Cable Modems

Of all the access technologies covered in this chapter, cable modems are the only one not using a phone line from the local telco for physical connectivity. Many homes also have a cable TV service supplied by an analog electrical signal entering the home over a coaxial cable—in other words, over the cable TV cabling. Cable modems provide an always-on Internet access service, while allowing you to surf the Internet over the cable and make all the phone calls you want over your telephone line—and you can watch TV at the same time!

Cable modems use some of the bandwidth that otherwise might have been allocated for new TV channels, using those frequency bands for transferring data. It's a little like having an "Internet" channel to go along with CNN, TBS, ESPN, The Cartoon Network, and all your other favorite cable channels.

To appreciate how cable modems work, you need a little perspective on some cable TV terminology. Cable TV traditionally has been a one-way service—the cable provider sends electrical signals down the cable for all the channels. All you have to do, after the physical installation is complete, is choose the channel you want to watch. While you are watching The Cartoon Network, the electrical signals for CNN still are coming into your house over the cable—your TV is just ignoring that part of the signal. If you have two TVs in your house, you can watch two different channels because the signals for all the channels are being sent down the cable.

Cable TV technology has its own set of terminology, just like most of the other access technologies covered in this chapter. Figure 15-15 outlines some of the key terms.

The cable head-end site is a main site that receives the programming. Programming typically is received via a satellite receiver dish. The head end converts the signals to match the correct encoding and frequencies used on the cable and transmits the signals. It also might scramble channels that require an extra fee from subscribers so that you have to get a descrambler— typically called a set-top box—from the CATV company.

Figure 15-15 Cable TV Terminology

Andy's PC

Andy's PC

Mayberry CATV


Mayberry CATV

Essentially, the CATV signal is broadcast over the rest of the cable plant, being amplified along the way. A drop cable taps into the distibution cable that runs near your house and then enters your home and connects to the back of a wall plate near your TV. You just need to run the short coax cable from the back of your TV to the wall plug, and the cabling is complete.

Because most people will want to watch TV as well, possibly multiple TVs, the drop cable must be split. Splitting does not mean literally taking the wire out of the cable and cutting it in half—instead, it means that you use a small device that passively lets the signal coming in from the street pass through to other cables in your house. You can use the same kind of line splitter when using a cable modem that you use when you connect two TVs to the cable TV line at the same time. In the figure, the splitter connects to the drop cable, as well as the two cables connecting to the cable modem and the TV. The splitter just takes the incoming signal from the drop cable and passes it out both of the other lines. Note that the connector, the round connector common on most CATV cabling, is called an f-connector.

When using a cable modem, the CATV company becomes your ISP. Everything between your house and the router at the head end is a single physical and data link. The PC in your home uses a router owned by the cable company, housed at the head-end site, as its default gateway. In fact, the PC typically uses DHCP to discover its IP address and the IP address of its default gateway; the DHCP server would be inside the cable company's IP network, typically at the head-end site.

Conceptually, what happens between the home and the cable head end is similar to a single LAN segment. The details, of course, are different, but the cable installation provides a combination of Layer 1 and Layer 2 protocols to let a PC deliver IP packets to a router inside the cable network. So, as you read about the details of what happens between the home and the router at the head end, keep in mind that the goal is simply to deliver IP packets between the home and the head-end router, and vice versa.

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