Making Data Sound Like Voice
First, let's cover a little bit about how voice works and how
it works over a phone line. Then, I'll cover how to send data over that same
phone line.
Sound waves travel through the air by vibrating the air. The
human ear hears the sound because the ear vibrates as a result of the air inside
the ear moving, which in turn causes the brain to process the sounds that the
ear hears.
The telco, however, cannot forward vibrating air particles over
its network. To send the voice over the phone line, something must convert the
sound (the vibrating air) into an electrical signal, because the telco local loop (the phone line
between your house and the CO) was designed to carry electrical signals. To
convert the sounds into electricity, a telephone includes a microphone. In case you've never
stopped to think about it, a microphone simply converts sound waves into an
analog electrical signal. The telco can send the electrical signal between one
phone and another. On the receiving side, the phone converts the electrical
signal back to sound waves using a speaker that is
inside the part of the phone that you put next to your ear.
Enough talk about this voice stuffour end goal is to be able to
send data over the phone line. To understand how that happens, consider what
occurs when a phone sends the electrical signal over a phone line, as shown in
Figure 16-2.
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