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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