4 microseconds! On Wednesday, October 19, 2022 3:23pm, "David Lang via Cake" <c...@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 <da...@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 > >> > > > > > _______________________________________________ > Cake mailing list > c...@lists.bufferbloat.net > https://lists.bufferbloat.net/listinfo/cake >
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