4 ms·

>This is approximately the same speed as SF to Hong Kong, which is quite surprising - I would expect SF to Hong Kong to be much faster than Denver to Chicago, s

>This is approximately the same speed as SF to Hong Kong, which is quite surprising - I would expect SF to Hong Kong to be much faster than Denver to Chicago, since a larger percentage of the time would be spent travelling over fiber. Undersea fiber cables have repeaters embedded in them. So even though it's only one hop from an L2 perspective, the light is being decoded, error corrected, and re-transmitted many times along the way to hong kong from SF.

rusk 9y ago
Back 20 years ago when I was doing my undergrad, optical amplifiers [0] were a thing. In theory it's possible to boost signal without repeating. Is it the case that they're still too expensive or are we talking old legacy long haul infrastructure that hasn't been updated yet? I'd say it's more likely to be some sort of routing or even filtering overhead between the two national jurisdictions. Plausibly there be some kind of layer 3 filtering going on ... [0] https://en.wikipedia.org/wiki/Optical_amplifier https://en.wikipedia.org/wiki/Optical_amplifier
- askvictor 9y ago
  Does the light signal in an fibre optic cable get noisy over distance? If so, then wouldn't an analog signal amp such as this simply amplify the noise along with the signal? Decoding and re-encoding would have the effect of 'cleaning' the signal in this case. If it does get noisy, where does the noise come from? The medium? Leakage from outside the cable?
  - rusk 9y ago
    I thought the point of fibre was that it was low noise ... EDIT There's a good discussion of what you're talking about here [0] Upon further reading, there's a such thing as an all optical regenerator which would appear to provide the best of both worlds, though there's nothing here about how widely these are used [1] [0] http://www.rfwireless-world.com/Terminology/Optical-Repeater-vs-Optical-Amplifier.html http://www.rfwireless-world.com/Terminology/Optical-Repeater... [1] https://en.wikipedia.org/wiki/Mamyshev_2R_regenerator https://en.wikipedia.org/wiki/Mamyshev_2R_regenerator
    - askvictor 9y ago
      That's pretty fascinating, but I don't understand how this works from a purely thermodynamic POV. If the signal is being amplified, where is the extra energy coming from? Does it deplete the material? Is it like boosting voltage at the expense of current?
      - amazon_not 9y ago
        Longhaul subsea fiber optic cables have a conductor in them that carries a high voltage DC current that powers the amplifiers on the cable route.
  - amazon_not 9y ago
    > Does the light signal in an fibre optic cable get noisy over distance? Yes. > If so, then wouldn't an analog signal amp such as this simply amplify the noise along with the signal? Yes. That's why there are limits on the reach of amplified networks. > Decoding and re-encoding would have the effect of 'cleaning' the signal in this case. Yes, but regenerating the signal introduces complexity, and thus reduces reliability, adds latency and only works for a specific encoding, which makes upgrades impossible. None of these things are wanted at the bottom of the sea for the next 25 years. > If it does get noisy, where does the noise come from? There are lots of noise sources. See: http://www.svphotonics.com/pub/pub029.pdf http://www.svphotonics.com/pub/pub029.pdf > The medium? Yes, partly. > Leakage from outside the cable? No. Although vibrations can cause noise.
- amazon_not 9y ago
  > Is it the case that they're still too expensive or are we talking old legacy long haul infrastructure that hasn't been updated yet? No, all long haul subsea fiber optic cables use all optical amplifiers. They are just colloquially called repeaters.
Hikikomori 9y ago
No, you use optical amplifiers for long haul fiber.
dfox 9y ago
There are various levels of intelligence in repeaters/regenerators in order to balance the additional gained noise margin with added latency. Simplest repeaters only do optical-electrical conversion to binary signal and then back to optical signal, more complex ones also do realigning of edges and only minority of the infrastructure actually decodes and understands the protocols involved. Most of the design of optical networks traces back to 90's telco requirements and not to computer networking and during that time telcos were extremely sensitive to latency (the idea being that when the latency of the digital part of the network is smaller than some limit the analog-digital transition can be done by somewhat cheaper hardware). The funky interleaved SDH/SONET frame formats are motivated by ability to implement muldexes that introduce less than entire frame (ie. 125us) of latency.
amazon_not 9y ago
> Undersea fiber cables have repeaters embedded in them. So even though it's only one hop from an L2 perspective, the light is being decoded, error corrected, and re-transmitted many times along the way to hong kong from SF. No, this is incorrect. Subsea fiber cables use all optical amplification at each repeater. There is no optical-electrical-optical conversion.
bogomipz 9y ago
> "So even though it's only one hop from an L2 perspective, the light is being decoded, error corrected, and re-transmitted many times along the way to hong kong from SF." This is not right. Submarine cable along the ocean floor is strictly Layer 1 ,there is no L2 down there. Also nothing is being decoded and retransmitted along that path. Signal regeneration only happens at the terminal stations. There's no repeaters along the ocean floor. The signal undergoes no conversion from optical to electrical, its optical the whole time it's under water. Submarine cables use an optical amplifier. Basically the fiber itself is doped with erbium creating an EDFA - erbium doped fiber amplifier. See: https://www.rp-photonics.com/fiber_amplifiers.html https://www.rp-photonics.com/fiber_amplifiers.html
- detaro 9y ago
  > Submarine cable along the ocean floor is strictly Layer 1 ,there is no L2 down there. Isn't that what "only one hop from a L2 perspective" would mean?
  - bogomipz 9y ago
    I understood perhaps mistakenly the OP's comment to mean each "repeater" point was an L2 hop. But my main point was that the undersea spans are not in fact "repeatered" and there is no conversion of signal once it leaves land.
    - hueving 9y ago
      Yes, you are mistaken. "only one hop" was actually intended to mean only one hop.
      - bogomipz 9y ago
        Well there's no such things as a "hop" at Layer 2. And as others also pointed out its an optical amplifier, there is no "decoded, error corrected, and re-transmitted" being done along the path of the submarine cable.

rusk 9y ago

Back 20 years ago when I was doing my undergrad, optical amplifiers [0] were a thing. In theory it's possible to boost signal without repeating. Is it the case that they're still too expensive or are we talking old legacy long haul infrastructure that hasn't been updated yet? I'd say it's more likely to be some sort of routing or even filtering overhead between the two national jurisdictions. Plausibly there be some kind of layer 3 filtering going on ... [0] https://en.wikipedia.org/wiki/Optical_amplifier https://en.wikipedia.org/wiki/Optical_amplifier

askvictor 9y ago
Does the light signal in an fibre optic cable get noisy over distance? If so, then wouldn't an analog signal amp such as this simply amplify the noise along with the signal? Decoding and re-encoding would have the effect of 'cleaning' the signal in this case. If it does get noisy, where does the noise come from? The medium? Leakage from outside the cable?
- rusk 9y ago
  I thought the point of fibre was that it was low noise ... EDIT There's a good discussion of what you're talking about here [0] Upon further reading, there's a such thing as an all optical regenerator which would appear to provide the best of both worlds, though there's nothing here about how widely these are used [1] [0] http://www.rfwireless-world.com/Terminology/Optical-Repeater-vs-Optical-Amplifier.html http://www.rfwireless-world.com/Terminology/Optical-Repeater... [1] https://en.wikipedia.org/wiki/Mamyshev_2R_regenerator https://en.wikipedia.org/wiki/Mamyshev_2R_regenerator
  - askvictor 9y ago
    That's pretty fascinating, but I don't understand how this works from a purely thermodynamic POV. If the signal is being amplified, where is the extra energy coming from? Does it deplete the material? Is it like boosting voltage at the expense of current?
    - amazon_not 9y ago
      Longhaul subsea fiber optic cables have a conductor in them that carries a high voltage DC current that powers the amplifiers on the cable route.
- amazon_not 9y ago
  > Does the light signal in an fibre optic cable get noisy over distance? Yes. > If so, then wouldn't an analog signal amp such as this simply amplify the noise along with the signal? Yes. That's why there are limits on the reach of amplified networks. > Decoding and re-encoding would have the effect of 'cleaning' the signal in this case. Yes, but regenerating the signal introduces complexity, and thus reduces reliability, adds latency and only works for a specific encoding, which makes upgrades impossible. None of these things are wanted at the bottom of the sea for the next 25 years. > If it does get noisy, where does the noise come from? There are lots of noise sources. See: http://www.svphotonics.com/pub/pub029.pdf http://www.svphotonics.com/pub/pub029.pdf > The medium? Yes, partly. > Leakage from outside the cable? No. Although vibrations can cause noise.
amazon_not 9y ago
> Is it the case that they're still too expensive or are we talking old legacy long haul infrastructure that hasn't been updated yet? No, all long haul subsea fiber optic cables use all optical amplifiers. They are just colloquially called repeaters.

Hikikomori 9y ago

No, you use optical amplifiers for long haul fiber.

dfox 9y ago

There are various levels of intelligence in repeaters/regenerators in order to balance the additional gained noise margin with added latency. Simplest repeaters only do optical-electrical conversion to binary signal and then back to optical signal, more complex ones also do realigning of edges and only minority of the infrastructure actually decodes and understands the protocols involved. Most of the design of optical networks traces back to 90's telco requirements and not to computer networking and during that time telcos were extremely sensitive to latency (the idea being that when the latency of the digital part of the network is smaller than some limit the analog-digital transition can be done by somewhat cheaper hardware). The funky interleaved SDH/SONET frame formats are motivated by ability to implement muldexes that introduce less than entire frame (ie. 125us) of latency.

amazon_not 9y ago

> Undersea fiber cables have repeaters embedded in them. So even though it's only one hop from an L2 perspective, the light is being decoded, error corrected, and re-transmitted many times along the way to hong kong from SF. No, this is incorrect. Subsea fiber cables use all optical amplification at each repeater. There is no optical-electrical-optical conversion.

bogomipz 9y ago

> "So even though it's only one hop from an L2 perspective, the light is being decoded, error corrected, and re-transmitted many times along the way to hong kong from SF." This is not right. Submarine cable along the ocean floor is strictly Layer 1 ,there is no L2 down there. Also nothing is being decoded and retransmitted along that path. Signal regeneration only happens at the terminal stations. There's no repeaters along the ocean floor. The signal undergoes no conversion from optical to electrical, its optical the whole time it's under water. Submarine cables use an optical amplifier. Basically the fiber itself is doped with erbium creating an EDFA - erbium doped fiber amplifier. See: https://www.rp-photonics.com/fiber_amplifiers.html https://www.rp-photonics.com/fiber_amplifiers.html

detaro 9y ago
> Submarine cable along the ocean floor is strictly Layer 1 ,there is no L2 down there. Isn't that what "only one hop from a L2 perspective" would mean?
- bogomipz 9y ago
  I understood perhaps mistakenly the OP's comment to mean each "repeater" point was an L2 hop. But my main point was that the undersea spans are not in fact "repeatered" and there is no conversion of signal once it leaves land.
  - hueving 9y ago
    Yes, you are mistaken. "only one hop" was actually intended to mean only one hop.
    - bogomipz 9y ago
      Well there's no such things as a "hop" at Layer 2. And as others also pointed out its an optical amplifier, there is no "decoded, error corrected, and re-transmitted" being done along the path of the submarine cable.