Hardware Reference
In-Depth Information
modem is several years old, it may only conform to the DOCSIS 1.1 or older standards, which
may be limiting your connection speeds. For example, I had been using a DOCSIS 1.1 modem
and (according to Speedtest.net) was experiencing download speeds of greater than 8Mbps and
upload speeds of greater than 2Mbps. After replacing my DOCSIS 1.1 modem with a newer
DOCSIS 2.0 model, my connection speeds increased to greater than 20Mbps download and
greater than 8Mbps upload—nearly triple the performance! Since DOCSIS 2.0 is limited to a
maximum of 38Mbps, when I subscribed to a 50Mbps connection I had to replace the modem
again. DOCSIS 3.0 modems supporting up to 304Mbps are required for most of the higher
connection speeds offered today.
Although most cable modems are now available for about $50-$100, you should check with your
CATV Internet provider before purchasing one to determine which models your 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 connecting 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-in router, although I prefer to keep them as separate
units. If the router is built in, you can disable it and use an external router if you choose. A typical
cable modem is shown in
Figure 16.2
.
Figure 16.2. Front (left) and rear (right) views of a typical cable modem showing status LEDS
on the front and power, LAN, USB and cable connections on the rear.
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 modem from the CATV network.
In this way, the cable modem functions as a tuner, just like your cable TV box, ensuring that your PC
