[asterisk-users] FAX Issues

[Lee Howard]

Ryan McGuire wrote:
> Unless your network is under load and you are seeing dropped packets
> and high jitter, I would absolutely not do T.38. The cheapest and
> easiest approach that I have found is to buy yourself an FXS gateway
> and just make sure you are using ulaw.

As SIP is usually running over UDP/IP it doesn't take much to produce 
dropped packets.  Dropped packets mean lost audio which means lost data 
and possible demodulation difficulties for the modems.  If you're in an 
environment where dropped UDP packets don't occur you're in a very rare 
scenario.

For the most part people who claim success when faxing over SIP G.711 
are being rescued by ECM (error correction) within the fax protocol.  
There are very, very few who really have mitigated UDP packet loss.

That said, all T.38 systems are not equal.  Certainly, the reliability 
of your T.38 provider may not be any better than that of G.711 fax over 
the SIP UDP.

I only recommend faxing over TDM everything else is at your own risk.

Thanks,

Lee.