[Bloat] Bufferbloat on 4G Connexion

[Toke Høiland-Jørgensen]

Luca Muscariello <muscariello at ieee.org> writes:

> 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 :)

Hehe, yeah, funny how that works.

IMO the LTE people went a little bit overboard on the complexity,
though... ;)

-Toke