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
_______________________________________________
Bloat mailing list
Bloat@lists.bufferbloat.net
https://lists.bufferbloat.net/listinfo/bloat
