[Asterisk-Users] Application Faxing using SIP

[Lee Howard]

Jerry Jones wrote:

> 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.... 


A 50-page non-ECM fax is similar to an average 20-page ECM fax in that 
there are repeated sections of V.17/V.29/V.27ter modulation occuring.  
That high-speed data communcation is the really sensitive part about 
faxing and getting a mistaken carrier drop during that time is the thing 
that kills.

If you're saying that a 50-page legal document will frequently have 
troubles, then you're talking about an error ratio that most businesses 
of my acquaintence would simply not tolerate.

In a typical lossless audio environtment, fax speeds with ECM should be 
the same or better due to additional compression mechanisms that require 
a lossless image type.  If you find that your fax speeds with ECM are 
frequently slower than without ECM... or if you find that ECM fails more 
frequently than non-ECM, then it would seem to indicate that the audio 
corruption that is occurring before audio gets to your fax machine is 
fairly severe.

Lee.