News Noise cancellation tech quadruples optical bandwidth

While I am all for advancements in technology, this is one area I am fed up with. We don't need significantly faster broadband, we need a significantly better network to provide what we already have to more people. The usual argument is that people in rural areas can not get high broadband speeds or even a regular connection but I think the problem is far worse than that. People seem to take it for granted that those that live in built up areas can access the full potential of services and that just is not the truth, there are vast areas of London, the capital city I'll remind you that have no fibre networks laid at all. I live near Victoria Park in the East End which is about 1.5 miles from the City's financial square mile and there is not fibre near me even if I wanted it. I live less than half a mile from a hub and the broadband speed I get is about 6-8Mb and even BT's high speed service is quoted to only give me around 15Mb (while costing an extra £30 a month).

So while this is good for future main lines to handle the business end of data transfer for the providers only those who already have more than enough speed wil see any benefit and those struggling will be left further behind.

 

I wonder if this requires Polarization Maintaining fiber, or if it still works with standard single/multi-mode fiber.

 

God I love technology.

 

Correct me if I'm wrong, but isn't this very similar to how noise is cancelled on copper Ethernet connects? The color pairs?

 

Strictly speaking, Ethernet (and USB, and firewire, and PCIe, etc, etc) does use differential signalling over its twisted pairs, which appears to be what they're talking about here. Or at least what you're talking about here. Are you sure that's actually what they're talking about here?

 

From the first page that's available free, what they're proposing is actually nothing to do with differential signalling, but they're using some fairly deep jargon to describe what might actually be reasonably simple quadrature techniques.

Although you could do differential, I suppose, with multispectral optical signals down a single fibre. I have no idea if that'd be in any way useful.

 

Sexy innit?

 

This doesn't make any sense. It sounds like it is an enhancement to error correction, and may allow longer distances between optical regenerators, but I totally fail to see how it could increase transmission capacity. I imagine that the 400gbit/s link speed was only mentioned to show that it works on a production DWDM network.

 

This sounds exactly like Common Mode Rejection, which has been used in professional audio equipment for decades. Why has it taken so long to apply this principle?

 

You're right, I overlooked that fundamental difference of the two mediums, electrical and optical. What I failed to get from the article snip-it link is what this optical "interference" is actually from? My, albeit limited, understanding of fiber optics is that the limiting factor in transmissions over distance is attenuation and optical dispersion/refraction along the cable. Wouldn't two slightly different frequencies or wave lengths travel slightly different paths in the cable and therefor get affected slightly differently?

 

In high-speed serial stuff (SATA, PCIe, USB, DVI...), inverse waveforms is exactly what you're doing. The two signals are 180 degrees out of phase.

Yup, this is the difference - but I'll explain why further down...

Not quite. Differential pairs are just there to increase your signal-to-noise ratio. If you just had one of the wires it would still work a lot of the time, as long as you had a way of maintaining DC balance. Adding the second, inverted, signal just helps reduce crosstalk (net field emitted is smaller) and gives you common-mode EMI rejection, allowing you to turn the data rate up.

If you reduce signal losses, your SNR increases, so your channel capacity increases. Shannon-Hartley_theorem

It is basically common-mode rejection. You can get common-mode rejection pretty much for free in differential signalling, but only because differential signals are all transmitted as baseband. USB, SATA, PCIe and DVI are just serialised versions of the data, with a bit of scrambling (either to increase the number of edges to help clock recovery or reduce the number of edges to save bandwidth) and sometimes some error detection/correction, but the transmitted bit rate is still about the same as the signal frequency. In optical stuff, you're not transmitting at baseband. The symbol rate is Mbit or Gbit/sec, but the frequency of the light is in THz. You therefore can't do a simple inversion to get rid of common-mode noise.

So, because you can't easily control the phase of your optical carrier, these people have found the next-best equivalent, which is to use the phase conjugate of the optical waves. They then found that it does work just the same as common-mode rejection once it gets to the far end. The reason that it's not been done before is presumably because it's much higher frequency and therefore quite hard to produce phase-conjugate signals, then re-combine them accurately.

 

What he said... lol.

 

Gareth, my final paragraph did say that it was better for the main lines... I still find it a joke that you can get up to 100MB from cable providers while others struggle to get bandwidth fast enough to stream the BBC iPlayer or YouTube properly. As for Smartphones, even many 3G services out perform many household connections. So my point was while this may help with the ever overloaded mainlines, when it comes to home users those who have will get more and those who don't, won't.