At 10:20 PM 10/6/00, whatshakin wrote:
>This makes it sound like there is actually something tangible being put on
>the wire. Bits are merely ones and zeros which are signaled by different
>voltages etc in the line encoding.
>
>Bits do not occupy line space.
Sure they do. Wasn't it Einstein that said time and distance are related?
(Don't quote me on that.) Seriously, the discussion of how much space a bit
takes on the wire has been going on since Ethernet was invented. See
Optimized Engineering's Technical Compendium for a discussion that brings
up the space issue.
http://www.optimized.com/COMPENDI/EN-Propa.htm
And here's some more related trivia. Do you know why a 32-bit jam is sent
when a station detects a collision? It's to increase the time and distance
of the collision event. It's to avoid the case where the collision happens
right next to you and you've stopped transmitting by the time the collision
event hits a repeater that could be 500 meters away.
Why didn't they use a 16-bit jam? Because on thick coax Ethernet cable the
signal travels at 231,000 kilometers per second. This means a bit occupies
23.1 meters on thick Ethernet. 16 x 23.1 wouldn't have been enough. 32 x
23.1 is 739 meters. An extension of 32 bits allows the sender to busy out a
maximum 500-meter segment. This explains both the jam size and the fact
that a repeater extends a received fragment by at least 32 bits.
I know this is a strange way of looking at things, but it is one way that
engineers, including the inventors of Ethernet, looked at the Ethernet
parameters.
Priscilla
>Measurements of how fast data can be moved over a wire are the time it takes
>for a signal at one end to be heard at the other. The amount of data
>(signals) which can be moved across a wire is ascertained by the line
>encoding method, and how many signals the encoding system can be made to
>produce in a second. Minus the delay factors between point A and B of
>course.
>
>I seem to recall reading some papers from folks at the US Berkley computer
>science dept a few years back that researched the various line encoding
>techniques etc that were quite interesting. I cannot find them now that I
>need them though!!
>
>BTW, my calculations for the speed of light resulted in 299,793,100 m/s
>
>
>----- Original Message -----
>From: Leigh Anne Chisholm <[EMAIL PROTECTED]>
>To: Kevin L. Kultgen <[EMAIL PROTECTED]>; Tim O'Brien
><[EMAIL PROTECTED]>; <[EMAIL PROTECTED]>
>Sent: Friday, October 06, 2000 4:15 PM
>Subject: RE: Ethernet Trivia
>
>
> > Ahh Kevin, your post reminds me of some research I did when I was putting
>together my paper on LAN Switching for CertificationZone. I was looking at
>how to calculate the round-trip propagation delay for 10BaseT networks.
>Here's a few technical numbers for you you (and possibly other Groupstudy
>members) might find interesting.
> >
> > --- Beginning of Calculations ---
> >
> > Electrical signals travel in a copper wire travel (propagate) at
>approximately two-thirds the speed of light. Remembering that the speed of
>10 Mbps Ethernet is 10,000,000 bits/second, we can determine the length of
>wire that one bit occupies, by using the following calculation:
> >
> > Speed of Light in a Vacuum = 300,000,000 meters/second
> >
> > Speed of Electricity in a Copper Cable = 200,000,000 meters/second
> >
> > 20,000,000 meters/second / 10,000,000 bits/second = 20 meters per bit
> >
> > The minimum size Ethernet frame consisting of 64 bytes (512 bits) occupies
>10,240 meters of cable.
> >
> > --- End ---
> >
> >
> > -- Leigh Anne
> >
> >
________________________
Priscilla Oppenheimer
http://www.priscilla.com
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