256
Macintosh
User's
Handbook
The second task which a modem must perform
is
to
transmit and receive data.
In
order
to accomplish this, the
modem must have some scheme for changing the bits into
audible tones. This scheme must be mutually agreed upon by
both
the local modem
and
the remote modem.
If
one modem
was speaking in one way
and
the other was speaking another
way, each would receive meaningless noise from the other.
It
would be like
an
American speakng
to
a Frenchman.
If
neither
knew the other's language, communication would be pointless.
Most modems use a technique known as
Tone Modulation.
Here, the modem will send one predetermined tone when
sending a
"0"
and
a different tone when sending a
"I."
On the
receive side, the modem will relay a
"O"to the computer when it
receives one specific tone
and
a
"I"
when it receives another
specific tone. As long as there
is
mutual agreement between the
two modems as to the interpretation
of
each tone, communica-
tion should
run
smoothly.
There
are
two types
of
modems which have been used
extensively for computer communications. These are acoustic
and
direct connect modems. Acoustic modems are designed to
connect with a standard telephone. An acoustic modem incor-
porates a cradle which
is
designed to hold a telephone handset
securely. The modem's speaking
and
receiving units use the
telephone as a person does. Signals are sent through the talking
portion
of
the phone
and
received through its listening portion.
An acoustic modem
is
depicted
in
figure 8.1.
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