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This HOWTO only deals with the common type of analog modem used to
connect PC's to ordinary analog telephone lines. There are also
higher speed analog modems that use special types of lines: cable and
DSL modems. There is also the ISDN "modem" which uses digital signals.
While this HOWTO doesn't cover such modems, some links to documents
that do may be found at the start of this HOWTO. The next 3
sub-sections: DSL, Cable, and ISDN, briefly discuss such modems. For
both DSL and Cable modems, the basic QAM modulation method is similar
to ordinary analog analog modems. See
Combination Modulation
DSL (often ADSL) uses the existing twisted pair line from your
home (etc.) to the local telephone office. This can be used if your
telephone line can accept significantly higher speeds than an ordinary
modem would use. It replaces the analog-to-digital converter at the
local telephone office with one which can accept a much faster flow of
data (in a different format of course). The spectrum of the twisted
pair line is divided up into various channels. Each channel uses QAM
modulation like ordinary modems do. Data is sent over multiple
channels. The device which converts the digital signals from your
computer to the analog signal used to represent digital data on what
was once an ordinary telephone line, is a DSL modem.
The coaxial cables that provide for cable television in homes have
additional bandwidth not used for television, mostly at frequencies
higher than used for cable TV. This extra bandwidth may be used for
connecting computers to ISP's. However, many computers need to share
the same cable. The spectrum of the free bandwidth is split up into
channels (frequency division multiplexing) and each channel is given
time slots to which individual computers are assigned (time division
multiplexing). The cable modem converts the digital date from your
computer (from a network card: NIC) to the required analog signal, and
only broadcasts within it's assigned time slots on it's assigned
channel.
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