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