[PLUG] Asterisk - echo with cisco 79xx handsets

[Aaron Burt]

On Thu, Jan 05, 2012 at 01:09:05PM -0800, Keith Lofstrom wrote:
> 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 assume you also don't get the echo when you do an echo test (*45).

It's been a while, so I had to refer to Cisco's fine documentation:
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080149a1f.shtml
http://www.cisco.com/en/US/products/sw/voicesw/ps1860/products_tech_note09186a00800b4a71.shtml#topicsub1
http://www.cisco.com/en/US/docs/ios/solutions_docs/voip_solutions/EA_ISD.html

Sounds like "hybrid echo" - an impedence mismatch is causing an echo (or
even oscillation) between the FXO on your end and the PSTN "hybrid".

I understand you can use the "fxotune" util to autotune settings.
Also check http://kb.digium.com/articles/Troubleshooting/General-Echo-HOWTO

> The rxgain was set by calling a 1004Hz 1mW test line in Seattle
> and calibrating with dahdi_monitor for a level of 14500 units.

You used something like this procedure?
http://www.mattgwatson.ca/2008/05/howto-tune-zaptel-dahdi-fxo-interfaces-on-asterisk-pbx/

I'm glad you're bringing this stuff up.  I just got the go-ahead to bring
my wife's business telephone system into the late 20th century.
I'm thinking Norstar key system...