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tom Holden · Guest

Just upgraded from the Nortel 1-meg modem to the Alcatel 3mbps modem
with Bell Canada's Sympatico high speed consumer service. Was
initially unimpressed with the 1.1 mbps download speed tests (cf 800
kbps on the old platform). Became singularly depressed with slowdowns
- sometimes dropping to below 15kbps - and virtual disconnects.
Suspected some issue with my SMC barricade g wireless router as it has
been flaky on the wireless front. Substituting my old 1-port Linksys
router initially showed speed tests up to 1.6 mbps but became
similarly sporadic. Re-booting modem or router after an extreme
slowdown might restore speed temporarily and might not.

The modems were both located right at the line entrance. One filter
(upgraded with the 3-meg) branched off to feed a couple of phones.
Another couple of lines branched off to phones with filter at the
phone (in support of my pre-router "Access" dial-up). Disconnecting
the local filter (and its phones) resulted in pretty consistent 2.6
mbps speed tests, without the fatal slowdowns. Great DSL but no
kitchen phone! Restoring the connection with a different filter
degraded back to the slow, unreliable connection so not a defective
filter. Relocated the modem an extra 30 ft or so to be adjacent to
router to see what was going on. With the local filter disconnected as
before, 2.6 mbps. Now here's the surprise: re-connecting the local
filter at the line entrance did not degrade performance. Now I've got
great DSL and a kitchen phone!

The only change is that the modem is now about 30 ft of phone wire
more distant from the line entrance and the filter that branches from
there. Why would adding cable improve performance when little change
or slight degradation might be reasonably expected?

I think transmission line theory provides an explanation. The 3 mbps
DSL spectrum must extend into the radio frequency range of at least a
few MegaHertz and maybe even above 10 MHz. The phone cable has a
characteristic impedance at these frequencies and behaves as a
transmission line. The filters and phones present a complex impedance
that is not a perfect match to the transmission line. At impedance
mismatches, a fraction of the energy proportional to the magnitude of
the mismatch is reflected. Such reflections also occur at the junction
of two different kinds of cable having different characteristic
impedances and wired in series. These reflections bounce back and
forth getting progressively weaker but the time delays of the echoes
distort the shape of the pulse train at the receiver and can cause
(jitter) errors in distinguishing between 1's and 0's. I suppose that
displacing the modem an extra 30 ft or so changed the timing
relationship of these echoes in a favourable way for me. The opposite
might be the case in another house, or another part of this house. As
they say, your mileage might vary.

Knowing more about the DSL spectrum and the transmission line
properties of household phone wiring might suggest optimal lengths for
modem or filter displacement that may be tried for possible
improvement.

Does anyone else share my theory and have more information on
optimising household phone wiring for DSL performance?

Tom

Tom · Guest

Found this thread to be of relevance:
http://groups.google.ca/groups?q=adsl+transmission+line&hl=en&lr=&selm=C3rKb.11437%246B.186%40newsread1.news.atl.earthlink.net&rnum=6

Talks about the adverse effect of open bridges, i.e., transmission line
stubs teed off as branches from the main line - typical of household
phone wiring. Unterminated ones (no device connected) have the worst
effects although all visible to the DSL spectrum will have some effect.

My next objective is to rearrange the wiring at the entrance so that
the line splits to two branches, one unfiltered to the DSL modem, the
other to one filter to feed all the voice branches. That should
minimise the effect of the now several branches. However, I'm unsure
about the effect of multiple branches off the one filter. Hopefully,
this configuration will be more stable and independent of which
branches have phones and other devices connected.

Tom

Robert Redelmeier · Guest

Tom <holden_family@sympatico.ca> wrote:

w_tom · Guest

Obtain basic facts first. Your phone line typically
connects to the telco network at an NID box. Open that box.
Temporarily unplug the wire to your house. Plug your DSL
modem directly into the NID. Now test. This is the best you
are going should you rewire the house.

Multiple branches off one filter does not cause any
problems. To those other POTS devices (fax, telephone, etc),
electrically, the filter does not even exist - is completely
transparent.

If you don't get sufficient performance with a direct
connection to NID, then you have put the 'burden of fixing' on
your DSL provider.

Tom wrote:

David Lesher · Guest

"Tom" <holden_family@sympatico.ca> writes:

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