[Asterisk-Users] Cisco's description of echo

[Keith O'Brien]

Yes, if you hear echo then the source of the echo is from the far end
equipment.   This is due to the fact that delay is what makes echo
noticeable.  Think of yelling in a small room.  Your voice bounces off all
of the walls and returns to your ears but you probably don't hear any echo
since the time from when you spoke to the time when the sound was returned
to your ears was too short.  Now think of yelling in a large canyon.   The
time it takes your voice to leave your mouth, bounce off the canyon walls
and return is large enough that you now notice the echo.
 
The same principle applies to VoIP.   If you are hearing echo on your
conversation it is because there is something at the far end equipment that
is reflecting the signal (typically a A/D conversion or 2/4 hybrid).    Now
although the echo is caused by the far end the *near end* echo canceller is
responsible for eliminating this echo which is why when you tweak
zapata.conf you notice a difference.   Essentially the near end echo
canceller is looking at the outgoing signal and analyzing the return signal
for the same wave form.   
 
Also, keep in mind that *delay* is what really causes echo.   As the delay
in your ISP increases the echo will become more noticeable (remember the
canyon analogy)   If you can reduce delay through QOS or some other
mechanism you should also be able to reduce echo.   
 
Another common problem that causes echo in networks is not setting your loss
plan correctly.    You need to be sure that you aren't coming in too hot at
any of your analog interfaces.   In general you should see a signal between
-20dbm and -12dbm when someone is talking on the line.   If it is
significantly hotter then you run the chance of having a larger reflected
signal resulting in echo.   I typically try padding down analog levels by
3dB at a time to see if echo is reduced.   In the canyon analogy this
equates to yelling louder which in turn makes the echo louder.
 
-----------------------------------------
It claims that echo always comes from the far end of the connection.  So if
I
hear echo, then the origin of the echo is in the equipment on the end of the
line near the person to whom I'm talking.

The description seems to make sense, but the zapata.conf setting for echo
cancellation seem to also help echo on the near end of the connection.

I have read about echo on the wiki and in the mailing list, but it almost
always discusses it with respect to the digium cards, not SIP alone
 
 
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