[Asterisk-Users] Echo Canceling

[Rich Adamson]

> > You know what.. I have sporadic echo issues too and I just checked my
> > dmesg and also see that!   What's this all about?
> 
> *STOP*
> 
> You will receive these messages if you send or receive faxes.  I asked for 
> this particular procedure to be executed because I was curious to see if 
> zaptel was seeing an echo cancel disable tone when calling the numbers with 
> extreme echo.
> 
> Again, this is NORMAL to see and EXPECTED if you are sending or receiving 
> faxes.  He's not calling a fax machine (I suspect he's not anyway) so I 
> wanted to make sure that the zaptel echocan was NOT hearing the disable tone.

Identifying why a echo cancel tone is occurring on a normal voice call
is reasonable, but why would a _local_ echo canceller be needed on a
four-wire full-duplex digital link?

If the end-to-end call is digital all the way, there really isn't a need
for it. So, isn't the issue one of who has responsibility for inserting
the echo canceller when a 4-wire to 2-wire hybrid is involved? (Obviously,
its not the originating site since one would have no idea what the 
destination site is doing.) Or, is there an assumption going on that says
an echo canceller is always needed on all pstn calls regardless of whether
is doing anything or not?