Using lasers to communicate

[tcmay@got.net (Timothy C. May)]

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
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