[Asterisk-Users] Application Faxing using SIP

[Rich Adamson]

> > Traditional faxing (not T.38) pretty much requires a lossless audio 
> > channel.  Normally the best way to get this is with PSTN channels/lines 
> > through a Zap device.  That said, VoIP channels can be configured such 
> > that they are also lossless.  IAXmodem, for example, functions on the 
> > premise that an IAX2 channel passing over the loopback device will be 
> > lossless.  I have also seen lossless SIP and IAX channels running over a 
> > WAN, but they were very specificially configured, and I wouldn't expect 
> > most connections with traditional VoIP providers to be anything near the 
> > kind of losslessness that is required for this to work well.
> 
> I have a PRI terminated in a TE110XP card on my Asterisk box.  Right now
> we are using a separate analog line for faxing, but (for a variety of
> reasons) I would like to switch to sending and receiving faxes over the
> PRI via Asterisk.
> 
> What's the recommended way to do this?  The three obvious options I can
> think of are:
> 
> 1. Connect the fax machine to an ATA and have it speak SIP or IAX to *
> 
> 2. Fit a TDM400P with FXS linecard into the * box and connect the fax
> machine to it directly.
> 
> 3. Replace the TE110XP with a multispan E1 card, connect a channel bank
> to the second span, and plug the fax machine into that.
> 
> Option 3 can be ruled out immediately for us due to cost.  Option 2 is
> quite appealing, but I've previously been told that running multiple Zap
> cards in a single machine is not a good idea.  Option 1 seems like the
> cheapest and easiest, but I have no idea how reliably faxing will work
> over an ATA.

A rather knowledgable person suggested (off list) that faxing via the
TDM400 analog card is more reliable "if" the fax machine is directly
connected to an fxs interface module located on the same TDM400 card
as the fxo module. I don't have any fxs modules to try it and have no
idea whether that might be true or not, but might be worth the effort
for someone that has both modules to test. If that is true, it would
certainly improve the image/usability of the TDM card a bunch for small
systems. (Its been a fairly major show-stopper for placing asterisk in
any small business environment.)

Using the TDM400 analog card with faxing via asterisk (eg, ata devices)
have an extremely high failure rate (would guess better then 90% of
those that have attempted it failed totally). It seems the TJ320 chip
on the card gets blamed for missed/dropped packets across the pci bus,
which would cause a total fax failure. To my knowledge, no one has
actually proven whether that statement is true or not.

The issues with all three options noted above essentially involves
dropped/missed packets, regardless of whether those packets are missed
sip packets across a lan/wan or via the system pci bus. Think of it as
an unreliable layer-2 communications issue that you have little or no
control over.

If someone can send me a fxs module (for the TDM analog card), I would
certainly validate the above with supporting documentation.

None of the above comments apply to digital fxo interfaces such as
PRI's, etc.