How Cable Modems Work
by Curt Franklin
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.
Photo courtesy Motorola, Inc.
Extra Space
Motorola SB5100E SURFboard
Cable Modem
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.
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
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.
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.
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.
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.
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
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.
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