[Asterisk-Users] Application Faxing using SIP

[Jerry Jones]

>
> Losing an audio packet here or there wouldn't normally be so bad  
> for fax.  Normally I would expect the fax protocol, especially ECM  
> protocol, to be able to recover from it.  However, Asterisk seems  
> to not work in an ideal fashion for this purpose.  Whenever  
> Asterisk encounters a lost audio packet something called "packet  
> loss concealment" is performed by placing a "PLC" frame there as a  
> placeholder.  When the audio is retransmitted the PLC frame is  
> supposed to be converted into synthesized audio.  Between what I  
> have been told and from what I have observed, this conversion of  
> PLC frames into synthesized audio does not happen with uLaw, alaw,  
> or slinear codecs (the only codecs suitable for fax).  Consequently  
> the PLC frame is converted into zero-data... or 20 ms of silence...  
> which is probably the worst-possible thing that could happen.

Turning ECM seems to cause most of my issues with FAX. Most newer  
machines have this on by default. However if there is any packet  
loss, then when ECM tries to resend and there is additional loss,  
then it gets in a loop and everything just fails. Whereas with ECM  
off, you may have an occasional extra or missing pixel, but most  
users never notice, and the speed is way faster.
Most complaints are solved by jsut turning ECM off. Of course this  
does not necesarily help mortgage companies who seem to enjoy faxing  
50page legal docs....