MODEM
This is an
acronym for
modulator-demodulator. A modem is a
device or
program that enables a
data transmission over
telephone lines. While computer information is stored
digitally, data sent over telephone lines is transmitted are in the
form of
analog waves. A modem
converts from one form to the other. To faciliate
compatibility between modems and computers, a standard interface, called
RS-232, has been established so that any computer can be be connected with any modem adhering to the standard. Modems in the form an expansion board that are placed in a
vacant expansion slot are called
onboard or
internal modems. There are a number of
protocols for formatting the data transmitted through the modem,
While the modem
interfaces are
standardized, a number of different protocols exist for formatting data to be transmitted over telephone lines, like the
CCITT1 V.34 and the
X25 for high speeds. Usually a modem has built-in support for the most common protocols.
The characteristics of a modem, in addition to the formatting
protocols, include:
- bps
-
The modem transmission speed for sending and receiving data. For slow rates, the unit of measurement is called a baud. The slowest rate (and at one time the only one) is 300 baud. When higher speeds are attained, speed is measured in bits per second (bps). Even higher speeds are available when the data is compressed before transmission. In some areas telephone lines cannot reliably transmit at high speeds.
- voice/data
-
Today most modems can switch between voice and data transmission. In voice mode, the modem acts like a telephone with onboard speaker and microphone. In data mode, the modem transmits only data.
- auto-answer
-
Ths feature allows the computer to receive calls automatically.
- compression
-
As mentioned before greater speeds of data transmission are obtained by compressing the data before it is sent. Obiously, the receiving modem must have the capability of decompressing the data, using the same rules.
- Fax capability
-
Most modern modems can also be used for sending and receiving fax messages.
_________________
1 =
Comité Consultatif International Téléphonique et Télégraphique or International Telegraph and Telephone Consultative Comittee.