Squeezing More Bandwidth Out of Fiber 185
EigenHombre writes "The New York Times reports on efforts underway to squeeze more bandwidth out of the fiber optic connections which form the backbone of the Internet. With traffic doubling every two years, the limits of current networks are getting close to saturating. The new technology from Lucent-Alcatel uses the polarization and phase of light (in addition to intensity) to double or quadruple current speeds."
Re:Dark Fiber (Score:5, Informative)
The shortage is almost entirely in the transcontinental links.
Re:what about color (Score:5, Informative)
In a way they already do this the different wave lengths are used in something called multiplexing, they can cram a lot of completely different signals down the same pipe at the same time with this technique.
http://en.wikipedia.org/wiki/Multiplexing [wikipedia.org]
That link probably explains it much better then I can.
Also they do not send bits, they send more then bytes, they send packets, or entire frames.
Re:what about color (Score:5, Informative)
Already being done; TFA mentions this (for "wavelength" read "color", as the light that is being used is in the infrared).
What limits the number of wavelengths in a single fiber is the bandwidth of the amplifiers: optical fibers slightly absorb light, and current long-haul links require reamplification ca. every 100km. This is done using EDFAs (erbium-doped fiber amplifiers), which work for wavelengths in the 1530-1560nm range (the "C band"; visible light is in the 400-800nm range). Adding wavelengths outside this band would require redeploying new amplifiers along the fiber, which would be expensive; besides, other types of amplifiers aren't quite as mature as EDFAs, and you would need more of them because the fiber attenuates more outside this window.
You could also try to squeeze these wavelengths tighter, to put more of them within the C band, but they are already packed at 0.4-nm intervals, corresponding to a 50-GHz frequency interval, which holds a 10- or 12.5-Gbit/s signal with little margin, as long as conventional optical techniques are used--that is, switching the light on or off for each bit.
There remains the possibility of using smarter ways of modulating the light, using its phase and polarization, to pack e.g. 100Gbit/s in a 50-GHz bandwidth; and that's what Alcatel are doing. They are not the only ones, of course, the field of "coherent optical transmissions" has been a hot topic in the past couple of years. Now commercial solutions are getting into the field.
Note that these techniques are already widely used in radio and DSL systems, and had been proposed for optical systems back in the 1980s, before EDFAs essentially solved the attenuation problem. Now, however, we have again reached a bandwidth limit and have to turn back to coherent transmission. In the 1980s, that meant complicated hardware at the receivers, impossible to deploy outside the labs; now all the complicated stuff can be done with DSP in software. Radio and DSL already do this, but only at a few tens of Gbit/s; doing it at 100Gbit/s for optics is more challenging, and is just now becoming possible.
tens of Mbit/s not Gbit/s (was:what about color) (Score:2, Informative)
Oh...of course! (Score:5, Informative)
The article implies that it's easy to do, there was simply never a need before. I seriously doubt that it's a trivial thing to accomplish a four-fold increase in bandwidth on existing infrastructure.
Polarization has a habit of wandering around in fiber. Temperature and physical movement of the fiber will change how the polarization is altered as it passes through the fiber. In a trans-oceanic fiber the effect could be dramatic; the polarization would likely wander around with quite a high frequency. This would need to be corrected for by periodically sending reference pulses though the fiber so that the receivers could be re-calibrated. Not too difficult, but any inaccessible repeaters would still need to be retrofitted. I also don't know if in-fiber amplifiers are polarization maintaining. They rely on a scattering process that might not be.
Phase-encoding has similar problems. Dispersion, the fact that different frequencies travel at different velocities (this leads to prisms separating white light into rainbows), will distort the pulse shape and shift the modulation envelope with respect to the phase. You either need very low dispersion fibers, and they already need to use the best available, or have some fancy processing at a receiver or repeater. Adding extra phase encoding simply implies that the current encoding method (probably straight-up, on-off encoding) is inefficient. That's not necessarily lack of foresight, that's because dense encoding is probably really hard to do in a dispersive medium like fiber. Again, it's not a trivial drop-in replacement.
The article downplays how hard these problems are. It implies that the engineers simply didn't think it through the first time around, but that's far from the case. A huge amount of money and effort goes into more efficiently encoding information in fiber. There probably is no drop in solution, but very clever design in new repeaters and amplifiers might squeeze some bonus bandwidth into existing cable.
Re:what about color (Score:5, Informative)
Yes, they're using the wavelength aspect of light there.
An electromagnetic waveform can be represented mathematecally as a function of time: s(t) = A * cos(wt + p)
Amplitude (A), or the intensity of light was always used to represent the on/off states.
Wavelength (related to w), or the 'colour' of the light is used in wavelength multiplexing. You just inject multiple signals at different wavelengths and filter them out into the different signals at the receiver.
Phase (p), or the starting position of the wave if you like, you can imagine left or right shifting a sine wave, is another aspect of a wave that can carry information. Phase Shift Keying (PSK) is already used in many radio frequency digital modulation schemes. Not sure how this method could be used to increase bandwidth through modulation though. Probably worth reading the paper.
Note, I've represented a very basic two dimensional wave here. Of course polarisation or the way a 3-D wave is aligned is another aspect that may be used to encode information. For multiplexing, I imaging the idea is to have multiple waves at the same wavelength but with different polarisations. You would then need to be able to filter out particular polarisations at the receiver.
As a side note, the second paragraph of the article says something about not being able to make light go any faster beyond a barrier to increased bandwidth. It in fact has no bearing on it. Even if the on/off effect along kilometres of fibre was instantaneous, you would still have to deal with noise and attenuation in the channel.
A bit more and a rough rule of thumb (Score:4, Informative)
Because the speed of flow (creep) is related to diffusion rate a very rough rule of thumb is that if the temperature is above 2/3 of the melting point in degrees Kelvin then it will happen given time and stress.
That's why it shows up in very old and large lead pipes (low melting point) but not in large windows (high melting point).
Re:Oh...of course! (Score:3, Informative)
Because it's more complicated to reach for a high spectral efficiency. Until now, on fiber, it was possible to just increase the spectral bandwidth (increase the number of wavelengths in a single fiber, in fact). In wireless, on the contrary, the spectrum is much more regulated--if only because it is shared among everybody, whereas what happens in a fiber doesn't affect anything outside it. Thus the drive for a high spectral efficiency in radio.