Thanks, and how long does it take to transmit the wifi header (at 1Mb/s and at
11Mb/s)? That's also airtime that's not availalbe to transmit user data.
And then compare that to the time it takes to transmit a 1500 byte ethernet
packet worth of data over a 160MHz wide channel
Going back to SM's question, there is per-transmission overhead that you want to
amatorize across multiple ethernet packets, not pay for each packet.
David Lang
On Wed, 19 Oct 2022, David P. Reed wrote:
4 microseconds!
On Wednesday, October 19, 2022 3:23pm, "David Lang via Cake"
<cake@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
>>
>
>
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