[asterisk-users] Zap channel faxing in or out fails but phone calls work.

[Lee Howard]

Jerry Jones wrote:

> without ecm -
>     line errors will cause slight imperfections (dots) on transmitted 
> image
>
> with ecm -
>     retry, retry, retry, fail 


In the ECM retransmissions only the frames that were not received 
properly are resent.  Once the data to send is assembled by the sender 
it is transmitted in exactly the same way as with non-ECM communications.

So as an example let's say that an ECM page consisting of a single block 
of 100 frames is sent, and that a "slight" imperfection is detected, so 
maybe 10 frames are bad: frames 5, 15, 25, 35, 45, 55, 65, 75, 85, and 
95.  ECM protocol will cause there to be three more retransmissions 
before the sender has a deliberate chance to abort retransmissions.  You 
can probably see that the same "slight" imperfection would have a 
difficult time persisting through three more iterations.  However, for 
the sake of argument, let's say that frames 5, 55, and 95 still aren't 
received properly by the 3rd retransmission (4th attempt).  The sender 
then has the choice to change the speed of transmission and continue 
another set of 4 retransmissions or to end retransmissions and (in 
theory) move on to the next block.

Most fax machines will abort the call if they do not choose to continue 
retransmissions... even if they do signal that they're going to move on 
to the next block.  So at that point the receiver is left with a page of 
image data with some "very slight" imperfections in frames 5, 55, and 
95.  Now it's up to the receiver to be able to make something out of 
that image.  If the image type is MH or MR then the receiver is going to 
be able to print out the full page... with maybe some dots as you say.  
If the image type is MMR then the image will be truncated at the data 
where frame 5 occurs (pretty much the top of the page).  If the image 
type is JBIG then the end result will depend upon the decoder... but 
generally the decoders are lightly tolerant of some slight data 
corruption, but too much corruption will be a problem.

So at the end of all of this contrived scenario the problem in not 
getting a "slightly imperfect" image out of an ECM-using fax machine 
would be due to the data type more than it woudl be due to the usage of 
ECM.  If you truly are experiencing the kind of, perhaps contrived, 
situation where "retry, retry, retry, fail" occurs then I would expect 
that disabling MMR+JBIG would be more effective than disabling ECM.  
(Yes, disabling ECM does also disable MMR and JBIG, but you can use ECM 
with MH and MR and thus benefit of a tolerant image type *and* a 
tolerant protocol.)

If you continue to experience problems with ECM without MMR/JBIG then 
you will (guaranteed) experience problems without ECM.

Lee.