7 ms·
How fast are those packets moving?
- siscia 9y agoI 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?
- noncoml 9y agoThe switch/router latency should be a couple of microseconds per “hub”.
- deleted 9y ago[deleted]
- abainbridge 9y agoI 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-trading-tower-near-dover-rejected-2017-1 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.
- d33 9y ago> which has a higher speed of light What? How'd that be possible?
- antirez 9y agoLight can travel in air much faster it can travel in glass.
- yusyusyus 9y agoSimilarly, 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.
- fridek 9y agoRefraction index of light in fiber is ~1.44. Rule of thumb for packets in transit is 200km/ms.
- kentiko 9y agoRadio takes less time because it takes a direct, straight path from point to point. An optic fiber will always have a longer path.
- woliveirajr 9y agoAnd 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.
- amazon_not 9y agoOnly in multimode fiber. Not so in single mode fiber. All subsea cables use singlemode fiber.
- Hikikomori 9y agoRadio waves in the atmosphere also travel faster than light in a fibre.
- antirez 9y agoI 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.
- kryptiskt 9y agoLight 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/ http://oklo.org/2015/01/01/lightspeed/
- nothrabannosir 9y agoHere 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 9y agoThis 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+Fall+2009+Fiber+basics https://garyherlache.wikispaces.com/Digital+Communications+F...
- gsnedders 9y agoThat 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.)
- rocqua 9y agoI believe the fiber acts as a waveguide, essentially keeping the wavefront perpendicular to the fiber at all times.
- amazon_not 9y ago> 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.
- amazon_not 9y ago
- eeZah7Ux 9y agoYou 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-Fiber-Latency http://www.m2optics.com/blog/bid/70587/Calculating-Optical-F...
- hueving 9y ago>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 agoBack 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 agoDoes 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 agoI 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
- mar77i 9y agoI found it enLIGHTening indeed. :D
- sudhirj 9y agoThe 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.
- userbinator 9y agoI 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.
- lnx01 9y agoReminds me of this apocryphal story: https://www.ibiblio.org/harris/500milemail.html https://www.ibiblio.org/harris/500milemail.html
- anon1253 9y agoThere 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?"'
- topranks 9y agoA 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 9y ago> the variance here ins't to do with the fibre. Which is basically what the article is about.
- preinheimer 9y agoWe've got a larger dataset of ping times here: http://wondernetwork.com/pings http://wondernetwork.com/pings
- d1p 9y agoThe 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.
- deleted 9y ago[deleted]