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 About Fax Modems

    Faxual II works with Class 1, 2, and 2.0 fax modems.  The 'Class N'
    labels were devised by the Electronics Industries Association (EIA) and
    the Telecommunications Industries Association (TIA), which publish
    standards describing the operation of these modems (among many other
    things).

    Class 1 modems are more sensitive to timing than Class 2 or 2.0.  This
    shows up during transmission, when a Class 2 or 2.0 modem can maintain a
    connection for several seconds (at least--possibly much longer) if not
    supplied with data, while a Class 1 modem usually gives up as soon as
    its output buffer runs empty.

    Class 1 timing sensitivity can usually be overcome by allocating a large
    enough output buffer for the serial port.  SerOpen() defaults to 2K for
    the buffer.  Most modems have at least 1.5K of internal buffer.  This
    gives 3.5K of buffer, which at the highest fax data rate of 14400 BPS,
    allows for just under two seconds of inactivity.

    If your program does not supply enough data to the modem, the result is
    a 4014 (DTE data underflow) error.  If these occur, you may need to
    increase the size of the output buffer.

 Class 2 and 2.0

    Class 2 and 2.0 (pronounced "two point oh") are similar, but not
    interchangable.  The difference between these two labels is worth a
    footnote, at least.

    The EIA defined three classes of fax modem, called Service Classes 1, 2,
    and 3.  Class 1 is the simplest; Class 2 is more complex and powerful,
    and Class 3 (for which no standard has yet been published) is the most
    powerful of the three.

    The EIA and TIA together published the Class 1 standard (EIA/TIA-578) in
    1990.  At the same time, their committee continued to change and revise
    drafts of the Class 2 standard, while several manufacturers felt that
    the drafts were good enough.  Some of those manufacturers decided that
    they were ready to build Class 2 modems--standard or no standard.  So
    the large number of 'Class 2' modems that began to appear in 1990 and
    1991 were based on a draft standard published in the middle of 1990.

    The EIA/TIA committee continued to work on the standard until early in
    1993, when they published it (EIA/TIA-592).  By that time, it had been
    so far changed from the 1990 draft that no existing 'Class 2' modem
    would meet the standard.

    Recognizing the large number of existing modems and amount of existing
    software, but unwilling to give up the Service Class labels, the
    committee decided that modems fitting the standard would identify
    themselves with the string '2.0' instead of the bare number '2' (which
    is "reserved for manufacturers" and used by all draft-standard Class 2
    modems).

    Thus, the published Class 2 standard is often called Class 2.0, and the
    label Class 2 reserved for the 1990 draft standard.