Hardware Reference
In-Depth Information
provider supports and whether your CATV Internet provider still requires you to lease the
cable modem.
If you plan to keep cable modem service for more than a year, I recommend purchasing
a CableLabs Certified cable modem, but if you're unsure of your long-term plans, leasing
isn't all that expensive. Typical lease costs for the device are about $5/month.
Cable modems normally come in an external box that has a cable connection for connect-
ing to the cable network, along with USB and Ethernet ports to connect to your network.
Although you can connect the cable modem directly to a single PC via USB, this is not
recommended for security reasons as well as for sharing the connection. By connecting
the cable modem to a router, you can share the connection among several PCs, with the
router's built-in firewall providing security and protection. Some cable modems include a
built-inrouter,althoughIprefertokeepthemasseparateunits.Iftherouterisbuiltin,you
candisableitanduseanexternalrouterifyouchoose.Atypicalcablemodemisshownin
Figure 16.2
Front (left) and rear (right) views of the RCA DCM425 Cable modem—CableLabs Certified
DOCSIS 2.0.
Cable Bandwidth
Cable TV uses what is known as a
broadband network
, meaning the bandwidth of the
connection is split to simultaneously carry many signals at different frequencies. These
signals correspond to the channels you see on your TV. A typical HFC network provides
approximately 750MHz of bandwidth.
For data networking purposes, cable systems typically allocate bandwidth in the
50MHz-750MHz range for downstream traffic—that is, traffic coming into the cable mo-
dem from the CATV network. In this way, the cable modem functions as a tuner, just like
your cable TV box, ensuring that your PC receives signals from the correct frequency.
