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(ASCEND) xDSL limitations



I'm putting this information here should you need it.  Ascend was not able
to figure this one out but Paradyne Systems was able to explain the
problem.

I've been unable to send an IDSL signal from my ISP to my house 13,200 ft
over 26 ga wire with the 32idsl card in a Max TNT.  There are no loads, no
taps and all the wire was installed in the last two years. The phone
company tested the circuit antenuation at 80K and pronounced it good.  The
signal should have gone 18,000 feet.

Trouble was, when the pipeline at the other end could see the TNT (rarely)
I kept getting inband timeouts and dropped connections anytime I tried to
transmit data.

Here is what was wrong.

First, the phone company used a pair that is in the 200 pair bundle coming
into our ISP.  This bundle contains several T1's lines using AMI line
coding.  Well, AMI and DSL don't get along very well.  Crosstalk from AMI
is serious noise to DSL and shortens the distance it can travel by 30% or
more.  Never mix AMI and DSL in the same bundle.

Second, crosstalk from regular phone lines increases dramatically at the
entry and exit points of a phone company CO.  This is not normally a
problem for signals originating in the CO because the noise level is low
compared to the signal level in the CO.  Signals originating from phone
users are normally only down 3-6 dB at the CO.  However, a DSL signal
originating at the ISP may be down 10-20 dB when it enters the CO and it is
not regenerated at the CO.  Therefore, the noise level is significant and
further reduces the total distance the signal can travel.

And one I hadn't run into yet (but would have) - there is a limit on how
many DSL circuits that you can run in a single bundle.  Typically a bundle
of 50 pairs will experience a 25% or greater reduction in distance as a
result of echo cancellation problems when too many DSL circuits are in
close proximity.

Solutions? See if the phone company will let you put the Max TNT in the CO
(ha, unless you have a team of lawyers and are a CLEC to boot) or have them
run 22 or 24 ga high twist pairs to the CO just for your use (very
expensive).

Just though someone else should know xDSL may not work well for an ISP
unless he is also the phone company.  Think I'll see if I can talk the
state regulators into granting ISP's the same access to the CO as CLEC's.

--Tony

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