[Asterisk-Users] FAX, IAX and *....Maybe I'm dreaming...:-)

Steven Critchfield critch at basesys.com
Sat Dec 13 01:30:17 MST 2003

On Fri, 2003-12-12 at 09:29, Dan wrote:
> Hi,
> ----- Original Message ----- 
> From: "Alastair Maw" <asterisk at almaw.com>
> To: <asterisk-users at lists.digium.com>
> Sent: Friday, December 12, 2003 4:58 PM
> Subject: Re: [Asterisk-Users] FAX, IAX and *....Maybe I'm dreaming...:-)
> > On 12/12/03 13:56, Dan wrote:
> > > This is because the fax is transmitted using the audio stream.
> > > It is not related to the signaling protocol (SIP/IAX etc.) but to the
> audio
> > > codec used.
> >
> > Fax uses FSK modulation to transmit the data. If you compress this in a
> > lossy way (GSM, MP3, whatever) then the integrity of the data is
> > affected (more or less seriously depending on the codec used). Fax
> > machines are generally quite picky, so compressing faxes is unlikely to
> > work.
> >
> > I'm wondering why on earth you want to push fax data over a VoIP link at
> > all. Fax compression isn't very efficient.
> Who wants that???
> By fax data I mean the data contained in a fax (basically a picture file),
> not the fax data audio stream.
> It can be converted (GIF or JPG) then sent reliable over a slow IP link.
> Just a special codec at both ends, able to pass the data to the fax app or a
> fax machine connected to a TDM400/ AT or whatever.
> > It would be much less
> > bandwidth intensive to decode the fax and send it over as proper data
> > rather than audio, compressed using gzip/gif/png/something else.
> This is exactly what would be great to have it.

Sounds like you want app_rcfax at the pstn to decode the fax data, then
it would somehow transfer the image file to your local machine via the
slow link and use app_tcfax to send the image to your local fax machine.

Steven Critchfield <critch at basesys.com>

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