|
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 driven diodes to
radio.
The most familiar example is a
voiceband modem
that turns the digital '1s and 0s'
of a personal
computer
into sounds that can be
transmitted over the
telephone lines of Plain Old
Telephone
Systems (POTS), and once
received on the
other side,converts those 1s and 0s back into
a form used by a USB, Serial, or Network
connection.
Modems are
generally classified by the amount
of data they can send in a given time,
normally
measured in bits per second, or "bps". They can
also be classified by Baud, the number of
distinct
symbols transmitted per second; these
numbers
are directly connected, but not necessarily
in
linear fashion (as discussed under
Baud.)
A modem (a
modulator/demodulator) lets
you connect your computer to a
standard
telephoneline so you can transmit and receive
electronically transmitted data. It is the key
that
unlocks the world of the Internet and its World
Wide
Web,commercial online services,
electronic mail (E-mail), and
bulletin board
systems (BBSes).
Types of Modems:-
Depending upon how your computer
is configured and
your preferences, you can have an external,
internal or
PC modem
card. All three types work the
same way, but
each has its
advantages and
disadvantages.
External modem:-
|
This is the simplest type of modem
to install
because you don't have to open the
computer.
External modems have their own power
supply
and connect with a cable to a computer's
serial
port. The telephone line plugs into a socket
on
the rear panel of the modem.
|
|
|
|
Because external modems have their
own power
supply, you can turn off the modem
to break an
online connection quickly without
powering down
the computer.
|
|
|
|
Another advantage over an internal
modem is
that an external modem's separate
power supply
does not drain any power from the
computer.
You also can monitor your modem's
connection
activity by watching the status
lights.
Internal modem:-
|
Most internal
modems come installed in the
computer
you buy. Internal
modems are more directly
integrated
into the computer system and,
therefore, do not need any
special attention. Internal
modems are activated
when
you run a communications program
and are turned
off when you exit the program.
This convenience is especially
useful for novice
users.
|
|
|
|
Internal
modems usually cost less
than external modems,
but the price difference is
usually small. The major
disadvantage with internal
modems is their
location:
inside the computer. When you want
to replace an internal
modem you have to
go inside the computer case to make the
switch.
|
|
PC Card modem:-
|
These modems, designed for
portable computers,
are the size of a credit card and
fit into the
PC
Card slot on notebook and
handheld
computers. These modems are
removed when
the modem is not needed. Except
for their size,
PC Card modems are like a combination
of
external and internal
modems.
|
|
|
|
These devices are plugged directly
into an
external slot in the portable
computer, so no
cable is required other than the telephone
line
connection. The cards are powered by
the
computer, which is fine unless the computer
is
battery-operated. Running a PC Card modem
while the portable computer is operating on
battery power
drastically decreases the life
of
your
batteries.
|
How Modem Works?
|
When a modem first makes a connection,
you
will hear screeching sounds coming
from
the modem. These are digital signals
coming
from the computer to which you are
connecting
being modulated into audible
sounds. The
modem sends a
higher-pitched tone to
represent the digit I and a
lower-pitched tone to
represent the digit
0.
|
|
|
|
At the other end of
your modem connection,
the computer attached to its modem
reverses
this process. The receiving modem
demodulates
the various tones into digital
signals and
sends them to the receiving
computer.
|
|
|
|
Actually, the process is a bit
more complicated
than sending and receiving signals
in one
direction and then another. Modems
simultan-
-eously send and receive signals in
small
chunks. The modems can tell incoming
from
outgoing data signals by the type of
standard
tones they use.
|
|
|
|
Another part of the translation
process involves
transmission integrity.
The modemsexchange
an added mathematical code along
the way.
This special code, called
achecksum, lets
both
computers know if the data
segments are coming
through properly.
|
If the mathematical sums do not
match, the
modems communicate with each other
by
resending the missing segments of
data.
Modems also have
special circuitry that allows
them to compress digital signals
before
modulating them and then
decompressing
them after demoduating the signals. The
compression/decompression process
compacts
the data so that it can travel
along telephone
lines more
efficiently.
|
|
|
|
Modems convert
analog data transmitted over
phone lines into digital data
computers can
read; they also convert digital
data into analog
data so it can be transmitted.
This process
involves modulating and demodulating
the
computer’s digital signals into
analog signals that
travel over the telephone
lines.
|
|
|
|
In other words, the modem translates computer
data into the language used by
telephones and
then reverses the process to
translate the
responding data back into
computer language.
What is the difference between
digital
and analog signals?
A computer performs its tasks by
turning on and off a
series of electronic switches
represented by the numerical
digits of 0 and 1. A 0 is the code
for off, and a 1 is the code
for on. Combinations of these
digital codes represent text,
computer commands, and graphics
inside the computer.
By comparison, the telephone works by sending sounds
in a continuous analog signal sent
along an electronic
current that varies in frequency and
strength.
|
|
|
|
| 
No comments:
Post a Comment