[Asterisk-Users] PRI echo issues: solvable?

[Andrew Kohlsmith]

On Tuesday 11 October 2005 11:49, alan wrote:
> 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

Add to the mix
Asterisk CVS HEAD (always within a couple of weeks of today)
Slackware 10.1
TE405 and TE406 (with and without VPM support)
Tried with various motherboards and chipsets (all P4)

> 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.

Yup.

> 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.

I've never seen that, it's always when we call out.  Certain numbers will 
always trigger it.  888-737-4787 (IPC Resistors, it dumps into an IVR so it's 
safe to call) is one such number, but we have local numbers that hit other 
COs which exhibit this problem as well so it's not a specific CO or switch 
problem.

I have just verified today with the 888 # mentioned above that zaptel is NOT 
disabling its echo canceller.  dmesg shows no "disabling echo canceller due 
to tone" message and "zap show channel [affectedchannel] shows that the echo 
canceller is configured correctly (128 taps for us) and is ON.

I hope to do further testing as I am made more aware of the numbers causing 
the issues.

> - 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

I think you want to achieve an effective value of -2 to -3dBm.  I'm unsure 
what "value" this is in ztmonitor -v.  I know this should not be needed at 
all on a PRI or other digital connection.

-A.