[asterisk-users] spandsp (foip)

[Jay R. Ashworth]

On Sun, Sep 24, 2006 at 05:17:22PM -0700, Lee Howard wrote:
> So, to answer your questions...
> 
> Why does using G.711 not require T.38?  Because from the viewpoint that 
> the question was given, G.711 and T.38 are competing approaches.  T.38 
> was designed to replace G.711.  You can packetize G.711 audio just fine 
> without converting it to anything else.  So when faxing with G.711 T.38 
> is not involved because its basically mimicking the old-style 
> traditional PSTN faxing, except that the audio is passing over a 
> different (less-reliable) medium.
> 
> So the reason that T.38 exists is because UDP/IP is lossy and is not 
> therefore reliable for the purposes of faxing with G.711 unless the 
> communication can be guaranteed to be nearly lossless.  For those that 
> work on lossy channels, G.711 will just not work reliably.

Lee's answer is more complete than mine (as you might expect from the
fact that he does this stuff for money, and I only speculate about it
:-).  I wasn't thinking about jitter and packet loss... though clearly
I should have, since Fax is a WAN app.

(I'm embroiled in a design project for a big Asterisk switch, and I'm
thinking "over the LAN" this week.  Sorry.  :-)

Cheers,
-- jra
-- 
Jay R. Ashworth                                                jra at baylink.com
Designer                          Baylink                             RFC 2100
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