| Author |
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Robert Redelmeier
Guest
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 Posted:
Fri Apr 29, 2005 4:20 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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In comp.dcom.cabling Floyd L. Davidson <floyd@barrow.com> wrote:
| Quote: | Robert Redelmeier <redelm@ev1.net.invalid> wrote:
In comp.dcom.cabling Floyd L. Davidson <floyd@barrow.com> wrote:
It doesn't have to be. Frequency is *not* what determines
whether it will interfere or not.
Then what does? Voltage?
The parameter that is modulated or encoded is the only
parameter which suffers interference.
|
Yes, but I didn't ask what was interfered with.
I asked what determines interference.
My answer is current. Most particularly HF changes in current.
Especially with sharp edges that have high freq components.
Voltage has nothing to do with it except as it _might_
increase current. But with modern semiconductor telephone
sets, I doubt even a peizoelectric ringer generates the varying
current (&arcing!) that a mechanical contact bellringer does.
| Quote: | Essentially, voltage noise can be filtered out *if* it has
some other parameter that allows you to selectively reduce
its level while not affecting the desired signal level.
|
Which is what balanced signals and twisted pair media provide.
| Quote: | Current causes induction. The field induced results in
a *voltage* being detected at the receiver (otherwise,
it doesn't exist!).
|
Not quite. Current produces a magnetic field. A _changing_
magnetic field induces voltage in a separate conductor that leads
to secondary current. So only changes in current cause induction.
| Quote: | Inside a shared sheath that is properly wired yes, and
that is *exactly* what the specifications are intended
to provide. The point here was originally stated as what
the effect will be if there are kinks or other damage to
the cable or miswired connectors that split a pair.
|
I didn't catch any reference to split pairs. They are horrible!
(Although splitting voice isn't as bad as splitting data).
Anyone pushing to the limit with shared sheath had best know what
they're doing.
-- Robert |
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Floyd L. Davidson
Guest
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| Posted:
Fri Apr 29, 2005 4:20 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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Tomi Holger Engdahl <then@solarflare.cs.hut.fi> wrote:
| Quote: | floyd@barrow.com (Floyd L. Davidson) writes:
True, but the typical *telephone* cable was CAT1 quality, not
CAT3 (and higher) until 10baseT came along.
What the typical telephone cable indide house is depends on the
country and the year when the building is built. Some countries
recommended better cabling than CAT1 on 1980's for example to
be prepared to future technologues (ISDN at that time).
Some of the cable installed to offices and residential buildings
from 1980's on has been pretty high quality (CAT3 rating or almost
like it). I know the situation in Finland best. I wrote one
paper for one course at Helsinki University of technology on ISDN
technology (around 1995), and when writing on that I read quite carefully
one publication from Helsinki telephone company that described
the suitablility of existing wirings for ISDN use.. gave idea
what kind of cable was use on buildings in the last decade or two..
|
Exactly. And you can perhaps remember that it didn't say that
CAT3 was the typical standard prior to about 1990. And for that
matter, it might even have mentioned what the ISDN acronym stands
for? "It Still Does Nothing"
Unfortunately the telecom industry as a whole simply ignored
ISDN, mostly because as a unit the industry was simply ignorant
of data networking and data communications. The typical
response to questions about implementation of ISDN was "Who's
going to pay for it? There simply is no market."
Of course for anyone who had half a thimble full's exposure to
computer networking at the time, that was the sound of pure
idiocy! And it was *exceedingly difficult* to deal with that
attitude throughout the industry while watching the market they
claimed not to exist went to others.
The telecom industry did not respond to the need for data
communications, and instead the modem industry went from 1200
bps modes to 2400, then it v.32, V.34, and finally to v.90, and
reaped millions, while the telephone companies all told
themselves that nobody needed or would buy a data line faster
than 1200 bps!
Not that it was equally bad everywhere, but there was no
momentum even where it was tried (Europe, for example), and it
was almost universally as bad as it gets virtually everywhere in
the US.
| Quote: | In Finland some people are using the old telephone wiring to run
10Base-T or 100Base-TX Ethernet on them for networking purposes.
There is information on that at http://www.helsinkiopen.net/
|
Sure. It will work at reduced capability. But if you want to
hit the limits (on things like distance and BER numbers), you
can't do that. (I used 75 ohm coax for ethernet for years,
because I had access to lots of it, and did not have any needs
that stressed the specs. I certainly would not recommend that
to anyone with critical requirements though!)
| Quote: | At some countries there has been practice to install higher
quality cable, and some used the cheapest you can get
(I have see some installations in Russia and i doub that
if they world even qualify even as CAT1).
|
Europe, and particularly in your part of Europe, has virtually
always had somewhat higher standards for local loops than those
used in the US. But of course all of us have seen poor
implementations... Someday I'll describe the telephone system I
saw, in a guy's damp basement, in Eagle Alaska. ;-)
| Quote: | Virtually *all* essential spectral components are at 5 and 10
MHz. The transmitter may generate a square wave,
The transmitted signal on 10Base-T is essentially square wave
with some pre-distortion added to it and then filtered through
the low pass filter that revoves all highest frequency components
(so that system passes the noise limits on above 30 MHz noise).
So the the signal from 10Base-T transmitter can be viewed as
slightly distorted swuare wave signal.
|
But that has nothing to do with the "essential spectral
components"!
All that is required are two sine waves, one at 5 MHz and one at
10 MHz, and the switching transients produced by keying from one
to the other. That is essentially 180 degree phase change at
the start of a bit, so it could be looked at as phase modulation
too. (I'm not sure what the modulation products would look
like, does 5 MHz and 10 MHz with harmonics sound right??? :-)
| Quote: | but the
receiver only requires a sine wave, therefore none of the other
components have significance at the receiver (or anywhere else).
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*That* is the point which is essential to the previous discussion,
not what the transmitted signal looks like.
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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Floyd L. Davidson
Guest
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| Posted:
Fri Apr 29, 2005 4:20 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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James Knott <james.knott@rogers.com> wrote:
| Quote: | Floyd L. Davidson wrote:
Simple; ringing is 300+ V peak to peak...
And could very easily have components at 1GHz!
Harmonics at 1 GHz from a 20 Hz signal???? That would be the 50 millionth
harmonic! Not likely.
Who said anything about harmonics?
Well, we were talking about ringing current which is normally 20 Hz.
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And not a word about harmonics...
| Quote: | You
said it could have "components" at 1 GHz. What could those components be,
if not harmonics.
|
Everything from switching transients to power supply noise.
| Quote: | Ringing generators may not produce perfect sine waves, but I'd
be very surprised, if they produced measurable amounts of energy at 1 GHz.
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Then you'll be very surprised.
| Quote: | A ringing current is
simply a 90 volt RMS sine wave at 20 Hz. It does not normally produce
noise or spikes of comparable amplitude.
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Don't bet the farm on that...
| Quote: | It
would take an enormously powerful signal in the audio range to have much
effect on ethernet.
|
I'd say that a 300 volt P-P signal is in fact "an enormously
powerful signal", compared to the desired signal, wouldn't you?
| Quote: | Please cite the specs for a form of ethernet where the receiver
can handle +/- 300 volt burst of random noise hits.
Perhaps Rich can provide the relevant specs, but ethernet cards must, for
safety reasons be able to sustain such voltages, without creating a safety
hazard.
|
So, you can't cite any spec to support what you've said.
Off hand I don't know what the dynamic range of the receiver
input is, but the desired signal is a maximum of 2.8 V P-P, so I
can't imagine that the dynamic range is required to be much more
than twice that. However, it should also be clear that *any
voltage* greater than 1.4 V at the receiver input is going to
absolutely interfere with the ability of the receiver to detect
a valid crossover in the desired signal.
Obviously a 300 P-P burst of noise has to be greatly attenuated
before it reaches the receiver, or it will cause errors. The
60+ dB of common mode rejection resulting from properly balanced
UTP transmission facilities is the major protection against ring
current. An impaired facility is not immune to interference
from 20 Hz ringing voltage from telephone equipment just because
it is not in the same frequency range as the baseband for
10baseT.
| Quote: | However, normal telephone ringing and voice does not produce a
300V burst of random noise hits.
|
Wanna bet the farm on that?
The ring voltage is also superimposed on the "-48 VDC" battery,
which can actually be anything from 24VDC to as much as 70-80
VDC. So add that to the 300 P-P signal. Obviously the *normal*
range of voltage can commonly approach 200 V absolute, and
typically spikes occur that go *much* higher.
| Quote: | Now lets get down to some technical
details.
....
"Current loop data" at 100bs that doesn't interfere with voice?
You're leaving something out. If you would like an example of
that, just pick up any phone that has a tone/pulse switch, and
switch it to pulse. Give it a try and see if you think 10 pps
is not interfering.
Use split winding transformers, to couple the data line to the voice line.
The two halves of the split winding are connected at voice frequencies, by
capacitors, to bypass the data line. That capacitor appears as a high
impedance to the data circuit. The data currents through the two halves of
the windings are such that they cancel out any interference they might
otherwise cause.
Here's a crude drawing. The "X" represents the capacitor
__________ ________________ Combined voice and data
) (_________
Voice ) _X______ Data
_________ ) (________________
|
Nice drawing. Let me show you how to accomplish that:
| Quote: | -----)||(--------------> T
)||( |
)||(
)||(----+---------> A
)|| |
)|| -----
)|| ----- 1Mfd
)|| |
)||(----+---------> B
)||(
)||(
| Quote: | -----)||(--------------> R
|
This circuit is of course so commonly used in telephone
signaling equipment that everyone has seen it a million times.
It appears to be very simple, and yet making it work properly is
not at all simple. (And no I'm not going to go into details.)
| Quote: | When you click the hook switch or use pulse dial, you're actually opening an
closing the voice circuit, as those switch contacts are wired in series
with the voice path.
|
The "off-normal" contacts short circuit the telset. That has
two advantages, one of which is to remove the extremely annoying
pulse interference from the receiver. But the other is to
remove the DC resistance of the telset from the loop to improve
the pulse characteristics.
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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Floyd L. Davidson
Guest
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| Posted:
Fri Apr 29, 2005 4:20 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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James Knott <james.knott@rogers.com> wrote:
| Quote: | Floyd L. Davidson wrote:
The intent is juxtapositioned. CAT3 cable was designed for
10baseT, and allows VF circuits to ride on the two unused pairs.
"CAT 3" refers to a quality standard, not number of pairs.
|
Who said *anything* about it referencing the number of pairs?
Just as I said above, the extra pairs are intended to be used
for voice pairs. We've been discussing the use of RJ-45
connectors, which have 8 pins for 4 pairs, and the above comment
of mine simply refers to that configuration, and has *nothing*
to do with what you are responding to. Regardless, most of
your response if incorrect anyway...
| Quote: | While 4 pairs is
commonly used for ethernet, 25 pairs is used in central offices to connect
24 circuit channel banks etc. 50 pair cable is also often used. Modern
home have 3 pair cat 3 for phone wiring. There are many other
configurations available.
|
What has any of that got to do with anything?
The 25 pair cable used for inside plant is *not* CAT3 (and does
not meet CAT3 specs in several respects). 50 pair cable is
fairly rare, and is *not* "also often used". The 25 pair cables
used to wire channel banks are of course *only* used for the analog
Voice Frequency side. ABAM is used on the digital side.
Anyone who uses 25 pair telephone cable for 10baseT data, or
even for T1/DS1 data, should be on the layoff list. T1/DS1, if
it spans more than 5 racks in the same row or to a different
row, is supposed to be individually shielded pairs (ABAM). Can
you imagine the effect of wiring an office with CAT3 instead?
(There is a fundamental difference between a CO and a customer
location, and the sheer volume of circuits is what makes it
significant. A few runs of CAT3 is one thing, and thousands of
them is entirely different.)
| Quote: | True, but the typical telephone cable was CAT1 quality, not
CAT3 (and higher) until 10baseT came along.
As I mentioned in other notes, I was buying, in my work, large quantities of
cat 3 cable in 1989, which (IIRC) predates twisted pair ethernet.
|
CAT3 dates at 1991. Sounds like someone was carried away with the
emerging standards, and not paying attention to either the required
specs or the price tag.
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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Floyd L. Davidson
Guest
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| Posted:
Fri Apr 29, 2005 4:20 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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James Knott <james.knott@rogers.com> wrote:
| Quote: | Floyd L. Davidson wrote:
With Manchester coding "every other bit pattern" is merely a
combination of one or the other of the two extremes. Every
bit has a transition, which is the 5 Mhz component. Any change
from either a 1 to an 0, or from a 0 to a 1, results in a 10 Mhz
component by adding a transition between the regular once per
bit transition.
Draw it out. You'll see that an alternating 1 0 pattern will have a
fundemental frequency of 5 MHz or 1/2 that of a steady 1 or 0.
|
Exactly. Your original statement was "Every other bit pattern
would fall between those extremes.", which is not correct. It
will be one or the other of those extremes, period. No in between.
| Quote: | Same as for ring current, except that since the levels are
significantly lower it is very unlikely that any VF signals
actually will cause interference.
If the ethernet circuits are design to operate in the 5 - 10 MHz range,
there won't be much effect from < 4 KHz.
|
Under normal circumstances, that is correct. We are talking about
what happens when there are impairments though. The idea that 20 Hz
ring voltage will not interfere just because it is a different frequency
is simply not true. If it gets to the 10baseT receiver it makes *no*
*difference* what the frequency is... only the voltage level counts.
With normally functional equipment, virtually no 20 Hz voltage gets to
the receiver. With kinks, split pairs, and other cable impairments, it
*can* get to the receiver, and when it does the fact that it is 20 Hz
simply has no significance at all.
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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Rick Merrill
Guest
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| Posted:
Fri Apr 29, 2005 4:20 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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DaveC wrote:
| Quote: | What you're saying is to be sure to correctly wire 1,2,3,6 as Ethernet
pairs,
and 4,5 as POTS, and all should go well. Yes?
On Wed, 27 Apr 2005 14:09:19 -0700, Floyd L. Davidson wrote
(in article <87vf682agw.fld@barrow.com>):
You clearly have not Googled that question! But we can talk
about it anyway... ;-)
Actually I didn't need to; I know the pin-out of 10baseT (see above). Your
answer was not clear to me, so I asked a 1-line clarification question...
And given the 10BaseT arrangement, you can use the 4,5 and/or
the 7/8 pins for something other than 10BaseT on the same cable.
...to which the answer is, apparently, "yes". :-)
|
I stand by my original (1st responder) reply :-) |
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Floyd L. Davidson
Guest
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| Posted:
Fri Apr 29, 2005 10:02 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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Rich Seifert <usenet@richseifert.com.invalid> wrote:
| Quote: | floyd@barrow.com (Floyd L. Davidson) wrote:
Off hand I don't know what the dynamic range of the receiver
input is, but the desired signal is a maximum of 2.8 V P-P, so I
can't imagine that the dynamic range is required to be much more
than twice that.
|
I think this statement of mine is particularly interesting in
light of the actual specification that Rich has so kindly
contributed.
| Quote: | However, it should also be clear that *any
voltage* greater than 1.4 V at the receiver input is going to
absolutely interfere with the ability of the receiver to detect
a valid crossover in the desired signal.
|
None of it addresses that point, and I'm not sure that James,
or perhaps others, yet understand the significance of it.
Regardless, here are comments as to how the 10baseT
specifications relate to telcom specifications and general
practice.
| Quote: | -It must be able to sustain (and decode) a maximum-amplitude transmitted
signal, which is 2.8 V peak. (IEEE 802.3 section 14.3.1.2.1.)
-It must be able to reject sinusoidal signals with frequencies under
2 MHz and amplitude up to 6.2 V peak-to-peak. (IEEE 802.3 section
14.3.1.3.2(b).)
|
That fits *precisely* what I expected, as stated originally. It
appears they gave it 4 dB of head room, and I suggested that it
wouldn't be much over 3 dB.
| Quote: | As to tolerance of traditional telephony signals inadvertently presented
to a 10BASE-T receiver, the standard notes the following:
-Battery voltage is generally 56 Vdc applied through 400 ohms.
|
"Generally" it would be 52-54 VDC, and "generally" would not
exceed 56 VDC (a 48 volt plant on boost charge). However, I've
seen subscriber line carrier systems that used 75 volts! And of
course that odd circumstance has to be designed for.
| Quote: | -Ringing voltage has an AC component of up to 175V peak at 20-60 Hz
through a 100 ohms source impedance (on top of the battery voltage).
|
Note that the 175V is peak with respect to ground, and what
David Lesher and I referred to was P to P. They are saying P-P
is as much as 350 VDC, which is even higher than we stated. (In
fact, maximum is 120 V RMS, which is 170 Peak, or 340 P-P. But
typically ring voltage is actually no more than 105 V RMS, or
149 Peak and 298 P-P.) They are citing the maximum possible,
and we cited the maximum commonly to be seen.
| Quote: | As to what might occur if there was a wiring error, the standard says:
"Although 10BASE-T equipment is not required to survive [much less
operate during] such wiring hazards without damage, application of any
of the above voltages shall not result in any safety hazard." (IEEE
802.3 section 14.2.7.4.)
That is, putting a ring signal across a 10BASE-T receiver may cause the
receiver to be permanently destroyed, but it should not be a safety
hazard, i.e., cause personal injury.
|
And under any circumstance the receiver is unlikely to function
while that voltage is present, even if it doesn't destroy the
device (which it may in fact do).
| Quote: | Most 10BASE-T receivers put a pair of back-to-back zener diodes across
the input, which clamp voltages far in excess of the expected signal
levels. However, if the source impedance of the high input voltage is
low enough (i.e., there is enough *energy* available), the zener diodes
will be destroyed by the clamping current, followed by the destruction
of most of the rest of the input circuitry. Puffs of smoke and faint
smells of burning carbon resistors may follow.
|
I doubt that most telephone lines would have a low enough
distributed resistance to allow that with interrupted ring
current, though it might well be possible if the line has a very
short loop (in the next room or the same closet with a PBX, or
in the CO equipment room). But if 60 Hz power line voltage was
the source, that would probably happen very quickly.
Thanks for posting the 802.3 specs Rich! I gave a half hearted
attempt at searching google, but of course there are too many
hits to sort through and I never had the time to look at enough
of them to see if any had the interesting part.
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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Floyd L. Davidson
Guest
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| Posted:
Fri Apr 29, 2005 10:31 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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Robert Redelmeier <redelm@ev1.net.invalid> wrote:
| Quote: | In comp.dcom.cabling Floyd L. Davidson <floyd@barrow.com> wrote:
Robert Redelmeier <redelm@ev1.net.invalid> wrote:
In comp.dcom.cabling Floyd L. Davidson <floyd@barrow.com> wrote:
It doesn't have to be. Frequency is *not* what determines
whether it will interfere or not.
Then what does? Voltage?
The parameter that is modulated or encoded is the only
parameter which suffers interference.
Yes, but I didn't ask what was interfered with.
I asked what determines interference.
|
The two are (necessarily) the same.
| Quote: | My answer is current. Most particularly HF changes in current.
Especially with sharp edges that have high freq components.
|
Current is correct... to the degree that it translates to
voltage! E=IR. Which is to say that current, with the right
impedance, will produce a voltage that does in fact interfere.
| Quote: | Voltage has nothing to do with it except as it _might_
increase current.
|
That just isn't true. The receiver is a voltage sensitive
device. End of story.
| Quote: | But with modern semiconductor telephone
sets, I doubt even a peizoelectric ringer generates the varying
current (&arcing!) that a mechanical contact bellringer does.
|
Arcing? From a ringer? I've never heard of such.
Ohhh! You are thinking of DC buzzers! But telset ringers don't
work that way. No contacts, no arcs, no sparks, and no signal
generation at all.
| Quote: | Essentially, voltage noise can be filtered out *if* it has
some other parameter that allows you to selectively reduce
its level while not affecting the desired signal level.
Which is what balanced signals and twisted pair media provide.
|
Exactly.
| Quote: | Current causes induction. The field induced results in
a *voltage* being detected at the receiver (otherwise,
it doesn't exist!).
Not quite. Current produces a magnetic field. A _changing_
magnetic field induces voltage in a separate conductor that leads
to secondary current. So only changes in current cause induction.
|
Stop playing word games. What you are saying is merely
repeating *exactly* what I just said, with one small added
detail that is insignificant in this context. What's the point?
I assumed we are all aware of just exactly how current causes
induction (with a varying magnetic field), and there is no need
to recite AC theory, as opposed to just saying "current" when
clearly most of the discussion is about current that varies at
20 Hz per second.
I don't need to detail all of this down to a gnat's ass level
just to see if you can try to knit pick at something I've left
out. We don't need to write the definitive text book with each
article.
| Quote: | Inside a shared sheath that is properly wired yes, and
that is *exactly* what the specifications are intended
to provide. The point here was originally stated as what
the effect will be if there are kinks or other damage to
the cable or miswired connectors that split a pair.
I didn't catch any reference to split pairs. They are horrible!
(Although splitting voice isn't as bad as splitting data).
|
Trust me, splitting voice is *precisely* the same as splitting
data: a total disaster.
| Quote: | Anyone pushing to the limit with shared sheath had best know what
they're doing.
|
I agree. One of the problems with this type of thread is people
who have hands on experience with pushing the limits but little
understanding of the theory involved, and thus have no
perception of why something worked in one case but might not in
another. So they say, "Hey... we did it. It works fine." and
someone believes that and spends many dollars and/or hours
finding out just exactly what the differences are!
However, for anyone who is perceptive, there is a lot in this
thread to learn from...
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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James Knott
Guest
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| Posted:
Sat Apr 30, 2005 12:20 am Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
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Floyd L. Davidson wrote:
| Quote: | You
said it could have "components" at 1 GHz. What could those components be,
if not harmonics.
Everything from switching transients to power supply noise.
|
Most such noise tends to fall off well below about 30 MHz. It's still a
long way to 1 GHz from there.
| Quote: | Perhaps Rich can provide the relevant specs, but ethernet cards must, for
safety reasons be able to sustain such voltages, without creating a
safety
hazard.
So, you can't cite any spec to support what you've said.
|
No, I don't have such specs handy, but all such equipment must be built such
that a safety hazard is not created. It applies to not just computer
equipment, but also communications equipment, appliances etc. However,
that has nothing to do with interference, only safety. As far as
interference goes, when using the same cable to carry voice and ethernet,
is interference from voice to ethernet. Voice circuits will be limited to
a few KHz and ringing is 20 Hz. The circuitry on the NIC will be built to
pass the ethernet signal, which is in the range of 5 -10 MHz. It is not
built to pass voice frequencies or lower. So in order for interference to
occur, you have to have enough of that voice enery coupled into the
ethernet cable, and then get through a transformer and other circuitry,
that's designed to pass signals 1000x higher. Now, IIRC, a single pole
(for example a simple RC network) filter, has a rolloff of 3 dB / octave
going from 4 KHz, to 5 MHz, is roughly 10 octaves or 30dB attenuation of
the highest audio frequency, when passes through that simple one capacitor,
one resisitor high pass filter. A network card would likely have more
filtering than that, even without the transformer, which simply won't work
very well at audio frequencies. Care to explain how an audio signal,
nominally in the range of a few dBm, after passing through so much
attenuation will interfere with a signal that's several dB higher than the
original audio signal? Incidentally, for the 20 Hz ringing, you'd be
looking at over 60 dB attenuation, when going throug a single pole high
pass filter. That's a 1,000,000 to 1 ratio. How much of that ringing
current, will appear at the receiver???
| Quote: | Off hand I don't know what the dynamic range of the receiver
input is, but the desired signal is a maximum of 2.8 V P-P, so I
can't imagine that the dynamic range is required to be much more
than twice that. However, it should also be clear that *any
voltage* greater than 1.4 V at the receiver input is going to
absolutely interfere with the ability of the receiver to detect
a valid crossover in the desired signal.
|
A voice signal is a few dBm or dB above one milliwatt, which would be on the
order of 1V, with 600 ohms impedance. Ethernet cable is roughly 1/4 that
impedance, so the voice at that level will be about 1/2 V. After you've
managed to couple the signal from one balanced pair to another, the induced
signal will be much lower. It will not the be original 1/2V. Ringing
current will be attenuted even more. So, please explain how you're going
to get crosstalk, from one pair to the next, that's powerful enough to
cause interference.
Also, given all the claims you've made about noise and interferece, I can
only conclude you've got some incredibly horrible phone lines in Alaska.
In fact I'd have to consider them useless.
However, the original question was about sharing ethernet sharing a cable
with phones. According to Rich, one of the ethernet designers, that sort
of service was part of the design for 10baseT. No amount of arguing from
you is going to change that fact. |
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James Knott
Guest
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| Posted:
Sat Apr 30, 2005 12:20 am Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
|
|
Floyd L. Davidson wrote:
| Quote: | None of it addresses that point, and I'm not sure that James,
or perhaps others, yet understand the significance of it.
|
I fully understand that point. My question has been how does a voice signal
typically around 0 dBm or 1 milliwatt or a 20 Hz ringing current, in
another pair in the cable, manage to induce such voltages in the detector?
You've got to consider that as a result of the twisted pair, there is very
little crosstalk to begin with, followed by a transformer that's designed
to pass ethernet signals and not voice, followed by whatever filtering is
in the receiver. How does a standard voice signal manage to get through
all that and still produce a signal of sufficient amplitude, to cause the
interference??? |
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James Knott
Guest
|
| Posted:
Sat Apr 30, 2005 12:20 am Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
|
|
Floyd L. Davidson wrote:
| Quote: | The 25 pair cable used for inside plant is not CAT3 (and does
not meet CAT3 specs in several respects). 50 pair cable is
fairly rare, and is not "also often used". The 25 pair cables
used to wire channel banks are of course only used for the analog
Voice Frequency side. ABAM is used on the digital side.
Anyone who uses 25 pair telephone cable for 10baseT data, or
even for T1/DS1 data, should be on the layoff list. T1/DS1, if
it spans more than 5 racks in the same row or to a different
row, is supposed to be individually shielded pairs (ABAM). Can
you imagine the effect of wiring an office with CAT3 instead?
(There is a fundamental difference between a CO and a customer
location, and the sheer volume of circuits is what makes it
significant. A few runs of CAT3 is one thing, and thousands of
them is entirely different.)
|
Take a look at http://www.national-tech.com/catalog/telephonecables.htm and
tell me that again.
| Quote: | As I mentioned in other notes, I was buying, in my work, large quantities
of
cat 3 cable in 1989, which (IIRC) predates twisted pair ethernet.
CAT3 dates at 1991. Sounds like someone was carried away with the
emerging standards, and not paying attention to either the required
specs or the price tag.
|
I bought cable, as specified by the engineers.
As for the date. I don't have hard evidence for when cat 3 was created or
evolved, but this little narrative describes it's application (and cat 4)
in plans made in 1990. So it was already in use for them to have been able
to specify it. |
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Eugene Blanchard
Guest
|
| Posted:
Sat Apr 30, 2005 8:20 am Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
|
|
A colleague of mine told me about one of his customers who had plugged in an
RJ11 phone line into a 10BaseT NIC RJ45 connector. Everything worked fine
until the phone range and the ringing voltage took out the NIC, motherboard
and power supply....
Floyd L. Davidson wrote:
| Quote: | Rich Seifert <usenet@richseifert.com.invalid> wrote:
floyd@barrow.com (Floyd L. Davidson) wrote:
Off hand I don't know what the dynamic range of the receiver
input is, but the desired signal is a maximum of 2.8 V P-P, so I
can't imagine that the dynamic range is required to be much more
than twice that.
I think this statement of mine is particularly interesting in
light of the actual specification that Rich has so kindly
contributed.
However, it should also be clear that *any
voltage* greater than 1.4 V at the receiver input is going to
absolutely interfere with the ability of the receiver to detect
a valid crossover in the desired signal.
None of it addresses that point, and I'm not sure that James,
or perhaps others, yet understand the significance of it.
Regardless, here are comments as to how the 10baseT
specifications relate to telcom specifications and general
practice.
-It must be able to sustain (and decode) a maximum-amplitude transmitted
signal, which is 2.8 V peak. (IEEE 802.3 section 14.3.1.2.1.)
-It must be able to reject sinusoidal signals with frequencies under
2 MHz and amplitude up to 6.2 V peak-to-peak. (IEEE 802.3 section
14.3.1.3.2(b).)
That fits *precisely* what I expected, as stated originally. It
appears they gave it 4 dB of head room, and I suggested that it
wouldn't be much over 3 dB.
As to tolerance of traditional telephony signals inadvertently presented
to a 10BASE-T receiver, the standard notes the following:
-Battery voltage is generally 56 Vdc applied through 400 ohms.
"Generally" it would be 52-54 VDC, and "generally" would not
exceed 56 VDC (a 48 volt plant on boost charge). However, I've
seen subscriber line carrier systems that used 75 volts! And of
course that odd circumstance has to be designed for.
-Ringing voltage has an AC component of up to 175V peak at 20-60 Hz
through a 100 ohms source impedance (on top of the battery voltage).
Note that the 175V is peak with respect to ground, and what
David Lesher and I referred to was P to P. They are saying P-P
is as much as 350 VDC, which is even higher than we stated. (In
fact, maximum is 120 V RMS, which is 170 Peak, or 340 P-P. But
typically ring voltage is actually no more than 105 V RMS, or
149 Peak and 298 P-P.) They are citing the maximum possible,
and we cited the maximum commonly to be seen.
As to what might occur if there was a wiring error, the standard says:
"Although 10BASE-T equipment is not required to survive [much less
operate during] such wiring hazards without damage, application of any
of the above voltages shall not result in any safety hazard." (IEEE
802.3 section 14.2.7.4.)
That is, putting a ring signal across a 10BASE-T receiver may cause the
receiver to be permanently destroyed, but it should not be a safety
hazard, i.e., cause personal injury.
And under any circumstance the receiver is unlikely to function
while that voltage is present, even if it doesn't destroy the
device (which it may in fact do).
Most 10BASE-T receivers put a pair of back-to-back zener diodes across
the input, which clamp voltages far in excess of the expected signal
levels. However, if the source impedance of the high input voltage is
low enough (i.e., there is enough *energy* available), the zener diodes
will be destroyed by the clamping current, followed by the destruction
of most of the rest of the input circuitry. Puffs of smoke and faint
smells of burning carbon resistors may follow.
I doubt that most telephone lines would have a low enough
distributed resistance to allow that with interrupted ring
current, though it might well be possible if the line has a very
short loop (in the next room or the same closet with a PBX, or
in the CO equipment room). But if 60 Hz power line voltage was
the source, that would probably happen very quickly.
Thanks for posting the 802.3 specs Rich! I gave a half hearted
attempt at searching google, but of course there are too many
hits to sort through and I never had the time to look at enough
of them to see if any had the interesting part.
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Floyd L. Davidson
Guest
|
| Posted:
Sat Apr 30, 2005 1:24 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
|
|
James Knott <james.knott@rogers.com> wrote:
| Quote: | Floyd L. Davidson wrote:
None of it addresses that point, and I'm not sure that James,
or perhaps others, yet understand the significance of it.
I fully understand that point. My question has been how does a voice signal
typically around 0 dBm or 1 milliwatt or a 20 Hz ringing current, in
another pair in the cable, manage to induce such voltages in the detector?
You've got to consider that as a result of the twisted pair, there is very
little crosstalk to begin with, followed by a transformer that's designed
to pass ethernet signals and not voice, followed by whatever filtering is
in the receiver. How does a standard voice signal manage to get through
all that and still produce a signal of sufficient amplitude, to cause the
interference???
|
Please re-read the thread to find the answers.
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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Floyd L. Davidson
Guest
|
| Posted:
Sat Apr 30, 2005 1:28 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
|
|
Eugene Blanchard <blanchae@telus.net> wrote:
| Quote: | A colleague of mine told me about one of his customers who had plugged in an
RJ11 phone line into a 10BaseT NIC RJ45 connector. Everything worked fine
until the phone range and the ringing voltage took out the NIC, motherboard
and power supply....
|
Wow.
| Quote: | Floyd L. Davidson wrote:
Rich Seifert <usenet@richseifert.com.invalid> wrote:
floyd@barrow.com (Floyd L. Davidson) wrote:
Off hand I don't know what the dynamic range of the receiver
input is, but the desired signal is a maximum of 2.8 V P-P, so I
can't imagine that the dynamic range is required to be much more
than twice that.
I think this statement of mine is particularly interesting in
light of the actual specification that Rich has so kindly
contributed.
However, it should also be clear that *any
voltage* greater than 1.4 V at the receiver input is going to
absolutely interfere with the ability of the receiver to detect
a valid crossover in the desired signal.
None of it addresses that point, and I'm not sure that James,
or perhaps others, yet understand the significance of it.
Regardless, here are comments as to how the 10baseT
specifications relate to telcom specifications and general
practice.
-It must be able to sustain (and decode) a maximum-amplitude transmitted
signal, which is 2.8 V peak. (IEEE 802.3 section 14.3.1.2.1.)
-It must be able to reject sinusoidal signals with frequencies under
2 MHz and amplitude up to 6.2 V peak-to-peak. (IEEE 802.3 section
14.3.1.3.2(b).)
That fits *precisely* what I expected, as stated originally. It
appears they gave it 4 dB of head room, and I suggested that it
wouldn't be much over 3 dB.
As to tolerance of traditional telephony signals inadvertently presented
to a 10BASE-T receiver, the standard notes the following:
-Battery voltage is generally 56 Vdc applied through 400 ohms.
"Generally" it would be 52-54 VDC, and "generally" would not
exceed 56 VDC (a 48 volt plant on boost charge). However, I've
seen subscriber line carrier systems that used 75 volts! And of
course that odd circumstance has to be designed for.
-Ringing voltage has an AC component of up to 175V peak at 20-60 Hz
through a 100 ohms source impedance (on top of the battery voltage).
Note that the 175V is peak with respect to ground, and what
David Lesher and I referred to was P to P. They are saying P-P
is as much as 350 VDC, which is even higher than we stated. (In
fact, maximum is 120 V RMS, which is 170 Peak, or 340 P-P. But
typically ring voltage is actually no more than 105 V RMS, or
149 Peak and 298 P-P.) They are citing the maximum possible,
and we cited the maximum commonly to be seen.
As to what might occur if there was a wiring error, the standard says:
"Although 10BASE-T equipment is not required to survive [much less
operate during] such wiring hazards without damage, application of any
of the above voltages shall not result in any safety hazard." (IEEE
802.3 section 14.2.7.4.)
That is, putting a ring signal across a 10BASE-T receiver may cause the
receiver to be permanently destroyed, but it should not be a safety
hazard, i.e., cause personal injury.
And under any circumstance the receiver is unlikely to function
while that voltage is present, even if it doesn't destroy the
device (which it may in fact do).
Most 10BASE-T receivers put a pair of back-to-back zener diodes across
the input, which clamp voltages far in excess of the expected signal
levels. However, if the source impedance of the high input voltage is
low enough (i.e., there is enough *energy* available), the zener diodes
will be destroyed by the clamping current, followed by the destruction
of most of the rest of the input circuitry. Puffs of smoke and faint
smells of burning carbon resistors may follow.
I doubt that most telephone lines would have a low enough
distributed resistance to allow that with interrupted ring
current, though it might well be possible if the line has a very
short loop (in the next room or the same closet with a PBX, or
in the CO equipment room). But if 60 Hz power line voltage was
the source, that would probably happen very quickly.
Thanks for posting the 802.3 specs Rich! I gave a half hearted
attempt at searching google, but of course there are too many
hits to sort through and I never had the time to look at enough
of them to see if any had the interesting part.
|
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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Floyd L. Davidson
Guest
|
| Posted:
Sat Apr 30, 2005 2:28 pm Post subject:
Re: 10base-T & POTS in same Cat-5 cable? |
|
|
James Knott <james.knott@rogers.com> wrote:
| Quote: | Floyd L. Davidson wrote:
You
said it could have "components" at 1 GHz. What could those components be,
if not harmonics.
Everything from switching transients to power supply noise.
Most such noise tends to fall off well below about 30 MHz. It's still a
long way to 1 GHz from there.
|
Nobody suggested that most of such noise was in the GHz range.
Why make unequivocally useless comments that lack all relevance?
Such noise components *do* exist, whether James Knott has yet
had enough experience to know about them, or not.
| Quote: | Perhaps Rich can provide the relevant specs, but ethernet cards must, for
safety reasons be able to sustain such voltages, without creating a safety
hazard.
So, you can't cite any spec to support what you've said.
No, I don't have such specs handy, but all such equipment must be built such
that a safety hazard is not created.
|
Sigh, another non sequitur.
| Quote: | However,
that has nothing to do with interference, only safety.
|
That doesn't even make a good strawman, so why beat on it?
| Quote: | As far as
interference goes, when using the same cable to carry voice and ethernet,
is interference from voice to ethernet. Voice circuits will be limited to
a few KHz and ringing is 20 Hz.
|
You are, again, confusing useful spectrum with actual spectrum.
| Quote: | The circuitry on the NIC will be built to
pass the ethernet signal, which is in the range of 5 -10 MHz. It is not
built to pass voice frequencies or lower.
|
No one suggested that in a properly designed, installed, and
functioning system such interference will occur. We have *all*
agreed from the start that 10baseT utilizing CAT3 or CAT5 cable
with RJ-45 connectors was *designed* to work with POTS lines on
the same cable.
The question and discussion point was what potentials exist for
problems from miswired connectors and defects such as kinked
cable. Not to mention the claims by some that 20 Hz ring
voltage from a POTS line could be applied directly to a pair
running 10baseT without even causing errors, much less damaging
the network interface. That particular subtopic resulted from
questions about how 20 Hz could ever cause any interference,
given the disparate frequency.
All of that has been discussed in detail, with specifications of
10baseT and telecom systems compared to demonstrate the likely
effects under various circumstances... and all despite your
unwillingness to understand most of it.
| Quote: | Also, given all the claims you've made about noise and interferece, I can
only conclude you've got some incredibly horrible phone lines in Alaska.
In fact I'd have to consider them useless.
|
Incredibly horrible phone lines exist almost everywhere. What
causes them might indeed mystify *you*... ;-)
On the other hand some of us actually do have enough experience
and the theoretical background to understand, and at least make
attempts at explaining, what causes impairments on telecom
circuits, whether they are voice or data.
| Quote: | However, the original question was about sharing ethernet sharing a cable
with phones. According to Rich, one of the ethernet designers, that sort
of service was part of the design for 10baseT.
|
My position on that was exceedingly clear, and was posted prior
to Rich's fine comments. Perhaps your confusion is caused by
not having understood it? Here's the exact quote, which I
really do not think is easy to misunderstand unintentionally:
"That is in fact what CAT5 was designed for!"
| Quote: | No amount of arguing from
you is going to change that fact.
|
No amount of arguing from you will convince anyone that you have
read or understood this thread if you continue to make such
misdirected statements.
--
Floyd L. Davidson <http://web.newsguy.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@barrow.com |
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