[Asterisk-Users] PRI echo issues: solvable?

[alan]

Hello,

After solving the other "low hanging fruit" audio issues in our Asterisk
PBX, we are left with occasional cases of severe echo which we have not
found a solution for yet.

Our system:
- Asterisk 1.2.0-beta1
- TE110P on a PRI
- TDM04 and TDM40, but these are unrelated to current echo issues
- Fedora core 3
- Echo canceller KB1

Most calls have minimal, acceptable echo levels. But occasionally, we
get a call where the echo is delayed by a substantial amount (sometimes
around 250ms), and sounds as loud as the remote party.

One example: when one number (local to the same CO as our PRI) calls us,
the echo on our end is unbearably bad. When we call them, No Problem.

Am I right in guessing that we're unlikely to solve this in a
system-wide manner on our end, and at best we'd have to convince
the phone company they're misconfigured, for one remote phone
number at a time?

Some other specific questions:

- Gain tuning: Is the ztmonitor quantitative target value 14500 or
  14844?  These two sources conflict on this point:
  http://www.voip-info.org/wiki/view/Asterisk+zapata+gain+adjustment
  http://lists.digium.com/pipermail/asterisk-users/2004-November/071301.html
- Is the difference between 14500 and 14844 big enough to worry about?
  If my gain settings are incorrect, am I going to be seeing
  quantitative values a few hundred away, or ten thousand away
  (for example)?
- Is gain tuning effective using CO or asterisk-local milliwatt sources
  useful on a PRI line? Presumably, the path to the local CO's milliwatt
  line is all digital, and the loopback path to call our own internal
  milliwatt source will almost definitely be all digital, so where would
  the loss come from?
- Assuming gains are tuned correctly, or don't matter for the PRI, is
  there any other hope I have for solving these echo issues?

Thanks,

Alan Ferrency
pairNIC
pair Networks, Inc.
alan at pair.com