[TriLUG] "IP Passthrough" and AT&T 1-Wire

Rob Lockhart rlockhar at gmail.com
Sun Mar 15 15:53:11 EDT 2009


On Sun, Mar 15, 2009 at 14:36, Brian McCullough <bdmc at bdmcc-us.com> wrote:

>
> That's an interesting thought.  My issue is a ( 25? ) year-old house
> with the original telephone wiring, among other things.  When the tech
> came out, he plugged into the box on the outside of the house and
> measured between 3.5 and 4 Mbps with a "very good" noise factor.
> Inside, where the modem is attached, he was just barely 3, with an
> "adequate" noise value.  Aside from rewiring things ( which is in the
> plan ), his suggestion was replacing the box with one that he had on his
> truck.
>
>
I don't know why this is so hard to find, but your best bet is to use a NID
filter and use one of the unused pairs of wires in your house.  Throw those
microfilters in the trash.  Most homes are wired as either one continuous
phone wire, from jack-to-jack (best) or a star network to each jack from the
NID (best) or a combination (nightmare).  In the ADSL frequencies, it looks
like a bridge tap, so imagine that the impedance mismatch (100-ohm at ADSL
frequencies) caused by multiple 100-ohm wires in parallel.  Causes a
reflection, and thus the ADSL modem has to compensate as best it can (by
"dumbing down" the bits/tone across the band).

So, best thing to do is to use a NID filter, which has three wire-pair
connections:
1) common, from AT&T
2) to phone lines (filtered at 4kHz or just above that)
3) to DSL modem (unfiltered)

So, if you have a star topology, you'd connect as follows:
1) common, from AT&T - plug this into the jack in the NID
2) filtered, connect this to all phone lines in the house.  Use Red/Green
pairs
3) to DSL, find which phone line you have connected to the DSL modem, and
ONLY connect that pair to the unfiltered side.  Use Black/Yellow wire only

Note that POTS does care about polarity (most phones have a bridge to
reverse it if it's wrongly wired), but DSL does not.  Tip/ring reversal is
part of the ADSL spec (T1.413 and g.992.1).



>
> > I assume if I need to get into the Westel modem for some reason,
> > I will need to reset it to factory defaults, since it no longer has
> > an IP address???
>
> I would expect so.  In most cases, "bridging" makes the box's innards
> inaccessable.
>

Don't know if this is true... it could be a transparent bridge.  In the case
of cable modems, they act like a bridge, but will respond if you ping or
browse to 192.168.100.1 address.

Regards,
  -Rob



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