[Asterisk-Dev] Fax support?

[Craig Southeren]

On Sat, 10 Jul 2004 11:21:36 -0700
Bruce Ferrell <bferrell at baywinds.org> wrote:

> Noodling around on the ITU site, google and packetizer, it would appear 
> T.38 is pretty tightly tied to H.323 as a "codec" (ITU puts it in there 
> with 261/263/723.1 etc)

Notionally, T.38 can be treated like a code, but it is not dependent on
H.323 in any way. Just like G.711, or H.261, T.38 is a stand-alone
encoding that can be used by itself or as part of a larger protocol.

> I suspect under SIP it would have to show up as a modem 
> passthrough/relay over G.711

T.38 certainly is certainly handled much like a codec under SIP, but is
completely independent of G.711 as the data encoding. However, it is
considered useful to provide G.711 an alternate codec when making a fax
call just in case the other end is not a fax machine, and because some
equipment (Cisco in particular) require the fax start tone as G.711
before they will switch into fax mode.

Even though T.38 is handled like a codec, the T.38 payload data is not
RTP at all but is actually the underlying T.30 data stream with a very
thin wrapper to allow resequencing of the data (and to offer redundancy
if using an unreliable transport like UDP). In other words, if you
consider that a normal fax call is actually just a two-way T.30
bitstream via modulated fsk audio tones (exactly like a old-style modem),
then T.38 defines how to perform the same exchange of the same pattern
of bits over a transport like UDP or Tcp that does not require the use
of audio modulation. 

That's how programs like t38modem (from the OpenH323 project) work -
they create pseudo-modem device that converts AT commands into T.30 data
and then wrap it with T.38 headers. No audio modulation or demodulation
ever takes place.

   Craig


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