[Asterisk-Dev] False echo created by large read buffer in chan_zap.c

[Kevin P. Fleming]

Joel Daniels wrote:
> We don't use IAX, so I haven't tried it, but I assume it would work
> (Technically Speaking).  However, I don't think it would do you any good.
> Because IAX is VoIP, and unless you have a very good network, and have tuned
> asterisk to send tiny packets (ie. less than 20ms) then there will still be a
> lot of latency between your PSTN gateway, and your PBX.  If you are using VoIP
> then the only choice I am aware of is an echo canceller.  If the echo canceller
> built into asterisk doesn't do the trick, then you can buy a hardware echo
> canceller that goes directly on your PRI line between your PSTN gateway and the
> Telco.  At least so I have read, I haven't actually used a hardware echo
> canceller, so I expect that those on the Asterisk Users list who have used one
> could better enlighten you.

We have a full-duplex gigabit link between those two servers, on the 
same switch. That would qualify as a "good network" :-)

We only have occasional echo problems, and the PRI server is running an 
Asterisk CVS pull from about two months ago, so I'm not really ready to 
do anything major until it gets upgraded to something more recent.

And, as you mention, your trick really applies mostly to pure Zaptel 
installations, not SIP or IAX.

>    Theoretically, there might be a way to force Asterisk IAX to have very low
> latency.  However this would only work on a very good LAN, and would represent
> a huge bandwidth wastage, because you would have to send a whole IP packet for
> every 2-5 milliseconds of data.  This would mean that you would have only 16-40
> bytes of audio on each packet!  You can always try, but I don't know how much
> success you will have.

Yeah, that would require a dedicated Ethernet between the machines I'm 
sure, and even then you'd be dramatically increasing the interrupt load 
on the PRI server, which already has to handle the Zaptel interrupts. I 
don't think I'll try that!