more on Response to David Reed and Brett and the FRC

[David Farber <dave () farber net>]

Begin forwarded message:

From: "David P. Reed" <dpreed () reed com>
Date: September 13, 2006 2:25:54 AM JST
To: dave () farber net
Cc: ip () v2 listbox com
Subject: Re: [IP] more on Response to David Reed and Brett and the FRC

Dave - though I could continue my discussion/argument with Brett, I  
think it would be a waste of your readers' time.  His response to my  
response continues to show his relatively weak grasp of the basics of  
electromagnetic waves, information theory, and the limits of current  
radio technology.   There is no need for me to point those out to  
people who understand such things.

I do want to respond to his misreading of two of my statements, in  
order again to twist their meaning.

First: when I said on this list ""The Shannon-Hartley Theorem does  
not provide a limit to the communications capacity of the  
electromagnetic field...." I was making a very precise statement.    
The theorem does not.   Brett's claim was that it did.  Any physical  
limits arise from physical circumstances, in other words, from the  
physical constraints of the universe.   SHT merely assumes that the  
physical environment of its channel consists of Gaussian White  
Noise.   That is, that there is a probability distribution that  
predicts a process by which noise arrives at the receiver, which  
satisfies a particular model.   The SHT does NOT assert that the  
world is built that way, and as I have said, physicists do not see  
that when they measure the world in experiments.   Some very narrow  
experiments of very narrow bands in very isolated circumstances see  
GWN-like phenomena.  But you really have to throw out datapoints and  
pretend that you are in a free, equilibrium vacuum (or construct a  
test chamber) to see GWN.

And of course, the "Shannon Limit" is just a relationship between  
signal power and achievable information rate given as a "known" the  
variance of Gaussian White Noise in its abstract channel (variance  
for Gaussian signals = power).

Second: he quotes from an interview "There's no scarcity of spectrum  
any more than there's a scarcity of the color green. We could  
instantly hook up to the Internet everyone who can pick up a radio  
signal, and they could pump through as many bits as they could ever  
want. We'd go from an economy of digital scarcity to an economy of  
digital abundance."

I speak and write as precisely as I can, and this quote was no  
exception.   I stated "no scarcity".   Brett would like that to be  
read as if I said there is "no limit to capacity".   But what I was  
saying is precisely that there is no current scarcity of spectrum for  
all communications that would prevent the entire Internet from  
becoming wireless.   Of course, the current form of regulation *does*  
prevent that very directly, by allocating frequencies to transmitters  
by service.   This quote had nothing to do with the ShannonHartley  
theorem.   It was referring to the idea of "interference" which says  
that two transmissions cannot be allowed to be in the same band at  
the same time, because information is "destroyed".   That notion of  
"interference" is incorrect.   Information is destroyed only by very  
complex, energetic suppression, in a system as linear as the  
electromagnetic field.  What happens when you have multiple signals  
from multiple transmitters is superposition.  Superposition leads to  
ambiguity when (and ONLY when) there is sufficiently unpredictable  
noise and insufficient sampling by the receiver.   Increase the  
sampling, decrease the noise's predictability, and the ambiguity can  
be resolved.

It's not the quantity of "spectrum" that limits communication.    
Instead it is the design of the receiver system (including its  
antenna), the characteristics of the noise process (perhaps best  
described in terms of noise entropy, or information content, of the  
field on which signals are superposed), and the computational  
capabilities of the receiver (analog or digital) that are available  
to disambiguate the signals.




David Farber wrote:
> Begin forwarded message:
>
> From: Brett Glass <brett () lariat net>
> Date: September 12, 2006 1:16:15 AM JST
> To: dave () farber net, ip () v2 listbox com
> Cc: Bob Frankston <bob () frankston com>
> Subject: Re: [IP] more on Response to David Reed and Brett and the FRC
>
> Bob Frankston writes:
>
> > Our understanding and technology has come a long way since then and
> > examples like the 25 Watt Voyager which is able to send kilobits of
> > data per second against the background noise of Jupiter and now all
> > the way from the Kuipers Belt demonstrate the spectrum allocation
> > isn’t the only coding system and, in fact, it would be difficult to
> > find one that is more wasteful. It’s analogous to allocating cattle
> > ranches into quarter acre plots to avoid risking any bovine getting
> > more than its “fair” share.
>
> It is not the allocation scheme, but rather the lack of will (and
> the lack of well developed technology) to enforce fair sharing of
> the plots that is the problem. To use a real estate analogy: There's
> no problem with setting aside some plots of land for exclusive private
> use and others for roads. But if there are no rules of the road, it's
> natural for everyone to want to use private roads because there is
> nothing to preclude antisocial behavior, malicious blocking of the
> roads, hogging of the roads, and/or other tragedies of the commons.
>
> Alas, in the world of spectrum, one of the reasons that good
> mechanisms for sharing have not been developed and deployed is that
> the private spectrum owners ("spectrum barons?") have actually
> discouraged it so as to make their private turf more valuable.
>
> > Even if one accepts the need for spectrum allocation why do we go to
> > the extreme of having to prove no possible interference to the
> > feeblest of radios  the analog ones that can’t get a clear signal
> > anyway?
>
> It is true that there is an odd double standard. Radio and TV -- the
> media of the past -- are so sacrosanct that nothing may interfere
> with them. But wireless broadband services such as ours, which the
> public would dearly love to replace them, enjoy no such protection.
> And licensed spectrum has been placed beyond our grasp.
>
> > I know Brett is using packet radios as a bypass and he depends on
> > relatively long distance connections compared with in-home use.
>
> We are not merely using radios as a "bypass." Our broadband reaches
> areas which the cable/telco duopoly has no financial motivation to
> serve -- and provides higher throughput than them in any area we
> cover. We're not just a workaround; we're a first class
> communications medium and deserve consideration as such.
>
> > The problems of using single frequency non-redundant signaling is
> > exacerbated by channeling them into assigned bands. That alone
> > requires a system for policing its use. The problems are deeper --
> > the frequency regimen is fragile because the messages are not  
> > self- identifying so you must depend on others policing the bands  
> > on your
> > behalf rather than taking responsibility for the relationships. There
> > are many other consequences of this system  any change requires the
> > intervention of congress and decades to effect.
>
> In my ISART paper (cited previously), I've already mentioned these
> drawbacks of allocation by frequency. It's easy to demonstrate that it
> would be in everyone's best interest to assign subsets of a different
> and more flexible "signal space" instead. But even if we continue to
> allocate by frequency, we can certainly implement much better  
> policies.
> Current policy is crippling not only my business (which I never
> expected to do as well as it has, considering the obstacles that are
> being erected in its path) but countless other existing and potential
> businesses.
>
> David Reed writes:
>
> > I don't think Brett's comments read on anything I said.
> >
> > The major points I made were: 1) The Shannon-Hartley theorem does not
> > define a limit to the capacity of the spectrum, which most definitely
> > is what Brett was asserting.
>
> As I mentioned in my earlier message, "the spectrum" does not have a
> capacity. Communications links, or "channels" (as Shannon calls them),
> do. The way we allocate spectrum (a policy issue) and implement radios
> (a technical issue) determines how many clear, high capacity channels
> we can create. Both engineering and policy are important to making
> effective use of the available spectrum. That's why the FCC (a
> policy making body) has an Office of Engineering Technology. It is
> a shame that, too often, the OET's recommendations are brushed
> aside in favor of the desires of politically powerful corporations.
>
> > 2) Brett's comments were properly read as political,
>
> Actually, my comments were (and are) mostly related to science and
> engineering. The policy recommendations which I've included merely
> follow from them.
>
> > I'm not sure where Brett concludes that I suggested that
> > sophisticated technology could remove all constraints on
> > electromagnetic communications.
>
> You wrote:
>
> "The Shannon-Hartley Theorem does not provide a limit to the
> communications capacity of the electromagnetic field...."
>
> You have also asserted that
>
> "Interference is a metaphor that paints an old limitation of
> technology as a fact of nature.... There's no scarcity of spectrum
> any more than there's a scarcity of the color green. We could  
> instantly
> hook up to the Internet everyone who can pick up a radio signal, and
> they could pump through as many bits as they could ever want."
>
> (See
>
> http://dir.salon.com/story/tech/feature/2003/03/12/spectrum/index.html
>
> for the full text of the article.) There and elsewhere, you have
> indeed claimed that sophisticated technology could remove all
> practical constraints on electromagnetic communications.
>
> > You can read textbooks on electromagnetic physics at any level, and
> > you will not find Shannon-Hartley.
>
> This is correct. This is because the Shannon-Hartley theorem relates
> to information theory and communications. Communications can be done
> using electromagnetic fields, but do not have to be. Also, an
> understanding of the theorem requires an understanding of the theory
> of signals and systems and of the concept of signals as vectors in an
> abstract "signal space." Covering these concepts in a physics class
> would take it too far afield of the ropes, pulleys, falling bricks,
> etc. that it must cover to introduce the basic concepts it treats.
>
> > A "channel" is a mathematical object only: a function that combines a
> > set of inputs to produce an output - not a physical phenomenon at
> > all, but the most information-free abstraction.
>
> Abstractions are useful to explain the behavior of physical systems.
>
> > Many people similarly probably believes that intro physics (college
> > Physics I) uses simple linear differential equations because the
> > universe has been proven to be continuous and differentiable.
>
> It is, to a sufficient degree of accuracy for those equations to be
> useful in everyday life. Despite the fact that freshman physics
> uses frictionless pulleys, massless ropes, and other accessories
> you're unlikely to find even in Wile E. Coyote's Acme Catalog, it
> nonetheless explains a lot about the world, and to a pretty good
> degree of accuracy. I use those equations frequently when doing
> carpentry.
>
> > The other problem with Brett's comments is his statement "from the
> > perspective of the receiver, the undesired signals are simply
> > noise".
>
> They are.
>
> > There is a difference between undesired signals and
> > noise. Noise is technically quite different from undesired
> > signals.
>
> Actually, it is not. In fact, as I've already mentioned, the
> more efficiently a signal uses spectrum, the more it looks
> like random noise to a third party.
>
> > What makes noise different is that noise is unpredictable,
> > whereas signals are indeed predictable.
>
> I cannot predict when someone else who is "stomping on" my radio
> signal is going to transmit, nor can I predict what type of modulation
> scheme he's going to use or for how long he'll transmit. In fact,
> tailoring my system to reject the type of interference I expect only
> invites someone to design a different system which does interfere.
>
> This has, in fact, happened in the world of unlicensed communications.
> Some manufacturers have designed systems which intentionally obey no
> spectrum etiquette, so as to "defeat" systems which try to be  
> courteous
> to other users of the band or to sidestep interference. The only
> practical way to design a system, therefore, is to design for the
> theoretical worst case: Gaussian white noise.
>
> > Signals are designed to be decoded.  Noise is not.
>
> One cannot reliably distinguish between the two, because one cannot
> divine intent. See the comments in my paper at
>
> http://www.brettglass.com/ISART
>
> regarding SETI and the feasibility of trying to recognize, much
> less interpret, signals generated by alien civilizations. Also note,
> again, the spectrum at
>
> http://www.brettglass.com/Laramie900.jpg
>
> The energy in that spectrum consists entirely of signals which are
> "meant to be decoded." Yet for all practical purposes it is just
> Gaussian white noise.
>
> > If you have two signals
> > plus noise, the limit of the receiver's capacity is not, as Brett
> > would imply in the "undesired signal is simply noise" in S-H theorem:
> >
> >   R = W log(1 + S1/(S2+N)).
>
> This is not correct, because the second signal is (by your own  
> assertion)
> not noise.
>
> > Instead the system of two transmitters and one receiver can achieve a
> > rate of:
> >
> >   R = W log(1+ (S1+S2)/N).
>
> I have already stated this myself. As I mentioned in my earlier  
> message,
> techniques which rely upon multiple transmitters (or multiple
> transmissions) to improve throughput can be accounted for
> by modifying the signal to noise ratio. Simply adding them together
> is a best case scenario, though. In most cases, you'll find that
> the numerator of the fraction is (S1+S2) times some constant which is
> substantially less than 1. We see this in orthogonal frequency
> division multiplexing (OFDM), which was originally developed by Paul
> Baran for use in Telebit's modems and is now used in many wireless
> systems. But to augment the signal to noise ratio in this way, the
> transmitters must be transmitting the same information or parts of the
> same message. This is very different from a situation in which
> independent transmitters are transmitting unrelated information and
> are not cooperating.
>
> > It has recently been demonstrated (using information theory and a
> > conservative physical propagation model) by Tse and others that the
> > communications capacity of a set of radios operating as an adhoc net
> > in a common medium can scale linearly in the number of radios.
>
> This model makes a number of flawed assumptions -- most particularly
> that all of the users of the same spectrum are cooperating. This is
> not the case in the real world (which brings us back to the politics
> and policy of spectrum allocation).
>
> When it comes to spectrum, I'm not just a theorist and scientist
> but also an engineer, technician, and user. Today, I was out
> supervising my crew on two roofs as they installed antennas for
> wireless Internet. I then helped users at a trailer park connect to
> a wireless network I set up there.
>
> I also do technical support for half a dozen wireless hotspots.
> (Hotspot support is among the most difficult kind of technical  
> support,
> because you are usually dealing with a naive user who arrives with
> a computer system which you have never seen before and which the owner
> himself does not understand. And because you are usually supporting
> this person over the phone, you are playing "Blind Man's Buff" because
> you cannot see his or her screen and he or she does not know the words
> for the GUI elements which are displayed on it.
>
> But I digress... Back to spectrum issues. I see the practical effects
> of Shannon's Law, the principles of electromagnetic communications,
> and spectrum policy every day when cordless phones, competitors'  
> signals,
> and other interference sources threaten the integrity of my  
> network. As an
> electrical engineer, I'm quite comfortable with the "ivory tower"  
> theory.
> But it's when one comes down from that tower and climbs a radio tower
> to build a real world system, one gains real insight into the policies
> that are needed to best use the valuable resource we call "spectrum."
>
> --Brett Glass
>
>
>
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