Why is there a minimum spacing?

I wonder if anyone can give a definitive answer as to why there is a
minimum spacing specified on (some) ethernet cable. The thick stuff
with markers every 2.5m for example which is 1 bit of delay at 10 MHz.

There is some mention of it on various web sites but the reasons for
it are not stated. Maximum lengths etc are simple enough to
understand: you need to be sure that collisions are not late. The only
reason I can think of for specifying a minimum distance is to maximise
the effect of a collision when two MAUs start transmitting at the same
time. Only I can't see that it would. They won't actually start
together. If they're waiting for the line to become free, the last
data going past them will make sure one starts after the other. So the
second will start up at the eaxct moment the first's one's data
arrives. So it will experience a zero time-difference collision. The
first one will have a two bit difference. Even if there's an advantage
in that - which I don't understand -it assumes exactly one 2.5m
section of cable. But the 2.5m is only a minimum: the spec doesn't
require exact multiplesof 2.5m over hundreds of metres! So I'm racking
my brains as to why it was ever specified at all.

Posted by William P. N. Smith on September 12, 2005, 6:06 am

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Our own Rich Seifert certainly can, but IIRC it has to do with keeping
impedance discontinuities caused by taps far enough apart that they
don't reinforce each other.

news is your friend.

Posted by Henry on September 12, 2005, 12:33 pm

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William P. N. Smith <> said

Oops, just realized there's a factor of 10 missing there. My attempted
guess at the reasoning was wrong... The mystery deepens.

Thanks. I've found some stuff from Rich Seifert going back to
1980-something which explains it, sort of, though it's a bit woolly -
not Rich's explanation but the thinking behind it.

Posted by Rich Seifert on September 12, 2005, 8:05 am

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As you realized, one bit-time at 10 Mb/s is 100 ns, which corresponds to
23.5 m of coaxial cable.

The basic problem is that transceiver taps appear to the transmission
line as discrete, lumped capacitive loads; the specification mandates a
maximum of 4 pf, but this is still significant. When the signal
encounters this capacitance, it creates an out-of-phase reflection of a
portion of the energy. To all other devices on the cable, this
reflection appears as asynchronous "noise," i.e., a signal that
interferes with the desired signal.

The situation to be avoided is where all of the transceiver taps are
spaced such that the reflections from each of them add up in phase, thus
combining *algebraically* (i.e., simple summation). The small reflection
from 99 transceivers added up could create enough interference to cause
bit errors. Ideally, one would want the transceivers to be *randomly*
spaced along the cable; this would ensure that the reflections added not
algebraically, but on a root-mean-squared basis, yielding much less
reflected energy. In fact, my original proposal was to do exactly that;
I even had a patent application prepared for a method of manufacturing
cables with randomly-distributed markings for this purpose!

As it turns out, random markings were neither practical (installers
didn't like the idea, and neither did the cable manufacturers) nor
necessary. I did extensive simulations of the resulting reflections from
transceivers at various spacings, and empirically determined that 2.5 m
was "good enough." It was relatively easy to mark the cables with a
uniform 2.5 m marking; as the cable comes flying out of the extruder, it
passes across a roller with a 2.5 m circumference, which places a mark
at every rotation.

The idea is not just a *minimum* 2.5 m spacing; it is that transceivers
are only placed at the 2.5 m markings. However, as another poster noted,
it's not all that critical; if a few transceivers are offset, or even
lumped together, it is unlikely to cause a noticeable problem. I was
just trying to design for the worst-case, figuring that it would surely
show up *somewhere*, and that one installer would have no idea what the
problem was.

By the way, that cable-spacing work, along with the work that defined
the proper lengths to use for concatenating short coaxial cables into
long runs, constituted a major part of my EE master's thesis some 25
years ago.

--
Rich Seifert              Networks and Communications Consulting
                          21885 Bear Creek Way
(408) 395-5700            Los Gatos, CA 95033
(408) 228-0803 FAX

Send replies to: usenet at richseifert dot com

Posted by glen herrmannsfeldt on September 12, 2005, 3:25 pm

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Rich Seifert wrote:

(snip)

I am not sure how accurate the velocity factor is, but...

Constructive interference would result from a half wavelength spacing,
so 11.75m. A 500m cable could have 43 taps with that spacing,
which could be significant. If you put 44 taps equally distributed
over the same distance they will pretty much cancel each other out.
If you put 43 taps spaced at 11.75m and the velocity factor is
off by 2% they also pretty much cancel out.

The first odd multiple of 11.75m that is close to a multiple of 2.5m
seems to be 82.25m.

It seems to me very unlikely that, unless someone intentionally spaced
them at 11.75m that they would cause problems, but it is nice to have
a rule with a known effect.

-- glen

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Ethernet LAN Why is there a minimum spacing?