[Asterisk-Users] PRI & Fax Passthrough

[Rich Adamson]

Callum McGillivray wrote:
> Haha - My director wants to do away with the analogue line that we have 
> for our fax machine at the moment !! I really don't want a whole PBX 
> sitting around just to handle a single fax machine....
> 
> Thanks for the info / experience though....
> 
> Has anyone else got any other experiences with this kind of stuff ?

The bottom line is that analog modem signals have to be accurately 
reproduced through the system with no fluctuations in timing (eg, 
jitter). That generally translates into pci bus quality, interrupts 
being handled accurately with no delays, clean ethernet infrastructure, etc.

Bringing fax analog signals into the system via a digital facility (eg, 
PRI) is well supported throughout the industry. However, when the PRI 
packet content is shipped across the pci bus, handled by asterisk, and 
sent to another device (eg, sip adapter), the success/failure is highly 
dependent upon exactly what/how your system is configured. If your 
motherboard has a poor pci implementation, or if you didn't address 
interrupt sharing, or if your ethernet infrastructure has issues, or if 
your asterisk system has other applications running that consume cycles, 
faxing will be less then reliable.

In the pure analog pstn world, the only reliable way to handle faxes 
within asterisk is to use an analog pstn card that switches packets from 
an fxo port to an fxs port (fax machine) on the same card. The sangoma 
A200D works fine. The digium TDM2400 card just had a patch applied in 
the last couple of weeks to do this as well. (I've not yet tested the 
TDM2400 to actually see if that works though.)

In the pure digital pstn world, others have reported success with:
- channel bank(s) with an attached fax machine
- some sip adapters (with a solid network infrastructure)
- combination of iaxmodem and hylafax

Pick one and see if it works in your environment.