[Asterisk-Users] TE405P and echo

[Peter Svensson]

On Tue, 22 Mar 2005, McQuiggan, Mark  xt46480 wrote:

> I am using a SIP softphone (X-lite, SIPPS or Firefly) connected to an
> Asterisk v 1.0.3 PBX.  The PBX is also connected via a ISDN-PRI crossover
> cable to a Avaya Definity Generic 3 PBX via a TE405P card.  All outside of
> the office calls go through the Definity.  Here's the issue:
>  
> Calls to internal SIP extensions, Definity extensions, other offices within
> our private network (through the Definity), and cell phones are great.  When
> I call outside of the office to POTS lines (like my home), there is a most
> noticeable echo of my voice.  The party on the line hears no echo.  Any
> efforts on the configuring the SIP softphones, and within zapata.conf, have
> been for naught.
>  
> Is this problem common for ISDN-PRI connections?

It is a problem you will see when calling an analogue subscriber over a 
link with a long latency (such as VoIP). The echo will most probably be 
generated by a 2- to 4-wire hybrid at the far end. In a pure amalogue/tdm 
path you would perceive the reflected energy as a plesent sidetone. As 
soon as the latency increases to 50-100ms the refelcted energy will be 
perceived as an echo instead.

The options available to you are to live with the echo of your own voice 
or to insert an echo canceller at the pstn interface. Asterisk includes an 
echo canceller that may or may not be good enough. It seems to like some 
pstn interfaces and not others. If the Asterisk echo canceller is not 
enough you may consider an expensive inline echo canceller. 

Peter