Fiberless Optical Networks

Alien54 writes "According to this Forbes Magazine article, the time for Fiberless Optical Networks may have arrived. Wireless optics have been given up for dead until very recently. But now better technology and lower product costs have enabled some to solve most of the problems. AirFiber (a company mentioned in the article above) is emerging as one of the favorites in wireless optics, and seems to have a set of good answers for the inevitable "bird and fog" questions: Can a flock of birds take down a network by flying through the lasers? Can a heavy fog send your precious information into the ether?"

Comments

Solution to the bird problem.

[Magus311X]

Can a flock of birds take down a network by flying through the lasers?

Well, if you increase the power of the lasers, you could then only need to pose this question:

Can a network take down a flock of birds flying through the lasers?

Problem solved. ;)

Glaring Technical Error

[Chic Pea]

From the article:
"And, with asynchronous transfer mode technology (ATM), the lasers have become intelligent enough to track the laser beams between the two optical transceivers, so they never get off target."

How the heck is ATM going to keep the lasers on target? I think the author confused this with ATM signallig setting up SVC's on the fly to provide reliable data transfer through the network in the case of a link going down.

Weather susceptibility still a problem?

[davidb54]

This is an exciting technology, but there are some unfortunate tradeoffs which have to be made when trying to achieve ultra-broadband wireless in not-so-thin air.

Bit rate is proportional to bandwidth times the logarithm of the signal-to-noise ratio. To maximize bandwidth, you go up to higher and higher transmission frequencies. To maximize signal to noise ratio, you step up the transmission power. But in a wireless laser network, both of these steps have their disadvantages.

The first problem is essentially that the higher frequencies (e.g. infrared, which is on the order of microns, as opposed to microwave, which is on the order of centimeters) are more susceptible to various scattering phenomena. The most frequently mentioned is, of course, fog, dust, smog, etc. These scatterers are far to small to have any significant effect on, for example, cellular communications (transmitted signal has a wavelength of tens of centimeters, not microns), but they are excellent scatterers in smaller wavelengths. In addition, the atmosphere itself scatters visible light more and more effectively as you go to higher and higher frequencies, reaching a maximum somewhere in the ultraviolet. This is due to the electronic properties of diatomic nitrogen and oxygen and cannot be avoided. (As a side note, it is also why the sky is blue and sunsets are red). So, one cannot step around the fog problem by going to even higher frequencies. I believe, but am not certain, that fiberless lasers still operate in the IR.

The second problem, of course, is that stepping up the power output of the transmitter is expensive. A tenfold increase in bandwidth requires a thousandfold increase in signal to noise ratio. To see why this is so, imagine that with a given signal to noise ratio, you can resolve 16 signal strengths with a bit error rate of less than, say, 10^-8. This means that you can transmit 4 bits of information per symbol. To get twice as many bits per symbol, or double the bit rate, you need to be able to resolve 256 signal strengths - i.e. square your signal to noise ratio. To get 12 (three times as many) bits per symbol, you need to cube your S/N, and so on. Essentially, you have to double your S/N for each additional bit per symbol you wish to be able to resolve at a certain bit error rate. Hence the need for enormously increased power to achieve relatively modest increases in bandwidth.

So, with these constraints in mind, it will be interesting to see what optimum is achieved by TeraBeam et al, and how resilient their systems turn out to be.

Dave Bailey

Reminds me of the "ArcLight"

[Ungrounded Lightning]

This reminds me of something more than 20 years back: Datapoint's "ArcLight", for their Arcnet.

Arcnet was a token-ring based network with a broadcast topology. Cut the connection between two parts and it immediately reconfigures into two nets. Plug it back in and it reconfigures into a single net.

Ran on 8080-based terminals.

To get between buildings they used a gadget with an infrared laser diode (which had just come out) and a photodiode - each behind a lens about 6 inches in diameter. The device looked somewat like a weatherproof half-height-full-width monitor case with a little bit of a lightshade and the screen replaced by a couple of big glass eyes.

In a city where most buildings weren't skyscrapers (so a little defocussing could deal with building sway and clear-air turbulence without too much energy loss and interference acceptance), clouds and fog were rare, and at a time when high-speed data lines were 300 baud, it was great. A LAN that spanned multiple buildings. If the fog rolled in the network partitioned until it went away (no data between the head office and the branch for a couple hours, but the nets WITHIN the buildings were still up. Birds were handled by retransmissions that were part of the normal protocol.

Something similar would be easy with IP these days: Run a low speed (56k, T1, whatever) between the buildings AND put up the high-speed link. On foggy days your bandwidth drops but your connection is still there. IP also understands flakey connections and rerouting around them, and TCP understands using retransmission to make a reliable connection over unreliable links.

Re:What about fog?

[Kris_J]

A unit stuck in the middle of the fog will be completely surrounded on all sides, so communication will be shot between whoever uses the particular node and the rest of the world.

Thank you, I thought exactly the same thing after reading the article. How does the system cope with fog?

I'd like to see what these things do when they are covered in a cardboard box!

Isolated from any control system they revert back to their basic programming -- kill all the humans. First they burn through the box, then they start killing anything moving...

Shannon's delight

[XNormal]

Most of Dave's comments are relevant to RF communication, not to optical.

Capacity is proportional to the logarithm of (1+signal to noise ratio). A small but significant difference. The result is that for a given power budget it is always better to use as much bandwidth as possible unless you are limited by arbitrary constraints such as the FCC's dumb frequency management practices.

There's no need to go to higher optical frequencies to increase capacity. The carrier frequency of a 1.3 micron infrared laser is 230 terahertz. It's easy to see that a few hundreds of megabits per seconds barely scratch the theoretical capacity.

You've got so much bandwidth in optical that more than one bit per symbol makes absolutely no sense. In fact, you want LESS than one information bit per symbol by using forward error correction codes.

The right frequency to choose is in the atmospheric window wavelengths - those least absorbed by water vapor.

In fog conditions the cumulative attenuation per meter is so high that even a hundredfold increase in laser power will not make a significant increase in the effective range. You are stuck with a few hundreds of meters. Deal with it. AirFiber's architecture looks like the right way to do it. Even if you ignore the bird problem, with a relay every few hundreds of meters the end-to-end reliability drops exponentially with the number of hops the signal has to go through. A mesh architecture can cover long distances while still maintaining adequate availability.

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