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

[Joel Daniels]

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.

P.S.
   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.

   Sincerely,
      Joel Daniels
----- Original Message ----- 
From: "Kevin P. Fleming" <kpfleming at starnetworks.us>
To: "Asterisk Developers Mailing List" <asterisk-dev at lists.digium.com>
Sent: Friday, November 19, 2004 12:57 PM
Subject: Re: [Asterisk-Dev] False echo created by large read buffer
inchan_zap.c


> Joel Daniels wrote:
>
> > Also, if this is a reasonable echo solution, would there be any interest in
a
> > patch that would make this option configurable.  It would simplify upgrade
> > procedures for us, and far more importantly, it would give other Asterisk
users
> > a hint that zaptel latency can be reduced.  The patch should be very easy
to
> > write, and if no one else wants to I'll be glad to write it.  If there is
some
> > reason why this should not be a configuration option, then should the trick
of
> > redefining it be posted on the wiki?
>
> Do you know of any problems that would be caused by using this
> configuration on an Asterisk server that is doing _nothing_ but Zaptel
> and IAX? In other words, our PSTN gateway box has Zaptel connected to
> PRIs, and IAX to other carriers, as well as IAX to our Asterisk PBX
> server. In this configuration, nothing is being directly played to
> Zaptel channels, it's all Zaptel<->IAX.
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