wireless connection

Dave Smith dave at thesmithfam.org
Sun Jun 4 16:32:56 MDT 2006


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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