Cable Modems

Last Updated on Fri, 01 Sep 2023 | Data Link

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

Head-end

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.

Continue reading here: Comparison of Remote Access Technologies

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Readers' Questions

  • thomas
    What type of connector is often used for cable tv (including cable modem) connections?
    1 year ago
    • The type of connector often used for cable TV and cable modem connections is called an "F-type connector." It is a coaxial RF connector with a threaded barrel, commonly used in the telecommunications industry.
    • Carlton
      What connects a personal computer to a cabletv system that offers an internet connection?
      1 year ago
      • A cable modem is the device that connects a personal computer to a cable TV system that offers an Internet connection.
      • j
        How to hook up cisco cable box to tv?
        1 year ago
        1. Connect one end of the coaxial cable to the “Cable In” port on the back of the Cisco cable box.
        2. Connect the other end of the coaxial cable to the “Cable Out” port on the back of your TV.
        3. Plug the power cord of your Cisco cable box into a power outlet.
        4. Turn the Cisco cable box on by pressing the “Power” button.
        5. On your TV, switch the input source to the “Cable” setting using the “Input” button on your remote.
        6. Use the “Menu” button on your remote to select the type of connection your cable company uses.
        7. Select “Auto Tuning” from the list of options.
        8. Your Cisco cable box should now be connected to your TV, and you can start watching TV.
        • sinead
          How to connect two modems in one house?
          1 year ago
          1. Connect the modems to different outlets. Each modem should be connected to its own individual outlet.
          2. Hook up one modem to your computer via an Ethernet cable.
          3. Connect the other modem to the first modem via an Ethernet cable.
          4. Log into both modems by typing the IP addresses into your web browser.
          5. Configure the modems so that they have different IP addresses and different subnets.
          6. Configure one modem as the main gateway router and the other as a secondary router.
          7. Set up each router with the same SSID and Wi-Fi password.
          8. Make sure both modems are connected to the internet.
          9. All done! Your two modems should now be connected and you should be able to use both modems in your home.
          • rosie
            Can you hook up 2 modems in 1 house?
            1 year ago
            • Yes, you can hook up two modems in one house. This can be done by connecting one modem to your router and the other to your computer. You will also need to configure the two modems with different IP addresses and make sure that each modem is compatible with the router.
            • Mackenzie
              Is it possible to have two cable providers in one house?
              1 year ago
              • Yes, it is possible to have two cable providers in one house. Depending on the location, there may be multiple companies that offer services in a certain area. The home owner must contact each provider to see what services they offer and what type of equipment is needed to install the services.
              • rowan
                How to connect two cable cords?
                1 year ago
                1. Determine the type of cable cords you are connecting. Different types of cables have different connectors and require different methods for connecting.
                2. Acquire the necessary tools and accessories for connecting the cables. This could include cable cutters, crimpers, and any adapters needed to match the connections of the cables.
                3. Cut the cable cords to the desired length, leaving enough slack for maneuvering the cords.
                4. Strip the outer cover off of each cable cord. Carefully expose the internal wires and make sure none of them come into contact with each other.
                5. Determine which of the exposed wires need to be connected together, depending on the cable type. Generally, solid wires are crimped together and stranded wires are twisted together.
                6. Connect the appropriate wiring. Use crimpers or a wire stripper to crimp solid wires together, or use a pair of pliers to twist stranded wires together.
                7. Connect each exposed wire to the appropriate terminal on the connection device. Double check your connections to make sure they are secure.
                8. Secure the connection device and cover the cord connections with electrical tape.
                9. Test the connection to make sure it works.
                • agenore moretti
                  What type of coaxial cable for modem?
                  1 year ago
                  • RG-6 coaxial cable is typically used for connecting modems to cable outlets in homes and businesses.
                  • wiseman noakes
                    Can you have multiple cable modems in one house?
                    1 year ago
                    • Yes, you can have multiple cable modems in one house. However, multiple cable modems may not be necessary or even recommended by your internet service provider (ISP). You may need more than one modem if you have multiple internet services, such as cable and DSL, or if you are running multiple devices on one connection.
                    • abby
                      What type of splitter for cable modem?
                      1 year ago
                      • A cable modem typically requires a coaxial cable splitter. This splitter will allow you to connect multiple devices to the modem, such as TVs, DVRs, cable boxes, and gaming consoles.
                      • evelyn
                        Do you need to connect coax cable for internet?
                        1 year ago
                        • Yes, in most cases you will need to connect a coax cable for internet service. Coax cables are usually used in cable TV and cable internet services, but some providers also offer DSL internet service over coax cables.
                        • anne
                          How to wire your house for cable tv?
                          1 year ago
                          1. Contact your local cable provider and request a cable connection, providing the necessary account information.
                          2. Mount the cable outlet closest to your television on an interior wall. This is where the coaxial cable (coax) will be fed in from the outside.
                          3. Drill a hole in the wall close to the outlet and feed the coax cable in from the outside, connecting it to the outlet.
                          4. Run the coax cable from the outlet down to the cable box or modem provided by the cable provider. If necessary, use wall anchors and fasteners to secure the cable along walls and ceilings.
                          5. Connect one end of the coax cable to the back of the cable box or modem, and the other end to the wall outlet.
                          6. Connect the cable box or modem to your television using either component or HDMI cables.
                          7. Make sure all the connections are secure, then power on the television and the cable box or modem.
                          8. Follow the on-screen instructions to complete the cable setup and enjoy your cable TV!
                          • Saku
                            Can you have two cable modems in one house?
                            1 year ago
                            • Yes, you can have two cable modems in one house. However, you will typically need to set up one modem in bridge mode for it to work correctly. This means that the modem is essentially disconnected from the network, allowing the other modem to handle all network traffic. Additionally, you may need to check with your internet service provider to make sure that having two cable modems is allowed.
                            • Haddas
                              How do cable modems work?
                              1 year ago
                              • Cable modems are used to connect computers and other devices to the internet via a coaxial cable line provided by a cable service provider such as a cable television provider. The modem receives a signal from the cable service provider’s head-end, which is the broadcast headquarters. The signal is then converted into a digital signal and sent to the computer. The modem then splits the digital signal into two signals: one for downloading information from the internet and one for uploading information from the computer. The downloading signal is sent to the modem’s demodulator, which translates the signal into a usable format for the computer. The uploading signal is sent to the modem’s modulator, which translates the information from the computer’s modem into a format that the head-end can receive. The head-end then sends the information through the cable line to its destination. The modem then reverses this process for data sent from the internet to the computer.