[Asterisk-Users] Will Echo problems EVER be solved, I'm scared

[Rich Adamson]

> First off, thank you *very* much for this unbelievably informative post!  I've 
> got it saved away now along with Kris Boutilier's adjusting rxgain/txgain 
> post.
> 
> On Wednesday 24 August 2005 17:14, Bruce Ferrell wrote:
> > At the point where the phone line get's to your demarc the is supposed
> > to ba a -2 to 3db reference point, sometimes called a -2 or -3 test
> > level point (TLP).  So that milliwatt tone at that point should read in
> > the range of -2 to -3 dbm.

If I read the above words exactly as written, the above is not true. Maybe
there was a different intent that I'm missing, or, maybe words left out?

I'm reading the words to say "if I put a transmission test set on the 
cable pair just before the pair leaves the central office, the reading
should be in the -2 to -3 dbm range." If that is what you meant, then 
its incorrect. Even the old analog step-by-step switch specs called 
for no more then .5db loss from the milliwatt generator to the cable 
pair (CO distribution frame).

If you mean placing a transmission test set at the customer's demarc (at
the customer's site), the -2 to -3 db is still incorrect for "analog"
pstn circuits. That level _will be_ the 0db generator tone minus the cable 
loss from the CO to the customer's demarc. That cable loss is 100% 
predictable if you know the length and gauge of the copper wires between
the central office and the customer's site. (That "is" exactly how the
engineering spec is set for the less technical telephone installers
to measure after installing a new pstn facility to a customer site.)

If you are referring to a Remote Line Module (where CO lines are extended
into a neighborhood using fiber, T1's, or whatever) and placing a transmission
test set on the customer's cable pair as it leaves that remote module,
then the -2 to -3 dbm range is correct "at the remote module". It is not
correct at the customer's demarc as you still have the physical copper
loss from the module to the customer's site. That loss is still 100%
dependent on the length and gauge of the copper cable to the customer's
demarc.

If you are referring to a customer site that is 100% digital (eg, T1, 
fiber) from the CO to the customer site demarc, then your words
are correct. But, the customer probably wouldn't be using an analog
asterisk interface if they had a clue.

FWIW, the majority of the larger telco's now store a variable in the
customer's online record that represents the length of copper cable
from the customer's physical address to the CO (or remote line module).
That length is used by less technical types to predict whether DSL can
be supported, and, is translated into an "expected loss" value that is
given to the installer (on their service order). That expected loss
value becomes the boggy for the installer to determine whether the
completed installation meets company standards. Any deviation of that
value is suppose to be reported as trouble and resolved before the
service order is completed.

> Ok so since -3dB is 1/2 power we should be expecting a reading of 7422 in 
> ztmonitor if it's a linear reading of the signal.
> 
> > If the milliwatt is arriving at the demarc at the nominal -2 to -3dbm
> > and getting into the asterisk to be measured at 8dBm (+8dbm0), I'd say
> > something is grossly mal-adjusted.  You're seeing 8db of gain!

Agreed, regardless of what type of facility exists between the CO and
the customer's site.
 
> No, he was saying he had to set his rxgain to 8 in order to get a level of 
> 14844 (0dBm) in ztmonitor.

Right, and if we knew the gauge of copper used to that customer's site,
we could calculate exactly how many feet of copper exists between the
customer and the CO (or remote module). An 8db loss is very very common,
and even 12 db of loss is acceptable by some telco's to distant customers.

FWIW, those individuals that are responsible for planning the location
of a new central office or remote line module use programs that calculate
the copper loss from several potential sites to each potential customer.
Sort of like drawing a circle around a potential module location and
asking the question "how many customer sites can be reached that are
within 12 dbm of that module location?" (Substitute some realistic 
engineering value for the 12db.) Part of that planning process actually
involves playing what-if games with things like: 1) if I use 26 gauge
cable to every customer from that potential site, what is the total
cost of deploying a remote module? 2) if I use 19 gauge copper, what's
the total cost?  (19 gauge copper has a much lower transmission
loss then does 26 gauge, and can reach more customers. However, the
cost of 19 gauge copper is much greater then 26 gauge so some sort of
economic trade off is obvious.)

I fully understand the TLP point referenced in the post, but the words
that were used is going to lead other readers to an incorrect conclusion.

The echo cancellation problem with the x100p and TDM cards are very
much related to the narrow operating range that the existing echo 
cancellation software can operate within.  In the above 8 db loss
example (assuming there is an actual 8 db of copper loss between the
CO and the customer's site), the _correct_ rxgain and txgain settings
would be to provide approximately +6 dbm of gain within asterisk/zaptel.
Those gain settings would fall within the stated -2 to -3 db TLP range.
However, that 6db of gain (in both directions) will fall outside the
operating range of the existing echo canceller, therefore smaller
gain values are almost always required in asterisk. It doesn't make
any difference whether the transmission level is measured with 
ztmonitor or a $10k transmission test set. Neither will make it work
right.

I think it was Steve Underwood that commented on the echo cancel
routines some time ago (and if you look in zaptel code you'll see
where he attempted to provide another alternative). The issue of
recognizing reflected energy (eg, echo) and providing some sort of
subtractive routine to remove that energy is very complex. Its very
easy if the routine only has to deal with a single tone (eg 1000 hz),
but is very difficult and processor intensive to deal with harmonics,
male vs female voices, soft vs loud speakers, local vs distant sources
of reflected energy (latency), etc. The companies that engineer dedicated
echo cancellers can throw processing power at the problem or use
dedicated chips (DSP's) to do that function. Not likely asterisk's
canceller will ever be equivalent to dedicated hardware cancellers.
(And, why does the new digium T1 card have it on board?)

One other point that may not be obvious from previous echo postings.
Those asterisk users that are physically located a greater distance
from the CO _always_ have greater echo issues. Those that are relatively
close don't have as big an issue. That _is_ due to the small operating 
range of asterisk's echo canceller. And, that is _one_ of the reasons 
why one implementor's settings don't work at another implementor's 
location. (Not to mention differences in motherboards, etc, etc.)

Rich