[Bloat] Bufferbloat on 4G Connexion

[Luca Muscariello]

On Thu, Oct 24, 2019 at 11:34 AM Toke Høiland-Jørgensen <toke at redhat.com>
wrote:

> Luca Muscariello <muscariello at ieee.org> writes:
>
> > On Wed, Oct 23, 2019 at 2:27 PM Toke Høiland-Jørgensen <toke at redhat.com>
> > wrote:
> >
> >> Rich Brown <richb.hanover at gmail.com> writes:
> >>
> >> >> On Oct 23, 2019, at 5:54 AM,<erik.taraldsen at telenor.com <mailto:
> >> erik.taraldsen at telenor.com>> wrote:
> >> >>
> >> >> If you could influence the 4G vendors to de-bloat their equipment,
> >> >> would you recommend BQL, L4S or codel/cake?
> >> >
> >> > I've been enjoying this discussion and wonder whether the work going
> >> > on in the make-wifi-fast
> >> > (https://lists.bufferbloat.net/pipermail/make-wifi-fast/) is
> relevant.
> >> >
> >> > I only have a 30,000 foot understanding of this work, but it seems the
> >> > use of AQL (Airtime Queue Limit) maps better onto the vagaries of
> >> > 4G/5G radio transmissions than BQL. Specifically, having a measurement
> >> > of the actual time it takes to transmit a packet might give additional
> >> > information about the current link speed, with the potential for
> >> > adjusting the codel target, etc.
> >>
> >> Indeed, I suspect something like AQL would work for LTE as well. At the
> >> right level; think this might need to be in the firmware (which in turn
> >> could push back on the host).
> >>
> >> > Separately, I also wonder whether the Air Time Fairness algorithm
> >> > might provide a benefit if the cellphone tower station manufacturers
> >> > chose to get into the game.
> >>
> >> LTE base stations already does TDMA scheduling (which they can do easily
> >> because they are centralised and own the license band); airtime fairness
> >> is about getting the same benefits into WiFi that LTE has been enjoying
> >> from the get-go :)
> >>
> >
> > There is one main difference between ATF and the kind of TDMA
> > realized by an LTE scheduler (but also HSDPA/HSUPA).
> > Toke correct me if I'm wrong.
> >
> > The current ATF scheduler for WiFi does airtime-DRR based on the
> > current PHY rates, is that right? Side question, how do you measure
> > current?
>
> s/current/last/. The ATF scheduler does everything after-the-fact, by
> accounting the actual TX time of a transmission after it has completed.
> So no fancy scheduling or prediction tricks are needed; with the
> tradeoff being coarser granularity of the fairness achieved (i.e., there
> can be unfairness on short timescales).
>
> In the airtime queue limit work that's ongoing, we do ahead-of-time
> airtime estimation to limit queueing in firmware. But this still just
> uses the last TX rate recorded for the given station to calculate the
> estimate.
>
> > In LTE TDMA makes use of what is called multi-user diversity gain
> > by scheduling users when they are at their relative best radio condition.
> > Typically the user with the best current radio condition NORMALIZED
> > over the average radio conditions. The average can be based on a
> > moving average or a sliding window. This is the case of the widely used
> > David Tse's proportional fair scheduler.
> >
> > This means that TDMA is still in place to share air-time fairly but the
> > scheduler will tend to avoid bad radio conditions.
> >
> > From a theoretical point of view if you do that the total capacity
> > of the AP can increase with the number of stations (I think
> logarithmically)
> > as the scheduler surfs across radio quality peaks and not the average
> radio
> > quality. Very smart.
> >
> > In LTE this is doable as the scheduling time slot is 1ms and the
> > feedback channel is as fast. Not all TDMAs are equal.
>
> Yeah, the LTE MAC is pretty cool. Just a shame that the equipment is so
> expensive :(
>

It looks like there is a positive correlation between the size
of the specifications and the cost to build the associated product :)




>
> > Maybe the current scheduler in WiFi can be improved to do that. Maybe.
>
> I think 802.11ax is going in that direction. Nothing nearly as advanced,
> but at least there's the possibility of a dedicated back channel...
>

That's right. ax does much better.



>
> -Toke
>
>
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