The
cable modem
is the industry’s response to DSL and other broadband network services from
competitors. It provides remote access connectivity by establishing a shared data channel across
the existing cable television network. In fact, it’s apt to call it a channel—the bandwidth provided
to customers is actually taken from one of the 6MHz channels that would normally be
used for a video feed such as CNN or ESPN.
This 6MHz channel (NTSC—the North American standard from the National Television
System Committee) can provide up to 40Mbps of downstream (to the user) bandwidth and
12Mbps of upstream bandwidth. This bandwidth, as noted in Chapter 27, “Remote Access
with Digital Subscriber Line,” is shared by all the customers within a specific area. As such, due
to the normal installation and design model, in addition to bandwidth rate limiting by the provider,
a typical user should expect less than 2Mbps downstream and 128Kbps to 256Kbps
upstream. The typical cable modem installation is illustrated in Figure 28.1.
As shown in this figure, each home is connected to the coax (coaxial cable) that is running
through the neighborhood and providing video services. At the head end, or cable service provider,
this cable is connected to a hybrid fiber-coax (HFC) device that might also provide the
cable modem termination system (CMTS). This device is connected to the router that links to
the Internet and to the video streams (greatly simplified in this figure). The CMTS is the electronic
engine that processes cable modem feeds comparable to the digital subscriber line access
multiplexer (DSLAM) in DSL.
The installation at the home requires the installation of a filter to service all the televisions
on the premises. An unfiltered connection is provided to the cable modem itself. Note that for
customers without cable modems, the filter is typically placed in the street. In residences with
cable data services, the filter can be installed anywhere between the head end and the televisions
that will be using the cable signal. Many customers, as a result, never have to concern themselves
with the filter, but it does complicate the installation of a cable modem, just as the splitter
complicates DSL installations.