Re: [Cake] [Rpm] [Make-wifi-fast] [Bloat] The most wonderful video ever about bufferbloat

[Bob McMahon <bob.mcmahon@broadcom.com>]

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I'm not sure where the gap in milliseconds is coming from. EDCA gaps are
mostly driven by probabilities
<https://link.springer.com/article/10.1007/s10270-020-00817-2>. If
energy detect (ED) indicates the medium is available then the gap prior to
transmit, assuming no others competing & winning at that moment in time, is
driven by AIFS and the CWMIN - CWMAX back offs which are simple probability
distributions. Things change a bit with 802.11ax and trigger frames but the
gap is still determined by the backoff and should be less than milliseconds
per that. Things like NAVs will impact the gap too but that happens when
another is transmitting.


[image: image.png]

Agreed that the PLCP preamble is at low MCS and the payload can be orders
of magnitude greater (per different QAM encodings and other signal
processing techniques.)

Bob

On Wed, Oct 19, 2022 at 12:09 AM David Lang <david@lang.hm> wrote:

> On Tue, 18 Oct 2022, Sebastian Moeller wrote:
> > Hi Bob,
> >
> >> Many network engineers typically, though incorrectly, perceive a
> transmit
> >> unit as one ethernet packet. With WiFi it's one Mu transmission or one
> Su
> >> transmission, with aggregation(s), which is a lot more than one ethernet
> >> packet but it depends on things like MCS, spatial stream powers, Mu
> peers,
> >> etc. and is variable. Some data center designs have optimized the
> >> forwarding plane for flow completion times so their equivalent transmit
> >> unit is a mouse flow.
> >
> > [SM] Is this driven more by the need to aggregate packets to amortize
> some cost over a larger payload or to reduce the scheduling overhead or to
> regularize things (as in fixed size DTUs used in DSL with G.INP
> retransmissions)?
>
> it's to amortize costs over a larger payload.
>
> the gap between transmissions is in ms, and the transmission header is
> transmitted at a slow data rate (both for backwards compatibility with
> older
> equipment that doesn't know about the higher data rate modulations)
>
> For a long time, the transmission header was transmitted at 1Mb (which is
> still
> the default in most equipment), but there is now an option to no longer
> support
> 802.11b equipment, which raises the header transmission time to 11Mb.
>
> These factors are so imbalanced compared to the top data rates available
> that
> you need to transmit several MB of data to have actual data use 50% of the
> airtime.
>
> David Lang
>

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