[lug] DSL Technical ?'s

Nate Duehr nate at natetech.com
Thu Sep 13 13:59:53 MDT 2001


On Fri, Aug 31, 2001 at 09:50:51AM -0500, Jeremy Hinegardner wrote:
> 1. Why can't Qwest provide any DSL service? Is it because (a)
>    They don't provide SDSL service at all and don't have the
>    equipment at the CO? or (b) bridges/tap/etc on my line
>    to prevent ADSL from working?  

To know for sure, you'd have to ask them.  And they probably won't tell
you, feeling that the information is "too technical" for you.  :-)
Else, you'd be working with a salesperson who just gets a "good/bad"
indication on the order -- no details from their provisioning database.

Most likely, Tim's response is correct -- however his use of the term
"pair gain line" is a bit incorrect.  PairGain is actually a company
who's products made it possbile to stuff two analog voice-grade lines
down the same pair of copper.  They kinda started a trend in residential
service, but were by no means the first to do such things.  There are
other services like channel banks and GR-303 capable units that would
preclude the use of the "extra bandwidth" on the copper pair for DSL or
DSL-style services.  (Ever see the warnings in your DSL boxes that if
you have a monitored security system you have to stop and call Qwest
before you install your DSL modem?  This is because some kinds of
security/fire-alarm systems use signals over the top of your copper
voice phone pair to alert your monitoring company.)

Basically, if your copper pair doesn't go all the way to the CO with
nothing inbetween, and then also has a way to get to your service
provider's DSLAM, you can't get DSL.  It's a complex rats nest of wiring
and in many cases (like my current house) 1960's cable that hasn't been
touched in years.  (I have no grounding point for my phone lines, they
come into the house UNDERGROUND and go through two carbon fuses before
continuing right to the phone jacks... I get a lightning strike in the
backyard, my DSL modem and anything else plugged into a phone jack is a
goner. -- Yes, I'll probably have the drop moved to a more convenient
location and a proper outside demarcation point with a good ground
installed eventually -- at my own expense.  Qwest doesn't care.)

This is getting long, but suffice to say there are LOTS of ways to carry
analog voice service to a neighborhood and provide DC voltage to power
your phone and AC ring voltage.  Many of these methods while great to
voice engineers, are incompatible with systems like ADSL which require
access to the "extra bandwidth" on the copper pair coming into your
home... and most telco's won't bother to explain what the problem is
when you put your phone number into their database and they come back
with only "it doesn't qualify for DSL".  

>    Part of this question is why cannot Qwest just use the
>    existing copper pair that is in place for my SDSL and
>    treat it as a phone line to run ADSL over?  Both types
>    can run over the same line configuration right? I mean
>    I basically have an additional phone line at my house
>    when they installed the SDSL circuit right?  It just
>    happens to plug into Rhythms' DSLAM instead of Qwest's.

SDSL usually requires its own copper pair with no voice on it.  At least
mine did when I had it.  It's not built as a "ride-along" service, or at
least Rhythms DSLAM in my old neighborhood wasn't.

>    An additional interesting note on this one is I was 
>    originally with Jato last fall (which also went out of
>    business) and from what they said, my SDSL circuit was
>    actually installed by Qwest.

Sure.  Qwest owns 99.999 (not a real number) of the outside plant and is
required by regulation to share that outside plant with CLEC-style carriers
like Jato was.  Jato and others rarely do their own outside plant work.
They just place orders with Qwest to do the work and then they ride over
the copper provided.

> 2. Why can't Covad provide SDSL service to me.  They do
>    provide SDSL service.  Is it because they just don't
>    take SDSL service to 16000'?  And if that is the case 
>    What is the technical reason why?  If Rhythms was able
>    to provide a satisfactory SDSL service to 16000' why
>    is Covad unable to?  They are both in the same CO right?

SDSL will drive to 16000' in most flavors, I'm pretty sure.  Covad may
not have an SDSL DSLAM in the CO that serves you, and/or may not have
any way to cross-connect to your copper pair.
 
-- 
Nate Duehr <nate at natetech.com>

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