A modem ( modulator- dem odulator) is a network
hardware device that modulates one or more carrier
wave signals to encode digital information for
transmission and demodulates signals to decode the
transmitted information. The goal is to produce a
signal that can be transmitted easily and decoded to
reproduce the original digital data. Modems can be
used with any means of transmitting analog signals,
from light emitting diodes to radio . A common type
of modem is one that turns the digital data of a
computer into modulated electrical signal for
transmission over telephone lines and demodulated
by another modem at the receiver side to recover
the digital data.
Modems are generally classified by the amount of
data they can send in a given unit of time, usually
expressed in bits per second (symbol bit/s ,
sometimes abbreviated "bps"), or bytes per second
(symbol B/s ). Modems can also be classified by
their symbol rate, measured in baud . The baud unit
denotes symbols per second, or the number of times
per second the modem sends a new signal. For
example, the ITU V.21 standard used audio
frequency shift keying with two possible frequencies,
corresponding to two distinct symbols (or one bit per
symbol), to carry 300 bits per second using 300
baud. By contrast, the original ITU V.22 standard,
which could transmit and receive four distinct
symbols (two bits per symbol), transmitted 1,200 bits
by sending 600 symbols per second (600 baud)
using phase shift keying .
