In the top half of the figure, the phones generate and receive
analog electrical signals. The equipment in the telco CO is called a telephone
switch because the device in the CO is built to support telephone traffic, and
it thinks that there's a telephone on the other end of the local loop cable. To
create a voice phone call, the telco causes the analog electrical signals to go
from one side of the network to the other.
Modems allow two computers
to send and receive a serial stream of bits over an analog phone circuit, with
no physical changes required on the typical analog local loop between a
residence and the telco CO. Because the telephone switch in the CO expects to
send and receive analog voice signals over the local loop, modems simply send an
analog signal to the PSTN and expect to receive an analog signal from the PSTN.
However, instead of voice that a human speaker creates, the analog signal
represents some bits that the computer needs to send to another computer.
Similar in concept to a phone converting sound waves into a representative
analog electrical signal, a modem converts a string of binary digits on a
computer into a representative analog electrical signal. In fact, although most
people simply use the term modem, you can also call these devices analog modems because they create
and interpret analog electrical signals.
Modems encode a binary 0 or 1 onto the analog signal by varying
the analog signal, for instance, by varying frequency or amplitude. Changing
these characteristics of the analog signal is referred to as modulation. For instance, one of
the earliest modem standards used an analog signal of 2250 Hertz for a binary 1
and 2100 Hz for a binary 0. (Remember: Frequency is measured in Hertz.) A modem
could modulate, or change, between the two
frequency levels to imply a binary 1 or 0. Figure 16-4 outlines the basics of how a modem can
transmit data.
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