Understanding Cable Modem Technology

Internet access via cable TV infrastructure uses technologies that are very different from those that provide DSL access. The physical cabling is, of course, quite different, but the differences go deeper than this.

Chapter 18

Shared Bandwidth

Cable TV technology allows for the distribution of very large amounts of data. A typical cable TV system provides about 750MHz of bandwidth, and each channel consumes about 6MHz. The first channel (2) consumes the frequency range from 50-56MHz, channel 3 consumes 56-62MHz, and so on. This pattern yields a total capacity of more than 100 channels.

Internet access using this system works by devoting one channel to digital data distribution. DSL systems work by using a mere 1MHz frequency range, so with 6MHz in a single cable TV channel, it might seem that cable TV systems have the edge in speed. This is true in some sense, but cable's advantage is reduced or eliminated by the fact that this 6MHz of bandwidth is shared.

When you tune in a particular cable TV station to watch your favorite show, that same signal is viewable by your neighbors. The same is true of the digital Internet data coming over the cable. If you and your neighbor both download large files, you must share that 6MHz of bandwidth. The more customers a cable company signs up for Internet access, the more people will share the available bandwidth and the slower the access becomes. The cable company doesn't pre-carve the bandwidth, however, so the slowdown varies with actual use. For instance, at 4:00 a.m., chances are that few people are using the network, so if you download the latest 18

Linux distribution at that time, you'll probably get a blazingly fast download. Try the same thing at a peak hour—say, 8:00 p.m.—and performance will be less stellar, because you'll be m competing against your neighbors reading their email and cruising the Web. o m

Your cable company can split up a neighborhood into several independent network segments, S

however, so if the network becomes too crowded, it's possible to relieve the congestion. When this happens, the channel devoted to Internet access carries entirely different signals in the different segments.

Like cable service, DSL service also becomes combined at some point. This combination occurs further from the subscriber in the case of DSL. Figure 18.7 illustrates this difference. Suppose that four of the five computers in the left group of the cable network shown in Figure 18.7 are downloading large files, and that one of the five in the right group is doing the same. This arrangement produces, at that moment, speed disparity between those two groups of subscribers. If the equivalent computers in the DSL network are downloading files, all the users experience the same speed.

One other aspect of the shared nature of cable connections deserves mention: security. Because you and your neighbors receive the exact same signals, it might be possible for your neighbors to snoop on your network traffic in a cable modem network. In practice, this isn't usually the case, because most cable companies encrypt traffic in such a way that a subscriber's cable modem only decodes the data destined for that modem. It's also possible to misconfigure a DSL network such that one subscriber receives another's traffic. You should definitely ask about encryption used on your cable company's network if you consider using a cable modem.

Part IV

Cable Shared Access

Cable Shared Access

DSL Dedicated Access
Computer in use

Figure 18.7

Minute-to-minute differences in network use have a greater influence on cable networks than on DSL networks.

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