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?
A
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,
etc.
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.Â
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
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
On
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.
Ethernet
The
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
downstream.
Universal
Serial Bus
Among
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).
Corp.
Cost
The
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
profitable.
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.
Speed
Cable
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.
Note:
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
modems.
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.
Online
Cable
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.
Competition
The
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.
V
Srivathsan is business
development manager (India) Eicon Technology