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

Simon Barber simon at superduper.net
Mon Aug 3 23:20:55 EDT 2015

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.


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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