Computer World. Матросова Т.А. - 28 стр.

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run faster and more reliably, pack more features, use less power in less space, and
cost less. It's the familiar computer pattern of moving toward more power in less
space for less money.
External modems sit on your desk or underneath your monitor, while internal
ones plug into an expans ion slot ins ide the computer case. There are also acoustic
modems, the oldest modem technology of all.
Mo de m Bas ics
At its simplest, all any modem needs to do is provide a translation service from
computer signal to telephone signal and back. Computers «think» in digital, binary
form, but copper telephone wires carry signals that correspond roughly to the human
voice in loudness, tone, and range. It's little wonder the two need a translator to
communicate.
The modem provides this necessary translation. When a computer sends out its
data, the modem turns the computer's ON/OFF electrical s ignals into a telephone's
varying or modulated audible signal. In other words, the modem modulates the signal
– which accounts for the modulate part of a modem's name.
The newly modulated signal sounds like a whistle or, at higher speeds, like
«fuzzy» noise to the human ear. It would make no sense whatever to a computer's
logic circuits, but it travels just fine on copper wire. When it arrives at the other end
of the phone connection, another modem must turn it back into a digital, ON/OFF (or
demodulated) signal. Otherwise the receiving computer won't be able to make sense
of it. Hence the rest of the name: modulate/demodulate, or modem.
To make this simple idea work, all modems share certain components, like a
transmitter and receiver. On a telephone, the transmitter is the mouthpiece and the
receiver is the earpiece. Why not just use those? The only other component really
needed is something to «talk» and «listen» to the phone while sending and receiving
data... аn electronic translator that can talk over the phone.
Acoustic Modems
Early modems were just about that simple. You placed a normal telephone
receiver into a pair of noise-reducing, cushioned cups. These modems, still in use, are
c alled acoustic couplers or acoustic modems, because they have no direct electrical
connection. Instead, they send an audible signal directly into a small speaker, which
«speaks» into the telephone mouthpiece.
Acoustic coupling is not very efficient. In an office environment, the short-
comings quickly become obvious. If someone puts down a coffee cup too hard or
drops a spoon during a transmission, it introduces random blips and bleeps into the
signal. These get translated at the other end into «garbage» or nonsense characters. At
a pay phone, passing traffic can have similar effects.
Direct-Connect Modems
A better solution is to connect the modem directly – electrically, that is, instead
of acoustically into the phone circuit. This eliminates noise, interference, and

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