Using lasers to communicate

[gw]

At 10:38 AM 2/14/96 -0500, you wrote:
>
>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. 

not as much, IMHO, as you would seem to suggest.  50% mirrors picking off
those laser beams would do it, and you could not detect the resulting
attenuation as distinct from naturally ocurring attenuation and variation.  


>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on the laser table are rapidly inserted and withdrawn from the light beam,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.").
>

KNOWN opposition ... hmmm.  you're back to obscurity=security.
It's always expensive to eavesdrop (tapped any fiber cables in pressure
jackets recently?) ... and (having been in the same business as yourself
once) alignment and bandwidth problems of them pesky little scanners is a
pretty hefty problem.

The concept seems to have merit ... but I don't think I'd want to have to
implement it!
Easily adapted .... ?????

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