[asterisk-users] Echo...

[Eric "ManxPower" Wieling]

Ken Williams wrote:
> I've never understood why people would think it's a PSTN issue. 

Echo *IS* a PSTN issue.  Telcos have been dealing with this for years. 
Telcos do have high latency paths (granted less so these days).

Cell companies use hardware echo cancelers to remove the echo.  Cell 
phones are high latency devices so users will hear the echo that is 
inherent in the PSTN unless it is removed.

Long distance companies use echo cancelers too.  These days latency on 
toll calls tends to be so low I doubt EC is still required in most 
situations.  When calls go over satellite or very, very long runs of 
copper (thousands of miles) there will be echo.

I get my hardware echo cancelers from eBay.  Many of them still have the 
telco sticker on them (usually AT&T).

Echo is not new and is not unique to Asterisk or VoIP.  It's just that 
Asterisk and VoIP are most people's exposure to high latency voice. 
Echo is usually canceled at the interface between the high latency leg 
of the call and the low latency leg of the call (Asterisk). If all legs 
of a call are digital (PRI into Asterisk and ISDN or cell or something 
else digital on the other end) there will be no echo.