[Asterisk-Users] RFD: With echo and other distortion, can ulaw/alaw line quality ever be good enough for faxing?

[Darren Nickerson]

Folks,

I've been following recent discussions regarding echo and echo training with
much interest, since it's a problem we've never been able to eliminate here.

We're facing two challenges presently, and they may be related (or not):

a) Cisco 7960s in the office here echo back to our staff, but the customer
hears decent sound.
b) We've yet to be able to pass faxes through asterisk, despite trying a
number of approaches

A bit about our setup:

Two analog lines come into the office, into the FXO card of an CAC ADIT 600.
These are 'connected' to the TDM interface of the ADIT, and go out to
Asterisk via 2 channels of a a 24-channel fxs_ks line into a port of a
TE405P. We have two other ports on the TE405P occupied by digital fax
boards, connected to asterisk via AT&T 5ESS PRI. And of course there's the
7960 handsets, connected directly to a switch at 100MBps (fdx).

Challenge a)
=========
In terms of looking into a), we started with:

http://asterisk.sohoskyway.net/Asterisk_Doc/current/docs-html/x939.html

Clearly some of the details are outdated, since the echo canceller options
live in zconfig.h now, not a Makefile, but it's a decent starting point.
Which gives rise to my first question - which echo canceler should we be
using? We have Steve, Steve 2, and Mark 1-3, with an aggressive option with
3 that could make calls scratchy ;-)

Is there any conventional wisdom here?

Next question ... we have control over gain at the zaptel level, and also at
the ADIT level apparently. Which should we use when trying to adjust things
with ztmonitor?

Next question ... it's not clear to me what the target is with ztmonitor.
Are we shooting for tx and rx levels that are balanced. and about 50% of the
scale at their max energy? I found that when I called a voicemail service
and listened to the auto-attendant that the RX meter of ztmonitor was about
50%, but I found the TX meter fluctuated wildly when I spoke. I could reduce
my voice level slightly and it would be very low energy, but with a slight
increase in volume it was pegged. The settings I made seemed to do nothing
to change this (I stop asterisk, unload and reload all zaptel modules, and
restart astertisk between each test).

Is this really supposed to work? I've managed to get the connection
extremely distorted with some settings, but have yet to make a change that
improved the quality or removed the echo.

Challenge b)
========

In some ways, this is the most worrying problem for us, since we would love
to be able to pass faxes through asterisk reliably, and from the traffic on
this list and the fine efforts of Mr. Underwood, it seems many others would
too. People have spoken here of the nearly immediate echo cancelation on
pure TDM circuits, and so I would have thought we'd escape the echo problems
above in either of the following setups:

i) Plain ole' HP fax machine plugged into TDM400 FXS card
ii) Brooktrout/Eicon fax board connected to TE405P

In practice we're rarely able to train with the remote fax machine when
dialing to an outside line through asterisk and the ADIT 600. If we do
manage to get connected and the connection supports ECM error correction, we
can see lots of retransmitted frames which again points to very poor line
quality. In contrast, by connecting directly to the analog lines we can send
faxes all day with the trusty little HP ... ie: the lines aren't inherently
bad.

I guess what I'm looking for here is a sanity check ... are we trying to
push the limits here, or should this stuff be working much better than this?
We're willing to invest some time getting faxing to work, but I'd hate to
ask people here to dedicate their time and energy to something that's never
really going to work well due to limitations in Asterisk/zaptel technology.

-Darren

--
Darren Nickerson
Senior Sales & Support Engineer
iFax Solutions, Inc. www.ifax.com
darren.nickerson at ifax.com
+1.215.438.4638
+1.215.243.8335 (fax