[asterisk-users] spandsp (foip)
Jay R. Ashworth
jra at baylink.com
Sun Sep 24 16:20:27 MST 2006
On Sun, Sep 24, 2006 at 01:58:21PM -0700, Christopher Corn wrote:
> I've been reading about FOIP and there something i dont undrestand. maybe
> someone can explain to me.
> A couple of faxing methods im confused about.
> The pass through method, sending fax data over G711 codec
> Relay method, t30 to t38 conversion
> Can someone explain to me why the pass through method doesn't
> require t30 to t38 conversion ( or does it do it?)? i believe
> the conversion to t38 is so that it can be routed through a
> packet network and then back to t30 so that the fax machine can
> understand. why is it that if you use a pass through method, and
> your still passing through a packet network, you dont need to
> convert to t38 and t30?
I believe the proper answer to your question is this:
Fax and modem tones will not be passed properly by some codecs because
they're optimized for the frequency patterns of human voices -- and
because those tones were themselves optimized for a traditional analog
or DS-0 PCM channel.
Since G.711 *is* an uncompressed DS-0, that's why you can pass fax
tones through it, without any help from spoofing protocols like t.38.
Jay R. Ashworth jra at baylink.com
Designer Baylink RFC 2100
Ashworth & Associates The Things I Think '87 e24
St Petersburg FL USA http://baylink.pitas.com +1 727 647 1274
"That's women for you; you divorce them, and 10 years later,
they stop having sex with you." -- Jennifer Crusie; _Fast_Women_
More information about the asterisk-users