Understanding Cable Modems

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.

External cable modems from 3Com and RCA. Courtesy: www.3com.com, www.rca.com

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.

Sachin Makhija