
How fast are those packets moving? - luu
http://blog.wesleyac.com/posts/ping-lightspeed
======
anon1253
There is an interesting urban legend / story about the 500 mile email that
covers this as well:
[https://www.ibiblio.org/harris/500milemail.html](https://www.ibiblio.org/harris/500milemail.html)

excerpt:

' "We're having a problem sending email out of the department."

"What's the problem?" I asked.

"We can't send mail more than 500 miles," the chairman explained.

I choked on my latte. "Come again?"'

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userbinator
_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._

Regular electrical signals can propagate just as fast in copper wires as light
does in fiber:

[https://en.wikipedia.org/wiki/Wave_propagation_speed](https://en.wikipedia.org/wiki/Wave_propagation_speed)

Also, if there are any wireless (microwave) links in the path, those will
actually be (almost, due to the atmosphere) at the speed of light.

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topranks
A few issues:

\- They don't factor the time the opto-electrical converstion takes at each
hop.

\- They don't factor queueing and buffering in any devices

\- They assume the router was able to instantly generate the TTL exceeded
message at each traceroute hop.

\- They don't seem to consider that the return path might be going via Japan
with a hop through France, which won't show on the traceroute at all.

\- They assume each non-passive device on the path will show in the traceroute
(this isn't true, L1/2 devices won't show, technologies like MPLS can
obfuscate entire network cores).

In terms of how fast light moves in single-mode fibre it's fairly constant,
the variance here ins't to do with the fibre.

~~~
mseebach
> the variance here ins't to do with the fibre.

Which is basically what the article is about.

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siscia
I may be completely wrong here but I would like to hear some one else opinion.

The speed of packets in fiber is exactly the one calculate doing the math
combining together the speed of light and the refraction coefficient of fiber.

What slowndown everything is reaching an hub, detect the light and trasmute
those impulses into bits, decide where to redirect the packet, and re-trasmute
the bits into light impulse.

It is not the light domain the one that dominate the speed of packets but is
the electronic domain.

Am I wrong?

~~~
nothrabannosir
Here is a suspicion I have, can someone who has experience in the field shed
some light on this?:

In addition to the refraction index, you’d expect fibre optics to have another
limiting factor: scatter. As light bounces off the side of the fibre, it gets
scattered back and forth. I.o.w.: you’d expect a single, “perfect” pulse to
eventually reach the other end as a slightly drawn out pulse (with its
intensity spread out over a bell curve?). Is that true? If so, does that mean
that in fibre optics , the longer the cable, the lower the frequency at which
you can send pulses, to avoid “bleeding” from one into the next? And is it
true radio doesn’t have that?

Or does fibre not work that way?

~~~
hansc
This is the case in Multimode fiber, which is used for short distances
(meters). For anything longer, single mode fiber is used and the core of the
fiber guides the light. See
[https://garyherlache.wikispaces.com/Digital+Communications+F...](https://garyherlache.wikispaces.com/Digital+Communications+Fall+2009+Fiber+basics)

~~~
gsnedders
That looks like single mode fiber never refracts… Surely it can't go round any
bend, then, though? (And I'm probably missing something obvious. It's been a
while since I've done optics of any sort.)

~~~
amazon_not
> That looks like single mode fiber never refracts…

No, it does.

> Surely it can't go round any bend, then, though?

No, if you bend the fiber too much the light leaks out into the cladding.

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hueving
>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
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
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
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...](http://www.rfwireless-world.com/Terminology/Optical-Repeater-vs-
Optical-Amplifier.html)

[1]
[https://en.wikipedia.org/wiki/Mamyshev_2R_regenerator](https://en.wikipedia.org/wiki/Mamyshev_2R_regenerator)

~~~
askvictor
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
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.

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lnx01
Reminds me of this apocryphal story:
[https://www.ibiblio.org/harris/500milemail.html](https://www.ibiblio.org/harris/500milemail.html)

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d1p
The author's calculations are based on flight distance which will be
significantly shorter than the actual fiber distance. Also, the actual network
path is much harder -if not impossible- to derive with conventional tools. Put
another way: the calculations are extremely rough approximations and should be
taken with a grain of salt.

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sudhirj
The [https://www.submarinecablemap.com/](https://www.submarinecablemap.com/)
is also a great resource - one can expect to hit almost the speed the light,
barring the repeaters / amplifiers, while going through the cables.

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preinheimer
We've got a larger dataset of ping times here:
[http://wondernetwork.com/pings](http://wondernetwork.com/pings)

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mar77i
I found it enLIGHTening indeed. :D

