[Make-wifi-fast] Thoughts on tackling airtime fairness

Luca Muscariello luca.muscariello at gmail.com
Wed May 11 10:48:36 EDT 2016

On Wed, May 11, 2016 at 3:45 PM, Toke Høiland-Jørgensen <toke at toke.dk>

> Luca Muscariello <luca.muscariello at gmail.com> writes:
> > Toke,
> >
> > I'd suggest to add this in you list of references:
> >
> > Godfrey Tan and John Guttag, Time-based fairness improves performance in
> multi-rate WLANs. In Proc of USENIX 2004
> >
> https://www.usenix.org/legacy/publications/library/proceedings/usenix04/tech/general/full_papers/tan/tan.pdf
> Awesome, was not aware of that. Thanks!
> > It's worth having a look to the APware project for freeBSD and Godfrey
> Tan PhD thesis at MIT.
> >
> > http://nms.csail.mit.edu/projects/apware/
> Hmm, that link is not working for me right now; will try again later.
> > this work predates 802.11n and aggregation.
> Yeah, I'm aware that there is a lot of stuff that predates 802.11n. In
> fact the article I linked (Kim et al) is the only one I've found that
> talks about 802.11n. We also had some people at my uni doing stuff with
> 802.11g.
> > Ten years ago I played with SFQ and madwifi for 802.11g to get max-min
> > time fairness (and so proportional rate fairness) with excellent
> > results. The hacking I made was based on using time quanta instead of
> > bytes. Which required me to get the current PHY rates (AP to all
> > STAtions) and dynamically compute/update SFQ time quanta.
> Do you happen to recall what precision you achieved or how much the
> precision was really important? Several papers seem to assume that very
> high precision is not terribly important since it all evens out in the
> end, and I can see how that could be true; but would like to have it
> confirmed :)

what do you mean with precision?
Do you mean in measuring the PHY rate?
Short term vs long term measurements? else?

The hard part was adaptiveness. Correlation between the speed of the STA,
PHY rate controller and quanta updates in SFQ.
Ideally we were trying to approach something like
downlink channel scheduling in HSDPA where you have slotted time TX
and polling. A slot in HSDPA (but also LTE) is a burst  of fixed time size
of several packets.
Similar to what you want to achieve in your email point 1.
in HSDPA/LTE you have UEs feedback every ms in a side channel. So rate
is a lot simpler.

As a side note,
there are several differences in aggregates in 802.11n and HSDPA/LTE
as in the latter the scheduler can send an aggregate containing packets
using different modulation/coding schemes to reach different stations with
a single aggregate transmission.
For the records, this feature was rejected in the 802.11n amendement but
discussed  by the group as it makes the chip more expensive.

> > It's surprising that 802.11 standard never considered time fairness in
> > the EDCF. A reason might be the time fairness might be enforced using
> > the PCF.
> Might be. Might also be that no one thought to measure for that? A lot
> of vendors seem to only test single-station raw throughput...
> Are you aware of any open source 802.11 stuff that uses PCF at all?

No. Sorry.

> > IMO, It's a very good topic.
> > Thanks for bringing this up.
> You're very welcome! ;)
> -Toke
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.bufferbloat.net/pipermail/make-wifi-fast/attachments/20160511/37d9fe0f/attachment.html>

More information about the Make-wifi-fast mailing list