Using lasers to communicate

At 5:43 PM 1/4/96, Jason Rentz wrote:
>Previous exchanges deleted...
>>
>>With a tightly focused beam (light is easy, I don't know about lower
>>frequencies), you can prevent interception except by very obvious physical
>>devices.  (e.g. Someone in a cherry picker truck.)  You may be able to
>>avoid the need to encrypt the link (and all the paranoia about key
>>management, advances in factoring etc. that that implies.)
>>
>>Bill
>
>The problem with this comes when you start creating links between much
>taller buildings like in San Fran.  Any give building over 30 stories might
>sway a foot or so at any given time.  Combine that with the other building
>and you might get a few feet of movement. (movement not including during an
>earthquake)  :)

Just a couple of points on this optical idea.

We were linking buildings a mile apart in the 70s, at Intel. We needed to
ship CAD data back and forth, and PacBell rates for a dedicated line were
outrageous, slow to be installed, etc. So, a commercially available laser
and modulator/demodulator (modem, but it bears sometimes using the longer
version, to remind people of what it is doing in general) were mounted on
the roofs of our buildings. I'm sure various packages are commercially
available to do this.

As to buildings swaying in earthquakes, somehow I don't think transient
loss of channel capacity during a quake is going to be a pressing concern!
:-} Swaying in ordinary wind is an easily-handled problem. (Any good
engineer can think of several fixes: paraboloidal dish receivers are cheap
(not even optical quality, just to get light pulses), compensation for
sway, acceptance of slightly reduced data rates as modem error correction
handles sway-induced dropouts, movement of the transmitters and receivers
to lower levels, etc.)

Also, nearly all high-tech buildings (or at least more than 95% of all
high-tech floorspace in the U.S.) are less than 3-4 stories tall; most are
1-2 stories. Building sway is nonexistent. And building sway only
approaches the multiple meter level in the highest floors of the tallest
buildings. I would guess that fewer than 1% of all offices are affected;
for them, a lower data rate is acceptable.

I'm actually more positive on low-level (below safety regs get interested
in) light than on free space RF, for bypassing of the local cable/phone
monopolies. There's just not enough "bandwidth of free space" available. Do
the math.

(Footnote: Some years back some of us got interested in the idea of using
lasers to communicate between San Diego/Chula Vista and Tijuana. Ordinary
phone lines turned out to be cheaper.)

--Tim May

We got computers, we're tapping phone lines, we know that that ain't allowed.
---------:---------:---------:---------:---------:---------:---------:----
Timothy C. May              | Crypto Anarchy: encryption, digital money,
tcmay@got.net  408-728-0152 | anonymous networks, digital pseudonyms, zero
W.A.S.T.E.: Corralitos, CA  | knowledge, reputations, information markets,
Higher Power: 2^756839 - 1  | black markets, collapse of governments.
"National borders aren't even speed bumps on the information superhighway."

This idea of sending data via laser beams across open spaces has some
very useful potential.  I want to suggest some motivation and some
enhancement.  [quotes below give a little background, from the list]

Eavesdropping and channel-blocking and physical-location-discovery are 
related threats to which most traditional data channels are susceptible.  
Any link which depends on a physical conduit (phone line, fiber, coax)
is relatively easy to interrupt and to trace to its end points.
RF links, even with frequency hopping, are subject to triangulation and
jamming.  All these kinds of links can be eavesdropped.

Point-to-point conduitless laser signalling, as envisioned by "Bill" and
Tim in their quotes below, eliminates or reduces these threats. 
Now consider an enhancement.  In show business, we sometimes entertain
the folks with "laser light shows".  The technology used is fairly
straightforward, mainly involving the use of mirrors (the effect also uses
smoke ususally, but please don't prematurely dismiss my remarks on this
basis).  The laser source is attached to a "laser table" which holds a 
number of small mirrors which may be individually inserted (via fast
solenoids) into the path of the laser beam.  Each of these mirrors is then
calibrated to aim at a particular place in the theatre, usually another
larger mirror.  Then (under computer control) the various small mirrors
on the laser table are rapidly inserted and withdrawn from the light beam,
causing the laser beam to follow first one path, then another, then another
through the (smoky) air -- all to the delight of the audience.

This technology could easily be adapted to make a communication channel
safer from the various threats of eavesdropping, interruption, and tracing.
A single point-to-point channel could be made to follow various paths 
having common elements only VERY close to the endpoints.  Better still,
a network of more than two nodes could be constructed without needing to
provide multiple transceivers at each node (and with possibly multiple 
beam paths between each pair).  With known methods of routing and
collision avoidance, we could thus not only route around any known opposition
but also make it very expensive to eavesdrop or even to discover that 
a signal exists.  ("Honey, call the EPA again -- those gubmint boys are back,
driving their oil-burning old van around Mr. May's house.").

[previous attribution unknown...:]
} >>With a tightly focused beam (light is easy, I don't know about lower
} >>frequencies), you can prevent interception except by very obvious physical
} >>devices.  (e.g. Someone in a cherry picker truck.)  You may be able to
} >>avoid the need to encrypt the link (and all the paranoia about key
} >>management, advances in factoring etc. that that implies.)
} >>
} >>Bill

On Thu, 4 Jan 1996 12:45:15 -0800, tcmay@got.net (Timothy C. May) wrote:
} 
} Just a couple of points on this optical idea.
} 
} We were linking buildings a mile apart in the 70s, at Intel. We needed to
} ship CAD data back and forth, and PacBell rates for a dedicated line were
} outrageous, slow to be installed, etc. So, a commercially available laser
} and modulator/demodulator (modem, but it bears sometimes using the longer
} version, to remind people of what it is doing in general) were mounted on
} the roofs of our buildings. I'm sure various packages are commercially
} available to do this.
[snip] 
} I'm actually more positive on low-level (below safety regs get interested
} in) light than on free space RF, for bypassing of the local cable/phone
} monopolies. There's just not enough "bandwidth of free space" available. Do
} the math.
[snip]


::::::::::::::::::::::::::::::::::::::
::  Lou Poppler <lwp@mail.msen.com> ::  " The more you drive, 
::      http://www.msen.com/~lwp/   ::    the less intelligent you are."
::::::::::::::::::::::::::::::::::::::    -- Repo man