[Asterisk-Users] Chaining more than one zap echo canceller?

[Rich Adamson]

Yes, regardless of what the DC resistance is, the impedance is relatively
close to 600 ohms. Impedance mismatches have to be relatively large to
impact echo, etc. Like in UK folks using the x100p (600 ohm impedance
only) on a 135 ohm line (or something like that). The typical telco
person that comes out to check customer loops really does not understand
impedance vs resistance. Most are only given very simple tools (something
like a VOM meter) with very little training. They are usually taught
what the limits are and anything that falls outside those limits
requires some sort of local proceduralized action. 

For the last 100 years, telco technicians used test equipment to measure
DC resistance (to the CO), resistance from tip-to-ground, ring-to-ground,
transmission loss (your -3.1 db from the central office milliwatt gen),
sometimes noise levels, sometimes induced AC levels (influence),
and a couple of other rather basic items. The test equipment does the
majority of that automatically (in the background), and he's only given
enough output from the test equipment to generally report a problem
(following their local procedures). Most don't have a clue what is
actually being tested under the face plate of the tester.

Highly unlikely that you really need any sort of complex impedance
matching device. That is so unlikely that it is extremely difficult
to actually find any manufacturer of such devices any more.

Since this topic has been discussed many many times in the last year,
lets take this discussion off-list. Send me an email directly with
the details of your config and what you've done thus far to isolate
the issues.

Rich

------------------------
> To be clear then, my subscriber loop may have a DC resistance of 150ohms due
> to the physical loop length but should have an AC impedance of 600ohm (+/- a
> few)? I have to assume that as the telco cable guy was performing the test
> he's telling me the AC impedance.
> 
> If so, then would I need to insert some sort of complex impedance network to
> build out a match rather than just a single resistor?
> 
> > -----Original Message-----
> > From: Rich Adamson [mailto:radamson at routers.com]
> > Sent: October 12, 2004 2:58 PM
> > To: Asterisk Users Mailing List - Non-Commercial Discussion
> > Subject: Re: [Asterisk-Users] Chaining more than one zap echo 
> > canceller?
> > 
> > 
> > Adding resistance to one side of the line only begs for problems
> > as it creates a tip-ring imbalance that will "cause" echo, etc,
> > when other imperfections exist.
> > 
> > If that approach works at all for anyone, its addressing a symptom
> > and not the root cause.
> > 
> > Try this one: Each customer loop is made up of copper and the longer 
> > the copper, the more resistance. Yet the impedance (in the US) is
> > consistently 600 ohms. A short loop might be a 100 ohms while a long
> > loop might be well over 1500 ohms; still both are 600 ohm impedance.
> > 
> > ------------------------
> > 
> > > Impedance is the measure of total opposition (resistance, 
> > capacitance, 
> > > and inductance) to alternating current flow.  Adding 
> > resistance will 
> > > raise the impedance of the line.
> > > 
> {clip}
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---------------End of Original Message-----------------