[hylafax-devel] Re: My RH6.2 RPM hack & Q: Failure to train at 2400 bps or +FMINSP value

[Harald Pollack <Harald.Pollack@Datanews.at>]

On Thu, 18 May 2000 15:35:55 -0400 (EDT), Robert Hardy wrote:

Hello Robert!

>Will attempting class 1 w/ the same modem in this case make any difference?

If your fax software is well implemented, than of course. An 'well implemented' means, that 
programmer of faxsoftware KNOWs meaning of each single bit in a DIS/DCS frame ... but can 
be read in ITU-T.30 :-) 

>You will excuse my ignorance on this topic. I have never found a good
>explanation of class 1 vs. class 2/2.0

The difference is ONLY the protocol (the language level) between the fax program itself and 
the modem. 

>which clearly defined the consequences/advantages of using class 1.

Command/response set of class 1 is as simple as possible. There are mainly TWO commands, 
one to switch on the MOdulator (V.17, V.29, V.27 or V.21 carrier) and one (mutal exclusiv 
to the former) to switch on the DEModulator.

For example:

Enable modem to send frames in V.21 (300 Bd):

AT+FTM=3 or AT+FTH=3 if modem should generate zerobit insertation and CRC self.

Enable modem to send fax data (in V.29, 9600 bps):

AT+FTM=96 or AT+FTH=96 if you want to send ECM data, encapsulated in HDLC frames.

Thats (almost) all. The ATD and ATA commands are the same as in other modes, and there are 
two 'helper' commands also:

AT+FTS=n which 'sends' n*10ms silence (both modulator and demodulator are switched off)

AT+FRS=n which lets modem hunt for silence (no carrier) for n*10ms.

If modem is in AT+FTM state, than each byte supplied from faxprogram is transmitted BITWISE 
to the phoneline...

If modem is in AT+FRM state, modem collects all bits coming from remote, bundles them into 
bytes and delivers them to faxprogram.

It is as simple as imaginable :-)

Timing consideration are of less importance, because the are in the magnitude of several 
seconds (in contrary to class 272.0, where often some milliseconds decide for death or 
alive :-)

All the timing assumption which are related to fax are under control of faxprogram in class 
1. In class 2/2.0, the programmer of the fax firmware has done all timing and the 
faxprogram in class 2 does not realy know, which timings are meaningfull (can only guess 
and fail :-)

>From what I have read, I deduced (perhaps incorrectly) that class 1 was a bit like using a 
winmodem.

No good comparison :-)

Class 2/2.0 makes a kind of mystery of fax. class 1 is clear and straight forward and has 
the advantage, that faxprogram 'knows' really ALL neccessary parameters of a fax connection 
and can handle each misbehaviour in the right way (in class 2/2.0, you can only hope or 
pray, that developer has done its homework well).

CLass 1 faxsoftware with open source is like to have sourcecode of modem firmware :-)

>They are ok if your computer isn't doing alot of other things while you are using it

keep in mind, that real fax machine have Z80 (or 8085) type CPU with some (4 - 10) MHz ...

It is in practice possible to have a slow 386 based PC (eg IBM PS/2 model 56) where class 1 
fax is possible with 2 faxmodems (one sends, the other receives) and incoming fax is shown 
in REALTIME on (graphic) screen!

>or if you have a real time operating system which can guarantee enough time
>to the modem

There is really NO DIFFERENCE, if you use class 1 or class 2/2.0, because you deliver the 
SAME faxdata over the SAME serial line interface. Difference is only in HOW you are 
'speaking' to your modem (and HOW the modems responses) BETWEEN the single pages, and in 
this state, timing is (in class 1) no real problem, because the T.30 protocol is sooo 
tolerant (but NOT your modem, if you 'speak' in class 2/2.0).

Worst case in class 1 (which is also true for two fax machines) between two pages is in the 
dimension of 15 to 20 seconds! Try such large times in class 2/2.0 (you will never deliver 
one single page :-)






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