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Wire and Wireless Communication Applications Chap. 8
to RF and transmitted to the home via coaxial cable. This allows the neighborhood coaxial
cable network to operate up to 800 MHz because the coaxial distribution lines are relatively
short and very few RF amplifiers are needed.
To deliver neighborhood data service in an over the coax network, one or more TV
channels in the 50 to 750 MHz range are allocated for downstream data traffic and an upstream
path is provided by one or more channels in the 5 to 42 MHz range. The user connects
the CATV cable that enters hisher home to the cable modem which demodulates the downstream
data and modulates the upstream data. The modem data is usually connected to the
local computer or local in-house data network via an ethernet line, or, less likely, a USB
connection. The downlink modem data speed is usually around 8 Mbs and the uplink speed
is around 4 Mbs.
Referring to Table 8–15, a single downlink 6 MHz-wide channel can support a
composite downstream data rate of 27 Mbs (which is shared among the subscribers in
the neighborhood) if 64 QAM is used. Up to 36 Mbs can be supported if 256 QAM is
used. Typically, one upstream channel supports a composite data rate of 10 Mbs using
QPSK modulation, or 30 Mbs if 16 QAM is used. The upstream channel may be broken
down into three 2 MHz-wide channels, each with a data rate of 3Mbs for QPSK or
10 Mbs for 16 QAM. A 2 MHz-wide channel reduces the possibility of ingress noise
sent upstream from each user, since there are fewer users on a 2 MHz channel than on a
6 MHz channel.
One downlink and one uplink coaxial 6 MHz-wide channel can support up to about
200 subscribers since statistical multiplexing is used. With statistical multiplexing, each
subscriber uses hisher assigned data rate (e.g., 3 Mbs for uplink) only when needed;
otherwise, it is used by another subscriber. Using multiple 6 MHz channels, 500 to 2,000
subscribers can be supported in a neighborhood. If capacity is needed for more subscribers,
the original neighborhood “cell” can be broken down into multiple smallerneighborhood
cells.
From the discussion above, it is seen that cable modems are much more complicated
than DSL modems. They have front-end circuits (that operate in the VHF and UHF range),
down converters, and IF sections, as well as modulators and demodulators.
TABLE 8–15
CABLE MODEM STANDARDS
Item Downstream Upstream
Carrier frequency range 50 to 750 MHz 5 to 42 MHz
Channel bandwidth 6 MHz 6 MHz or 2 MHz a
Modulation 64 QAM or 256 QAM QPSK or 16 QAM
Composite data rate
27 Mb> s or 36 Mb>
s
10 Mb>
s to 30 Mb>
s
Subscriber data rate
1.5 to 18 Mb> s
256 kb>
s to 5 Mb>
s
Coding Reed Solomon Reed Solomon
Encryption DES DES
a If the uplink bandwidth is 2 MHz, then the composite data rate will be 13 that given the table.
DES = Data Encryption Standard