Asterisk-Users Digest, Vol 23, Issue 135

[Michael Wallette <mike (at) gecko-ak.org>]

There's an excellent tutorial on Cisco's web page at 
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_white_paper09186a00800d6b68.shtml

It will tell you just about everything you wanted to know about echo and 
more :)

The short answer to your question, however, is that echo is comprised of 
two components: volume and delay. Increase either one and the problem 
gets worse. In the PSTN, the delay tends to be rather short (25ms or 
less) for most calls, and therefore, tends not to be noticed by the 
average listener.

In a VoIP call, on the other hand, you must first translate the audio 
signal into digital, with a subsequent delay, pass the digital data 
through various routers, each of which add additional delay, then 
reconvert the digital data back into analog audio, which again adds 
still more delay. The sum of all of these delays is more than the 
threshold of 25ms, and as a result, what may have been an imperceptible 
echo on the PSTN is now a very apparent -- and very annoying -- echo in 
the VoIP call.

--Mike

> Subject: [Asterisk-Users] ECHO Tutorial
>
> Is there anyone that could explain to me the phenomenon of Echo or at  
> least point me where I can learn more? Why is this affecting the VoIP  
> world so much and not the regular PSTN analog world? What does the  
> PSTN industry have that they can handle such high volume of calls and  
> there is "no" echo problem?
>
> Thanks,
> Daniel



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