Vadim Antonov wrote:
Richard Irving wrote:
Nathan Stratton wrote:
On Thu, 11 Sep 1997, Vadim Antonov wrote:
Quote from Jim Steinhardt's <jsteinha@cisco.com> personal message:
a) light is slower in dense media
The index of refraction of glass is 1.5 vs 1.0 for a vacuum. Hence, the speed of light in glass is 2 * 10 **8 m/s.
Ok, now we have some information to work with. People have mailed me with all sorts of data. I can only hope to rely upon it for this argument. C = 299,792,458 Meters per second. - or - C = 186,355 Miles per second. Now for 4000 miles that is: 20 ms Now, one gentleman posted that North America is really only about < 2500 miles across, so round trip would be 2 x 2500 miles, or ~ 5000 miles. (~ means approx.) 4000 Miles on my part was just a wild gross figure pulled out of the air late at night....... And, we were useing 4000 miles *one way* in our earlier calculations. So inserting real data, versus bogus assumptions, we have .030 or 30 ms in a vaccuum.
Refractive Index for Vaccuum : 1.0 Refractive Index for Glass: 1.5
Now lets insert the refractive index, and we get an estimated journey time at the speed of light at 45ms, through glass. (Round Trip) Now, another individual was so polite as to send in his actual ping timings. They were ~ 74 ms. (I think the gentleman was mailing in a 1 hop ping. Point to Point, cross continent, but it was live, and it is an accurate number) So now we have a theoretical RTT of 45 ms, through glass, and a real RTT of 74ms. This implies that 39% of the actual timing is overhead. ---Or------ The glass accumlates 39% more length due to the reality of the sonet physical. Certainly a possibility.... But at the price per mile for that stuff.... I don't know. However, I am having a cross nation circuit delivered, and a condition of the sale was to know to the actual length of the physical, to the mile..... (I always did have to experiment... ;)
That gives 60 ms RTT on 4000 mile line.
Uhhh, please see the above for a little more accurate set of numbers.
You didn't understand, did you? All your calculations were to disprove Sean's point that the electronics and switching delays are pretty small (so as to be insignificant) as compared to the signal propagation delay. Your (wildly inaccurate) estimate was that more than 50% of time is spent in electronics.
My wildly innacurate was an "off the cuff", not researched. (My hypothesis is maturing ) However, 39% does not stand that far off from 50%, as to be *wildly* innacurate. You are right, there is way more delay than I expected. Over the years, I have heard all sorts of claims to be "limited by the speed of light", including we couldn't get much faster processors than the 386 33Mhz , due to the "speed of light". (* snicker* ) I had to question that one, I couldn't help it. I should have been raised in the show-me state, really. ;) I have to admit, to some extent, that it was bait...... I learned. (Which was the point of the exercise) I still say that we are comparing "perfect world" core routes, One or Two hops cross-continent. I am still interested in what happens when we get multiple hops. The typical route through the internet is about 7 or 8 hops for clients. (Your mileage may vary)(Please no flames because your median hops are more , or less, it really is not that important.) I would be interested in "accumulated hop latency" statistics. ATM Switch versus Router. But, no. I never expected the attack I received regarding the speed of light issue. I was hoping for something of this nature, a presentation of argument.... I did not expect, however, the information to come in with such /*gusto*/. But, the original point was regarding ATM... Not the speed of light. Remember ATM ? Light was a sub-issue. However, am I to conclude that better QOS, than exists today, cannot be achieved ? That the router is capable of delivering QOS better than ATM switches ? Or, are we trying to conclude that all these loss and variability issue's cannot be resolved because of the speed of light ? The original posture still stands, (about ATM), for now.... And No, I am not so dense as to "not get it". I will just have to take a scientists approach... Show me. If someone has actual "published" data, that is not of questionable origin, my quest for truth would be over. Otherwise, just to prove it to myself, I am going to test that circuit. ;) Richard
The real figures, however, show that it is at most 5%. I strongly suspect that if you figure in that fibers aren't going in straight line, you'll get that down to 0.1-0.5%.
--vadim