Many residences in the North America and elsewhere receive hundreds of broadcast television channels over coaxial cable networks. (We will discuss coaxial cable later in this section.) In a traditional cable television system, a cable head end broadcasts television channels through a distribution network of coaxial cable and amplifiers to residences.

While DSL and dial-up make use of the telco's existing local telephone infrastructure, cable Internet access makes use the cable television company's existing cable television infrastructure. A residence gets cable Internet access from the same company that provides it cable television As illustrated in the following figure, fiber optics connect the cable head end to neighborhood-level junctions, from which traditional coaxial cable is then used to reach individual houses and apartments. Each neighborhood junction typically supports 500 lo 5,000 homes. Because both fiber and coaxial cable are used in this system, it is often referred to as hybrid fiber coax (HFC).

A hybrid fiber-coaxial access network

Cable Internet access needs special modems, called cable modems. As with a DSL modem, the cable modem is normally an external device and connects to the home PC through an Ethernet port. (We will discuss Ethernet in great detail in "The Link Layer and Local Area Networks".) Cable modems divide the HFC network into two channels, a downstream and an upstream channel. As with DSL, access is usually asymmetric, with the downstream channel usually allocated at a higher transmission rate than the upstream channel.

One important feature of cable Internet access is that it is a shared broadcast medium. Particularly, every packet sent by the head end travels downstream on every link to every home; and every packet sent by a home travels on the upstream channel to the head end. For this reason, if some users are simultaneously downloading a video file on the downstream channel, the actual rate at which each user receives its video file will be significantly lower than the aggregate cable downstream rate. On the other hand, if there are only a few active users and they are all Web surfing, then each of the users may in fact receive Web pages at the full cable downstream rate, because the users will seldom quest a Web page at exactly the same time. Because the upstream channel is also shared, a distributed multiple-access protocol is required to coordinate transmissions and avoid collisions. (We'll discuss this collision issue in some detail when we discuss Ethernet in "The Link Layer and Local Area Networks".)

Supporters of DSL are quick to point out that DSL is a point-to-pint connection between the home and ISP, and therefore, the entire transmission capacity of the DSL link between the home and the ISP is dedicated rather than shared. Cable supporters, however, argue that a reasonably dimensioned HFC network provides higher transmission rates than DSL. The battle between DSL and HFC for high-speed residential access is raging, specifically in North America. In rural areas, where neither DSL nor HFC is available, a satellite link can be used to connect a residence to the Internet at speeds of more than 1 Mbps; StarBand and HughesNet are two such satellite access providers.


distribution network, dsl, protocol

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