Apple Express Fax/Modem - Manual (Page 24)

What modems do
Before two computers can exchange data via a modem connection, they must
be able to recognize each other and agree on how data will be transmitted.
Without modems, computers cannot exchange data over long distances
because the signal will degrade after a few meters. Therefore, to pass across
the PSTN (Public Switched Telephone Network), digital information from
your computer is converted to a sound modulated signal. The signal is
reconverted to digital by the receiving modem.
Modems also dial and establish connections between computers. They control
the flow of data across the telephone line so transmission takes place with
optimal speed and accuracy.
Setting up a connection
When two computers connect through their modems, the modems’ first task is
to reach agreement on speed, error control, and data compression. Reaching
agreement is called handshaking.
Handshaking is equivalent to saying, “I’m German; you’re French, but we
both speak English. So let’s speak English a little slowly. If we’re both
English, we can speak more quickly.”
Your modem is already set to automatically make the best possible
handshaking agreement. See Appendixes A and B for a complete list of
specifications and default settings.
In general, it is best to accept your modem’s automatic settings. If you want to
set your own speed, error control method, and data compression type, see the
user’s guide that came with your communications software and the AT
command set in Appendix B.
12 C H A P T E R T W O