by March 1, 2001 0 comments

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

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