[Asterisk-Users] Why echo occurs

[Eric Bishop]

Hi all,

Can someone give me a simple rational explanation why a $5 analog
handset  gives me no echo whatsoever on an analog PSTN line, but
PSTN-VoIP devices such as the TDM400 and Sipuras do and thus require
software-based echo cancellation. Surely a $5 analog handset does not
have an "echo canceller".

The echo I mean is when I hear myself while talking to another party.

I have heard it said it is because of some slowdown of the signal. But
where is this mysterious bottleneck?

1. It is not in the Asterisk box because IP to IP calls do not suffer
this malady

2. It is not from the Central Office to my premesis because my $5
analogue handset works without echo. Also PRI ISDN works without echo.

Can anyone explain what I am missing?