How modems work 31
• Many network services have their own proprietary
protocols. To connect to these services, you often
have to buy the protocol from the service. Other net-
work services rely on V.42bis or MNP 4 and 5.
Fax modems
Fax modems send and receive facsimiles and generally
transmit and receive pictures better than text or other
data. In a fax machine, a page is read by a scanner and
broken into a series of dots representing light and dark
portions of the page. These dots compose the pictures,
graphics, letters, and numbers printed by the fax
machine.
Important:
All error detection and correction protocols com-
press data to avoid slowing the transmission rate.
However, combining protocols will not increase your
transmission speed or significantly reduce transmis-
sion errors.
For example, running V.42bis and X-modem together
will not give you a greater compression rate or trans-
mission speed than running V.42bis alone. Use only
one protocol.
Important:
The scan rate option in your fax software deter-
mines the size of a fax and how much time it takes
to transmit the file.
The fax machine scans a page line-by-line and records
the number and position of each dot in a scan line. This
information is converted into a special code and trans-
mitted to another computer or fax machine. This trans-
mission uses a set of transfer and modulation
specifications called protocols.
Fax machines and modems are connected in Groups.
The Group a fax machine belongs to indicates its speed
and level of technological advancement. The original fax
machines were Group 1 machines. These are the slow-
est machines and are almost obsolete today. Faster
Group 2 fax machines replaced the Group 1 machines.
Group 2 machines are also nearly obsolete today. Most
fax machines sold today are Group 3 machines.
Generally, a Group 3 fax machine can connect most effi-
ciently to another Group 3 machine and may also be
able to connect to many Group 2 machines. However, a
Group 2 machine cannot connect to a Group 3 machine.
The modem in your computer works with most Group 3
fax machines but not Group 2 or group 1 fax machines.
Choosing fax software applications
If your computer did not come with fax software, you
must buy a fax software application to operate your fax
modem. Also, if you add a fax modem to your computer,
you need to purchase fax software.
bi2gntce.bk : bi2giod.doc Page 31 Saturday, June 3, 1995 6:54 AM
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