How Modems Work; Modem Connection Process; Data Modems - IBM Aptiva Hardware Handbook

How modems work

If you are having problems connecting to other
modems, or are interested in learning more about how
modems work, the information in this section can help
you. This section discusses the modem connection pro-
cess, data modems, connecting to other data modems,
fax modems, and choosing fax software applications.

Modem connection process

For two modems to connect, they must be of the exact
same type and function. A data modem can only con-
nect to a data modem and a fax modem can only con-
nect to a fax modem. Many modern modems are
designed with both data and fax capabilities. The data
components of these data/fax modems can communi-
cate with data modems, and the fax components can
communicate with fax modems.
The connection process is controlled by software built
into the modem. This connection process is called a pro-
tocol. Modems communicate by protocol and negotiate
to achieve a connection. These protocols must be com-
patible for the modems to connect.
The result of this negotiation determines the speed at
which the two modems connect. Normally a 14,400 bits
per second (bps) modem connects with a 9,600 bps
modem at the lower speed of 9,600. But, because of
phone line noise, they may connect at 2,400 bps. If you
use a modem communications software application
(such as Windows Terminal or Microsoft Works for Win-
dows), you should see the final connection speed on the
screen. This rate remains constant throughout the ses-
sion.

Data modems

Data modems send and receive data in the form of
codes that represent letters, numbers, and spaces. Each
letter of a word is represented by a code. These codes
are sent in a series with a different code representing a
space between words. Special applications, like word
processors, use additional code sequences to indicate
font type, bold, italic, and so on.
You use the Attention command set (sometimes called
the AT or Hayes command set) to send commands to
your data modem. The AT command set has evolved
over the years, and some of its "standards" have also
changed. Modem default settings and the more sophisti-
cated modem commands, in particular, have changed
over time.
Modem protocols govern the way a modem converts
digital information from the computer into analog sig-
nals that can be transmitted over the telephone lines.
Over time, standards have been created for these proto-
cols. These standards are necessary so that modems
made by different companies can exchange data.
How modems work 29