[Asterisk-Users] spandsp and app_rxfax (alternative topic: t38modem)

[Lee Howard]

On 2005.01.07 08:13 Ryan wrote:
> Hmmmm. Did I just ask in the wrong forum, or has _nobody_ experienced
> image
> corruption using app_rxfax that was NOT due to using the wrong version
> of
> libtiff?

Oh, you can get image corruption on any non-ECM fax, and that doesn't 
have anything to do with anything other than data corruption 
over-the-wire.  Since spandsp doesn't support ECM, you can get immage 
corruption and not have it be anything's fault except for the nature of 
non-ECM faxes.  During fax Phase C (the part where the TIFF image data 
is communicated) if some data gets garbled due to whatever reason, then 
the image data will be messed up.

> If that's the case, then my secondary approach is going to have to be:
>   PSTN <-> Asterisk + chan_h323 <-> t38modem + Hylafax

Well, that would be a lovely thought... if only you could get Asterisk 
to talk T.38 through chan_h323, and Asterisk does not support T.38 in 
any way, shape, or form right now.

> Is there anybody that could help me with either of these solutions?

As far as I'm aware, the only way to get HylaFAX working behind 
Asterisk is to connect a HylaFAX-controlled hardmodem either into an 
FXS port or a passthru span.  For example:

   PSTN - X100P - Asterisk - SPA-2000 - analog modem - HylaFAX

or

   T1 - TE405P - Asterisk - TE405P - T1 modem - HylaFAX

The first configuration, which will generally work tolerably well with 
modern HylaFAX and most fax machines, is subject to a fair risk in data 
corruption due to the combined analog-to-digital (digital === ulaw) and 
digital-to-analog conversions.  A mere 20ms delay in faxing can make a 
huge impact.  The only reason that this configuration works tolerably 
well is due to the ECM support in modern HylaFAX and most fax 
machines.  The ECM protocol is able to recover the corrupted data 
through retransmission attempts.  Now, if the sender doesn't support 
ECM, then you're generally stuck with whatever corruption occurs (maybe 
none, but probably some).

The second configuration seems to be quite a flawless way to do this as 
Asterisk is merely forwarding the already-digital signal.  The downside 
is, of course, that it's probably not really an option unless you have 
a T1, a TE405P, and a T1 modem (either an Eicon Diva Server or a Patton 
DataFire 2977).

Lee.