kayser at sympatico.ca
Wed May 7 15:02:39 CDT 2003
>DSL Line conditioners, or filters, are installed between the incoming
>telephone line and each telephone set. The blurb states that "the
>in-line DSL filter eliminates all erratic impedance from telephone
>equipment that interfere with DSL".
>See for instance:
>Does anyone actually know what this is supposed to mean?
Yes, the narrative description, "the in-line DSL filter eliminates all
erratic impedance from telephone
equipment that interfere with DSL", is really quite accurate. The problem
is really to isolate the on and off hook analogue telephone connection(s)
from the echo canceller in the front end of the DSL box. This was always
the major problem from the very beginning of digital loops, even back in
the ISDN days and before that, was to get adequate performance out of the
hybrid echo canceller.
Back so many years ago, in fact back to January 1980 when I returned to the
telephone industry my first action was to quickly fund a digital loop
product into Northern Telecom using a Ping Pong technology. This came out
in Northern DMS products as Datapath and it was done with Ping Pong
transmission, time compression multiplexing, because we simply could not
get enough performance out of the then A/D and D/A devices to do an echo
Later, before the ISDN standards were settled, we managed to get a decent
ISDN hybrid (it was a huge thing physically) working with the 2B1Q code,
just in time to severely test the silicon manufacturers with their ability
to put A/D's with low enough noise performance into DMS line card chips for
the ISDN era.
No sooner had the NT manufacturing people solved the silicon yield problems
that we raised the bar and made the first HDSL boxes using 2B1Q coding at
about 800 KHz so we had a complete T1 on two copper loops for local
distribution. This was done in 1987so long ago in todays terms. AT&T was
promoting a multi tone concept for HDSL which required a few dB of
additional A/D performance which was eventually achieved and we all know
what happened with the ADSL market.
So in fact the issue remains that every pots circuit action, if it is seen
by the ADSL hybrid process, causes the ADSL up top to have to what we used
to call it "retrain" - that is balance the echo canceller again. The
canceller range in the vastly cost reduced silicon being used now has such
a narrow training range that the analogue filters are essential to separate
the impedance changes and the make and break noise burst from being seen by
the canceller. The worst impedance problems on pots lines come from the
random occurrence of ON and OFF connections of multiple POTS telephone
sets. There is another effect as well but at least it is a constant, that
is the effect of random pots sets going ON and OFF and the "end effect" of
the bridge tap created by wire in the telephone cable plant that keeps on
going past the subscribers termination point. If the bridge tap is long
enough it will kill almost any digital loop transmission system.
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