal-03420681/document
--
I tried to build a better future, a few times:
https://wayforward.archive.org/?site=https%3A%2F%2Fwww.icei.org
Dave Täht CEO, TekLibre, LLC
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