# [Bloat] [Starlink] [Cake] [Make-wifi-fast] [Cerowrt-devel] Due Aug 2: Internet Quality workshop CFP for the internet architecture board

Sun Aug 8 01:07:56 EDT 2021

Thanks - your wording is more accurate. The path loss matrix is hollow
symmetric while the RF channel is reciprocal.

The challenge comes when adding phase shifters. Then it's not just a path
loss matrix anymore.

Bob

On Sat, Aug 7, 2021 at 10:04 PM Dick Roy <dickroy at alum.mit.edu> wrote:

>
>
>
> ------------------------------
>
> Behalf Of *Bob McMahon
> *Sent:* Monday, August 2, 2021 8:23 PM
> *To:* David Lang
> *Cc:* starlink at lists.bufferbloat.net; Make-Wifi-fast; Cake List;
> codel at lists.bufferbloat.net; cerowrt-devel; bloat
> *Subject:* Re: [Starlink] [Cake] [Make-wifi-fast] [Cerowrt-devel] Due Aug
> 2: Internet Quality workshop CFP for the internet architecture board
>
>
>
> The distance matrix defines signal attenuations/loss between pairs.
>
> *[RR] Which makes it a path loss matrix rather than a distance matrix
> actually.*
>
> It's straightforward to create a distance matrix that has hidden nodes
> because all "signal  loss" between pairs is defined.  Let's say a 120dB
> attenuation path will cause a node to be hidden as an example.
>
>      A    B     C    D
>
> A   -   35   120   65
>
> B         -      65   65
>
> C               -       65
>
> D                         -
>
> So in the above, AC are hidden from each other but nobody else is. It does
> assume symmetry between pairs but that's typically true.
>
> *[RR] I’m guessing you really mean reciprocal rather than symmetric. An RF
> channel is reciprocal if the loss when A is transmitting to B is the same
> as that when B is transmitting to A. When the tx powers and rx
> sensitivities are such that when combined with the path loss(es) the “link
> budget” is  the same in both directions, the links are balanced and
> therefore have the same capacity. *
>
>
>
> The RF device takes these distance matrices as settings and calculates the
> five branch tree values (as demonstrated in the video).
>
> There are limitations to solutions though but I've found those not to be
> an issue to date. I've been able to produce hidden nodes quite readily. Add
> the phase shifters and spatial stream powers can also be affected, but this
> isn't shown in this simple example.
>
> Bob
>
>
>
> On Mon, Aug 2, 2021 at 8:12 PM David Lang <david at lang.hm> wrote:
>
> I guess it depends on what you are intending to test. If you are not going
> to
> tinker with any of the over-the-air settings (including the number of
> packets
> transmitted in one aggregate), the details of what happen over the air
> don't
> matter much.
>
> But if you are going to be doing any tinkering with what is getting sent,
> and
> you ignore the hidden transmitter type problems, you will create a
> solution that
> seems to work really well in the lab and falls on it's face out in the
> wild
> where spectrum overload and hidden transmitters are the norm (at least in
> urban
> areas), not rare corner cases.
>
> you don't need to include them in every test, but you need to have a way
> to
> configure your lab to include them before you consider any
> settings/algorithm
> ready to try in the wild.
>
> David Lang
>
> On Mon, 2 Aug 2021, Bob McMahon wrote:
>
> > We find four nodes, a primary BSS and an adjunct one quite good for lots
> of
> > testing.  The six nodes allows for a primary BSS and two adjacent ones.
> We
> > want to minimize complexity to necessary and sufficient.
> >
> > The challenge we find is having variability (e.g. montecarlos) that's
> > reproducible and has relevant information. Basically, the distance
> matrices
> > have h-matrices as their elements. Our chips can provide these
> h-matrices.
> >
> > The parts for solid state programmable attenuators and phase shifters
> > aren't very expensive. A device that supports a five branch tree and 2x2
> > MIMO seems a very good starting point.
> >
> > Bob
> >
> > On Mon, Aug 2, 2021 at 4:55 PM Ben Greear <greearb at candelatech.com>
> wrote:
> >
> >> On 8/2/21 4:16 PM, David Lang wrote:
> >>> If you are going to setup a test environment for wifi, you need to
> >> include the ability to make a fe cases that only happen with RF, not
> with
> >> wired networks and
> >>> are commonly overlooked
> >>>
> >>> 1. station A can hear station B and C but they cannot hear each other
> >>> 2. station A can hear station B but station B cannot hear station A 3.
> >> station A can hear that station B is transmitting, but not with a strong
> >> enough signal to
> >>> decode the signal (yes in theory you can work around interference, but
> >> in practice interference is still a real thing)
> >>>
> >>> David Lang
> >>>
> >>
> >> To add to this, I think you need lots of different station devices,
> >> different capabilities (/n, /ac, /ax, etc)
> >> different numbers of spatial streams, and different distances from the
> >> the capabilities may be sufficient while keeping all stations at same
> >> distance.  This assumes you are not
> >> actually testing the wifi rate-ctrl alg. itself, so different throughput
> >> levels for different stations would be enough.
> >>
> >> So, a good station emulator setup (and/or pile of real stations) and a
> few
> >> RF chambers and
> >> programmable attenuators and you can test that setup...
> >>
> >>  From upload perspective, I guess same setup would do the job.
> >> Queuing/fairness might depend a bit more on the
> >> station devices, emulated or otherwise, but I guess a clever AP could
> >> enforce fairness in upstream direction
> >> too by implementing per-sta queues.
> >>
> >> Thanks,
> >> Ben
> >>
> >> --
> >> Ben Greear <greearb at candelatech.com>
> >> Candela Technologies Inc  http://www.candelatech.com
> >>
> >
> >
>
>
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