[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
>    versus
>    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.

-- jra
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,
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