Lee Howard wrote:
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.
TxFAX is doing the right thing, according to the spec. If you don't know
how to handle RTN you should disconect. spandsp has not yet been taught
how to retrain and retransmit properly. However, this is largely
irrelevant. On modern lines a retransmit should be rare. Lately lots of
people are reporting this problem to me, and I have not had time to
investigate why it happens. Some say it happens with specific machines,
and some with all machines.
Regards,
Steve
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