With a recent discussion of What exacticlt is or is not a Modem as it
concernes DSL and Cable Devices I found the following.

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DSL
Officaly there are several names for the DSL "adapter". The Telephone
CEntral Office people call them DSL Transcievers. The Engineers call
them ATU-C and ATU-R units. The ATU stands fro ASDL Termination Unit.
And the one in the Home is the R unit, Remote and the one for that line
in the Central Office is the C unit.
They operate very similar but in a very more complex way as the Analog
Modems we al are fimilar with so they can Correctly be called a Modem.
There are two methods used. One breaks up the DSL line into three
sections. the 0 to 4 Hkz area is the Analog Phone service portion. The
25 to 160 Khz portion is for Upstream Transmissions and the 240 Khz to
1.5 Mhz portion is for the Downstream Transmissions. This is
called CAP(Carrierless Amplitude/Phase)
Teh other method is called DMT(Discrete Multitone). In this method the
bandwidth is broken up into 247 seperate 4 Khz wide channels. Eac
channel is monitored and if quality goes down o none chennel
that data is shifted to another channel Several of the lower 4
Khz channels are Bi Directional and carry the Upstream Data.
the rest are for Downstream data. This system is more complex
but has more flexibility on lines of differing quality.
Becasue of Signal Cross talk and line losses there is a Maximum
transmissions distance of 18 Kilo-Feet form the Central Switching
Station having yuor ATU-C unit.

*
Cable
The Cable Unit is a REAL Modem as it has an actual internal Modulator
and De-Modualtor for placing the Information on an RF carrier for
outgoing data and detecteding(De-Modulating) it from the RF carrier for
incoming data. Inside the modem there is also other sections including a
Tuner, CPU, USB port and or Net Card and a MAC(Media Access Control
chip.
The cable system uses qalmost al lof the practical usable RF spectrum.
For the TV they need to divide it into 6 Mhz Channels just like it is
don for Off the Air Television.  For Internet uses with the CAble Moden
they assign 2 Mhz wide channels for Upstream pueposes and 6Mhz Channels
for Down stream purposes. the systems vary but a lot of CAble Systems
assign a 2 Mhz channel between 5 Mhz and 42 MKhs for the upstream flow
of data and a 6 Mhz channel between 42 Mhz and 850 Mhz fro Downstream
Data. Unlike the DSL where you are the sole user of the Bandwidth the
cable system has it's area broken down into "Nodes" and you Share the
Bandwidth with others in your Area. They try to monotor typical usage on
the nodes and reassign as necessary to maintain some reasonable
Speed.Althouhg at some times during the day or night you might notice a
slow down during high activity periods on your node.
Here is short bits o nthe modulator and De-modualtor.
**
Inside the Cable Modem: 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.
**
Inside the Cable Modem: 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

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 þ SLMR 2.1a þ This note from El Cajon California USA