[PLUG] Asterisk - echo with cisco 79xx handsets

Keith Lofstrom keithl at kl-ic.com
Thu Jan 5 21:09:05 UTC 2012


I expect Wes and Aaron will have useful opinions, but there might
be other lurking Asterisk PBX users on the list.  I'm close to 
finished configuring a small-but-complicated Asterisk setup for
my wife's office - a few logic/dialplan/workflow tweaks remain.

The system connects three Cisco 7940/60g phones to three Centurylink
POTs lines.  I believe we are about three miles from the CO, but
there is likely a nearby remote switch, because the fourth DSL/FAX
line is running 7Mbps, and the loop must be short for that.  We
are in a commercial office building and don't get to tweak on
the wiring onwards from jacks in the office.

=== The biggest remaining issue is echo. ===

The echo may be due to the Cisco handsets, which are functionally
opaque.  The echo on at our end, which sounds like very ringy
sidetone listening on the handsets, ranges from briefly annoying
to completely unacceptable.  There is no detectable echo on
either the voicemail or for those calling in.  I'm guessing
there is some second echo cancellation process that I need
to turn off.

I am running Asterisk 1.8.7.1 - pretty recent, but I am hoping
to never update it unless a major flaw emerges.  The analog
FXO modules are chinaroby via ebay.

I set /etc/dahdi/system.conf with echocanceller=mg2 on all the
incoming analog lines.  I set /etc/asterisk/chan_dahdi.conf with

echocancel=yes
echocancelwhenbridged=yes
echotraining=128
rxgain=9.2

The rxgain was set by calling a 1004Hz 1mW test line in Seattle
and calibrating with dahdi_monitor for a level of 14500 units.
I couldn't find a closer test line, and the service techs at
CenturyLink in Portland don't know what I'm talking about, but
I assume that with digital trunks connecting these Centurylink
COs I am getting the same level of signal.  9.2dB seems like a
lot of receive gain, but it doesn't seem to affect the echo,
and again, the folks at the other end hear us just fine.  

99% of our calls are to the Portland/Vancouver area, we don't
need three way calling, and we need to keep POTS/analog for
reasons I won't dwell on here.  If needs be, we can replace
the Cisco phones, but I am not fond of "buy and try".  

Any suggestions?  The woman answering the phones is ready
to kill me, and she probably knows how to do that in a
forensically undetectable way.  I wouldn't want to subject
her to the remorse, my wife to the burial expense, and you
folks to the subsequent peaceful/boring mailing list.  :-)

Keith

-- 
Keith Lofstrom          keithl at keithl.com         Voice (503)-520-1993
KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon"
Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs



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