Modem Connections And Standards - IBM Aptiva Handbook

At the other end of the line, the receiving fax machine,
or fax software, reverses the process. The page is
reconstructed one line at a time.
The number of lines a fax machine or program uses to
scan a page affects the quality of the reproduced image
and the time required to transmit the facsimile. The
more lines per page, the higher the image quality and
Note:
the longer the transmission time.
Many fax software programs offer an option for
changing this scanning resolution.

Modem connections and standards

When two modems first start to communicate, they
negotiate to establish a connection through which both
the sending modem and the receiving modem can
dependably transmit and receive data. Each modem
identifies the highest speed at which it can transmit and
receive data. If the speeds do not match, the lower of the
two speeds is agreed upon as the transmission rate.
If two modems do not have any transmission speeds in
common, they cannot successfully establish a
Note: connection. Otherwise, they communicate using the
highest transmission rate at which they both can
operate reliably.
Before the digital data on a computer can be transmitted
through analog telephone lines, the modem must
convert it to analog signals. Methods for this
conversion, and the associated transmission rates, have
been standardized so that modems manufactured by
different companies can work with each other. These
standards are called modulation protocols. Primarily,
modulation protocols have been set by three standards
organizations:
•
AT&T set the Bell standards.
•
The International Telegraph and Telephone
Consultative Committee (CCITT) set the V. (Vee Dot)
recommendations.
•
The International Telecommunication Union (ITU)
has superceded the CCITT and continues to set
standards.
Newer modems, like your Aptiva modem, incorporate a
variety of protocols so they can connect to other
modems at various transmission speeds (baud rates).
Data modems use these modulation protocols for baud
rates of up to 33,600 bits per second (bps):
•
Bell 103
•
Bell 212A
•
ITU V.21, V.22, V.22bis, V.23, V.32, V.32bis, and V.34
Similarly, a modem that also has fax capabilities may
also support these protocols:
•
ITU V.17, V.29, and V.29ter
Usually a modem attempts a connection using the
modulation protocol that provides the best
performance. However, the protocol actually used
depends upon the protocols the two modems have in
common. When a higher-speed modem agrees to
communicate using a lower speed modulation protocol,
the process is called auto negotiation. For example, a
Modem connections and standards 153