For millions of people, television
brings news, entertainment and educational programs
into their homes. Many people get their TV signal
from cable
television (CATV) because cable TV provides
a clearer picture and more channels. See How
Cable TV Works for details.
Many people who have cable TV can now get a
high-speed connection to the Internet from their
cable provider. Cable modems compete with technologies
like asymmetrical
digital subscriber lines (ADSL). If you have
ever wondered what the differences between DSL
and cable modems are, or if you have ever wondered
how a computer network can share a cable with
dozens of television channels, then read on. In
this article, we'll look at how a cable modem
works and see how 100 cable television channels
and any Web site out there can flow over a single
coaxial cable into your home.
Extra Space
You might think that a television channel would
take up quite a bit of electrical "space," or
bandwidth, on a cable. In reality, each
television signal is given a 6-megahertz (MHz,
millions of cycles per second) channel on the
cable. The coaxial cable used to carry
cable television can carry hundreds of megahertz
of signals -- all the channels you could want
to watch and more. (For more information, see
How
Television Works.)
In a cable TV system, signals from the various
channels are each given a 6-MHz slice of the cable's
available bandwidth and then sent down the cable
to your house. In some systems, coaxial cable
is the only medium used for distributing signals.
In other systems, fiber-optic
cable goes from the cable company to different
neighborhoods or areas. Then the fiber is terminated
and the signals move onto coaxial cable for distribution
to individual houses.
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When a cable company offers Internet access
over the cable, Internet information can use the
same cables because the cable modem system puts
downstream data -- data sent from the Internet
to an individual computer -- into a 6-MHz channel.
On the cable, the data looks just like a TV channel.
So Internet downstream data takes up the same
amount of cable space as any single channel of
programming. Upstream data -- information
sent from an individual back to the Internet --
requires even less of the cable's bandwidth, just
2 MHz, since the assumption is that most people
download far more information than they upload.
Putting both upstream and downstream data on
the cable television system requires two types
of equipment: a cable modem on the customer
end and a cable modem termination system
(CMTS) at the cable provider's end. Between these
two types of equipment, all the computer networking,
security and management of Internet access over
cable television is put into place.
Inside the Cable
Modem
Cable modems can be either internal or external
to the computer.
In some cases, the cable modem can be part of
a set-top cable box, requiring that only a keyboard
and mouse
be added for Internet access. In fact, if your
cable system has upgraded to digital cable, the
new set-top box the cable company provides will
be capable of connecting to the Internet, whether
or not you receive Internet access through your
CATV connection. Regardless of their outward appearance,
all cable modems contain certain key components:
- A tuner
- A demodulator
- A modulator
- A media access control (MAC) device
- A microprocessor
Tuner
The tuner connects to the cable outlet, sometimes
with the addition of a splitter that separates
the Internet data channel from normal CATV programming.
Since the Internet data comes through an otherwise
unused cable channel, the tuner simply receives
the modulated digital signal and passes it to
the demodulator.
In some cases, the tuner will contain a diplexer,
which allows the tuner to make use of one set
of frequencies (generally between 42 and 850 MHz)
for downstream traffic, and another set of frequencies
(between 5 and 42 MHz) for the upstream data.
Other systems, most often those with more limited
capacity for channels, will use the cable modem
tuner for downstream data and a dial-up
telephone modem for upstream traffic. In either
case, after the tuner receives a signal, it is
passed to the demodulator.
Demodulator
The most common demodulators have four functions.
A quadrature amplitude modulation (QAM) demodulator
takes a radio-frequency
signal that has had information encoded in it
by varying both the amplitude and phase of the
wave, and turns it into a simple signal that can
be processed by the analog-to-digital (A/D) converter.
The A/D converter takes the signal, which varies
in voltage, and turns it into a series of digital
1s and 0s. An error correction module then checks
the received information against a known standard,
so that problems in transmission can be found
and fixed. In most cases, the network frames,
or groups of data, are in MPEG
format, so an MPEG synchronizer is used to
make sure the data groups stay in line and in
order.
Modulator
In cable modems that use the cable system for
upstream traffic, a modulator is used to convert
the digital computer network data into radio-frequency
signals for transmission. This component is sometimes
called a burst modulator, because of the
irregular nature of most traffic between a user
and the Internet, and consists of three parts:
- A section to insert information used for error
correction on the receiving end
- A QAM modulator
- A digital-to-analog (D/A) converter
Media Access Control
(MAC)
The MAC sits between the upstream and downstream
portions of the cable modem, and acts as the interface
between the hardware and software portions of
the various network protocols. All computer
network devices have MACs, but in the case of
a cable modem the tasks are more complex than
those of a normal network interface card. For
this reason, in most cases, some of the MAC functions
will be assigned to a central processing unit
(CPU) -- either the CPU in the cable modem or
the CPU of the user's system.
Microprocessor
The microprocessor's job depends somewhat on whether
the cable modem is designed to be part of a larger
computer system or to provide Internet access
with no additional computer support. In situations
calling for an attached computer, the internal
microprocessor
still picks up much of the MAC function from the
dedicated MAC module. In systems where the cable
modem is the sole unit required for Internet access,
the microprocessor picks up MAC slack and much
more. In either case, Motorola's
PowerPC processor is one of the common choices
for system designers.
Cable Modem Termination
System
At the cable provider's head-end, the CMTS provides
many of the same functions provided by the DSLAM
in a DSL
system. The CMTS takes the traffic coming in from
a group of customers on a single channel and routes
it to an Internet service provider (ISP) for connection
to the Internet. At the head-end, the cable providers
will have, or lease space for a third-party ISP
to have, servers
for accounting and logging, Dynamic
Host Configuration Protocol (DHCP) for assigning
and administering the IP
addresses of all the cable system's users,
and control servers for a protocol called CableLabs
Certified Cable Modems -- formerly Data
Over Cable Service Interface Specifications
(DOCSIS), the major standard used by U.S. cable
systems in providing Internet access to users.
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The downstream information flows to all connected
users, just like in an Ethernet
network -- it's up to the individual network connection
to decide whether a particular block of data is
intended for it or not. On the upstream side,
information is sent from the user to the CMTS
-- other users don't see that data at all. The
narrower upstream bandwidth is divided into slices
of time, measured in milliseconds, in which users
can transmit one "burst" at a time to the Internet.
The division by time works well for the very short
commands, queries and addresses that form the
bulk of most users' traffic back to the Internet.
A CMTS will enable as many as 1,000 users to
connect to the Internet through a single 6-MHz
channel. Since a single channel is capable of
30 to 40 megabits per second (Mbps) of total throughput,
this means that users may see far better performance
than is available with standard dial-up
modems. The single channel aspect, though,
can also lead to one of the issues some users
experience with cable modems.
If you are one of the first users to connect
to the Internet through a particular cable channel,
then you may have nearly the entire bandwidth
of the channel available for your use. As new
users, especially heavy-access users, are connected
to the channel, you will have to share that bandwidth,
and may see your performance degrade as a result.
It is possible that, in times of heavy usage with
many connected users, performance will be far
below the theoretical maximums. The good news
is that this particular performance issue can
be resolved by the cable company adding a new
channel and splitting the base of users.
Another benefit of the cable modem for Internet
access is that, unlike ADSL,
its performance doesn't depend on distance from
the central cable office. A digital CATV system
is designed to provide digital signals at a particular
quality to customer households. On the upstream
side, the burst modulator in cable modems is programmed
with the distance from the head-end, and provides
the proper signal strength for accurate transmission.
