Understanding Cable Modems

A look at the different types of cable modems and how they transfer Internet data over your cable TV network

You will agree that accessing the Internet through normal modems, be it from office or home, is not the easiest thing in the world to do. One of the alternatives that’s becoming popular among home users is a cable modem. In this article, we’ll take a closer look at cable modems and see what makes them tick.

Cable Internet means accessing the Internet through the same cable that brings TV channels like Star, Zee, and MTV into your homes. The two main devices which make this possible are a Cable Modem Termination System (CMTS), which has to be installed at your cablewallah or broadband service provider’s end, and a cable modem, which has to be installed in your home. Simply put, a cable modem is a device that lets you access the Internet through your Cable TV (CATV) network. Cable modems come in three different flavors:

External cable modems

The most common type of cable modem available today, this is a small device, similar to ordinary dial-up external modems. But that’s where the similarity ends. One side of the cable modem connects to the coaxial cable coming from your local cable operator, and the other side connects to your PC through an Ethernet interface. So you’ll need an additional network card inside your PC to connect this. An Ethernet cable will connect your PC to the cable modem. An obvious advantage of the Ethernet interface is that you can easily connect more computers to the cable modem by attaching a hub to it.

When everything else is going USB, it’s not surprising that cable modems are also available with a USB interface. This means you don’t need to buy an additional network card and installation will also be easier. On the flip side, you can only connect a single PC to a USB cable modem.

Internal cable modems

These cable modems fit inside your PC and usually have a PCI interface. They’re cheaper than external cable modems, and being PCI, will only fit inside an ordinary desktop PC. Mac and notebook users will probably need an external cable modem.

Set-top box

You may have seen set-top boxes (STBs) from companies like Jadoonet and Samsung, which allow you to access the Internet using your TV and a keyboard. Till now, these boxes contained a regular modem that would dial to an ISP and connect over normal telephone lines, but the cable modem is also available in set-top boxes now. The STB connects to the cable coming from your cable operator at one end and a TV at the other end.

Inside a cable modem

Although the various types of cable modems we’ve mentioned are different in appearance, they all contain the same key components needed to make them work. Let’s look at these components and see how they function.

Tuner

Internet data travels in the form of radio frequency signals over a cable network. Therefore, a cable modem needs something to be able to send and receive these signals. That’s what a tuner does. It sits inside the cable modem and connects to the cable coming from your cable operator. This cable has to go through a splitter before it reaches the cable modem, which separates your Internet data from normal TV programs. Internet data is transmitted at different frequencies for uploading (upstream) and downloading (downstream). For this, the tuner contains a diplexer, which allows it to handle both downstream (between 42–850 MHz) and upstream frequencies (between 5–42 MHz). The tuner receives digitally modulated QAM signals and passes them on to the demodulator. QAM stands for Quadrature Amplitude Modulation and is a method of modulating digital signals into radio-frequency signals by varying both amplitude and phase of the wave signal.

Demodulator

This part of a cable modem converts radio-frequency signals received from the tuner into signals that can be fed to an analog to digital (A/D) converter. This in turn converts these analog signals into a series of 0s and 1s. An error correction module then goes through these 0s and 1s to check for any problems in transmission. Finally, an MPEG synchronizer is used to make sure that the digital signal data stays in order.

Modulator

The modulator does the reverse of what a demodulator does. It converts digital computer data (upstream data sent from your PC to the Internet) into radio-frequency signals, which can be transferred over the cable. It is also known as a burst modulator because of the irregular nature of traffic flowing between the user and the Internet. The modulator has three basic parts: an error correction module, a QAM modulator and a D/A (Digital to Analog) converter.

MAC

The MAC (Media Access Control) mechanism sits between the upstream and downstream data paths. It’s used to share the media in a controlled and reasonable way, so that all users are able to access the Internet without any problems. For example, the cable modem service provider can control the bandwidth assigned to a particular cable modem using its MAC address. Other functions of MAC in a cable modem are far more complex than in other network devices such a LAN card, etc, which also have a MAC address. Therefore, some of the cable modem’s MAC functions will be assigned to the processor in the cable modem or to the CPU in your PC.

Interface

The interface, which can be Ethernet, PCI, or USB, transfers data between your PC and the cable modem. STBs don’t have a PC interface, but connect directly to your cable operator’s cable. You then attach your TV and a keyboard to the STB to access the Internet.