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

Dick Roy dickroy at alum.mit.edu
Sun Aug 8 01:04:07 EDT 2021




From: Starlink [mailto:starlink-bounces at lists.bufferbloat.net] On 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

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

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.



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
tinker with any of the over-the-air settings (including the number of
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,
you ignore the hidden transmitter type problems, you will create a solution
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
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
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
> 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
> 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
>> AP.  From download queueing perspective, changing
>> 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
>> 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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