Fax Modems - IBM Aptiva Hardware Handbook

Recently developed protocols include the Microcom
Network Protocol (MNP) and V.42bis. These protocols
are performed by "firmware." Firmware is software built
into the modem. V.42bis is the newer of these two proto-
cols. It has its own methods of error detection and cor-
rection, and also incorporates the MNP 4 and 5
protocols. As a result, modems with MNP 4 and 5 and
modems with V.42bis can connect with each other.
If you cannot connect to another modem, it may be
because the protocol you are using is not compatible
with the protocol of the modem on the other end. If
you're having trouble connecting to other modems, try
following these guidelines.
•
If you know what protocols are available on the
modem you are connecting to, enable a protocol on
your modem that is supported by the other modem.
Usually V.42bis works best if both modems support
it.
•
If you frequently connect to bulletin boards, you
should buy a modem communications software
application that includes many different software
protocols. Different bulletin boards use different pro-
tocols.
28 How modems work
•
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.
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-
Important: 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.

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 pic-
tures, graphics, letters, and numbers printed by the fax
machine.
The scan rate option in your fax software deter-
Important: 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.