Re: [Bloat] Bufferbloat on 4G Connexion

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Luca Muscariello <muscariello@ieee.org> writes:

> On Wed, Oct 23, 2019 at 2:27 PM Toke Høiland-Jørgensen <toke@redhat.com>
> wrote:
>
>> Rich Brown <richb.hanover@gmail.com> writes:
>>
>> >> On Oct 23, 2019, at 5:54 AM,<erik.taraldsen@telenor.com <mailto:
>> erik.taraldsen@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 :(

> 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...

-Toke