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 existing CATV (cable TV) connection.
Regardless of their outward appearance, all cable modems contain certain key
components:
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 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:
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 Media Access Control mechanism is
normally implemented in hardware or in a combination of hardware and software.
The primary purpose of the MAC is to share the media in a reasonable way. Both
the CMTS and the Cable Modem implements protocols to do
·
Ranging to
compensate for different cable losses.
It is essential that the upstream bursts from all Cable Modems are received in
the Head-End at the same level. If two Cable Modems transmit at the same time,
but one is much weaker than the other one, the CMTS will only hear the strong
signal and assume everything is okay. If the two signals are same strength, the
signal will garble and the CMTS will know a collision occurred.
·
Ranging to
compensate for the different cable delays. The size of a CATV network calls for fairly large delays in the
millisecond range.
·
Assigns frequencies
etc. to the Cable Modems. The Cable
Modem first listens to the downstream to collect information about where and
how to answer. The it signs on to the system using the assigned upstream
frequency etc.
·
Allocate the
time-slots for the upstream.
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.