Re: Again on ADSL line


"sycochkn" <sycochkn@xxxxxxxxxxxxx> wrote in message
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"Ross Herbert" <rherber1@xxxxxxxxxxxxxx> wrote in message
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On Fri, 29 Feb 2008 20:29:15 GMT, "Epsilon Rho" <epsinlonrho@xxxxxxxxxx>
wrote:

:A few weeks ago I posted a message regarding some problems with my ADSL
:line. Following the intervention of the AT&T technician, the line is now
OK.
:In an effort to understand the original problem (a mismatch between the
Ring
:and Tip impedance) I measured again such impedances. Now that everything
:seems to be operating properly, I measured approx. 2,200 ohm (DC) on the
Tip
:and on the Ring line. I know that at the central office (CO) there is a
200
:ohm resistance, so each wire resistance should be 2,000 ohm. AT&T told
me
:that they cannot upgrade the speed to my location because I am too far
from
:the CO. I learned that this distance is 18,000 feet.
:
:
:
:Armed with these numbers, I tried to calculate how far I am from the CO.
And
:this is what is puzzling. Assuming that I am at 18,000 feet away, the
AWG
:that would give a reading of 2,000 ohm at 18,000 feet is pegged at
around
:30-31 AWG and that is impossible, because this kind of gauge is far too
thin
:for a phone line. As a matter of fact, I read that the smallest AWG used
in
:the phone line is 24 AWG.
:
:
:
:Has anybody an idea of what is going on?
:
:
:
:TIA
:
:Gene

As BobW has indicated you can't measure the actual dc line loop
resistance
without disconnecting the line at the MDF and placing a short circuit
across it
while measuring at the distant end. Looking down the line towards the
exchange
you effectively have a 48V battery connected between the wires at the
exchange
end plus the battery feed retard windings (200+200ohms) plus the line
resistance. Depending upon the type of exchange (electromechanical analog
or
digital) the line interface arrangements may differ slightly.

In most cases a 26AWG line loop resistance of not more than 1800ohms is
allowable for reliable telephone service on a POTS line, but this can
vary
depending upon the exchange type and line construction. It is usual for a
POTS
line construction using 26AWG which has a dc resistance of
133.8568ohms/km
(total length = 13.4km). For a loop resistance of 1800ohms this would put
the
maximum distance at about 6.7km (straight line) from the exchange. In
practice,
because the cable route will follow street layout, this would limit the
radial
distance to probably around 5.5km max from the exchange, give or take.
ADSL1 is
generally limited to a line length of 3.5km from the exchange while ADSL2
has an
even shorter distance limit.

What would you need? a time domain reflectometer?

Bob

TDRs have their limitations on long loops. The high frequency loss of the
loop is large enough such that you can't see a "sharp" reflection. A long
pulse TDR will get there, and back, but the "hump" that returns if very
tough to pick out-of-the-noise.

Also, If the other end of the loop happens to be terminated with 100ohms at
high frequencies (as the case if that line is served with DSL) and 600ohms
at lower frequencies (depending on the POTS line card when it's "on hook"),
then you won't get a farthest-end reflection, anyway. So, you won't be able
to deduce its length with a TDR. Either way, you need to do something at the
other end of the loop.

The phone companies spend big bucks on test equipment. I used to work for a
company that made some of this stuff. We made large amounts of money selling
to the telcos -- at least until the market got saturated with our equipment.
If they (telcos) could have figured out a way to do things like loop length
determination WITHOUT the expensive central office equipment or some
craftsperson sitting at the MDF then they would have done it.

BobW



.

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