PE> Yeah, in the absence of some scientific evidence
PE> from Rod about why modem's impedence is constant,
PE> the guesses seem equally as valid, yours more so.

RS> Welp they aint, tho in your case you can be excused because you
RS> dont know that much about the finer detail of electronic hardware.

RS> The short story is that the 'modem impedance' cant be anything, and is
RS> in fact under the control of the modem with a modem echo cancelling modem.
RS> Its not that easy to summarise the detail in a para or two, but there is
RS> a thing called a hybrid which combines the receive and transmit onto the
RS> single pair of wires that are the phone line. How well the modem can cancel
RS> out its OWN outgoing data so it can here the data from the other end at 
RS> all,
RS> and remember they are on the SAME frequency with modern modems, depends on
RS> how close the modems impedance is to the lines impedance for complicated
RS> reasons. And thats why you see the near end and far end echo figures in
RS> the stats, they are a measure of that impedance imbalance.

[and later]

RS> Well, its a bit more complicated than that when the modems can vary the
RS> impedance they present to the line to balance the hybrid. Part of what
RS> So Bills waffle about modem impedance is just that, waffle.

Looks like another fence-jump to me, Rod.  Sure, I may be 
misunderstanding you, but that's the way it comes across.  Your
theory was that the impedance doesn't change, you then say it
"can't be anything" (why not be more definitive and say "is always
constant" so we know you're not jumping fences), then you say "it's
under the control of the modem", and then you finally come out and
say that "modems can vary the impedance they present to the line",
which is exactly what Bill was saying.

I'm just telling you how your messages come across TO ME sometimes,
and I bet they come across TO BILL exactly the same.  If your goal
is to communicate, you have a problem.  Who is violating the
English language spec is a different story, could be me+Bill, could
be you.

PE> I find it hard to believe that the USR Courier can conform to
PE> the V.34 spec AND simultaneously decide that the other end is
PE> a non-Courier so it should start faking away at the numbers.

RS> Thats just playing silly buggers. No one is suggesting that it does
RS> that on purpose. Its clear from the MASSIVE difference of 6db, which
RS> is a HELL of a fucking lot, that it claims to see with and M34F on
RS> the other end, its having a massive brain fart. Just why is currently
RS> harder to say. One obvious possibility is just a bug in the USR code
RS> in that particular situation. 

In what particular situation?

RS> It would be interesting to see what
RS> happens with a non rockwell non USR on the other end, like a Motorola.

To keep the test consistent would mean using my phone line, which
means I need the Motorola, so that test can't be done accurately.

PE> I guess if we're guessing, we should ask the question, if YOU
PE> were designing a modem, how would YOU measure the line quality.

RS> The proper way is to use proper test equipment, and then compare
RS> those results with what the Courier/Courier session claims. Ditto

The question I was asking was how to design a modem.

PE> So if YOU were manufacturing a modem, and you couldn't figure
PE> out a way of getting the other end to emulate a 600-ohm resistor

RS> Thats precisely what the line probe is supposed to be doing. It really
RS> does attempt to do a full test of the line characteristics, then it
RS> adjusts the various stuff it can adjust to do the best with what its
RS> just measured the line to be.

Ok, so how does it perform the line probe?

PE> (after a connect even), then what would YOU measure INSTEAD of line quality

RS> That question is meaningless. There are a variety of parameters like
RS> noise, impedance, the variation in loss with frequency, and other more
RS> complex stuff like group delay which are in total the line quality.

And which ones can be measured?  Or can you only measure the sum,
or what?  And is your measurement method dependant in any way on
the other modem?

RS> There is nothing BUT the line quality that matters
RS> when attempting to achieve maximum thruput.

RS> And thats all a completely separate issue to the gross handshaking
RS> failures that Bill is seeing on Courier/M34F sessions on SOME calls.

It could be related.  It might be that the failures only happen
on noisy lines, never on the good lines.  

BTW, my line to the exchange is presumably constant, ditto for
Bill's, and the STD link should be perfect, which means that for
us to have noisy lines, would mean that my wire to the exchange
would need to have some crosstalk from someone else's wire.  And
presumably at 3am there is less chance of interference from
others?  BFN.  Paul.
@EOT:

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