[Asterisk-Users] Echo in CAPI channels

[Derek Conniffe]

I'm telephone company connections only (due to only having a 64Kbps 
fixed internet connection).

Its definitely relating to the far end because it only happens when I'm 
talking to a person using an analogue line on the far end but the 
question asked by Robert Rozman a couple of hours ago is quite 
interesting: how come the echo doesn't happen with other ISDN telephone 
equipment and only with Asterisk?  I've been using ISDN for years now 
and the first time I ever heard any echo was with Asterisk.  Unless you 
are right about the echo only being noticable because of the additional 
delay by having Asterisk in the system (I'm running Asterisk on a 
Pentium III 500Mhz with SCSI disks and the server doesn't do anything 
else in life) this could be it.

Thanks,

Derek

Peter Svensson wrote:

>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
>
>
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-- 


Derek Conniffe
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