[Asterisk-Users] Echo in CAPI channels

[Peter Svensson]

On Fri, 29 Oct 2004, Derek Conniffe wrote:

> I've been wondering about this too.  I've now got two telephone systems 
> side by side - my old system is an analogue PBX connected to ZyXel 
> routers (Prestige 100s) which give me POTS lines from the ISDN NT1 boxes 
> and its only since I've started with Asterisk that I've come across echo 
> (I have echo problems with X100P cards and analogue lines too). 

Are you using TDM all the way, or do you have a voip hop somewhere? When 
running a pure tdm connection (i.e. one with only isdn, t1,e1 or analog 
lines) the round trip time is usually so short that any echo mixes with 
the sidetone (immediate feedback of the microphone to the earphone) that 
it is not perceived as an echo. For longer links the telco will switch in 
an echo canceler unless the fax/data guardtone is detected.

When a voip link is introduced you get a lot longer latency due to the
packetization, making the echo noticable. 

Echo that you can hear and find obejctionable is usually introduced by 
imperfections in the transmission path *at the far end*. Most of the time 
there is nothing that can be done about the far end. As long as 2-wire 
analog connections are in use some will reflect part of the energy from 
you back towards you. An echo canceler can help.

Peter