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During the lecture unit, professor clearly states that if we could replace fiber optics with vacuum, we could accomplish the a great speed increase. I realize that in the current state of technology this is not currently possible (and might never be possible in the future) but the way homework question was phrased, we cannot assume that. I believe the correct answer to the question should be "Replacing all the fiber optics with vacuum". |
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The decrease in time for fibre optics -> vacuum is very small compared with the time it takes for the packets to get through the routers. For the 100 ms journey in the videos, approximately 72 ms is taken at routers and 28 ms 'on the wire' since the speed of light along optic fibre is about half what it is in a vacuum.. There would be a saving of approximately 14 ms if the fibre optics were replaced by vacuums. If the number of routers is cut to 4, then that's reducing the time spend at routers to 1/5 of the original 72 seconds, which is around 14 ms. That saves around 58 ms in contrast to 14 ms for the vacuums. No one was mentioning the time for the packets going through. Assume 0 and it will depend on the line only. 1
@TooMuchIdeas The whole point is that most of the latency is due to delays at routing points, he clearly states this in the video. 2
The answer totally depends on the speed of the routers, which he is not mentioning in the video. Take fnenu's example but just other numbers: For the 100 ms journey in the videos, approximately 10 ms is taken at routers and 28 ms 'on the wire' since the speed of light along optic fibre is about half what it is in a vacuum.. There would be a saving of approximately 14 ms if the fibre optics were replaced by vacuums. If the number of routers is cut to 4, then that's reducing the time spend at routers to 1/5 of the original 10 seconds, which is around 2 ms. That saves around 8 ms in contrast to 14 ms for the vacuums. 1
In Unit 4.18 (after the quiz), he states that most of the latency is not on the wires, but at routing points. Yes, but that was not the question. It was about reducing 20 hops to 4 without giving any numbers for those hops. Do the math yourself. I just knew nothing about the speed savings for reducing those 20 hops to 4. The math I was giving to you is not lying. As fnenu was giving an example where the answer is correct I was giving one where it is wrong. I asked a similar question, maybe my solution can help you out: http://www.udacity-forums.com/cs101/questions/43766/latency-question Yes but your numbers are not realistic, that example would never occur. I don't mean to be arguing back and forth with you pointlessly. The whole point is that the information processing done at routing points is always going to be relatively slowly compared to transmitting the information from point to that, if you didn't understand that before, hopefully you do now. Of course they are. Just read the post of XXnerd: @TooMuchIdeas, yes, 100 ms, you were suggesting 10ms. @curiousborg 100 microseconds are 0.1 milliseconds. I haven't seen that article before, I'll read it. I'll be very surprised if it concludes that fibre optic cables will ever be the bottleneck on any network. Edit: I don't know enough about the hardware to really comment on this, but there are other considerations including network load that will affect the latency. I'm pretty sure though that it's safe to say, as in the answer video, that networks are not limited by the speed of light but by the routing points. May you take the words from CISCO: "Latency with a hardware-assisted switch will be in the 4-to-20 microsecond range. The most reasonable processing delay that you can expect in practice should be 25 microseconds per hop. The processing delay on a software-based router can be considerably higher." http://www.cisco.com/application/pdf/en/us/guest/netsol/ns407/c654/ccmigration_09186a008091d542.pdf or may you should also read this article: http://www86.homepage.villanova.edu/christopher.salembier/documents/physics.pdf I'm inclined to think that Dave and Peter know what they are talking about, but nevertheless I'll certainly ask this question in the next office hours. "For the 100 ms journey in the videos, approximately 10 ms is taken at routers and 28 ms 'on the wire'" Where did you put the other 62 ms? It was an example for showing that routers latencies are not in the range of milliseconds, but microseconds.
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The answer to this question can be found on Unit 4 Lecture 12 (Latency). If you listen carefully to what David says, he stated that,
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I think this question should be thrown out. I got it "right" based on the example in the video, but the "right" answer assumes a few things that aren't right:
Here are some documents about latency in routers: This one compares IPv4 versus IPv6, but is still relevant. Note that the latency numbers for somewhat high-end routers (covered on the last page) is 28 microseconds. Of course this is a vendor measuring it, and they admit that the routers have pretty basic configs, but still, I bet that's orders of magnitude less than what our professor is assuming: http://www.cisco.com/web/strategy/docs/gov/IPv6perf_wp1f.pdf This one compares Cisco and Juniper, supposedly written by an impartial 3rd party. Notice the 16 microsecond latency in the chart on page 4. http://www.bradreese.com/core-router-comparison-cisco-juniper.pdf This ACM article says, "Modern routers such as the Cisco CRS-1 exhibit average latencies of about 100 microseconds." http://queue.acm.org/detail.cfm?id=1530063 So, I don't think the answer is as clear-cut as our professor assumes. Reducing the number of routers in a lot of cases won't help? What do others think? 2
I totally agree with you. It totally depends on the speed of the routers. If it falls under a given threshold cutting down the 20 hops to 4 it wouldn't be as much saving as a vacuum line. "Latency with a hardware-assisted switch will be in the 4-to-20 microsecond range. The most reasonable processing delay that you can expect in practice should be 25 microseconds per hop. The processing delay on a software-based router can be considerably higher." http://www.cisco.com/application/pdf/en/us/guest/netsol/ns407/c654/ccmigration_09186a008091d542.pdf |
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The speed of transmission will largely be based on the number of routers along the way, regardless of how they are interconnected. Correct, those connection points are slowing the whole thing down much more. Not really. Just assume the time a station has to work on a packet is 0, the number of stations do not matter at all! The answer was dealing with vacuum lines, so we have to also consider stations with a transmission time of 0. Additionally it was not ask to work in ALL cases. If you put just zero it doesnt make any sense. I can also say if the time the data needs per mile in Fibre optic is 0, it doesnt change anything if you change the wires. I got your point but it doesn't even make sense about asking If the line is a vacuum as it is not reality, but was still asked for. Got MY point? Instead of 0 just use lightspeed routers. In this case it also wouldn't matter (point to point) distance. @TooMuchIdeas You can't assume zero time at a station, since that's actually where most of the time gets lost! It could be reality its just very expensive. And if you JUST assume zero it JUST doesnt make any sense. Try 0.001 as a number and you could make real calculations. As there were no constraints regarding the speed of the stations I can assume it. I just want to point out, that the questions is ambiguous cause of the lack of those constraints. No constraints - All assumptions are possible. F.ex. The bandwidth question. If I assume there is a compression algorithm, which compresses all files to zero I can have 0 bandwidth ;-) And this is why there has to be constraints for such questions. @TooMuchIdeas No, I'm afraid that assumption does not make any sense. If you paid close attention to the video after the quiz in Unit4.18, the "constraints" you mention would have been clear to you. Then just assume other numbers as in the example above: 10ms for the routers. @TooMuchIdeas The time spent passing through the routers will always be longer than the time spent on the wires. We're talking about the real world here, where the information processing that takes place at routing points takes a lot longer than just sending bits from one point to another. The argument that we could assume that the routers work faster than the fiber optic cables is just silly. 1
@couriousborg It's not a silly argument and it's not true that the time spent in routers "will always be longer." Do the math for high speed carrier class routers (with microsecond latency). Also keep in mind that traceroute isn't a good test. Routers put ICMP packets in a low priotiy queue. In addition, retuning an ICMP TTL Exceeded message is more "expensive" (higher latency) than simply forwarding an IP packet. The information processing is minimal and the routers are built to optimize this task. In another comment you said you would bring this up in office hours. Thanks for doing that. @curiousborg Yes, we ARE talking about the real world here. And in the real world switching latency per hop is about 25 microseconds. I never said routers work faster than the fibers. I just said they are working in the range of microseconds and not milliseconds.
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My figures (rounded) where slightly differents, but the result is exactly the same. From the lectures you know that 75% of the travel time is lost in routers. You know (lectures) that vaccum speed doubles fiber optic. For a travel time of 100ms we have 75 ms in routers and 25 ms travelling. b) we still have 75 ms in the 20 routers + 25 / 2 ms replacing fiber optic = 87,5 ms c) we still have 25 ms of travelling + 75 ms / 5 reducing routers by 1/5 = 40 ms Then solution is c. |
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I think the distance between router to router matters. For example, in the lecture video, when 'tracerouting' on locations far apart, there were clear differences when one moved from a hop within the region to a hop outside that region. Wouldn't the replacement with vacuum, for instance between a router in the US and one in Antananarivo(madagascar) result into better transmission than lowering the number of routers? |
And I agree with you Mike. He made the point in the lecture/demo and it was a good one. And, that on top of the parenthetical note on the question, which seemed to be stressing the huge improvement.
Hmmm, I heard David say that if it were one vacuum - the question was to replace ALL fiber optics with vacuums - and that leaves still the 20 routers that have to handle the transfers. The in-between travel speed would be tremendous, but you still have to wait around while the router does its thing.
@clair He clearly states in the video that most of the latency is due to delays at routing points. The point is that the decrease in latency achieved by replacing the cables with vacuums would be negligible since the longest delays would still occur at routing points.
I fell for it too. My brain told me the answer was decreasing the routing points, but for some reason I took the question as a "trick" question and chose the vacuum answer because of what I thought I heard during the lecture. Got an 86% homework grade because of it :-) Listening to the video explanation, however, reducing the routing points is correct.
Yep - I fell for it also - Kicking myself now :/
@Joel. The same thing happened to me -- including the 86% grade. What still annoys me is that I think I would have gotten it right had he not unhelpfully qualified the statement of the question with his parenthetical remark: "Note: several of them would improve the latency a little bit, but only the one answer which would produce the biggest improvement is correct." I remembered him making such a big to-do about how dramatically faster vacuum tubes are that I didn't think they would only make a "little" improvement in the latency. So I picked that answer. However, when I went back to review his exact wording (see the Traveling Data question from Unit 4), I discovered that the point you and I both thought he had made (vacuum-tube connections are super fast) was very different from the point he had actually made (direct vacuum-tube connections are super fast; in fact, direct fiber-optic connections are super fast, too). I've been really happy with the course, so I hate to quibble, but I think the fact that so many people made that mistake means that he could have been a bit clearer in emphasizing that the true limiting factors are the routers, not the wires. Of course, having gotten the question wrong and really thought about why, I probably learned the material better than I otherwise would have!
I got the answer right but unfortunetly I clicked the wrong one when submitting the answer :\ I also got 86% because of that :\