flexfax: Meaning of RTN (was: Hylafax question)

[David Woolley <david@djwhome.demon.co.uk>]

> >Can you explain what the RTN problem is?
> 
> In fact I'm not too sure ! As far as I understand, after sending a page, modems
> of both sides will do some hand shake to check the connection (retrain), if the

RTN is sent *before* any retraining.  At the end of a page, the modems
switch back from a typical, phase C, 9600 bps (1200 baud) to 300 bps
(and baud), of phase D, and the sender sends a message saying whether
there is any more to send (typically MPS - "more pages, same document")
and the receiver sends one saying what it wants to do next.  The receiver
can say continue normally, but, if it thinks the analogue signal quality
has degraded a bit can send "retrain positive" to indicate that the
sender should send training signals to recalibrate the receiving modem's
equaliser before continuing.

If it thinks that, in addition, the image quality was unacceptably poor,
it can, instead, send "retrain negative" to indicate that the page should
be resent after the retraining process.  Other possibilities include an
indication that the operator has interrupted reception, with or without
successful receipt of the page.

Many fax machines don't actually monitor the true analogue signal quality
and use protocol violations in the recovered image data to determine
whether the line quality is degrading.  If there is a systematic error
in the coding of the image, they will repeatedly treat this as requiring
retraining, but retraining won't help.  If the receiving system is 
unreasonably fussy about image quality, it may send RTN when the
human user of the system cannot see a problem, with the result that 
the first page is sent multiple times for no apparent reason.

One such systematic error is caused by incorrect splicing of the tag line 
into the top of the image.  A patch for this has existed for a long time,
but is not in the released version of the Hylafax, although probably in the
current beta.  The basis of the recent patch is that the "return to
control" code at the end of image data is duplicated if the program that
created the TIFF file also included one.  Some fax receivers appear
to require an immediate carrier drop after the first one for them to
consider the protocol valid.

> handshake failed, the RTN will report. Please check the HylaFAX home page
> for details or ask Dmitry.

The original subject would cover almost every posting on this list, so I
have changed it.