Data Modems; Connecting To Other Data Modems - IBM Aptiva Hardware Handbook

OS/2 Warp), 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 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.
Primarily, modem standards and recommendations in
the United States have been set by two organizations:
•
The International Telephone and Telegraph Corpora-
tion set the "Bell" standards.
•
The International Consultative Committee on Tele-
phony and Telegraphy (CCITT) set the "V." (Vee Dot)
recommendations.
Data modems use these standards for speeds up to
14,400 bps:
•
Bell 103
•
Bell 212A
•
CCITT V.21, V.22, V.22bis,V.23, V.32, and V.32bis
Many modems support all of these standards. Older
modems may support some but not all of these stan-
dards.

Connecting to other data modems

Phone line noise can cause loss of data and connection
when modem speeds rise above 1200 bps. Over time,
many different software transmission protocols, such as
X-modem, MNP, and V.42bis, have been developed
solely to detect and correct the errors caused by phone
line noise. Not all of these protocols are compatible.
This may be the reason you are unable to connect to
another modem.
If your data modem uses any error detection and correc-
tion protocol, the modem on the other end must also
have the same protocol. For example, if your data
modem uses X-modem (one of the early protocols), the
modem you're connecting to must also use X-modem.
This is why you are asked to choose a transfer type in
your software before downloading a file from a bulletin
board.
Many different error detection and correction protocols
are commonly used today. Examples of the earliest error
detection and correction protocols include X-modem,
Y-modem, and Kermit. These protocols are performed
by software on your computer.
How modems work 27