Data Modems - IBM Aptiva Handbook

negotiate to achieve a connection. These protocols must
be compatible for the modems to connect.
The result of this negotiation determines the speed at
which the two modems connect. For example, a 14,400
bits per second (bps) modem connects with a 9,600 bps
modem at the lower speed of 9,600 bps. But, because of
phone line noise, they may connect at 2,400 bps. If you
use a modem communications software program, such
as Windows HyperTerminal, you should see the final
connection speed on the screen. This rate remains
constant throughout the session.

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 programs, 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
sophisticated modem commands, in particular, have
changed over time.
Modem protocols govern the way a modem converts
digital information from the computer into analog
signals that can be transmitted over the telephone lines.
Over time, standards have been created for these
166 Appendix C: Modem information
protocols. These standards are necessary so that
modems made by different companies can exchange
data.
Primarily, modem standards and recommendations in
the United States have been set by two organizations:
•
The International Telephone and Telegraph
Corporation set the Bell standards.
•
The International Consultative Committee on
Telephony and Telegraphy (CCITT) set the V. (Vee
Dot) recommendations.
Data modems use these standards for speeds up to
28,800 bps:
•
Bell 103
•
Bell 212A
•
CCITT V.21, V.22, V.22bis,V.23, V.32, V.32bis, and V.34
Many modems support all of these standards. Older
modems may support some, but not all of these
standards.