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<DIV><SPAN class=279361615-26032005><FONT size=2>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.</FONT></SPAN></DIV>
<DIV><SPAN class=279361615-26032005><FONT size=2></FONT></SPAN><SPAN
class=279361615-26032005><FONT size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=279361615-26032005><FONT size=2>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. </FONT></SPAN></DIV>
<DIV><SPAN class=279361615-26032005><FONT size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=279361615-26032005><FONT size=2>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. </FONT></SPAN></DIV>
<DIV><SPAN class=279361615-26032005><FONT size=2></FONT></SPAN> </DIV>
<DIV><SPAN class=279361615-26032005><FONT size=2>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.</FONT></SPAN></DIV>
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<DIV><SPAN class=279361615-26032005><FONT
size=2>-----------------------------------------</FONT></SPAN></DIV>
<DIV>It claims that echo always comes from the far end of the connection.
So if I<BR>hear echo, then the origin of the echo is in the equipment on the end
of the<BR>line near the person to whom I'm talking.<BR><BR>The description seems
to make sense, but the zapata.conf setting for echo<BR>cancellation seem to also
help echo on the near end of the connection.<BR><BR>I have read about echo on
the wiki and in the mailing list, but it almost<BR>always discusses it with
respect to the digium cards, not SIP alone</DIV>
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