How fast are those packets moving?(blog.wesleyac.com)
blog.wesleyac.com
How fast are those packets moving?
http://blog.wesleyac.com/posts/ping-lightspeed
52 comments
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
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.
Regular electrical signals can propagate just as fast in copper wires as light does in fiber:
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.
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.
- 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.
> the variance here ins't to do with the fibre.
Which is basically what the article is about.
Which is basically what the article is about.
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?
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?
I submit as counter evidence the masts that high frequency traders want to build in Kent, UK. I believe the point is that they can replace some of the fibre in the link between London and Frankfurt with radio, which has a higher speed of light.
http://uk.businessinsider.com/plans-for-high-frequency-tradi...
I also reckon that a modern router can forward a packet in a microsecond. Once you don't involve software in the data plane, routing a packet should only take a few clock cycles.
http://uk.businessinsider.com/plans-for-high-frequency-tradi...
I also reckon that a modern router can forward a packet in a microsecond. Once you don't involve software in the data plane, routing a packet should only take a few clock cycles.
Radio takes less time because it takes a direct, straight path from point to point. An optic fiber will always have a longer path.
And that's correct... inside the optic fiber, the light doesn't go straight in the core of the fiber. It bounces and is reflected, making the path longer.
Only in multimode fiber. Not so in single mode fiber. All subsea cables use singlemode fiber.
Radio waves in the atmosphere also travel faster than light in a fibre.
> which has a higher speed of light
What? How'd that be possible?
What? How'd that be possible?
Light can travel in air much faster it can travel in glass.
Similarly, copper is faster than glass too (but not faster than air). The trade folks refer to copper in terms of "velocity factor". Example, some arbitrary RG-6 I'm looking at has a velocity factor of 81%. Equivalent refractive index is 1/.81 for ~ 1.23.
Refraction index of light in fiber is ~1.44. Rule of thumb for packets in transit is 200km/ms.
Light takes a millisecond to go 300km in air (it will only travel about 200 km in a fiber in that time), at long distances that time will dominate.
An example: http://oklo.org/2015/01/01/lightspeed/
An example: http://oklo.org/2015/01/01/lightspeed/
You have 3 components that can be comparable with each other or not depending on many conditions:
- travel time on fiber
- transceiver conversion time
- time spent sitting in some buffer. This can dwarf other components on "common" networks!
http://www.m2optics.com/blog/bid/70587/Calculating-Optical-F...
- travel time on fiber
- transceiver conversion time
- time spent sitting in some buffer. This can dwarf other components on "common" networks!
http://www.m2optics.com/blog/bid/70587/Calculating-Optical-F...
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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?
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?
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...
Single mode fiber isn't immune to dispersion or other impairments, it just does better than multimode.
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.)
> 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.
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.
I believe the fiber acts as a waveguide, essentially keeping the wavefront perpendicular to the fiber at all times.
> In addition to the refraction index, you’d expect fibre optics to have another limiting factor: scatter.
Fiber optics have lots of limiting factors: power limits, chromatic and polarization mode dispersion, spectral attenuation, manufacturing imperfections, crosstalk, ...
Fiber optics are just better than a lot of other mediums, but that does not mean there aren't limitations.
Fiber optics have lots of limiting factors: power limits, chromatic and polarization mode dispersion, spectral attenuation, manufacturing imperfections, crosstalk, ...
Fiber optics are just better than a lot of other mediums, but that does not mean there aren't limitations.
The switch/router latency should be a couple of microseconds per “hub”.
I think you are right, there is no way you can slowdown light inside the fiber :-) So for sure everything we see as added latency compared to the baseline of traveling the cable distance must be something else, like software or other hardware.
>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.
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.
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.
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.
> "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
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
> 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?
Isn't that what "only one hop from a L2 perspective" would mean?
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.
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.
Yes, you are mistaken. "only one hop" was actually intended to mean only one hop.
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.
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.
>Well there's no such things as a "hop" at Layer 2.
Only if you're being obtuse and don't actually work in networking. A hop in L2 is a device that does L2 forwarding and participates in L2 protocols like STP.
>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.
This is only true for newer stuff. Look up OEO fiber repeaters. Those cables are still in use.
Only if you're being obtuse and don't actually work in networking. A hop in L2 is a device that does L2 forwarding and participates in L2 protocols like STP.
>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.
This is only true for newer stuff. Look up OEO fiber repeaters. Those cables are still in use.
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
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
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?
> 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
> The medium?
Yes, partly.
> Leakage from outside the cable?
No. Although vibrations can cause noise.
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
> The medium?
Yes, partly.
> Leakage from outside the cable?
No. Although vibrations can cause noise.
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...
[1] https://en.wikipedia.org/wiki/Mamyshev_2R_regenerator
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...
[1] https://en.wikipedia.org/wiki/Mamyshev_2R_regenerator
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?
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.
> 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.
No, all long haul subsea fiber optic cables use all optical amplifiers. They are just colloquially called repeaters.
> 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.
No, this is incorrect. Subsea fiber cables use all optical amplification at each repeater.
There is no optical-electrical-optical conversion.
No, you use optical amplifiers for long haul fiber.
Reminds me of this apocryphal story: https://www.ibiblio.org/harris/500milemail.html
[deleted]
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.
The 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.
We've got a larger dataset of ping times here: http://wondernetwork.com/pings
I found it enLIGHTening indeed. :D
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?"'