Re: FAX/Modem speeds

[dempson@atlantis.actrix.gen.nz (David Empson)]

In article <3gm4b1$kcq@nps.navy.mil>,
Mark A. Schooley <schooley@nps.navy.mil> wrote:
> 
> I have a question that I am hoping someone will be able to enlighten
> me with an answer which makes sense.
> Why is it that several FAX/Modems have two different speeds (i.e.
> 9600 FAX and 2400 Modem).

A fax (or fax modem operating in fax mode) and a data modem use
different communications protocols (over the telephone line).

Data modems are inherently full-duplex, i.e. data can be transmitted
and received at the same time.  (Some very old modems are only half
duplex.)

With a fax or fax modem, data is only sent one way at a time (half
duplex).

A half duplex connection over a telephone line requires less
bandwidth (frequency range) than a full duplex connection at the same
speed.  A moderately simple modem with cheap modulation/demodulation
circuitry may be able to implement much higher fax speeds than data
speeds, because of the lower complexity of the fax communication.

It is really determined by the capabilities that the modem chipset
manufacturer decided were worthwhile, and the chipset selected by the
modem manufacturer.


It is interesting to note that modern modems can now transmit data
faster than faxes (the highest official data rate is 28,800 bps, as
provided by V.34 or V.FC), but faxes are limited to 14,400 bps.  There
should be a 28,800 bps fax standard at some point in the
not-too-distant future.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand