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(modulator-demodulator) is a device that modulates an analog carrier
signal to encode digital information, and also demodulates such a
carrier signal 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 over any means of transmitting analog signals, from light emitting
diodes to radio.
familiar example is a voice band modem that turns the digital data of a
personal computer into modulated electrical signals in the voice
frequency range of a telephone channel.
can be transmitted 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 (bit/s, or bps).
can alternatively be classified by their symbol rate, measured in baud.
unit denotes symbols per second, or the number of times per second the
modem sends a new signal.
the process of changing the form of the signal carrying the information.
demodulation process does the task of extracting information from the
signals that are modulated. Analog signals can be transmitted by using
devices such as a radio or a diode.
are classified on the basis of two criteria, data sent per unit time,
change in the state of the signal per unit time computer sends
information in the form of digital signals.
information over the telephone lines needs to be transmitted in the form
of analog signals to solve this problem, the functionality of a modem
comes into play converts the digital signals into analog signals.
signals are carried over the telephone line these signals reach another
computer, the analog signals are converted back to the digital form by
In addition to
converting digital signals into analog signals, the modems carry out
many other tasks. modems minimize the errors that occur while the
transmission of signals.
They also have
the functionality of compressing the data sent via signals modems also
do the task of regulating the information sent over a network.
Correction: In this process the modem checks if the information they
receive is undamaged.
involved in error correction divide the information into packets called
this information, the modems tag each of the frames with checksums.
checksum is a method of checking redundancy in the data present on the
that receive the information, verify if the information matches with
checksums, sent by the error-correcting modem.
If it fails to
match with the checksum, the information is sent back.
the Data: For compressing the data, it is sent together in many bits the
bits are grouped together by the modem, in order to compress them.
Different modems vary in their speed of sending signals thus, it creates
problems in receiving the signals if either one of the modems is slow.
In the flow
control mechanism, the slower modem signals the faster one to pause, by
sending a 'character'. when it is ready to catch up with the faster
modem, a different character is sent, which in turn resumes the flow of
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