[Asterisk-Users] Echo Issues

[Kris Boutilier]

> -----Original Message-----
> From: asterisk-users-bounces at lists.digium.com
> [mailto:asterisk-users-bounces at lists.digium.com]On Behalf Of Callum
> McGillivray
> Sent: Tuesday, June 21, 2005 6:38 PM
> To: asterisk-users at lists.digium.com
> Subject: [Asterisk-Users] Echo Issues
> 
> 
> Hi all,
> 
{clip}
> Some of the calls we are receiving have echo at the our end 
> (we can hear ourselves speak).

This is expected behaviour for a call that has been placed through the PSTN where the echo canceller on your side that 'faces' the PSTN (ie. the zaptel mec2 module) is misbehaving and allowing the reflected signal that is originating in the PSTN to bleed through. Normally if you're using ISDN termination on your end then, with the exception of acoustic coupling within the handset or within the room (if using a speakerphone), you will never generate an echo audiable by the party remote to you.

> We have a traditional ISDN telephone system here as well and 
> when I make a call from one of the handsets asterisk answers, directs the call to 
> one of our representatives and the calls are completely 
> clear, however when we receive calls from customers at home, we often hear a lot of 
> echo (not all the time, just sometimes).

I assume you mean that you're calling out to the PSTN and then back into your Asterisk system via the public ISDN line. This suggests that the signal coming in from the PSTN is intermittantly distorted or otherwise malformed, relative to mec2's ability to process it. Common issues include signal levels that are too 'hot' (ie. the echo canceller cannot distinguish the echo from legitimate far end speech), echo endpath lengths that are 'longer' than the maximum length of the echo canceller (ie. calls that are passing over a digital cordless phone), excessive (relative to the mathmatics ;-) noise being imposed on the returning echo and so on...

Additionally there may be issues with your Asterisk host that are affecting the ability of the mec2 code to correctly process the echo. Others have suggested success with: resolving zaptel IRQ issues, resolving processor load issues, correcting module version issues and, often, replacing the hosts motherboard with an entirely different manufacturers unit.

> We have echo cancellation turned on and aren't really sure how we can get rid of this.
> 
> Can anyone offer any suggestions ?

mec2 echo cancellation is a dark art. For some it works just fine, for others it's entirely broken, for those such as yourselves it's misbehaviour is intermittant. Enough people have problems that it seems there is something subtly wrong or not accounted for in the code - the basic pattern of that type of echo canceller performs well enough under lab conditions... I made a stab at documenting mec2 last year in the hopes someone more knowledgable would pick it up and discover the root problem - you can find a patch at http://bugs.digium.com/view.php?id=2820.

Alternately, explore deploying a hardware echo canceller. There are notes on the wiki regarding installing a Tellabs 257x with Asterisk.

Hope that helps.

Kris Boutilier
Information Services Coordinator
Sunshine Coast Regional District