[Asterisk-Users] Channel Bank Echo

[Lyle Giese]

A T1 is a two way transmission media.  There is sound going both ways over a
'4 wire' interface.  4-wire means that Xmitt is seperate from Recv.  In the
channel bank there is a 4 wire to 2 wire conversion.  It's this junction
that introduces echo as some of the 4 wire recv gets feed back into the 4
wire xmitt direction.    Echo cancelling has to take place in the 4 wire
media path.

Lyle

----- Original Message ----- 
From: "Matt Schulte" <mschulte at netlogic.net>
To: "Asterisk Users Mailing List - Non-Commercial Discussion"
<asterisk-users at lists.digium.com>
Sent: Saturday, January 29, 2005 12:55 PM
Subject: RE: [Asterisk-Users] Channel Bank Echo


Agh, what I meant was the echo is heard from the PSTN side. It seems
echo canceling on the T1 (going to channelbank) does nothing, I'm
assuming because the T1 is "digital" and the channelbank is the
traversal from digital to analog.


-----Original Message-----
From: Peter Svensson [mailto:psvasterisk at psv.nu]
Sent: Saturday, January 29, 2005 12:49 PM
To: Asterisk Users Mailing List - Non-Commercial Discussion
Subject: Re: [Asterisk-Users] Channel Bank Echo


On Sat, 29 Jan 2005, Matt Schulte wrote:

> We are a voip terminating company, we're using Channelbank with FXS
> modules, Rhino, CAC, etc.. What we're wondering is, is how to would
> you echo cancel a channelbank. Of course we're realizing that
> cancel'ing on the T1 (on Ast) does no good (we think?) because the
> analog conversion is at the channelbank. Suggestions? Lowering the
> gain helps but we're looking for a real solution to this. Thanks.
>
> <PSTN?> -- <VOIP Network> -- <Asterisk> -(T1)- <channel bank> -- 
> <analog>
>   ^echo heard


If you hear the echo at the marked analog endpoint then it is almost
certainly far end echo. This is nearly almost present when calling an
analog phone at the far end. On short links without VoIP the reflected
energy will sound like a nice sidetone. For longer links (e.g.
international) and VoIP you need an echo canceler in the call path.

Since you have an analog phone attached to an endpoint there may be an
echo heared from the pstn as well.

Both these echos can be reduced by adding an echo canceler that has its
tail (i.e. subtracts the right amount of the slightly delayed
transmitted
signal from the received signal) into both directions. Asterisk can act
as
such an echo canceler. Asterisk may not be the very best echo canceler
available, but it may be good enough. Try it and see.

Peter


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