Dialup UK Article - Internal Working Of Modem
Modems are devices used to access internet in a dial up internet
connection. The word “Modem” is actually Derived
from 2 terms, Modulator and Demodulator. The primary function
of a modem is made up of two processes. One is to convert
data from digital computer signals to analog signals to send
over a phone line and other one is to convert analog signals
into digital data when it is received through phone line.
The process in which digital computer signals are converted
to analog signals is called modulation. The process in which
analog signals are converted back into digital is called demodulation.
The primary function of the modem seems very simple when
you read the description given above. However, the actual
process is much more intricate. It is divided into some smaller
functions as explained below.
The process begins when the computer transmits data to modem
to send over internet. However, there is a significant gap
between ability of data transfer between computer and modem.
Computers are capable of transmitting information to modems
at a faster rate then the modems are able to the same over
a phone line. Hence, modems use technique called Data Compression.
Modems collect bits of information together and compresses
then before transmitting them over a phone line. The modem
then group bits together and applies compression algorithms
to them. The data is compressed and sent over to the phone
line. This allows the transmission of large number of bits
of data at the same time.
The modem also takes responsibility to maintain the integrity
of the data sent through dialup connection. This function
is called Error Correction. This function helps modems to
authenticate that the information sent has remained intact
during the transfer. In error correction, modems break up
the data into small packets called frames. These frames of
information are then transmitted after adding a checksum to
each of these frames. The receiving modem checks whether the
checksum matches the information sent. If the checksum does
not match then the entire frame is resent. This ensures that
the only the valid data is transferred and integrity of the
data is preserved.
The final task is to ensure that the flow of the data transferred
over internet is maintained consistently. It is possible that
the modem at the sending end is much faster then the modem
at the receiving end. Here, another function called the Flow
Control comes into play. To maintain the rate of data transfer,
if the sending modem is capable of sending data much faster
than the receiving modem then the flow control at receiving
modem allows the receiving modem to tell the sending modem
to pause while it catches up. The flow control can be either
software or hardware flow control. With software flow control,
the modem sends a certain character that signals pause when
it is time to halt. When it is ready to resume, it sends a
different character. Hardware flow control uses wires in the
modem cable. Overall, the Hardware flow control is faster
and much more reliable than software flow control.
Overall, these three functions makes up most of the internal
working of a modem and help modem to transfer data over the
dial up connection in an efficient and reliable way.