ATM (was Re: too many routes)
rirving at onecall.net
Fri Sep 12 23:01:55 UTC 1997
Vadim Antonov wrote:
> Richard Irving wrote:
> > Nathan Stratton wrote:
> > >
> > > On Thu, 11 Sep 1997, Vadim Antonov wrote:
> > >
> > > > Quote from Jim Steinhardt's <jsteinha at 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:
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
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.
The glass accumlates 39% more length due to the reality of the
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
> 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*
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
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.
> 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%.
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