Hello everyone.
I am having some very bad time due to my ISP's poor last mile (in India).
DSL is loosing sync. consistently and this time problem seems quite
interesting so I though to ask how ISPs across world managing it. Problem
is high attenuation & low SNR because of "lot of free pairs" in the cable.
My connection is coming from something like 100 pair > 50 pair > 20 pair >
5 pair. Now 100 pair has less then 30 active lines but based on testing it
seems like at 100 pair DP there's very low noise and everything is pretty
good (usual BSNL pillars in India have 100 pair terminations). Next 20 pair
has just 4 active lines (and 16 free lines causing issues for those 4
working lines) and at the end my line comes from 20 > 5 with only one
(which is my) line active on one of 5 pairs.
Now argument of my ISP (BSNL) is that due to excessive number of free
pairs, they are causing huge noise and they likely need to reduce these
DP's by putting 1-2 line wire from my home till 100 pair pillar termination
(which is down in other street and so needs effort in digging and putting
new wire). But I just never heard about this problem anywhere else. Do DSL
providers really suffer due to free pairs? Assuming other pairs are all
crossed/shorted, can they still produce significant noise in other working
lines? Also, what exactly was "bonding" used by AT&T in US? I thought it
was actually making use of free pairs, bonding them together and having
more bandwidth for end user, isn't it?
If someone can pass me some detailed whitepaper or document explaining
about this noise, it will be very much helpful.
Thanks.
Hello Anurag.
I have not heard of this problem before, but I imagine that the
non-terminated pairs could be acting like antennas and picking up noise.
Have you considered grounding one end (or both) of the free pairs? Perhaps
this would reduce the amount of noise they pick up.
Regards,
John
John Souvestre \- New Orleans LA \- \(504\) 454\-0899
Grounding both ends will probably result in "hilarity ensues". And I suspect that Anurag
can't ground the free pairs, because the copper belongs to the provider.
Yeah, grounding both ends will result in some current traversing across
the pairs all the time because of differences in ground potential over
long-ish distances.
Ken Matlock
Network Analyst
303-467-4671
matlockk@exempla.org
Yes, but would this result in more or less noise than an open end acting
like an antenna? And would the ground loop noise be in the DSL spectrum?
John
John Souvestre \- New Orleans LA \- \(504\) 454\-0899
No, it will be strictly a DC current, with the amperage easily calculated from
the voltage difference between the two ends and the resistance of however many
cable-feet of wire is involved. Not usually a big deal, unless your
termination design didn't include the ability to sink a DC current 24/7.
(Of course, actually measuring the voltage and resistance may be non-trivial 
No, it will be strictly a DC current, with the amperage easily
calculated from the voltage difference between the two ends and the
resistance of
however many cable-feet of wire is involved. Not usually a big deal,
unless your termination design didn't include the ability to sink a DC
current
24/7.
(Of course, actually measuring the voltage and resistance may be
non-trivial 
That brings up an interesting question. I assumed the ground potential
stays the same between 2 points, but have there been any studies to see
if it's actually DC, or if there's an AC component to it?
If there's an AC component in the ground at either end (or both) that
may introduce EM into adjacent pairs across the cable. And are they more
or less than the EM ungrounded pairs would pick up?
Ken Matlock
Network Analyst
303-467-4671
matlockk@exempla.org
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That brings up an interesting question. I assumed the ground potential
stays the same between 2 points, but have there been any studies to see
if it's actually DC, or if there's an AC component to it?
Thaat's not a safe assumption, since most power companies use earth
grounds for their distribution systems. That means that potential
between two points, and the current through the ground between those two
points, may vary depending on what's happening in the electrically-near
parts of the power distribution system. That's not a happy thought, but
it is Real Life.
It's one of the reasons we went to fiber between widely-separated
buildings in our field sites.
In my experience, there are AC and DC components both. They're generally
-- but not always -- negligible, unless something goes wrong or one end
of the line takes a lightning strike, in which case "ground" can rise to
bunchty KV.
If there's an AC component in the ground at either end (or both) that
may introduce EM into adjacent pairs across the cable. And are they more
or less than the EM ungrounded pairs would pick up?
Whatever is picked up by ungrounded pairs should be common-mode -- the
same on both wires in the pair. Even if it is induced into the "live"
pairs in the bundle, it shouldn't affect signalling. In theory, that is.
You could "ground" then via some small capacitors. This would block DC and
the low frequency power line trash and even act somewhat as a fuse should
there be a lightning strike.
John
John Souvestre \- New Orleans LA \- \(504\) 454\-0899