From: goldberg@mars.superlink.net
Date: Sun, 21 Jan 1996 12:27:56 +0800
To: cypherpunks@toad.com
Subject: Re: Guerilla Internet Service Providers
Message-ID: <199601210258.VAA17721@mars.superlink.net>
MIME-Version: 1.0
Content-Type: text/plain


At 09:21 PM 1/4/96 -0800, jim bell wrote:
>At 10:54 AM 1/4/96 -0800, you wrote:
>>>Previous exchanges deleted...

>>
>>(1) No single communication technology is appropriate for every problem.
>>
>>(2) A technical fix could include having the receiver send steering orders
>>to the transmitter.  This solution would, of course, be a long way from the
>>low tech scavenged lens and 1/2 meter cardboard mailing tube technology I
>>was thinking of.
>
>I think you guys (further up the reply chain) are missing the point.  While
>IR does have stealth advantages in, say, wartime, for routine network usage
>everyone can be assumed to know where everyone else is, and where all the
>optical links are, etc.  There's no point trying to use link-location
>secrecy.  And presumably, encryption will provide all the
>message-secrecy/anti-spoofing functions required.  Simply ASSUME that the
>beams can be intercepted (although probably not intentionally cut).  That's
>why we're "cypherpunks," right?!?
>
>Secondly, IR beams can be plenty narrow enough to avoid inter-link
>interference, but at the same time wide enough to avoid beam-steering
>problems. Note: I'm assuming link distances of under, say 300 meters here.
>
>Previously, a point was made about the effects of fog cutting links:  Due to
>scattering, one of the reasons automobile fog lamps are 550 nanometer
>yellow/orange is to minimize the scattering that shorter wavelengths (400 nm
>blue, 450 nm green) are more prone to.  I would imagine that near IR at,
>say, 890 nm would be dramatically less sensitive to such scattering.  1400
>nm might be even better.  Rain might be a different story.  But then again,
>if we're limiting the links to around 300 meters, the total amount of water
>between "here" and "there" CAN'T be all that great.  And in addition, one of
>the advantages of computer networking over telephone-type networking is that
>we can "tolerate" (although, not LIKE) the occasional necessity of
>re-transmitting data.  And dynamic re-routing is probably far easier than
>for real-time telephone-type data.
>
>>From the standpoint of computer networking, the main benefit of IR is to
>cross rights-of-way without permission or trenching (or stringing cables
>from telephone poles) in urban and suburban areas, allowing data transfer
>near-fiber speeds.   In an urban setting, a single tall building could
>become a central hub for most of its nearest neighbors.   I don't anticipate
>IR being used "to the home" (especially since residential areas have trees,
>etc); rather, I would imagine that it would be used to feed the occasional
>top-of-the-telephone-pole microcell, with very-low-milliwatt (or high
>microwatt) RF going the last 100 meters or so to the home.  This would allow
>a non-phoneco, non-cableco company to offer bidirectional networking in an
>entire residential area with an absolute minimum of costs/rights aquisition.
>
>
I can give you the benefit of some experience I have had with optical data
transmission systems.  We used IR lasers to span an approximately 800 meter
distance between buildings, and the results were dreadful.  Never again!
Fog took the system down completely, on a regular basis here in New Jersey,
as did even moderate snow.  Rain was much less of a problem, surprisingly,
even heavy rain rarely did more than raise the retransmission rate and lower
throughput somewhat.  Further, the beams were very narrow, and over that
distance minute changes in transmitter orientation would cause the link to
go down.  I am talking about changes due to expansion and contraction of
metal mountings with temperature, for instance.  Mounting direct to masonry
would probably have helped a lot.  Then, there are the things like trees
growing into the path over the course of the summer, telephone cables
swinging into it intermittently in high winds, etc.  Shorter paths allow
greater control over environment, certainly, but I would be very careful
about deploying large numbers of these types of systems.  Spread spectrum
microwave radio is a great improvement, but nothing seems to beat properly
installed glass fiber for reliability.
Frederic M. Goldberg   WA2BJZ   EMT-D