Understanding the working of
Modem is an abbreviation for Modulator Demodulator. A modem converts
data from digital computer signals to analog signals that can
be sent over a phone line (modulation). The analog signals are
then converted back into digital data by the receiving modem (demodulation).
A modem is given digital information in the form of ones and zeros
by the computer. The modem converts it to analog signals and sends
over the phone line. Another modem then receives these signals,
converts them back into digital data and sends the data to the
The actual process is much more complicated then it seems. Here
we discuss some internal functions of modem that helps in the
modulation and demodulation process.
1. Data Compression
Computers are capable of transmitting information to
modems much faster than the modems are able to transmit the same
information over a phone line. However, in order to transmit data
at a speed greater than 600 bits per second (bps), it is necessary
for modems to collect bits of information together and transmit
them via a more complicated sound. This allows the transmission
of many bits of data at the same time. This gives the modem time
to group bits together and apply compression algorithms to them.
Modem compresses them and sends over.
2. Error Correction
Error correction is the method by which modems verify
if the information sent to them has been undamaged during the
transfer. Error correcting modems break up information into small
packets, called frames and send over after adding a checksum to
each of these frames. The receiving modem checks whether the checksum
matches the information sent. If not, the entire frame is resent.
Though error correction data transfer integrity is preserved.
3. Flow Control
If one modem in a dial up connection is capable of sending
data much faster than the other can receive then flow control
allows the receiving modem to tell the other to pause while it
catches up. Flow control exists as either software or hardware
flow control. With software flow control, when a modem needs to
tell the other to pause, it sends a certain character signaling
pause. When it is ready to resume, it sends a different character.
Since software flow control regulates transmissions by sending
certain characters, line noise could generate the character commanding
a pause, thus hanging the transfer until the proper character
is sent. Hardware flow control uses wires in the modem cable.
This is faster and much more reliable than software flow control.
4. Data Buffering
Data buffering is done using a UART. A UART (Universal
Asynchronous Receiver/Transmitters) is an integrated circuit that
converts parallel input into serial output. UART is used by computers
to send information to a serial device such as a modem. The computer
communicates with the serial device by writing in the UART's registers.
UARTs have buffers through which this communication occurs on
First in First out basis. It means that the first data to enter
the buffer is the first to leave. Without the FIFO, information
would be scrambled when sent by a modem. This basically helps
the CPU to catch up if it has been busy dealing with other tasks.
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