asterisk users - Dec 2003 - FAX, IAX and *....Maybe I'm dreaming...:-)

Hi,

It would be great if the IAX protocol will be able to tranfer fax data (even
converted in another format) between Asterisk boxes, using low bandwidth
codecs like GSM.
I know that this is possible only with the G.711 now (passing faxes using
the audio stream), but.... maybe in the future...some native support will
permit this.

Just a thought,
Dan

Dan wrote:
> Hi,
> 
> It would be great if the IAX protocol will be able to tranfer fax
> data (even converted in another format) between Asterisk boxes, using
> low bandwidth codecs like GSM. I know that this is possible only with
> the G.711 now (passing faxes using the audio stream), but.... maybe
> in the future...some native support will permit this.    
> 
> Just a thought,
> Dan
> 
> _______________________________________________
> Asterisk-Users mailing list
> Asterisk-Users@lists.digium.com
> http://lists.digium.com/mailman/listinfo/asterisk-users 
My understanding is that fax calls are ONLY possible with G711/SIP not
IAX.
Anyone?

Ta
SJ

Hi,

It would be great if the IAX protocol will be able to tranfer fax data 
(even
converted in another format) between Asterisk boxes, using low bandwidth
codecs like GSM.
I know that this is possible only with the G.711 now (passing faxes 
using
the audio stream), but.... maybe in the future...some native support 
will
permit this.

I suppose that with SteveU's fax receive stuff, it should be possible 
to convert the fax to an image, and then send the images along in IAX 
image frame types..

I.e.  AST_FRAME_IMAGE/AST_FORMAT_PNG.

You'd need to figure out how to get the call to filter through the fax 
stuff, such that the multiple audio frames on one side ended up as a 
set of Image frames on the other..

-SteveK

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----- Original Message ----- 
From: "Dan" <dtoma@fx.ro>
To: "Asterisk Users" <asterisk-users@lists.digium.com>
Sent: Friday, December 12, 2003 8:25 AM
Subject: [Asterisk-Users] FAX, IAX and *....Maybe I'm dreaming...:-)

> It would be great if the IAX protocol will be able to tranfer fax data
(even
> converted in another format) between Asterisk boxes, using low bandwidth
> codecs like GSM.
> I know that this is possible only with the G.711 now (passing faxes using
> the audio stream), but.... maybe in the future...some native support will
> permit this.
Sounds like you are thinking of something like T.38.  T.38 is a FAX over IP
protocol but my guess is that there would be licensing issues using it with
Asterisk.

It probably would not be that hard for someone to come up with a completely 
open FAX over IP protocol.  A FAX over IP protocol would not have to
operate in real time like VOIP.  You could actually use acknowledge signals
and retransmit packets that are dropped so that you get a perfect fax
transmission. This can not be done with VOIP because by the time you
realize you have missed a packet it is too late to re-transmit the packet.
With FAX over IP it just means it takes a little longer to transmit the page.
You could also design the protocol so that you could transmit at whatever
rate you wanted.  Slower links would use less bandwidth but take longer to
send.

On Friday 12 December 2003 07:25, Dan wrote:
> Hi,
>
> It would be great if the IAX protocol will be able to tranfer fax
> data (even converted in another format) between Asterisk boxes,
> using low bandwidth codecs like GSM.
> I know that this is possible only with the G.711 now (passing faxes
> using the audio stream), but.... maybe in the future...some native
> support will permit this.
You're not going to get that working because GSM is a lossy codec.
It is able to get extreme savings in size, because it optimizes out
parts of the sound that most humans don't hear.  However, that same
bandwidth that humans don't hear is exactly the bandwidth that the fax
application uses to transmit valuable portions of the image.

Therefore, the GSM codec is never going to be appropriate for sending
faxes.  Besides, if you need low bitrates for your IP connection,
you're likely to experience delays in the fax negotiation -- which
will probably result in a failed fax attempt.  If you want to be able
to send faxes in this way, then negotiate the fax at one end, and
email the resulting TIFF to the other end.

-Tilghman

> ----- Original Message ----- 
> From: "Alastair Maw" <asterisk@almaw.com>
> To: <asterisk-users@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.
The question asked here, "why on earth you want to push fax data over a
VoIP link at
all. Fax compression isn't very efficient." may speak volumes about the
future role of VOIP. My plans are to role out a VOIP connection to thousands of
Customers. Many have legacy fax equipment. Am I to assume that they will toss
out their fax equipment and join the PC based faxing crowd? I don't think I
can control this. If I am going to offer an aternative to the legacy wire
providers then I have to offer a comparable service. One that for example allows
a customer to use a legacy fax machine in the same way.

If this thought sidetracks the intent of this thread, you have my apologies, but
I do think that legacy fax functionality is essential.

Jeff

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>Did DVD players have to accommodate VHS tapes? Did VHS players have to
>accept beta? 
>Why does VoIP have to deal with an accent protocol that can't handle
>lossy audio, nor irregular delays?
>Also why should we be soo wasteful when fax machines need a 80K codec to
>get the data across IP, and the faster machines I see say 15 secs per
>page. So why should we send 1.2meg when 150k is fine? 
>Also who says Fax should ever be required on IP? My office has been
>using VoIP for all voice traffic for over a year now, but always left
>the fax machine on a analog line. The analog line was cheap enough to
>not be a concern. 
>-- 
>Steven Critchfield <critch@basesys.com>
I'm fairly new here and don't mean to be contentious. We all have
different perspectives as to what VOIP should be. My goal is to replace analog
lines, not supplement them. I'm talking residential installations. I
don't think I can ask these folks to leave their fax on an anolog line? I
think that if we start deciding things for the Customer, then VOIP will be seen
as an elitist toy for digitally inclined, instead of an acceptable alternative
for the masses.
No offense to the anti-fax coalition.
Jeff

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