[asterisk-users] Question about echo cancelation

[Tilghman Lesher]

On Tuesday 14 October 2008 02:10:39 Olivier wrote:
> 2008/10/13 Tilghman Lesher <tilghman at mail.jeffandtilghman.com>
>
> > <snip>
> >
> > Pray tell, how do you echo cancel in both directions?  Wouldn't that
> > necessitate cancelling echo before it occurs on the line (sort of a white
> > noise/pink noise kind of operation)?  Seems like modelling a projectile
> > such
> > that when it reaches its target, the atmospheric stresses during its
> > flight turn it into a perfect sphere (and with about the same likelihood
> > of success).
>
> So, bottom line is "you can't cancel echo in both directions", right ?
>
> I read from
> http://lists.digium.com/pipermail/asterisk-dev/2006-January/017774.html
>
> : <http://lists.digium.com/pipermail/asterisk-dev/2006-January/017774.html>
>
> "Generally speaking there is only one direction of echo cancellation
> needed: towards the 4-wire (digital) party. Any echo heard by the 2-wire
> party is normally acoustic echo, which we don't try to alleviate."
>
> It seems it could also have been written this way :
> "Echo cancellation in both directions can't be done and generally speaking
> there is only one direction of echo cancellation needed: towards the 4-wire
> (digital) party. Any echo heard by the 2-wire party is normally acoustic
> echo, which we don't try to alleviate."
>
>
> Anyway, excuse me if my question seems naive but, when using a media
> gateway between PRI (digital) and VoIP, which are respectively the 2-wire
> and the 4-wire sides as both technologies use 4-wire connections ?
> If Alice is on the VoIP side and Bob on the ISDN/PSTN side, which one will
> benefit from EC ?

Neither.  As both sides are digital, no 2-wire echo is possible, only acoustic
echo.

-- 
Tilghman