Cable
Modems
o Data
rate limitation of
traditional modems is
mostly due to
the narrow BW
of the
local
loop telephone line (4Khz)
o If
higher BWs are
available, one can
design a modem
that can handle
much higher
data
rates
o Fortunately, cable
TV provides residential
premises with a
coaxial cable that
has a
BW
of up to 750 MHz and sometimes even more
o This BW is normally divided into 6MHz
bands using FDM
o Each band provides a TV Channel
o Two bands can be left aside to allow a
user to download and upload the information
from
the internet
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o Instead of the traditional cable box, we
show a splitter
o The splitter directs the TV bands to the
TV set and the Internet access bands to the PC
™ DOWNLOADING: Downloading requires
a 6 MHz
BW in the
range above
40MHz.
The demodulation technique used is 64 QAM (6 bits at a time)
o This means that a user can download info
at a rate of 6MHz * 6 = 36Mbps
o However PCs are not yet capable of
receiving data at this rate
9 Currently rate is b/w 3 and 10 Mbps
™ UPLOADING: Requires a
6MHz BW in a range below 40MHz
o At
this low frequency,
home appliances can
create a noise
environment that
effects
modulation
o The
modulation technique uses is QPSK (4 bits at a time)
o This means that user can Upload info at a
rate of 6 MHz * 2=12 MHz
o Presently
uploading rate is b/w 500Kbps and 1Mbps
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