[Starlink] [Bloat] [Cake] [Make-wifi-fast] [Cerowrt-devel] Due Aug 2: Internet Quality workshop CFP for the internet architecture board
Matt Mathis
mattmathis at google.com
Fri Sep 3 10:35:57 EDT 2021
I am very wary of a generalization of this problem: software engineers who
believe that they can code around arbitrary idosynchronies of network
hardware. They often succeed, but generally at a severe performance
penalty.
How much do we know about the actual hardware? As far as I understand the
math, some of the prime calculations used in Machine Learning are
isomorphic to multidimensional correlators and convolutions, which are
the same computations as needed to do phased array beam steering. One can
imagine scenarios where Tesla (plans to) substantially overbuild the
computational HW by recycling some ML technology, and then beefing up the
SW over time as they better understand reality.
Also note that the problem really only needs to be solved in areas where
they will eventually have high density. Most of the early deployment will
never have this problem.
Thanks,
--MM--
The best way to predict the future is to create it. - Alan Kay
We must not tolerate intolerance;
however our response must be carefully measured:
too strong would be hypocritical and risks spiraling out of
control;
too weak risks being mistaken for tacit approval.
On Thu, Sep 2, 2021 at 10:36 AM David P. Reed <dpreed at deepplum.com> wrote:
> I just want to thank Dick Roy for backing up the arguments I've been
> making about physical RF communications for many years, and clarifying
> terminology here. I'm not the expert - Dick is an expert with real
> practical and theoretical experience - but what I've found over the years
> is that many who consider themselves "experts" say things that are actually
> nonsense about radio systems.
>
>
>
> It seems to me that Starlink is based on a propagation model that is quite
> simplistic, and probably far enough from correct that what seems "obvious"
> will turn out not to be true. That doesn't stop Musk and cronies from
> asserting these things as absolute truths (backed by actual professors,
> especially professors of Economics like Coase, but also CS professors,
> network protocol experts, etc. who aren't physicists or practicing RF
> engineers).
>
>
>
> The fact is that we don't really know how to build a scalable LEO system.
> Models can be useful, but a model can be a trap that causes even engineers
> to be cocky. Or as the saying goes, a Clear View doesn't mean a Short
> Distance.
>
>
>
> If there are 40 satellites serving 10,000 ground terminals simultaneously,
> exactly what is the propagation environment like? I can tell you one thing:
> if the phased array is digitized at some sample rate and some equalization
> and some quantization, the propagation REALLY matters in serving those
> 10,000 ground terminals scattered randomly on terrain that is not optically
> flat and not fully absorbent.
>
>
>
> So how will Starlink scale? I think we literally don't know. And the
> modeling matters.
>
>
>
> Recently a real propagation expert (Ted Rapaport and his students) did a
> study of how well 70 GHz RF signals propagate in an urban environment -
> Brooklyn. The standard model would say that coverage would be terrible!
> Why? Because supposedly 70 GHz is like visible light - line of sight is
> required or nothing works.
>
>
>
> But in fact, Ted, whom I've known from being on the FCC Technological
> Advisory Committee (TAC) together when it was actually populated with
> engineers and scientists, not lobbyists, discovered that scattering and
> diffraction at 70 GHz in an urban environment significantly expands
> coverage of a single transmitter. Remarkably so. Enough that "cellular
> architecture" doesn't make sense in that propagation environment.
>
>
>
> So all the professional experts are starting from the wrong place, and
> amateurs perhaps even more so.
>
>
>
> I hope Starlink views itself as a "research project". I'm afraid it
> doesn't - partly driven by Musk, but equally driven by the FCC itself,
> which demands that before a system is deployed that the entire plan be
> shown to work (which would require a "model" that is actually unknowable
> because something like this has never been tried). This is a problem with
> today's regulation of spectrum - experiments are barred, both by law, and
> by competitors who can claim your system will destroy theirs and not work.
>
>
>
> But it is also a problem when "fans" start setting expectations way too
> high. Like claiming that Starlink will eliminate any need for fiber. We
> don't know that at all!
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
> On Tuesday, August 10, 2021 2:11pm, "Dick Roy" <dickroy at alum.mit.edu>
> said:
>
> To add a bit more, as is easily seen below, the amplitudes of each of the
> transfer functions between the three transmit and three receive antennas
> are extremely similar. This is to be expected, of course, since the
> “aperture” of each array is very small compared to the distance between
> them. What is much more interesting and revealing is the relative phases.
> Obviously this requires coherent receivers, and ultimately if you want to
> control the spatial distribution of power (aka SDMA (or MIMO in some
> circles) coherent transmitters. It turns out that just knowing the
> amplitude of the transfer functions is not really all that useful for
> anything other than detecting a broken solder joint:^)))
>
>
>
> Also, do not forget that depending how these experiments were conducted,
> the estimates are either of the RF channel itself (aka path loss),or of the
> RF channel in combination with the transfer functions of the transmitters
> and//or receivers. What this means is the CALIBRATION is CRUCIAL! Those
> who do not calibrate, are doomed to fail!!!! I suspect that it is in
> calibration where the major difference in performance between vendors’’
> products can be found :^))))
>
>
>
> It’s complicated …
>
>
> ------------------------------
>
> *From:* Bob McMahon [mailto:bob.mcmahon at broadcom.com]
> *Sent:* Tuesday, August 10, 2021 10:07 AM
> *To:* dickroy at alum.mit.edu
> *Cc:* Rodney W. Grimes; Cake List; Make-Wifi-fast;
> starlink at lists.bufferbloat.net; codel; 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 slides show that for WiFi every transmission produces a complex
> frequency response, aka the h-matrix. This is valid for that one
> transmission only. The slides show an amplitude plot for a 3 radio device
> hence the 9 elements per the h-matrix. It's assumed that the WiFi STA/AP is
> stationary such that doppler effects aren't a consideration. WiFi isn't a
> car trying to connect to a cell tower. The plot doesn't show the phase
> effects but they are included as the output of the channel estimate is a
> complex frequency response. Each RX produces the h-matrix ahead of the MAC.
> These may not be symmetric in the real world but that's ok as
> transmission and reception is one way only, i.e. the treating them as
> repcripocol and the matrix as hollows symmetric isn't going to be a "test
> blocker" as the goal is to be able to use software and programmable devices
> to change them in near real time. The current approach used by many using
> butler matrices to produce off-diagonal effects is woefully inadequate.
> And we're paying about $2.5K per each butler.
>
> Bob
>
>
>
> On Tue, Aug 10, 2021 at 9:13 AM Dick Roy <dickroy at alum.mit.edu> wrote:
>
> Well, I hesitate to drag this out, however Maxwell's equations and the
> invariance of the laws of physics ensure that all path loss matrices are
> reciprocal. What that means is that at any for any given set of fixed
> boundary conditions (nothing moving/changing!), the propagation loss
> between
> any two points in the domain is the same in both directions. The
> "multipathing" in one direction is the same in the other because the
> two-parameter (angle1,angle2) scattering cross sections of all objects
> (remember they are fixed here) are independent of the ordering of the
> angles.
>
> Very importantly, path loss is NOT the same as the link loss (aka link
> budget) which involves tx power and rx noise figure (and in the case of
> smart antennas, there is a link per spatial stream and how those links are
> managed/controlled really matters, but let's just keep it simple for this
> discussion) and these generally are different on both ends of a link for a
> variety of reasons. The other very important issue is that of the
> ""measurement plane", or "where tx power and rx noise figure are being
> measured/referenced to and how well the interface at that plane is
> "matched". We generally assume that the matching is perfect, however it
> never is. All of these effects contribute to the link loss which determines
> the strength of the signal coming out of the receiver (not the receive
> antenna, the receiver) for a given signal strength coming out of the
> transmitter (not the transmit antenna, the tx output port).
>
> In the real world, things change. Sources and sinks move as do many of the
> objects around them. This creates a time-varying RF environment, and now
> the path loss matrix is a function of time and a few others things, so it
> matters WHEN something is transmitted, and WHEN it is received, and the two
> WHEN's are generally separated by "the speed of light" which is a ft/ns
> roughly. As important is the fact that it's no longer really a path loss
> matrix containing a single scalar because among other things, the time
> varying environment induces change in the transmitted waveform on its way
> to
> the receiver most commonly referred to as the Doppler effect which means
> there is a frequency translation/shift for each (multi-)path of which there
> are in general an uncountably infinite number because this is a continuous
> world in which we live (the space quantization experiment being conducted
> in
> the central US aside:^)). As a consequence of these physical laws, the
> entries in the path loss matrix become complex functions of a number of
> variables including time. These functions are quite often characterized in
> terms of Doppler and delay-spread, terms used to describe in just a few
> parameters the amount of "distortion" a complex function causes.
>
> Hope this helps ... probably a bit more than you really wanted to know as
> queuing theorists, but ...
>
> -----Original Message-----
> From: Starlink [mailto:starlink-bounces at lists.bufferbloat.net] On Behalf
> Of
> Rodney W. Grimes
> Sent: Tuesday, August 10, 2021 7:10 AM
> To: Bob McMahon
> Cc: Cake List; Make-Wifi-fast; starlink at lists.bufferbloat.net;
> 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. 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.
>
> That is not correct, symmetry in the RF world, especially wifi, is rare
> due to topology issues. A high transmitter, A, and a low receiver, B,
> has a good path A - > B, but a very weak path B -> A. Multipathing
> is another major issue that causes assymtry.
>
> >
> > 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
> > > >> 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
> > > 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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