[Asterisk-Users] Get faxed you faxing faxer!

[Adam Donnison]

Fax is not as easy as it may first appear.  First of all there
are two parts to the protocol, one of which would be relatively
easy to accomplish if you have built your softmodem as it uses
flow-controlled error detecting style conversations.  But this
is just the call setup, and inter-page negotiation.  The
actual sending of the page data uses a non-flow controlled
protocol that uses bit stuffing to make sure the receiving fax
receives a line of data just in time to print it.  The timings
are _very_ tight, from memory all you need is a delay of well
under 100ms for the remote machine to give up.  Even with
TR29.2 modems (where the T.4 data transfer protocol timings
are handled by the modem) you can get all sorts of problems unless
you can make sure you keep the modem buffer full.  Doing it
all in software means you have to manage the timings yourself.

 From my perspective, unless you are using a real-time OS, you
may as well forget implementing T.4 as your interrupt latency
is likely to kill you.  Either that or using a SMP machine
so that there is always a CPU available. Either way, it is
a big ask.

Adam

Eric Wieling wrote:
> software fax modem dsp
> 
> There doesn't seem to be many software fax modems out there, but I found
> these few when doing a Google search a few months ago and bookmarked
> them:
> 
> http://www.softmodem.org/
> 
> http://fabrice.bellard.free.fr/linmodem.html
> 
> http://artax.karlin.mff.cuni.cz/~pacoo0bm/archive/new_fischer/
> 
> http://www-users.cs.york.ac.uk/~fisher/modem/
> 
> I just realized that softmodem.org links to all the other URLs.
> 
> On Mon, 2003-08-11 at 19:23, Jamie Carl wrote:
> 
>>All this talk of faxing has started me thinking (this is 
>>always a bad thing) and I've come up with a question.
>>
>>Now, I know Asterisk can detect and route faxes, to a 
>>'fax' extension and all that.  But can Asterisk be used to 
>>'receive' faxes?  
>>
>>I know there was some talk about this just over a year ago 
>>and I'm wondering if anything came of it.  It would be 
>>nice to be able to receive faxes with asterisk and then 
>>have them filer through and AGI script and emailed or 
>>dumped in a directory/database/whatever.
>>
>>I mean for me, currently, I have a fax/modem hooked up to 
>>the same machine asterisk is on, and it get's sent faxes 
>>via my FXS port.  It would be nice to be able to pull off 
>>the modem and throw it away. (or auction it on ebay)
>>
>>
>> --
>>BTEL Consulting
>>850-484-4535 x2111 (Office)
>>504-595-3916 x2111 (Experimental)
>>877-552-0838 (Backup Phone)
> 
> 
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-- 
Adam Donnison                                  email: adam at saki.com.au
Saki Computer Services Pty. Ltd.
93 Kallista-Emerald Road                        phone: +61 3 9752 1512
THE PATCH  VIC 3792    AUSTRALIA                fax:   +61 3 9752 1098