CABLE MODEM: Your Handy Tool

VoicenData Bureau
New Update

The term "cable modem" is quite new and refers to a modem

that operates over the ordinary cable TV (CATV) network.

Basically you just connect this modem to the TV outlet for your

cable TV and the cable TV operator connects a Cable Modem

Termination System (CMTS) at his end.


Actually this term is a

bit misleading as it works more like a Local Area Network (LAN)

interface than as a modem.

What Is It?


cable modem connection is something in-between. The speed is

typically 3 to 50 Mbps and the distance can be 100 km or more.

The CMTS can talk to all cable modems but the modems

can talk only to the CMTS. If two such modems need to talk to
each other, the CMTS will have to relay the message.


External box cable modems

with Ethernet interface normally acts as either MAC-layer

bridges (low-end models) or as routers (high-end SOHO models).

A CATV network is

designed and used for cable TV distribution. With an upgrade of

the system, it is normally possible to allow signals to flow in

both directions. Higher frequencies flow forward to the

subscriber (you) and the lower frequencies go in the other

direction. This is done by upgrades to the amplifiers in the

cable distribution network, etc.

Most CATV networks are

Hybrid Fibre-Coax (HFC) networks. The signals run in Optical

Fibre Cables (OFC) from the head-end centre to locations near

the subscriber. At that point the signal is converted to coaxial

cables that run to the subscriber premises.


One CMTS will normally

drive about 1,000-2,000 simultaneous cable modem users on a

single TV channel. If more of these modems are required, the

number of TV channels can be increased by adding more channels

to the CMTS.

A number of different

cable modem configurations are possible. The three

configurations discussed below are the main as of now. More

systems will arrive with time.

External Cable Modem


External cable modem is

the small external box that connects to your computer normally

through an ordinary Ethernet connection. The downside is that

you need to add a (cheap) Ethernet card to your computer before

you can connect it. A plus point is that you can connect more

computers to the Ethernet. Also it works with most operating

systems and hardware platforms, including Mac, UNIX, laptop,


Another interface for

external cable modems is Universal Serial Bus (USB), which has

the advantage of installing much faster (something that matters

because cable operators are normally sending technicians to

install each and every cable modem). The downside is that you

can only connect one PC to one USB-based cable modem. 

Internal Cable Modem


This is typically a PCI

bus add-in card for a PC. That might be the cheapest

implementation possible but it has a number of drawbacks. First

problem is that it can only be used in desktop PCs. It is

possible to use Macs and laptops but they require a different

design. Second problem is that the cable connector is not

galvanic isolated from AC mains. This may pose a problem in some

CATV networks, requiring a more expensive upgrade of the network

installations. Some countries and/or CATV networks may not be

able to use internal cable modems at all for technical and/or

regulatory reasons.

Interactive Set-top Box

Interactive Set-top Box

The interactive set-top

box is really a cable modem in disguise. The primary function of

the set-top box is to provide more TV channels on the same

limited number of frequencies. This is possible with the use of

digital television encoding. An interactive set-top box provides

a return channel–often through the ordinary Plain Old

Telephone System (POTS)—that allows the user access to Web

browsing, e-mail, etc. directly on the TV screen.


When installing a cable

modem, a power splitter and a new cable are usually required.

The splitter divides the signal for the "old"

installations and the new segment that connects it. No TV sets

are accepted on the new string that goes to it.

The transmitted signal

from it can be so strong that any TV set connected on the same

string might be disturbed. The isolation of the splitter may not

be sufficient. So an extra high-pass filter might be needed in

the string that goes to the TV sets. The high-pass filter allows

only the TV-channel frequencies to pass, and blocks the upstream

frequency band. The other reason for

the filter is to block ingress in the low upstream frequency
range from the in-house wiring. Noise injected at each

individual residence accumulates in the upstream path

towards the head-end, so it is essential to keep it at a minimum

at every single residence that needs this service.

Data Interface



any kind of external cable modem (the majority of what is in use

today), you obviously need some kind of data interface to

connect the computer and the cable modem.



data-port interface on most external modems is 10 Mbps Ethernet.

Some might argue that you need 100 Mbps Ethernet to keep up with

the maximum 27-56 Mbps downstream capability of a cable modem.

This is not true. Even in a very good installation a cable modem

cannot keep up with a 10 Mbps Ethernet as many users share the



Serial Bus


others, Intel recently announced that it is working with

Broadcom on cable modems with Universal Serial Bus (USB)

interface. This is expected to bring down the installation

hassle for many users with lesser computer skills. Obviously you

do not need to open the box to install an Ethernet card if the

computer has an USB interface. If the computer does not have an

USB interface, you will need to install that (and you are back

to about the same hassle-level as with the Ethernet interface).





installation cost is a significant issue as this is something

that needs to be done in the house of every subscriber. The CATV

operators and equipment manufacturers need to try really hard to

push down the installation cost to keep the whole operation


A cable modem costs Rs

15,000 approximately and generally a cable operator provides

this on a leased basis.

Basically, these modems

are for ordinary people–just like analog modems and ISDN.



modem is much faster. Speeds from x25 to x1000 are possible

today and we are now only at the beginning of the cable modem

era! Remember 1200 baud analog modems? Compare the

speed you get to that of E1/T1 or Ethernet.


This is the speed from you to the CATV operator (ISP).

Provided he does not sell too many cable modems in your area

and/or upgrade his equipment to keep up with the number of cable


To provide high-speed

access to other sites on the Internet, the CATV operator also

uses fairly large proxy cache servers and a very fast connection

to the Net. We will see the CATV operators put a lot of (local)

content to which you will have very fast access.



modems are online whenever you turn on your computer much like

the network (LAN) used in most offices. This allows a whole new

range of applications.

Some cable operators do

not like you to run Web servers or FTP servers through your

cable modem, but that may change when they realize that they are

selling bandwidth.



phone companies will now have serious competition as you have an

alternative to connection through the phone lines. But telcos do

have alternative technologies like ADSL, which can provide very

high-speed secured connectivity to Internet to compete with the

less secured cable modem connectivity.

In India, few companies

are planning to provide Internet-over-cable services and ISPs

also see this as a good option to reach home segment in a big

way. Cable modem looks like a reality in near future and

probably your cable operator soon may knock at your door and

give a pleasant surprise by offering you Internet-over-cable.

Let’s wait and watch

what happens to this amazing technology.


Srivathsan is business

development manager (India) Eicon Technology