4 ms·

Does this mean anything for actual latency, or only bandwidth? e.g. speed of light could mean a ~40ms ping between LA and Sydney, but best we get today is prob

by sjm 2y ago

Does this mean anything for actual latency, or only bandwidth? e.g. speed of light could mean a ~40ms ping between LA and Sydney, but best we get today is probably around 150ms?

greggyb 2y ago
The speed of light in fiber (or electrical signal in copper) is less than the speed of light in vacuum. There are delays (very small) converting a signal from electrical on one end of the fiber to light and back to fiber on the other end. For this reason, DAC tends to have measurably lower latency compared to fiber for in-rack networking. The length of an undersea cable is greater than both the straight line and the great circle distance between two points on the earth's surface. These things do not explain all (probably not even most) of the difference between the latency you suggested and that in the real world, but I hope they help to suggest why the naïve calculation is not achievable.
- denotational 2y ago
  > both the straight line and the great circle distance Curious what you mean by “straight line”; Rhumb line?
  - greggyb 2y ago
    I meant the equivalent of boring a very deep and long tunnel (: I should have been more specific.
    - denotational 2y ago
      Ah, you’re literally thinking in a higher dimension to me!
cycomanic 2y ago
This is only throughput. The latency is given by the speed of light in the fibre (~c/1.5). That said Microsoft bought a company that develops hollow core fibre which yields a factor 1.5 improvement in latency. They just presented their latest results which is a 0.11 dB/km loss. This is actually the biggest result from the conference, because it is a massive improvement over regular fibre which has been hovering at about 0.15 dB/km loss for the last 40 years, with improvements below 1% over that time.
- foobiekr 2y ago
  I’m not so sure it will matter much. The earth is 42ms across at light speed, 66ms if traversing a great circle. Network hops are notoriously slow. In the datacenter the best I have ever seen is 200ns or so per packet which is very rare, most in DC hops are closer to 3-9 usec (especially modular chassis); then you hit the routers. With moderate congestion your routing hops are going to be twice that or more ignoring queuing, and you are likely six hops at least between two points in each direction. The hollow core stuff mostly will jot help since it gains with distance, but distance means more hops on average, so we are talking about an application where low latency is required but distances are high (where the improvement applies) but the minimum latency achievable is still tens of ms. It is interesting technology but I think it’s more interesting for hypothetical materials savings than for latency improvement.

by sjm 2y ago

Does this mean anything for actual latency, or only bandwidth? e.g. speed of light could mean a ~40ms ping between LA and Sydney, but best we get today is probably around 150ms?

greggyb 2y ago

The speed of light in fiber (or electrical signal in copper) is less than the speed of light in vacuum. There are delays (very small) converting a signal from electrical on one end of the fiber to light and back to fiber on the other end. For this reason, DAC tends to have measurably lower latency compared to fiber for in-rack networking. The length of an undersea cable is greater than both the straight line and the great circle distance between two points on the earth's surface. These things do not explain all (probably not even most) of the difference between the latency you suggested and that in the real world, but I hope they help to suggest why the naïve calculation is not achievable.

denotational 2y ago
> both the straight line and the great circle distance Curious what you mean by “straight line”; Rhumb line?
- greggyb 2y ago
  I meant the equivalent of boring a very deep and long tunnel (: I should have been more specific.
  - denotational 2y ago
    Ah, you’re literally thinking in a higher dimension to me!

cycomanic 2y ago

This is only throughput. The latency is given by the speed of light in the fibre (~c/1.5). That said Microsoft bought a company that develops hollow core fibre which yields a factor 1.5 improvement in latency. They just presented their latest results which is a 0.11 dB/km loss. This is actually the biggest result from the conference, because it is a massive improvement over regular fibre which has been hovering at about 0.15 dB/km loss for the last 40 years, with improvements below 1% over that time.

foobiekr 2y ago
I’m not so sure it will matter much. The earth is 42ms across at light speed, 66ms if traversing a great circle. Network hops are notoriously slow. In the datacenter the best I have ever seen is 200ns or so per packet which is very rare, most in DC hops are closer to 3-9 usec (especially modular chassis); then you hit the routers. With moderate congestion your routing hops are going to be twice that or more ignoring queuing, and you are likely six hops at least between two points in each direction. The hollow core stuff mostly will jot help since it gains with distance, but distance means more hops on average, so we are talking about an application where low latency is required but distances are high (where the improvement applies) but the minimum latency achievable is still tens of ms. It is interesting technology but I think it’s more interesting for hypothetical materials savings than for latency improvement.