[Asterisk-Users] TxFax: can't get a fax to destination (log inside)

[Lee Howard]

Marco Parmeggiani wrote:

> Can someone explain me what's going on and why the receiver of this 
> fax guives up saying communication error?
>
> Start tx page 1
> >>> EOP: 2f
> <<< RTN: 4c
> >>> DCN: fb
> Disconnecting


The receiver says communication error because txfax's response to the 
receiver's RTN signal was a DCN (disconnect) signal instead of a usual 
Phase B signal (i.e. DCS), also known as "retraining".

In non-ECM mode a receiver analyzes the "quality" of the incoming image 
data and determines whether or not it is of an "acceptable quality".  
Then, after the sender transmits the post-page signal (EOP in this case, 
meaning "last page"), the receiver transmits either MCF (confirmation, 
meaning the received page quality was acceptable) or RTN (meaning the 
page quality was unacceptable and that retraining is required before the 
next page is received).

My understanding of T.30 (and I'm not alone in this thought) was that 
RTN is meant to be the antithesis of MCF.  So not only does it mean that 
retraining is required for the next page, but it also means that the 
previous page was *not* received properly (i.e. not printed).  So, 
basically the page will need to get resent for the page to get through.  
However, many fax senders (especially old fax machines) were incapable 
of retransmitting a page because they didn't have the memory to store a 
full page of image data and so they sent data as they scanned it.  So of 
necessity these senders had to treat RTN as a confirmation and they left 
it up to the receiver to call back and request a retransmission of 
whatever pages came through uncleanly.  Some senders may have even 
produced a transmission report indicating to the sender which pages 
should be retransmitted in a new call.  Ultimately what this did for the 
fax community, though, was that it gave us a very confused understanding 
of an RTN signal.  And so various things were programmed with this 
confusing understanding programmed-in, and this programming often 
persists in "modern" fax machines because manufacturers don't rewrite 
their fax code very often if ever.

So txfax is behaving as many fax senders out there behave, although my 
opinion is that it is behaving incorrectly... and apparently this 
receiver that you are working with shares that opinion.

Lee.