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Henry
Guest
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 Posted:
Mon Sep 12, 2005 2:27 pm Post subject:
Why is there a minimum spacing? |
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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. |
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William P. N. Smith
Guest
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| Posted:
Mon Sep 12, 2005 3:06 pm Post subject:
Re: Why is there a minimum spacing? |
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Henry <me@privacy.net> wrote:
| Quote: | 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.
|
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.
{google,deja} news is your friend. |
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Henry
Guest
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| Posted:
Mon Sep 12, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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William P. N. Smith <> said
| Quote: | Henry <me@privacy.net> wrote:
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.
|
Oops, just realized there's a factor of 10 missing there. My attempted
guess at the reasoning was wrong... The mystery deepens.
| Quote: | 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.
{google,deja} news is your friend.
|
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. |
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James Knott
Guest
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| Posted:
Mon Sep 12, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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Henry wrote:
| Quote: | 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.
|
IIRC, the idea is that each connection to the cable causes an impedance
discontinuity. Spreading them out minimizes the problems caused by those
connections. |
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James Knott
Guest
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| Posted:
Mon Sep 12, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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Henry wrote:
| Quote: | 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.
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Why not post the link here? |
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Henry
Guest
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| Posted:
Mon Sep 12, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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James Knott <james.knott@rogers.com> said
| Quote: | Henry wrote:
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.
Why not post the link here?
|
I was going to do exactly that but I tried to retrace my search and
couldn't find it :( I may have been mixing two posts in my head.
Still, this is short and simple:
<rich-ya023060042802011315570001@nntp.ix.netcom.com>
Unfortunately it seems 802.3 is ambiguous (anyone got the thing?) and
can't make up its mind whether 2.5m is a minimum or whether you're
supposed to tap in ONLY at multiples of it.
<sysrick.713115865@starbase.spd.louisville.edu>
The fact it talks about non-alignment suggests someone must have
thought there were significant and potentially troublesome components
in the waveform up to 100s of MHz. I can believe that, since the small
mismatches in resistive impedance become very large mismatches,
effectively a short circuit, for very fast edges which encounter a
capacitive tap. However, then it doesn't make sense to insist on using
exact multiples as this will tend to create standing waves.
But at least I know it's nothing to do with collision detection. |
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Guest
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| Posted:
Mon Sep 12, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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| Quote: | 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.
|
According to the Spurgeon book, the spacing is a guideline to help
avoid signal reflections resulting from too many transceiver taps being
clumped together.
He goes on to say that maintaining an even 2.5m spacing isn't critical:
when joining cable sections you can ignore the marks, and if two taps
happen to land close together when cables are joined, that's okay too.
/chris |
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DHP
Guest
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| Posted:
Mon Sep 12, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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googlegroups@marget.com said
| Quote: | 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.
According to the Spurgeon book, the spacing is a guideline to help
avoid signal reflections resulting from too many transceiver taps being
clumped together.
He goes on to say that maintaining an even 2.5m spacing isn't critical:
|
I can't see why it would matter even an itsy-witsy little bit. But
that's where people seem to have different theories.
| Quote: | when joining cable sections you can ignore the marks, and if two taps
happen to land close together when cables are joined, that's okay too. |
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Rich Seifert
Guest
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| Posted:
Mon Sep 12, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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In article <jlmai1pi08p6do3jo0p0veditvn4ra7v8m@4ax.com>,
Henry <me@privacy.net> wrote:
| Quote: | William P. N. Smith <> said
Henry <me@privacy.net> wrote:
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.
Oops, just realized there's a factor of 10 missing there. My attempted
guess at the reasoning was wrong... The mystery deepens.
|
As you realized, one bit-time at 10 Mb/s is 100 ns, which corresponds to
23.5 m of coaxial cable.
| Quote: | 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.
{google,deja} news is your friend.
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.
|
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 |
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DHP
Guest
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| Posted:
Mon Sep 12, 2005 4:20 pm Post subject:
Re: Why is there a minimum spacing? |
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Rich Seifert <usenet@richseifert.com.invalid> said
| Quote: | In article <jlmai1pi08p6do3jo0p0veditvn4ra7v8m@4ax.com>,
Henry <me@privacy.net> wrote:
William P. N. Smith <> said
Henry <me@privacy.net> wrote:
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.
Oops, just realized there's a factor of 10 missing there. My attempted
guess at the reasoning was wrong... The mystery deepens.
As you realized, one bit-time at 10 Mb/s is 100 ns, which corresponds to
23.5 m of coaxial cable.
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.
{google,deja} news is your friend.
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.
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.
|
Hi Rich, thanks for all that.
Could I now quiz you a bit more? 4pF on a 50 ohm system gives a
characteristic time of some 200ps or a frequency of about 800MHz. So
I'm guessing (having forgotten the theory ages ago), without doing a
phasor diagram, that you'd get a reflection coefficient ~f/800 for
each component. But at the same time, you only need to worry about
reflections that interfere constructively, i.e. over about half a
wavelength = 117m/f.
So if the allowable reflection is 5%, the number of taps in 117/f m of
cable is 5/100 * 800/f, which is about 1 tap per 2.5m, though there
should be the odd fudge factor to upset the convenient result. Anyway
I can see the point of having a lowish average density of taps! Would
I be right in thinking that the requirement to place taps at equal
spacing is a result of needing to cater for the higher frequencies?
My thinking is that the allowable density of taps taken over a
fraction of a wavelength brings you down to just a small handful of
taps so you may as well just space them equally rather than worsen the
noise with a cluster? Is that the "real" criterion - to avoid clusters
over short distances? It would seem to assume that NICs are sensitive
to out-of-band noise.
Thanks for your time in answering this, it seems to crop up regularly
- though the google archive seems to peak in the early 90's :)
Oh yeah, my Masters is even older than yours! |
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DHP
Guest
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| Posted:
Mon Sep 12, 2005 9:39 pm Post subject:
Re: Why is there a minimum spacing? |
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| Sorry, should have said, I'm "Henry". |
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James Knott
Guest
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| Posted:
Mon Sep 12, 2005 9:53 pm Post subject:
Re: Why is there a minimum spacing? |
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Henry wrote:
| Quote: | James Knott <james.knott@rogers.com> said
Henry wrote:
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.
Why not post the link here?
I was going to do exactly that but I tried to retrace my search and
couldn't find it :( I may have been mixing two posts in my head.
Still, this is short and simple:
rich-ya023060042802011315570001@nntp.ix.netcom.com
|
That doesn't take me anywhere.
| Quote: |
Unfortunately it seems 802.3 is ambiguous (anyone got the thing?) and
can't make up its mind whether 2.5m is a minimum or whether you're
supposed to tap in ONLY at multiples of it.
sysrick.713115865@starbase.spd.louisville.edu
|
It's minimum distance, though the old thicknet cables had specific points
marked on the sheath, where a vampire tap could be attached.
| Quote: |
The fact it talks about non-alignment suggests someone must have
thought there were significant and potentially troublesome components
in the waveform up to 100s of MHz. I can believe that, since the small
mismatches in resistive impedance become very large mismatches,
effectively a short circuit, for very fast edges which encounter a
capacitive tap. However, then it doesn't make sense to insist on using
exact multiples as this will tend to create standing waves.
But at least I know it's nothing to do with collision detection.
|
Only in that impedance discontinuities will create standing waves, which can
interfere with the signal. |
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James Knott
Guest
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| Posted:
Mon Sep 12, 2005 9:58 pm Post subject:
Re: Why is there a minimum spacing? |
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DHP wrote:
| Quote: | He goes on to say that maintaining an even 2.5m spacing isn't critical:
I can't see why it would matter even an itsy-witsy little bit. But
that's where people seem to have different theories.
|
In theory, practice follows theory. In practice, it doesn't. ;-) |
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DHP
Guest
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| Posted:
Mon Sep 12, 2005 10:50 pm Post subject:
Re: Why is there a minimum spacing? |
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James Knott <james.knott@rogers.com> said
| Quote: | Henry wrote:
James Knott <james.knott@rogers.com> said
Henry wrote:
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.
Why not post the link here?
I was going to do exactly that but I tried to retrace my search and
couldn't find it :( I may have been mixing two posts in my head.
Still, this is short and simple:
rich-ya023060042802011315570001@nntp.ix.netcom.com
That doesn't take me anywhere.
|
Pop it into the box at the bottom of Google Groups Advanced Search -
minus the angle brackets.
| Quote: | Unfortunately it seems 802.3 is ambiguous (anyone got the thing?) and
can't make up its mind whether 2.5m is a minimum or whether you're
supposed to tap in ONLY at multiples of it.
sysrick.713115865@starbase.spd.louisville.edu
It's minimum distance, though the old thicknet cables had specific points
marked on the sheath, where a vampire tap could be attached.
|
Ah, well, Rich Seifert has joined the thread and says otherwise.
| Quote: | The fact it talks about non-alignment suggests someone must have
thought there were significant and potentially troublesome components
in the waveform up to 100s of MHz. I can believe that, since the small
mismatches in resistive impedance become very large mismatches,
effectively a short circuit, for very fast edges which encounter a
capacitive tap. However, then it doesn't make sense to insist on using
exact multiples as this will tend to create standing waves.
But at least I know it's nothing to do with collision detection.
Only in that impedance discontinuities will create standing waves, which can
interfere with the signal.
|
That may give you data corruption but it shouldn't trigger the
collision detector unless the level is ridiculous. |
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DHP
Guest
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| Posted:
Mon Sep 12, 2005 10:52 pm Post subject:
Re: Why is there a minimum spacing? |
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James Knott <james.knott@rogers.com> said
| Quote: | DHP wrote:
He goes on to say that maintaining an even 2.5m spacing isn't critical:
I can't see why it would matter even an itsy-witsy little bit. But
that's where people seem to have different theories.
In theory, practice follows theory. In practice, it doesn't. ;-)
|
At the risk of getting controversial, where do standards fit in? :) |
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