Guilty til proven Guilty!
I take it the measurements you have done that show 15db isolation
using the network analyzer, terminated 4,5 and 7,8 with 100 ohms,
differential and the proper common mode impedance to ground?
I trust your measurement at this point, but as we hang the DTE
on there, Are we not terminating with a different impedance than
100ohms, and what does that do to the measurement?, I don't
expect a lot of drama but worth a look...
How does "Noise, Inluding RF and xyz..." injection on the used pairs get worth
because I have a dc power on there?, and whatever happened to the awsome
40db common mode rejection of the magnetic and the differential input stage
of the receiver? This is a twisted pair after all isn't? all right I get real unlucky
and my smart "noise" source injects differential noise now, I am in trouble
with or without current saved by how good of a phy I have chosen what Am I
Larry, I ate drunk and slept phys ask your favorite vendors.
at the end of the day, PHYs are HUMAN, they are not perfect, and will never be.
the hope is that if the phy has 30-40m of margin built in above 100m physical
length that all this worries still apply but is not as dramatic.
I hear you, the "rules" are you need a proof, how many Phys and how many revs
must I go through, and in what system? I have seen great phys act way better
in some boxes, it is all in the eye of the "Beholder"....
As for the rules, I once played checkers with this person, and as I started to win
here and there he kept changing the rules, I tell you that was more fun than winning
at the end of the day...
At 10:50 AM 5/15/00 -0700, Larry Miller wrote:
>Roger and all,
>What I am saying is that whatever ill effects (not just noise, but rf-type
>things such as parasitic phenomena such as resonances, impedance matching,
>and the like) of attaching power components to the signal pairs are
>intrinsically isolated from the signaling pairs by using the other pairs
>This is a basic consequence of the guaranteed properties of CAT-5 cable.
>The amount of isolation guaranteed as a minimum is per the numbers quoted
>in my little drama.
>Even with things like RJ-45 connectors in the link, the isolation that is
>obtained is at least 15 dB. You can confirm this by putting some CAT-5
>cable on a network analyzer. (Maybe we will have to do that for the
>What this means is that whatever dirties of ANY kind your power attachment
>does to the wires you use (4-5,7-8) is isolated from the signal wires
>(1-2,3-6) by at least 15 dB over the 2-100 MHz frequency range. This is a
>factor of more than 5:1 and represents a risk reduction of that amount.
>15 dB covers quite a lot of sins!
>In the early meetings of the DTE Committee this was discussed and deemed to
>be a worthwile thing.
>I am not saying that it is impossible to successfully attach power to the
>signal wires, OK?
>But what I am saying is that if you want to take the riskier path and throw
>away this 15 dB cushion, you will have to show that you did not mess up the
>system, at least if you want my vote.
>***** Slight topic shift *****
>The present UTP universe is defined by the clauses in IEEE802.3 and ANSI
>TP-PMD that I cited in the list I put on the IEEE Web site. This is a list
>of tests that we use to evaluate any new technology variant.
>In particular, we (our company) have insisted that PHY vendors go through
>this hoop. A subset of this same list of tests is what the UNH IOL test
>suites uses for qualifying new PHY designs.
>The reason for doing this is that it is possible (even probable) for a link
>to still operate even though it did not meet these criteria; i.e., a plain
>old functional test or even general scope pictures do not give any idea of
>the margins (if any) present. If you have not been doing these tests on
>your own designs and have been getting away with it, then you can thank
>others who did do these characterizations so that by simply buying and
>hooking up "approved" parts you got a compliant design.
>Now we are going into another dark corner of the PMD ("new technology"), so
>we have to go back through the whole list of parameters to make sure that
>we did not inadvertently tank something.
>There is a definite proof that this is so: some of the test methods sitting
>in full force and effect in IEEE 802.3 will not work with powered signal
>wires, and if this is adopted the existing standards will have to be
>overhauled in some areas.
>As an analogy, this is a little bit like the Gigabit fiber committee
>(802.3z) discovering far, far into the standards process that the paucity
>of modes in laser outputs caused very severe effects on some kinds of
>multi-mode fiber. This was nothing new, and in fact it was in my
>20-year-old optics handbooks, with pictures even. But it had gotten
>overlooked. The result was that it delayed the standard by some 6 months to
>So, to repeat:
>You wanna power the signal pairs? Then you gotta show that this does not
>goof up anything of importance, and in fact, the standard will have to
>enumerate what you have to do to achieve this.
>At 09:50 AM 5/15/00 -0700, R karam wrote:
>> Hi Larry
>>Please clarify the statement below.
>>>I hope that it was shown that sending power over the signal pairs raises
>>>the technical difficulty bar by the amount of isolation between pairs. (15
>>>dB at least)
>>are you saying that because power is on the used signal pairs, I get
>>possibly up to 15db of noise injected on there, what about the supply
>>filter that insures DC- Pure DC on the wire, I am not clear on this?