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

[David P. Reed]

4 microseconds!
 
On Wednesday, October 19, 2022 3:23pm, "David Lang via Cake" <cake at lists.bufferbloat.net> said:



> you have to listen and hear nothing for some timeframe before you transmit, that
> listening time is define in the standard. (isn't it??)
> 
> David Lang
> 
> On Wed, 19 Oct 2022, Bob McMahon wrote:
> 
> > 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 at 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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> Cake at lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/cake
> 
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