Levi Pearson wrote:
Hans is correct; a DSL modem is a modem. A cable modem is also a
modem. Here's why: Modem stands for MODulate-DEModulate, and
modulation is the process of changing the frequency, amplitude, and
phase of an analog signal in order to encode a digital signal. Both
DSL and Cable internet are digital information carried through an
analog medium, so a modem is required on both ends. I believe ISDN,
on the other hand, is transmitted digitally, as are most network
protocols that go over twisted pair or fiber.
Cable modems are most definitely modems in every sense of the word. Most
cable modems use the 64- or 256-QAM modulation types. Wikipedia has a
good article describing QAM here (don't fear the math -- it's not that bad):
http://en.wikipedia.org/wiki/Quadrature_amplitude_modulation
Cable modems are very much like regular dial-up modems. The difference
is that the medium (high frequency coax cable) is much more capable than
a single voice channel (think 10Mbps vs. 56Kbps), and therefore allows
them to use a higher-bandwidth modulation type (QAM).
DSL modems are usually bridges in addition to modems. My DSL modem takes
Ethernet carrying IP data, strips off the Ethernet framing, wraps it in
PPP (Point-to-Point Protocol) over ATM, and then modulates it over the
twisted-pair phone line at an analog frequency that must be filtered by
the rest of the phones in the house. DSL, unlike cable, does not use
QAM, but rather DMT or CAP, or something else I'm not familiar with.
Enjoy.
--Dave
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