[Asterisk-Users] Echo cancellation again ...

[Rich Adamson]

> I have been reading with great interest the posts on trouble shooting 
> echo cancellation with *.  Is it just coincidence that all of this 
> discussion has been with analog lines.  Are PRI's susceptible to echo 
> problem like POTS lines.
> 

Keep reading. Echo _can_ occur whenever a two-wire circuit is converted to
a four-wire circuit (eg, hybrid involved). There is no way for you to identify
where (in a pstn call) that might occur even with a PRI. You could have 
a PRI (four-wire) leaving your facility, but the telco (or another pstn
customer) may have a hybrid involved in that end-to-end call.

Hybrids are most often associated with analog circuits, so you'll see a large
number of postings relative to echo on X100P and TDM cards. But, that certainly
does not support 'echo is only a problem on analog circuits'.

It doesn't help that a majority of current telco technicians wouldn't recognize
a hybrid (or echo canceller) if it hit them in the face. Add to that the number
of readers that repeat other postings without any reasonable knowledge of what
they are talking about, and you get a substantial variation in the cause of
echo (and how to troubleshoot it).

Its probably one of the most difficult telephony issues to troubleshoot as there
aren't any real tools available to pin-point the source. It just happens that
asterisk has more then its fair share of echo problems, and you'll find a large
number of postings that suggest asterisk's echo canceller does not have a very
wide operating range.  (E.g., the exact same asterisk system moved from one 
location to another will exhibit different echo characteristics due to the 
differences in pstn lines, etc. Analog interfaces having more echo problems
by far then digital PRI interfaces.)

The companies that specialize in digital to analog interfaces (eg, channel banks,
external pstn adapters) typically have more dedicated processing power to perform
echo cancellation, plus those companies spend a fair amount of research and
development time to make cancellation work correctly. If they didn't, they
wouldn't be able to sell their products.

A true end-to-end PRI will _never_ generate echo; but, the stuff that customer's
(large and small) attach on the other end of a PRI certainly can cause echo.
And, believe it or not, there are still a large number of telco switches that
are purely analog, thus requiring a digital to analog converter (hybrid).