Re: [Make-wifi-fast] [Cerowrt-devel] [tsvwg] Comments on draft-szigeti-tsvwg-ieee-802-11e

[Simon Barber <simon@superduper.net>]

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Indeed - to get maximum performance with WiFi you must form large 
aggregates. They are limited to 5.3ms of data, which is a theoretical 
4MBytes of data for 11ac radios. (8x8, MCS9). In practice, today it's 
very rare to see > 3x3 and MCS8 or 9, so somewhere between 1.5 and 
2MBytes of data in an aggregate. You need sufficient queues to form 
these aggregates, or performance is very poor.

Simon

On 7/31/2015 10:04 AM, Jonathan Morton wrote:
>
> > I think that is achievable, *even if there is a WiFi network in the 
> middle*, by thinking about the fact that the shared airwaves in a WiFi 
> network behaves like a single link, so all the queues on individual 
> stations are really *one queue*, and that the optimal behavior of that 
> link will be achieved if there is at most one packet queued at a time.
>
> I agree that queues should be kept short in general. However I don't 
> think single packet queues are achievable in the general case.
>
> The general case includes Wi-Fi networks, whose TXOP overhead is so 
> ruinously heavy that sending single MTU sized packets is inefficient. 
> Aggregating multiple packets into one TXOP requires those several 
> packets to be present in the buffer at that moment.
>
> The general case includes links which vary in throughput frequently, 
> perhaps on shorter timescales than an RTT, so either packets must be 
> buffered or capacity is left unused. This also happens to include 
> Wi-Fi, but could easily include a standard wired link whose competing 
> load varies.
>
> The endpoints do not have and do not receive sufficient information in 
> sufficient time to reliably make packets arrive at nodes just in time 
> to be transmitted. Not even with ECN, not even with the wet dreams of 
> the DCTCP folks, and not even with ELR (though ELR should be able to 
> make it happen under steady conditions, there are still transient 
> conditions in the general case).
>
> - Jonathan Morton
>
>
>
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