Managing free pairs to prevent DSL sync. loss

John Souvestre johns at
Tue Jul 17 19:27:01 UTC 2012

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 Souvestre - New Orleans LA - (504) 454-0899

-----Original Message-----
From: Mike Andrews [mailto:mikea at] 
Sent: Tuesday, July 17, 2012 12:46 pm
To: NANOG Mailing List
Subject: Re: Managing free pairs to prevent DSL sync. loss

On Tue, Jul 17, 2012 at 11:16:07AM -0600, Matlock, Kenneth L wrote:

> 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. 

Mike Andrews, W5EGO
mikea at
Tired old sysadmin 

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