Causes of echo
In Brief:
Its most likely your echo is caused by the far-end point. The loudness of the echo on VoIP calls is no worse than PSTN calls. The difference is that because of the inherent delay induced by VoIP, echo is much more noticeable.
Remember, for echo to be noticeable it has to be both loud AND delayed. In the normal PSTN world echo is loud, but NOT delayed therefore you don't notice it. Its for this reason telcos almost NEVER have echo cancel for local calls (hardware echo cancellers are expensive so why put them where you don't need them?). This is why in many cases you will notice bad echo on local calls but not on long distance or calls to cell phones. Long distance and cell phones always have echo cancellation. Some local calls may also have it which is why echo is often intermittent.
Solution: The only thing you can do is implement echo cancellation as close to the far end point as possible. Since you don't control the telco, most likely that is whatever you have connected to the PSTN (Asterisk box).
This page http://www.aussievoip.com/wiki/index.php?page=EchoInfo has a very good explanation.
Its most likely your echo is caused by the far-end point. The loudness of the echo on VoIP calls is no worse than PSTN calls. The difference is that because of the inherent delay induced by VoIP, echo is much more noticeable.
Remember, for echo to be noticeable it has to be both loud AND delayed. In the normal PSTN world echo is loud, but NOT delayed therefore you don't notice it. Its for this reason telcos almost NEVER have echo cancel for local calls (hardware echo cancellers are expensive so why put them where you don't need them?). This is why in many cases you will notice bad echo on local calls but not on long distance or calls to cell phones. Long distance and cell phones always have echo cancellation. Some local calls may also have it which is why echo is often intermittent.
Solution: The only thing you can do is implement echo cancellation as close to the far end point as possible. Since you don't control the telco, most likely that is whatever you have connected to the PSTN (Asterisk box).
This page http://www.aussievoip.com/wiki/index.php?page=EchoInfo has a very good explanation.
The phenomenon known as echo - whereby one or both parties hear what they just said a few milliseconds later - is one that seems to affect VoIP users frequently. It can range from being mildly irritating, to being utterly unacceptable.
It's important to fully understand the causes of this effect before you can take measures to eliminate it. This page attempts to explain clearly where echo comes from. Only once you've read and understood the explanation given here should you consider Asterisk echo cancellation.
Typical telephone connection
Leaving VoIP out of the equation for a momrny, a typical peer-to-peer telephone conversation looks a lot like this:
>)--<-+ 4-wire trunk +->--(<
| +---------------->--------------+ |
| 2-wire local loop | | 2-wire local loop |
Hybrid <---------------> Hybrid Hybrid <---------------> Hybrid
| | | |
| +----------------<--------------+ |
o)-->-+ +-<--(o
Caller Callee
This shows a (slightly simplified) connection, with two-wire segments between the two subscribers and their local exchanges, and a 4-wire (transmit and receive pairs) line between the exchanges, and within the telephones themselves. There are "hybrids" at each end to divide the 2-wire local line into the 4-wire trunk, and to connect the telephone microphone and earpiece to the 2-wire local lines.
The 2-wire local loop is often known as a POTS (plain old telephone service) line.
Sources of echo
There are several points in the above system where echo can be introduced.
From the caller's perspective, these are:
- Within the caller's telephone; a certain amount of the signal from the microphone is fed straight back to the earpiece, called "sidetone." This is often done by design (see also echo and sidetone ), and in any case, is not a problem here - more on why later. A particular special-case of this is a poorly-configured analogue (eg TDM400P) card - for example, the default (FCC) is not suitable for the UK.
- At the hybrid at the callee's end. An improperly balanced hybrid won't correctly filter out all of the transmitted signal, and will reflect some of it back down the trunk. Imbalance may be from poor design (common) or unpredictable impedance conditions on the POTS line (very common). ...