Modem Terminology Overview
Modem Terminology Overview - Page
- Page 33
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SPOT 14.4
SPOT 14.4 User's Guide
User's Guide
38GDE00001SCF - Issue 4 (11/96)
38GDE00001SCF - Issue 4 (11/96)
Fax Modems
Fax Modems
A fax machine scans a page as a series of fine lines, up to 196 lines per
inch down an A4 page in fine mode. Each line consists of a series of
dots, up to 203 dots per inch across an A4 page in fine mode. A dot is
present for each piece of text or drawing and absent for a space. Already
you should see that this is very similar to digital bits. The way two fax
machines send data between each other is very similar to modems,
turning the line dots into audible tones.
A series of additional Hayes modem commands have been developed
that allow a modem to operate as a fax machine to either receive or send
faxes.
– Group 1 & 2
Group 1 & 2::
These are two old protocols for communication between fax
machines. They are rarely encountered these days.
– Group 3
Group 3::
This is the most common fax standard in use. It includes standard
(203 x 98 dpi) and fine resolution (203 x 196 dpi) options.
– Group 3bis
Group 3bis::
This standard is an update to Group 3. It adds a super-fine resolution
(406 x 196 dpi) option.
– Class 2
Class 2::
This is a specification for AT (modem) commands to control a fax
modem (it follows Class 1 which allowed a computer to control a fax
machine).
There are several CCITT VV standards for fax. These relate to the speed
of transmission and the methods of compression/error correction used.
– V.27ter
V.27ter:: Covers the transmission of faxes at 2400 and 4800 bps.
– V.29
V.29:: Covers the transmission of faxes at 7200 and 9600 bps.
– V.17
V.17:: Covers the transmission of faxes at 14400 bps over exchange
lines.
