[Make-wifi-fast] Where is the bloat in WiFi?

[Luca Muscariello]

Hi Michael,

In my case (WAN = GPON DL/UP 2Gbps/500Mbps) some devices are bottlenecked
at the AP, others are
bottlenecked at the switch, like my NAS which is multi-homed with 2 x 1GE
ports to the switch.
WAN downlink is hardly ever the bottleneck. However, backhaul links are
10GE and there may be
congestion there depending on the level of over subscription.

(From a container in the NAS)

     Server: ORANGE FRANCE - Paris (id = 24215)
        ISP: Orange
    Latency:     0.92 ms   (0.03 ms jitter)
   Download:   935.31 Mbps (data used: 433.1 MB)
     Upload:   599.02 Mbps (data used: 645.4 MB)
Packet Loss:     0.0%

For a laptop connected to one AP (line of sight available) I get 500Mbps
DL/UL speedtest.
For the uplink it is hard to say if the bottleneck is the uplink of the
router or one of the three APs,
or the switch? At 5GHz no device goes below ~700Mbps (PHY) rate according
to the WLC.

My case is not really common now, but with fiber adoption growth and more
people working from
home I expect this to become more common. WMM helps with all the online
meetings we do these days.
For video end-points (like Cisco DX) I only use ethernet.

With fiber growth in the home, investments in the backhaul become critical
to keep up with demand.

For xDSL the bottleneck is the WAN with high probability.

Luca




On Tue, Oct 6, 2020 at 2:15 PM Michael Welzl <michawe at ifi.uio.no> wrote:

> Hi, and thanks for a quick answer!
>
> But, it's not quite what I was looking for.... see below:
>
> > On 6 Oct 2020, at 13:47, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
> >
> > Michael Welzl <michawe at ifi.uio.no> writes:
> >
> >> Hi all,
> >>
> >> A simple question to y'all who spent so much time on Cake and things
> >> ... in a household using WiFi, which buffer is usually bloated? Where
> >> does the latency really come from?
> >>
> >> Is it:
> >> 1. the access point's downlink queue, feeding into the WiFi network,
> >
> > This we mostly fixed, but only if you're on a recent OpenWrt with the
> > right WiFi drivers.
>
> Well okay... I was curious about where the bottleneck is. I can translate
> my question into: "if Cake is installed everywhere, where does it have the
> most work to do?".
>
>
> > Otherwise, this is a major source of latency *if*
> > the WiFi link is faster than the downlink from the internet.
>
> Huh? Slower, you mean?
>
>
> > This
> > depends on both the internet connection and the current rate each WiFi
> > station operates at, so it can vary wildly, and on very short time
> > scales.
>
> Sure... I was asking for the "if" in your statement above - since this is
> an operationally-oriented list: what do people see? What is the more common
> case?
>
>
> >> 2. the modem's downlink queue, feeding into the access point,
> >
> > If your internet (downlink) connection is slower than your WiFi link,
> > this is where you'll get the queueing.
>
> Yes sure  :-)     see above: I wanted to know what the more common case
> is, in households that people on this list have dealt with.
>
>
> >> 3. the modem's uplink queue,
> >
> > As above, but in the other direction - but as uplinks tend to be
> > asymmetric, this direction is often more of a problem.
> >
> >> 4. the access point's uplink queue towards the modem   (hm, that seems
> >> silly, surely the AP-modem connection is fast... so perhaps, instead:
> >> the queue in the host, as it wants to send data towards the access
> >> point)
> >
> > Yeah, that would be in the host; but host drivers can suffer from severe
> > bufferbloat as well, especially as rates drop (since the buffers are
> > often tuned for the maximum throughput the device can deliver in
> > best-case signal conditions).
> >
> >> or is it a combination of these?
> >
> > Usually it's a combination; especially since the WiFi capacity varies
> > wildly with signal conditions (as devices move around relative to the
> > AP), general link usage (more devices active mean less available
> > capacity for each device, exacerbated by airtime unfairness), and
> > interference. Also there are things like excessive retries causing HoL
> > blocking.
>
> Man, what an academic answer!    Makes me think you have a PhD, or
> something!  What *theoretically* can happen is not what I was fishing for
>  :-D
>
>
> >> I guess that, with openwrt, Cake is operating on the queue that's
> >> feeding the wifi network, as the modem's queue is out of its
> >> control... so: is this where the bottleneck usually is?
> >
> > It certainly used to be; but as uplink connection speeds improve, the
> > bottleneck moves to the WiFi link.
>
> Yessss, that's why I was asking....
>
>
> > The extent to which this happens
> > depends on where you are in the world; personally I've been bottlenecked
> > on the WiFi link ever since I got a fibre upstream (and with 802.11ax
> > rates maxing at >1Gbps, maybe that'll change again?).
>
> THIS is what I was after   :)   one data point, cool - so far, so good...
>
> Cheers,
> Michael
>
> _______________________________________________
> Make-wifi-fast mailing list
> Make-wifi-fast at lists.bufferbloat.net
> https://lists.bufferbloat.net/listinfo/make-wifi-fast
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.bufferbloat.net/pipermail/make-wifi-fast/attachments/20201006/ea07634d/attachment-0001.html>