[hylafax-devel] Re: Hylafax's Goals & Suitable Modems

[Dmitry Bely <dbely@mail.ru>]

"Harald Pollack" <Harald.Pollack@DATAnews.at> writes:

> >-- just remember "GSM" discussion here regarding Class 1 timing issues.
> 
> I have never understand, what the real problem was :-)

I think I understand the problem now.

T.30 specifies T4 timer, that starts in the begining of "Responce received" 
subroutine and stops when first HDLC flag is received from the remote. I
think this cannot be implemented via AT+FRH command, because it prints
"CONNECT" message immediately after detecting the carrier (and *before* the 
first flag has arrived), and then outputs the whole HDLC frame *after* all
flags has been detected and stripped:

[ from EIA/TIA-578 ]
...  If the 
specified carrier is detected, the modem generates the CONNECT message 
and enters the HDLC receive mode.

In HDLC receive mode, the modem receives HDLC frames from the remote fax 
machine, strips the flags, performs zero-bit deletion, performs error 
checking, and handles <DLE><chr> character pairs before passing the data 
to the DTE.  The modem prefixes each <DLE> character with another <DLE> 
character before sending it to the DTE.  After the last byte in the 
frame, the modem sends <DLE><ETX> to the DTE marking the end of the 
frame.  The modem then generates either the OK message if errors were 
detected (FCS is incorrect), and returns to command mode.
[ end of quote ]

As a result, Hylafax stops T4 timer, when the modem outputs first non-flag
HDLC byte after CONNECT -- not acceptable, because this way T4 in fact
limits the reception of the whole frame. But we cannot also stop T4 on
CONNECT, because the modem still has not detected any flag at this moment.

AFAIR the is no quite correct solution for this problem, but which one is
acceptable in your point of view?

Hope to hear from you soon,
Dmitry

P.S. I will also comment other your messages, but a little bit later :-)



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