HFC/CMTS HFC/CMTS
Router
Broadcast
Video
Internet
Note that cable modems have a perceived disadvantage of shared bandwidth for all users
on a particular link—there are two distinct shared domains shown in Figure 28.1. As noted in
Chapter 27, this is not a significant issue from a bandwidth perspective. It could be a security concern
however, as data from one home is viewable from all other homes within that domain. This
is addressed by the
Data Over Cable Service Interface Specification (DOCSIS)
ratified by Cable-
Labs, a nonprofit organization composed of cable service providers in the Americas. DOCSIS is
described in the following section; it provides customer data protection over the shared medium.
The biggest advantage to cable modems is their capability to provide high per-user bandwidth
over long distances, often significantly greater than DSL. Although the cable is capable
of providing up to 40Mbps of downstream bandwidth, the network is provisioned so that each
user can obtain only a predefined rate—typically less than 2Mbps. For consumer installations,
this is sufficient and leads to a very economical solution. However, the provider could easily
increase the bandwidth to an individual user, although they would need to have a dedicated
coax connection to attain the full capacity.