# Why is there a minimum spacing?

#### henry

##### Distinguished

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.

G

#### Guest

##### Guest

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.

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.

G

#### Guest

##### Guest

Henry 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.
>

IIRC, the idea is that each connection to the cable causes an impedance
discontinuity. Spreading them out minimizes the problems caused by those
connections.

G

#### Guest

##### Guest

> 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

#### henry

##### Distinguished

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.

>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.
>

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.

G

#### Guest

##### Guest

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?

G

#### Guest

##### Guest

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.
> >
>
> 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

#### henry

##### Distinguished

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>

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.

G

#### Guest

##### Guest

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.
>
> 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.
>
> 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.

G

#### Guest

##### Guest

Rich Seifert wrote:

(snip)

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

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

>>>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.
>>>
>>
>>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

G

#### Guest

##### Guest

>> 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.

> 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.

G

#### Guest

##### Guest

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. ;-)

G

#### Guest

##### Guest

Rich Seifert <usenet@richseifert.com.invalid> said

>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.
>> >
>>
>> 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!

G

#### Guest

##### Guest

In article <re6bi1pu62s7ndelmdqgnoq77c7lil7fou@4ax.com>,
DHP <me@privacy.net> wrote:

>
> 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.
>

Actually, I did all of the analysis in the time-domain, rather than the
frequency-domain, although of course they are fully interchangeable.

I started where a communications systems designer SHOULD start--with a
requirement for a maximum bit-error rate (which translates into a
frame-loss rate). For the specified BER of 10^-9 (worst-case), using
Manchester encoding, the minimum signal-to-noise ratio turns out to be
14 db, which is a factor of 5:1. You then take the worst-case minimum
transmit level and attenuate it by the maximum amount possible
(worst-case cables, longest specified lengths) to calculate the minimum
received signal level. The allowable noise at that point must be no more
than one-fifth of the minimum received signal to achieve the desired BER.

(I could re-create the actual numbers, or even find my old notebooks if
I looked, but my point here is to show methodology, which should apply
to a wide variety of communications systems, rather than show the
specific numbers for a now-obsolete system like coaxial Ethernet.)

I then apportioned the allowable noise among the various contributors:
tap reflections, reflections from cable impedance variations, external
EMI, etc. The tap reflection allowance resulted in the specification for
maximum shunt capacitance and the "2.5 meter" rule. The cable impedance
allowance resulted in the specification for maximum deviation from
nominal impedance (50 +/- 2 ohms), and the rules for concatenating long
lengths from shorter pieces. The EMI allowance resulted in the
specification for transfer impedance of the cable shield (effectively

Our motto was always that the system had to work in the worst-case.
Sure, most environments were much more benign than we assumed for the
design criteria; those environments would experience a much better BER
than worst-case. But even the worst environment would behave acceptably.
When you are planning for millions of networks, and tens-of-millions of
installed devices, even 99.9% assurance means a lot of angry customers.

--
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

G

#### Guest

##### Guest

Sorry, should have said, I'm "Henry".

G

#### Guest

##### Guest

James Knott <james.knott@rogers.com> said

>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.

>> 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.

>> 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.

G

#### Guest

##### Guest

DHP wrote:

> That may give you data corruption but it shouldn't trigger the
> collision detector unless the level is ridiculous.

What's the difference, between two signals colliding and a signal and it's
reflection colliding?

G

#### Guest

##### Guest

DHP wrote:

(snip)

>>>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.

There is physics, and then there are rules. The rules are set so
that the system will work within the physical limitations.

In many cases the rules are more strict than necessary to make
them simpler. The 2.5m tap rule is simple to state, not too
restrictive for actual use, and allows the system to work.

In many cases you can't see all of the cable, so you couldn't
guarantee a minimum. With cable marked at 2.5m you can be
sure that if you tap at marks you meet the requirement.

For thin ethernet the rule is 0.5m minimum. As BNC cables
commonly come premade in lengths that are not multiples of
0.5m it is good that it isn't required to be multiples.
Also, in the thin ethernet case, the real restriction is
against the lumped impedance effect of many taps close
together as seen from some distance away. A very short cable
with a (relatively) large number of taps will work just fine.

-- glen

G

#### Guest

##### Guest

In article <Za2dnZTNBNErT7jeRVn-qQ@rogers.com>,
James Knott <james.knott@rogers.com> wrote:

> DHP wrote:
>
> > That may give you data corruption but it shouldn't trigger the
> > collision detector unless the level is ridiculous.
>
> What's the difference, between two signals colliding and a signal and it's
> reflection colliding?

In coaxial Ethernet (the subject of the original post), collisions are
detected by measuring the average DC voltage on the cable, NOT by
comparison between the transmitted and received signal. A tap reflection
does not change the average DC; thus, while it might cause data
corruption, it will never cause a false collision.

--
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

G

#### Guest

##### Guest

James Knott <james.knott@rogers.com> said

>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?

G

#### Guest

##### Guest

Rich Seifert <usenet@richseifert.com.invalid> said

>In article <re6bi1pu62s7ndelmdqgnoq77c7lil7fou@4ax.com>,
> DHP <me@privacy.net> wrote:
>
>>
>> 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.
>>
>
>Actually, I did all of the analysis in the time-domain, rather than the
>frequency-domain, although of course they are fully interchangeable.

>I started where a communications systems designer SHOULD start--with a
>requirement for a maximum bit-error rate (which translates into a
>frame-loss rate). For the specified BER of 10^-9 (worst-case), using
>Manchester encoding, the minimum signal-to-noise ratio turns out to be
>14 db, which is a factor of 5:1. You then take the worst-case minimum
>transmit level and attenuate it by the maximum amount possible
>(worst-case cables, longest specified lengths) to calculate the minimum
>received signal level. The allowable noise at that point must be no more
>than one-fifth of the minimum received signal to achieve the desired BER.

>(I could re-create the actual numbers, or even find my old notebooks if
>I looked, but my point here is to show methodology, which should apply
>to a wide variety of communications systems, rather than show the
>specific numbers for a now-obsolete system like coaxial Ethernet.)
>
>I then apportioned the allowable noise among the various contributors:
>tap reflections, reflections from cable impedance variations, external
>EMI, etc. The tap reflection allowance resulted in the specification for
>maximum shunt capacitance and the "2.5 meter" rule. The cable impedance
>allowance resulted in the specification for maximum deviation from
>nominal impedance (50 +/- 2 ohms), and the rules for concatenating long
>lengths from shorter pieces. The EMI allowance resulted in the
>specification for transfer impedance of the cable shield (effectively
>
>Our motto was always that the system had to work in the worst-case.
>Sure, most environments were much more benign than we assumed for the
>design criteria; those environments would experience a much better BER
>than worst-case. But even the worst environment would behave acceptably.
>When you are planning for millions of networks, and tens-of-millions of
>installed devices, even 99.9% assurance means a lot of angry customers.

I appreciate the design philosophy! I was just trying to get a handle
on why you went for multiples of 2.5m rather than have it as a simple
minimum.

G

#### Guest

##### Guest

DHP <me@privacy.net> wrote:
>on why you went for multiples of 2.5m rather than have it as a simple
>minimum.

It's not a minimum or maximum, it's the spacing that minimizes
reflection-based bit errors. 2M and 3M (for instance) are both worse
than 2.5M.

G

#### Guest

##### Guest

DHP wrote:

(snip regarding tap spacing on thick ethernet)

> I appreciate the design philosophy! I was just trying to get a handle
> on why you went for multiples of 2.5m rather than have it as a simple
> minimum.

Having actually put taps into cables in cable trays and suspended
ceilings, it is sometimes hard to know where the other taps are.
Sometimes I have done it by feel when I could barely see the cable.
(Well, enough to know it was the right one.)

Then again, it is hard to know that there aren't more than 100.

As previously discussed here, random spacing would be even better,
but guaranteeing it is hard, and it seems that making a machine to
mark cables at random spacings is also hard.

-- glen

G

#### Guest

##### Guest

James Knott <james.knott@rogers.com> said

>DHP wrote:
>
>> That may give you data corruption but it shouldn't trigger the
>> collision detector unless the level is ridiculous.
>
>What's the difference, between two signals colliding and a signal and it's
>reflection colliding?

A collision actually means two units transmitting at once, not two
signals superimposing. So a true collision results in two
similarly-sized signals superimposing. This can be detected almost
instantly by purely electrical means.

The reflections we are talking about here are much smaller than the
original signal so they don't trigger the collision detection
circuitry. They produce quasi-random noise which occasionally causes a
data bit to be mis-read. The corruption won't be detected by this
layer but will be spotted by the next layer when it does a CRC check.

That's what I meant by the level being ridiculous. You can create huge
reflections by not terminating the line. In that case you may very
well get the collision detection circuitry triggering - when the
transmitting unit gets its own data back. You could even get it
happening if it was the only unit on the line. But it's a bit academic
as the data would be so coruupted that CRC would not be able to mend
it.

Is that what you wanted to know?

#### henry

##### Distinguished

James Knott <james.knott@rogers.com> said

>DHP wrote:
>
>> That may give you data corruption but it shouldn't trigger the
>> collision detector unless the level is ridiculous.
>
>What's the difference, between two signals colliding and a signal and it's
>reflection colliding?

One's the sound of one hand clapping, the other's a clash of symbols