[IP] more on Response to David Reed and Brett and the FRC
Begin forwarded message:
From: Brett Glass <brett@xxxxxxxxxx>
Date: September 12, 2006 1:16:15 AM JST
To: dave@xxxxxxxxxx, ip@xxxxxxxxxxxxxx
Cc: Bob Frankston <bob@xxxxxxxxxxxxx>
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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