4 ms·

why would a radiowave that is reflected off the atmosphere (and therefore taking the longer route) be faster than a direct fibre cable?

deleted 2y ago
[deleted]
scrlk 2y ago
Radio waves travel at nearly the speed of light, whereas light in an fiber optic cable travels at ~67% of the speed of light due to the refractive index of glass.
- cypherpunks01 2y ago
  Ericsson blog wrote: In a vacuum, electro-magnetic waves travel at a speed of 3.336 microseconds (μs) per kilometer (km). Through the air, that speed is a tiny fraction slower, clocking in at 3.337 μs per km, while through a fiber-optic cable it takes 4.937 μs to travel one kilometer – this means that microwave transport is actually 48% faster than fiber-optic, all other things being equal.
- JumpCrisscross 2y ago
  More bluntly: light in a fibre is still bouncing around a lot.
  - analog31 2y ago
    Not in single mode fibers, which still exhibit the effect. It's just about the refractive index of glass.
    - bcrl 2y ago
      Yes, even in single mode fibers light is bouncing around between the layers of glass with different diffraction indicies. The pulses just aren't dispersing while doing so.
- stephen_g 2y ago
  I worked for three years designing custom low-latency point-to-point microwave radios for HFT for this very reason. They didn't need very high bandwidths (their long-haul network was less than 200 Mbit, whereas in New York/New Jersey we had about 5 Gbps because the hops were much shorter and they had licenses for more RF bandwidth at a higher frequency). At those time scales, the difference is so large, it was incredible what they were willing to pay to build these networks!
  - scrlk 2y ago
    I somewhat regret not specialising in RF/comms in my EE degree - this side of HFT sounds like a fascinating line of work (Trading at the Speed of Light was a great read).
    - thijson 2y ago
      I doubt there's much here that's cutting edge. Any digital processing that's done in typical radio's to correct for channel impairments is avoided as it just adds latency. Meanwhile LTE is using as many digital techniques as possible to maximize bandwidth (MIMO, HARQ, OFMDA)
      - stephen_g 2y ago
        Haha, you got us :) - in terms of the digital side yes, kind of. We’d even try to not have any digital in the path if possible on some hops! We did have things like LDPC (and different FEC on control packets) but it was definitely not as complex as LTE or newer cellular or WiFi standards. But what was avoided digitally meant far more work going into the analogue side to improve SNR, dynamic range, NPR etc. through the signal chain.
FredFS456 2y ago
Speed of light in an optical fibre is about 2/3 that of the speed in air
_qua 2y ago
Light doesn’t go at light speed through optical fiber.
- AStonesThrow 2y ago
  By definition, it does, because the maximum speed is qualified by "the speed of light in a vacuum", so the speed of light [in other media] is simply a function of how much the medium slows it down, yet it is still the speed of light. Funny how that works!
- nimish 2y ago
  Sure it does. It's just that the speed of light in non-hollow optical fiber is slower than light in a vacuum. Microsoft bought a hollow optical fiber company for a reason.
  - cycomanic 2y ago
    Yes, funnily enough Microsofts reason was not HFT but AI. Essentially inter-datacentre training is limited by latency between the datacentres. Generally they want to build the datacentres close to metro areas, by using hollow core fibre the radius of where to place the data centres has essentially increased by 3/2. This significantly reduces land acquisition costs, and supposedly MS has already made back the acquisition cost for Lumenisity, through those savings.
    - 3eb7988a1663 2y ago
      That feels somewhat implausible. I assume a Microsoft sized data center starts at over $100 million. Moving the footprint X miles away might be cheaper, but is probably a drop in the bucket given everything else required for a build out. I would further assume that they were already some distance away from the top tier expensive real estate to accommodate the size of the facility.
      - donavanm 2y ago
        Its reality. Its generally about site and infra access, including power and fiber paths. The bigger providers (eg AWS) simply dont have more feasible sites that are within a few ms of the existing region DCs. Expect to see more infrastructure like “local zones” or AZs that are tens of ms away from the rest of the region.
indiosmo 2y ago
In addition to the radio signal being faster as noted by the other commenters, for long distances the radiowave is actually the shorter route. If you take one of the routes in the article, Chicago to Sao Paulo. The distance is about 8,400km in a straight line. According to https://en.wikipedia.org/wiki/Skywave https://en.wikipedia.org/wiki/Skywave a single shortwave hop can reach 3,500km, so 3 hops are required, or about 30ms. The shortest commercially available submarine cable between the US and Sao Paulo alone is significantly higher than that (almost double), and it comes out of the east coast, so you'd still have to factor in the latency between Chicago and New York. Even specialized low latency networks that mix wireless and fiber will still have much higher latency than the radio. The tradeoff is that shortwave radio has very little bandwidth so you're restricted to simple signals.

deleted 2y ago

[deleted]

scrlk 2y ago

Radio waves travel at nearly the speed of light, whereas light in an fiber optic cable travels at ~67% of the speed of light due to the refractive index of glass.

cypherpunks01 2y ago
Ericsson blog wrote: In a vacuum, electro-magnetic waves travel at a speed of 3.336 microseconds (μs) per kilometer (km). Through the air, that speed is a tiny fraction slower, clocking in at 3.337 μs per km, while through a fiber-optic cable it takes 4.937 μs to travel one kilometer – this means that microwave transport is actually 48% faster than fiber-optic, all other things being equal.
JumpCrisscross 2y ago
More bluntly: light in a fibre is still bouncing around a lot.
- analog31 2y ago
  Not in single mode fibers, which still exhibit the effect. It's just about the refractive index of glass.
  - bcrl 2y ago
    Yes, even in single mode fibers light is bouncing around between the layers of glass with different diffraction indicies. The pulses just aren't dispersing while doing so.
stephen_g 2y ago
I worked for three years designing custom low-latency point-to-point microwave radios for HFT for this very reason. They didn't need very high bandwidths (their long-haul network was less than 200 Mbit, whereas in New York/New Jersey we had about 5 Gbps because the hops were much shorter and they had licenses for more RF bandwidth at a higher frequency). At those time scales, the difference is so large, it was incredible what they were willing to pay to build these networks!
- scrlk 2y ago
  I somewhat regret not specialising in RF/comms in my EE degree - this side of HFT sounds like a fascinating line of work (Trading at the Speed of Light was a great read).
  - thijson 2y ago
    I doubt there's much here that's cutting edge. Any digital processing that's done in typical radio's to correct for channel impairments is avoided as it just adds latency. Meanwhile LTE is using as many digital techniques as possible to maximize bandwidth (MIMO, HARQ, OFMDA)
    - stephen_g 2y ago
      Haha, you got us :) - in terms of the digital side yes, kind of. We’d even try to not have any digital in the path if possible on some hops! We did have things like LDPC (and different FEC on control packets) but it was definitely not as complex as LTE or newer cellular or WiFi standards. But what was avoided digitally meant far more work going into the analogue side to improve SNR, dynamic range, NPR etc. through the signal chain.

FredFS456 2y ago

Speed of light in an optical fibre is about 2/3 that of the speed in air

_qua 2y ago

Light doesn’t go at light speed through optical fiber.

AStonesThrow 2y ago
By definition, it does, because the maximum speed is qualified by "the speed of light in a vacuum", so the speed of light [in other media] is simply a function of how much the medium slows it down, yet it is still the speed of light. Funny how that works!
nimish 2y ago
Sure it does. It's just that the speed of light in non-hollow optical fiber is slower than light in a vacuum. Microsoft bought a hollow optical fiber company for a reason.
- cycomanic 2y ago
  Yes, funnily enough Microsofts reason was not HFT but AI. Essentially inter-datacentre training is limited by latency between the datacentres. Generally they want to build the datacentres close to metro areas, by using hollow core fibre the radius of where to place the data centres has essentially increased by 3/2. This significantly reduces land acquisition costs, and supposedly MS has already made back the acquisition cost for Lumenisity, through those savings.
  - 3eb7988a1663 2y ago
    That feels somewhat implausible. I assume a Microsoft sized data center starts at over $100 million. Moving the footprint X miles away might be cheaper, but is probably a drop in the bucket given everything else required for a build out. I would further assume that they were already some distance away from the top tier expensive real estate to accommodate the size of the facility.
    - donavanm 2y ago
      Its reality. Its generally about site and infra access, including power and fiber paths. The bigger providers (eg AWS) simply dont have more feasible sites that are within a few ms of the existing region DCs. Expect to see more infrastructure like “local zones” or AZs that are tens of ms away from the rest of the region.

indiosmo 2y ago

In addition to the radio signal being faster as noted by the other commenters, for long distances the radiowave is actually the shorter route. If you take one of the routes in the article, Chicago to Sao Paulo. The distance is about 8,400km in a straight line. According to https://en.wikipedia.org/wiki/Skywave https://en.wikipedia.org/wiki/Skywave a single shortwave hop can reach 3,500km, so 3 hops are required, or about 30ms. The shortest commercially available submarine cable between the US and Sao Paulo alone is significantly higher than that (almost double), and it comes out of the east coast, so you'd still have to factor in the latency between Chicago and New York. Even specialized low latency networks that mix wireless and fiber will still have much higher latency than the radio. The tradeoff is that shortwave radio has very little bandwidth so you're restricted to simple signals.