4 ms·

16 fibre pairs doesn't seem like very much? I wonder what the constraining factors are, as surely the material costs of having hundreds of pairs are negligible

16 fibre pairs doesn't seem like very much? I wonder what the constraining factors are, as surely the material costs of having hundreds of pairs are negligible compared to the install costs? Is it amplification?

bluedino 6y ago
You need repeaters every so often. Also, someone will "buy" each cable, like Microsoft/Google/etc. You need one cable for each fiber, they aren't all in one bundle like fiber might come into your office.
centimeter 6y ago
Yeah, you periodically need amplification/signal conditioning hardware in the cable. If it was just fibers you could put a ton in there. The amplification is obviously there for losses in the cable, but you also need to do clock recovery and thresholding because the light pulse gets spread out as it travels. Higher data rate => More analog bandwidth (distribution of frequencies from the laser) More analog bandwidth => Worse chromatic dispersion You need to either: * Increase the frequency of the laser (so BW/center frequency is smaller, hopefully reducing dispersion), which might not be possible with current tech or be very expensive * Decrease the dispersion of the fiber (more expensive or lossy fiber) * Have periodic signal conditioners
- mmmBacon 6y ago
  It sounds like you are describing direct detect systems. These are no longer deployed in subsea. Due to the advent of coherent optical transmission and DSP, dispersion is no longer an issue for long distance optical communication. The limitation becomes nonlinear “crosstalk” between wavelengths and lower dispersion makes the nonlinear crosstalk worse as the phase matching condition for nonlinear processes can be reached more easily. To avoid this most new subsea cables use a high dispersion design as this helps mitigate nonlinear effects. Additionally coherent systems have a higher spectral efficiency and are transmitting >2bits/symbol because the information is encoded in the electric field’s phase and not just intensity. Depending on the reach, the number of bits per symbol can be increased. The advent of probabilistic shaping, allows more bits per symbol to be transmitted in higher order constellations. Probabilistic shaping produces a more ‘random-like’ coding that gets us closer to the Shannon Limit and also reduces the optical intensity rms further reducing nonlinear penalties. In fact in optical communications there is something called the nonlinear Shannon limit that takes into account the additional “noise” produced by nonlinearities in the fiber. See: https://arxiv.org/abs/1802.09120 https://arxiv.org/abs/1802.09120
  - nicpottier 6y ago
    I consider myself fairly technical but this read like something out of Silicon Valley (Weissman score!) to me. Hats off!
  - centimeter 6y ago
    TIL! Thanks for sharing.
  - est31 6y ago
    Wow this is the kind of comments I'm visiting this site for. This includes your parent comment as it made you write the comment :).
  - generatorguy 6y ago
    Dispersion is no longer an issue but lower dispersion increases crosstalk so new subsea cables are high dispersion? High compared to early days when dispersion was a problem or just high compared to low dispersion cables which suffer from crosstalk? So there is the perfect amount of dispersion now? It never occurred to me that fiber optic communication in glass actually used intensity to transmit bits. This sounds like AM radio albeit digital I guess the laser could be on or off. How long has it worked like that? I always assumed it was like CDMA radio where like you say phase shifts correspond to a point in a constellation which represents more than 1 bit.
    - mmmBacon 6y ago
      When we did intensity modulation, there was a time when some cables, particularly in Japan used NZDSF (Non Zero dispersion shifted fiber) type fiber. This was useful when we used OOK (on-off keying) and direct detection. With direct detection, we didn’t have access to the phase and could not compensate for dispersion all that well. So we used a DSF (dispersion shifted fiber) that has the opposite slope (sign) and cancels out the dispersion. This was imperfect and DSF was a source of nonlinearity due to its narrower core (nonlinearity goes as the time derivative of the intensity per unit area). There is no perfect amount of dispersion. However allowing the dispersion to increase in modern cables also allows us to make the fiber core a little bigger. This reduces the intensity per unit area, reducing the fiber nonlinearity. Additionally there’s some benefit for highly dispersed signals as they look like noise. For long distances we encode the information on the electric field phase and amplitude as us done in wireless. In actuality most of what’s done in optical from a modulation point of view and most of the theory comes from wireless originally. A difference is that we use a laser as a local oscillator to shift the data back down to baseband just like in wireless. We did intensity modulation for a long time because the signal speeds are very high and the semiconductor technology wasn’t there to do ADC and DACs at the rates we needed at low enough power and cost. This changed around 2008 when the first coherent systems became practical due to the availability of 45nm CMOS. Now most everything above 25Gb/s and 80km is coherent. Below that we still do intensity modulation and inside data centers it’s all intensity modulation. This is due to cost and power reasons. Even in 7nm, coherent is still higher power on a bit/s basis. It’s also more expensive due to complexity of the transmitter and receiver as well as the need for better quality lasers.
bogomipz 6y ago
The "Optical Pumps" that repeater the spans are the biggest constraint. See: https://www.popsci.com/google-fastest-internet-cable-atlantic-dunant/ https://www.popsci.com/google-fastest-internet-cable-atlanti... Also keep in a mind that fiber itself is theoretically unlimited bandwidth wise. So as the transmission gear on each end that lights the fiber improves so does the available bandwidth each fiber pair can't provide. The other issue is that the cable plant itself has a limited life as well generally around 20 years although that number has gone up recently to 25 or so years. So anything that's fiber not lit for the majority of that 25 year life span is potentially money wasted.

bluedino 6y ago

You need repeaters every so often. Also, someone will "buy" each cable, like Microsoft/Google/etc. You need one cable for each fiber, they aren't all in one bundle like fiber might come into your office.

centimeter 6y ago

Yeah, you periodically need amplification/signal conditioning hardware in the cable. If it was just fibers you could put a ton in there. The amplification is obviously there for losses in the cable, but you also need to do clock recovery and thresholding because the light pulse gets spread out as it travels. Higher data rate => More analog bandwidth (distribution of frequencies from the laser) More analog bandwidth => Worse chromatic dispersion You need to either: * Increase the frequency of the laser (so BW/center frequency is smaller, hopefully reducing dispersion), which might not be possible with current tech or be very expensive * Decrease the dispersion of the fiber (more expensive or lossy fiber) * Have periodic signal conditioners

mmmBacon 6y ago
It sounds like you are describing direct detect systems. These are no longer deployed in subsea. Due to the advent of coherent optical transmission and DSP, dispersion is no longer an issue for long distance optical communication. The limitation becomes nonlinear “crosstalk” between wavelengths and lower dispersion makes the nonlinear crosstalk worse as the phase matching condition for nonlinear processes can be reached more easily. To avoid this most new subsea cables use a high dispersion design as this helps mitigate nonlinear effects. Additionally coherent systems have a higher spectral efficiency and are transmitting >2bits/symbol because the information is encoded in the electric field’s phase and not just intensity. Depending on the reach, the number of bits per symbol can be increased. The advent of probabilistic shaping, allows more bits per symbol to be transmitted in higher order constellations. Probabilistic shaping produces a more ‘random-like’ coding that gets us closer to the Shannon Limit and also reduces the optical intensity rms further reducing nonlinear penalties. In fact in optical communications there is something called the nonlinear Shannon limit that takes into account the additional “noise” produced by nonlinearities in the fiber. See: https://arxiv.org/abs/1802.09120 https://arxiv.org/abs/1802.09120
- nicpottier 6y ago
  I consider myself fairly technical but this read like something out of Silicon Valley (Weissman score!) to me. Hats off!
- centimeter 6y ago
  TIL! Thanks for sharing.
- est31 6y ago
  Wow this is the kind of comments I'm visiting this site for. This includes your parent comment as it made you write the comment :).
- generatorguy 6y ago
  Dispersion is no longer an issue but lower dispersion increases crosstalk so new subsea cables are high dispersion? High compared to early days when dispersion was a problem or just high compared to low dispersion cables which suffer from crosstalk? So there is the perfect amount of dispersion now? It never occurred to me that fiber optic communication in glass actually used intensity to transmit bits. This sounds like AM radio albeit digital I guess the laser could be on or off. How long has it worked like that? I always assumed it was like CDMA radio where like you say phase shifts correspond to a point in a constellation which represents more than 1 bit.
  - mmmBacon 6y ago
    When we did intensity modulation, there was a time when some cables, particularly in Japan used NZDSF (Non Zero dispersion shifted fiber) type fiber. This was useful when we used OOK (on-off keying) and direct detection. With direct detection, we didn’t have access to the phase and could not compensate for dispersion all that well. So we used a DSF (dispersion shifted fiber) that has the opposite slope (sign) and cancels out the dispersion. This was imperfect and DSF was a source of nonlinearity due to its narrower core (nonlinearity goes as the time derivative of the intensity per unit area). There is no perfect amount of dispersion. However allowing the dispersion to increase in modern cables also allows us to make the fiber core a little bigger. This reduces the intensity per unit area, reducing the fiber nonlinearity. Additionally there’s some benefit for highly dispersed signals as they look like noise. For long distances we encode the information on the electric field phase and amplitude as us done in wireless. In actuality most of what’s done in optical from a modulation point of view and most of the theory comes from wireless originally. A difference is that we use a laser as a local oscillator to shift the data back down to baseband just like in wireless. We did intensity modulation for a long time because the signal speeds are very high and the semiconductor technology wasn’t there to do ADC and DACs at the rates we needed at low enough power and cost. This changed around 2008 when the first coherent systems became practical due to the availability of 45nm CMOS. Now most everything above 25Gb/s and 80km is coherent. Below that we still do intensity modulation and inside data centers it’s all intensity modulation. This is due to cost and power reasons. Even in 7nm, coherent is still higher power on a bit/s basis. It’s also more expensive due to complexity of the transmitter and receiver as well as the need for better quality lasers.

bogomipz 6y ago

The "Optical Pumps" that repeater the spans are the biggest constraint. See: https://www.popsci.com/google-fastest-internet-cable-atlantic-dunant/ https://www.popsci.com/google-fastest-internet-cable-atlanti... Also keep in a mind that fiber itself is theoretically unlimited bandwidth wise. So as the transmission gear on each end that lights the fiber improves so does the available bandwidth each fiber pair can't provide. The other issue is that the cable plant itself has a limited life as well generally around 20 years although that number has gone up recently to 25 or so years. So anything that's fiber not lit for the majority of that 25 year life span is potentially money wasted.