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Re: [hylafax-users] Difference in classes

Thanks everyone for their help. It sounds like I should stick with class 1.
I thought maybe the class was causing some of my problems, but it probably
is just the modem. I just bought a Rocketmodem II, so that will hopefully
fix some of my modem woes. Thanks again everyone.


-----Original Message-----
From: Lee Howard [mailto:faxguy@xxxxxxxxxxxxxxxx] 
Sent: Monday, November 03, 2003 1:12 PM
To: Matt Yeager
Cc: 'HylaFax Users Mailing List (hylafax-users@xxxxxxxxxxx)'
Subject: Re: [hylafax-users] Difference in classes

On 2003.11.03 07:55 Matt Yeager wrote:
> What's the difference exactly between class 1, class 1.0, class 2, 
> class 2.0, and class 2.1 . I've looked around some online, but can't 
> find any
> definative comparison.

Class 1 and 1.0 are protocols currently defined by T.31.

T.31 protocols give a low-level interface mechanism between the DTE 
(software - HylaFAX) and the DCE (modem).  Not all modems' firmware 
implement all of the command aspects found in T.31.  The basic 
difference between Class 1 and Class 1.0 is V.34-fax (SuperG3) 
capability.  Also there are a small few effectually minor command 
enhancements.  Actual Class 1.0 protocol varies quite a bit from Class 
1 in the DTE implementation (which is why HylaFAX doesn't support Class 
1.0 yet).

Class 2.0 and 2.1 are protocols currently defined by T.32.

Back when the CCITT first began formulating the T.32 standard Rockwell 
had a proposal and US Robotics had a proposal.  Ultimately USR's 
proposal was accepted as the standard and became Class 2.0.  However, 
Rockwell's proposal had been in production-use with many modems for a 
long time already and was called Class 2.  Class 2.0 has more commands 
than Class 2, but for the most part it is not consequential to basic 
faxing.  Class 2 will generally lack features such as subaddressing, 
extended-resolutions, and ECM... however, that is not always true as 
some firmwares have customized commands which try to provide those 
features in the Class 2 manner (so as to maintain compatibility with 
Class 2-only software).  So the effectual differences between Class 2.0 
and Class 2 are very vague, sometimes... it just depends on the modem 
manufacturer's customization of the firmware.  The difference between 
Class 2.0 and Class 2.1 is the same as between Class 1 and Class 1.0, 
that is, V.34-fax capability.

Class 2/2.0/2.1 provides a high-level interface mechanism between the 
DTE and the DCE.  This makes it very easy to develop fax software for 
the modem, but it therefore relies very heavily on the firmware for 
features and proper functionality.  Most faxing problems resulting in 
+FHNG or +FHS errors will require the fix to come from the modem
manufacturer.  Usage of Class 2 generally ensures that the modem will 
function identically accross different software.

By comparison, usage of Class 1 ensures that the software will function 
identically with various modems.  Software that uses Class 1 has a 
great deal of control over the actual faxing and it may therefore 
implement Class 1 with very few features or it may implement it with a 
very many features.  A full-featured Class 1-supporting software can 
support more faxing features than most Class 2 modems.  Faxing problems 
in Class 1 are generally the responsibility of the DTE software.

HylaFAX 4.1.7 has about the same feature-set in Class 1 as do most 
modems' Class 2 firmware.  HylaFAX CVS has a much greater feature-set 
in Class 1 than do most modems' Class 2 firmware (ECM and extended 

> Which one is the better one to use for faxing?

The one that works best for you.  In my case, since I know fax protocol 
and since I like the control I get with Class 1, I use Class 1.  I 
could also argue some technical aspects such as modulation control 
where Class 2 will never have the capability that Class 1 can, but I 
think that those arguments are irrelevant for most people's faxing 
needs.  Of course, if you have a problem in Class 2 you should switch 
to Class 1 and report the problem to the manufacturer.  If you have a 
problem in Class 1 you should report the problem here (log) and you 
could try Class 2.

As for buying a modem... it's advisable to choose one that supports 
both Class 1 and Class 2 that is actively supported by the manufacturer.


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