[attachment=1467]


Micro Controllers
(Cable Modems)


ABSTRACT

The present need to access internet in homes, offices, companies
etc in affordable way results in selection of efficient technology and
as a result cable modems are used. Cable modem technology brings high-
speed broadband networking to desired places through cable television
lines. Cable modems are devices that allow high-speed access to
information at a distant server may it be Internet server or video on
demand server, via a cable television network. While similar to the
traditional (analog) dialup modems, a cable modem is significantly more
powerful, capable of delivering data approximately 500 times faster.
This not only gave subscribers an opportunity to experience new level
of speeds of browsing the net but also eventually make it affordable.
The Tele Vision signals are usually spread in the 50 MHz to 750 MHz
spectrum. With each TV Channel such as CNN, BBC, and CNBC occupy 6MHz.
Similarly the Internet services via the cable (downstream and upstream
separately) occupy a 6 MHz channel too.

The paper further discusses the following topics on the subject
of cable modems: working, the factors based on which the cable modem
speed varies, traditional cable system, hybrid fiber coaxial system,
types of cable modems, external cable modem, internal cable modem, and
interactive set-top box. Inside the cable modem, tuner, demodulator,
burst modulator, MAC (Media Access Control), interface and CPU.


Table of Contents
Introduction...........................................................
......................................01
Working ¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦..02
Speed¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦...03
Traditional Cable
System.................................................................
...........04
Hybrid Fiber Coaxial System¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦¦..04
Types of Cable
Modems.................................................................
.............05
External Cable
Modem..................................................................
..............05
Internal Cable
Modem..................................................................
...............06
Interactive Set-Top
Box....................................................................
...........06
Inside the Cable
Modem..................................................................
............07
Tuner..................................................................
..........................................07
Demodulator............................................................
....................................07
Burst
modulator..............................................................
.............................07
MAC....................................................................
........................................07
Interface..............................................................
.........................................08
CPU....................................................................
.........................................08
Some
Acronyms...............................................................
...........................08
Conclusion.............................................................
......................................09

INTRODUCTION
During the early to mid - 1990s, the two-way interactive
communication capability of the existing cable network infrastructure
was realized which would imply a much greater bandwidth for accessing
the Internet. This realization resulted in several field trials being
conducted in various areas of United States and western European
countries. As a result of these field trials, several types of cable
modems were developed.
The idea is simple - To use the existing cable networks to hook up to
the net. This not only gave the subscribers an opportunity to
experience new level of speeds of browsing the net but also eventually
make it affordable. Along with this would come Video on demand and
Media Home Platform (MHP). Entrepreneurs immediately realized the
immense revenue potential of this technology by the sheer number of
subscribers that would be able to log on to the net via their already
existing cables connections and make technologies like MHP a reality.
The Technology: A Cable Modem is a device that allows high-speed data
access (such as to the Internet) via a cable TV network. It operates
over the ordinary TV network cables. The subscriber just connects the
Cable Modem to the Cable TV outlet at his end and the cable TV operator
connects a Cable Modem Termination System (CMTS) in his end (the head
end).
A cable modem will typically have two connections, one to the cable
wall outlet and the other to a computer (PC) or a set top box. Most
cable modems are external devices that connect to the PC through a
standard 10Base-T Ethernet card (or 100Base-T Ethernet card) and
twisted-pair wiring. Alternatively they could be connected via the
Universal
Serial Bus (USB) or may be available as internal PCI modem cards.


Working
The fact that the word modem is used to describe this device
can be a bit misleading. It is a modem in the sense it modulates and
demodulates signals. But the similarity to a typical telephone dial-up
modem ends there because cable modems are more complicated than their
telephone counterparts. Cable modems in addition can be
* Modem
* Tuner
* Encryption/decryption device
* Bridge
* Router
* Network interface card
* SNMP agent
* Ethernet hub
The Tele Vision signals are usually spread in the 50 MHz to 750
MHz spectrum. With each TV Channel occupying 6 MHz. In the same way as
we receive MTV or CNN or BBC as a channel. All of these occupy 6 MHz
bandwidth. Similarly the Internet services via the cable (downstream
and upstream separately) occupy a 6 MHz channel too.

A cable modem sends and receives data in two slightly different
fashions.
* Downstream
* Upstream



In the downstream direction, the digital data is modulated and
then placed on a typical 6 MHz television channel, somewhere between 50
MHz and 750 MHz (refer Table 2 - Frequency Band Utilization).
Currently, 64 QAM   is the preferred downstream modulation technique,
offering up to 27 Mbps per 6 MHz channel.
The upstream channel is more complex. Typically, in a two-way
activated cable network, the upstream (also known as the reverse path)
is transmitted between 5 and 42 MHz (Refer Table 2 - Frequency Band
Utilization). This tends to be a noisy environment, with RF
interference and impulse noise. Additionally, interference is easily
introduced in the home, due to loose connectors or poor cabling. Since
cable networks are tree and branch networks, all this noise gets
added together as the signals travel upstream, combining and
increasing. Due to this problem, most manufacturers use QPSK or a
similar modulation scheme in the upstream modulation techniques in a
noisy direction, because QPSK is more robust scheme than higher order
environment. The drawback is that QPSK is slower than QAM.
Table 1 - Common Television Broadcast Bands, lists the common
television broadcast bands and their frequency assignments. A TV
channel can be considered to fall into one of the seven predefined
frequency bands.
Table 1 - Common Television Broadcast Bands

To provide a meaningful relationship between the common television
broadcast bands listed in Table 1 and use of the frequency spectrum for
different applications, consider Table 2 - Frequency Band Utilization,
which indicates the typical use of different frequency bands. The
assignment of frequencies for channels varies by global location. The
following table lists U.S. off-air channel frequencies.
Speed
Cable modem speeds vary widely, depending on the following
parameters
* Cable modem system
* Cable network architecture
* Traffic load
In the downstream direction (from the server to the user or from
CMTS to user Cable Modem), network speeds can be up to 27 Mbps. This is
an aggregate amount of bandwidth that is shared by users.
Table 2 - Frequency Band Utilization
Only a few user systems will be capable of connecting at such high
speeds. A more realistic number is 1 to 3 Mbps. In the upstream
direction (from user system to network), speeds can be up to 10 Mbps.
However, most modem producers have selected a more a Kbps and 2.5 Mbps.
An asymmetric cable modem scheme is most common. The downstream
channel has a much higher bandwidth allocation (faster data rate) than
the upstream; this suit the common usage of cable modems. Activities
such as World Wide Web (http) navigation and newsgroups reading (nntp)
send much more data down to the computer than to the network. Mouse
clicks (URL requests) and e-mail messages are not bandwidth intensive,
(which are primarily in the upstream direction). Image files and
streaming media (audio and video) are very bandwidth intensive, (which
are primarily in the downstream direction).

bandwidth intensive, (which are primarily in the upstream direction).
Image files and streaming media (audio and video) are very bandwidth
intensive, (which are primarily in the downstream direction).
Traditional Cable System
Traditional Cable System
Each TV cable box listens to all the downstream transmissions
that is, transmission coming down from the network (CMTS). Each
transmission, or TV channel, is transmitted at a different frequency.
The original cable system was based on coaxial cable, end to end.
The cable TV head end takes video feeds from various sources
and introduces the signal onto the coax cable.


Within the coax cable system signal level the amplifiers are placed at
regular intervals in the system. There may be as many as 35 amplifiers
cascaded between the head end and the subscriber station. The following
Figure illustrates a conventional cable system.
Hybrid Fiber Coaxial System
Currently cable operators are in the process of modifying the
cable infrastructure by introducing fiber optic cables, replacing the
analog signals with digital transmissions, and replacing the amplifiers
so that the system becomes a two-way system.
The new infrastructure is referred to as Hybrid Fiber Coaxial (HFC)
system. With HFC, the cable TV head end has a fiber interface instead
of conventional coaxial interface, and with the introduction of fiber,
many of the characteristics of a conventional cable network change.

Most importantly, the fiber optic amplifiers are capable of
regenerating signals rather than simply amplifying it. Also, base band
filter RF amplifiers replace the conventional echo cancellation
amplifiers. These filters allow a certain frequency range to traverse
the network in one direction and a different frequency range to
traverse the network in the opposite direction. Thus, the new HFC
system is a two-way system. Note that upgrading to HFC is a nontrivial
task. It is essentially a complete rebuild of a significant portion of
the cable system. Fortunately, the last drop, from the fiber node to
the neighborhoods and into the homes, does not change, it is still coax
based.
Types of Cable Modems

A number of different Cable Modem configurations are possible.
The following 3 configurations are the main products that we see now.
* External Cable Modem
* Internal Cable Modem
*Interactive Set-Top box
Over a period of time more systems will arrive.
External Cable Modem

The external Cable Modem is a small external box that connects
to your computer normally through an ordinary Ethernet connection. The
downside is that you need to add a (inexpensive) Ethernet card to your
computer before you can connect the Cable Modem. A plus is that you can
connect more computers to the Ethernet. The available Cable Modems work
with most of the operating systems and hardware platforms, including
Mac, UNIX, laptop computers etc.
Another possible interface for external Cable Modems is USB, which has
the advantage of installing much faster (something that matters,
because the cable operators are normally sending technicians out to
install each and every Cable Modem). The downside is that you can only
connect one PC to a USB based Cable Modem.
The following Figure depicts an External Cable Modem. The following
Figure depicts an External Cable Modem.

Internal Cable Modem
The internal Cable Modem 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 PCâ„¢s. Macâ„¢s and laptops are possible, but require a different
design. The following Figure depicts an Internal Cable Modem.

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 (DVB).

A Second problem is that the cable connector is not galvanic ally
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 Cable TV system) networks may not be able to use
internal cable modems at all for technical and/or regulatory reasons.
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, email etc. directly on the TV screen. Though
this technology is now obsolete and the latest set top boxes allow the
return path via the cable itself. The following Figure depicts an
Interactive Set-Top Box.
Inside the Cable Modem
Figure gives a brief idea about a typical Cable Modem architecture
Tuner
The tuner connects directly to the CATV outlet. Normally a tuner
with built-in diplexer is used, to provide both upstream and downstream
signals through the same tuner. Then tuner must be of sufficiently good
quality to be able to receive the digitally modulated QAM signals. A
new concept of a silicon tuner is in the works. This is basically a
tuner on a chip, and is expected to cut the cost down quite a bit
compared to a more conventional tuner module.
Demodulator
In the receive direction, the IF signal feeds a demodulator. The
de modulator normally consists of A/D converter, QAM-64/256
demodulator, MPEG frame synchronization, and Reed Solomon error
correction. The demodulator component is required both in a cable modem
and in the more mature product, the digital (receive-only) set-top box;
so many companies have developed products for this part of the game.
Burst modulator
In the transmit direction, a burst modulator feeds the tuner.
The burst modulator does Reed Solomon encoding of each burst,
modulation of the QPSK/QAM-16 on the selected frequency and D/A
conversion. The output signal is fed through a driver with variable
output level, so the signal level can be adjusted to compensate for the
unknown cable loss. The burst modulator is unique to the cable modem
(and some two-way set-top boxes) chips are also available as the
integration race drives more and more functions into a single chip.
MAC
A Media Access Control mechanism sits between the, receive and
transmit paths. This can be implemented in hardware or split between
hardware and software. The MAC is pretty complex compared to an
Ethernet MAC, and in reality no MACâ„¢s are able to handle all of the MAC
layer function without some microprocessor help". For DOCSIS cable
modems, many companies are known to have MAC ASICâ„¢s available as a
standard products some are also in the market with a MAC that rely more
on software to handle the various functions, supposedly giving more
flexibility. Other companies are known to be working on various MAC
chips for both DOCSIS and DVB/DAVIC, with different partitions of what
goes in software and hardware. Some cable modem manufacturers even
develop their own MAC apparently in an attempt to be more competitive
or to differentiate their products.
Interface
The data that pass through the MAC goes into the computer or
set-top box interface of the Cable Modem, be it Ethernet, USB, PCI bus
or whatever. These have been discussed in detail earlier in this
document.
CPU
The microprocessor is not explicitly shown in the diagram, but
for external cable modems a CPU is required. Some work is being done on
host based processing cable modems, which use the processor in the host
(PC or Mac) to do all (or almost all) processing. Much like how dial-up
(analog telephony) modems (WinModem - a controller only modem) rely on
the PC processor to do the processing. Single devices combining MAC,
demodulator, burst modulator, processor, Ethernet/PCI/USB interfaces
and more are emerging, in effect integration the guts of a cable modem
in a single chip. There will still be some additional parts for memory,
tuner, analog stuff, lower supply etc. so we are still no-where near
the true single-chip cable modem.
Some Acronyms
A short list of some of the other technical terms and acronyms
that you may stumble across in trying to understand the cable modem
world.
1. CATV: Cable TV system. Can either be all coaxial or HFC (Hybrid
Fiber Coax) based.
2. CMTS: Cable Modem Termination System. Central device for connecting
the cable TV network to a data network like the Internet. Normally
placed in the head end of
the cable TV system.
3. CPE: Customer Premises Equipment. Used to describe the PC and/or
other equipment that the customer may want to connect to the cable
modem.
4. Downstream: The data flowing from the CMTS to the cable modem.
5. Downstream frequency or Downstream bandwidth: The frequency used for
transmitting data from the CMTS to the cable modem. Normally in the
42/65-850 MHz range depending on the actual cable plant capabilities.
6. Head end: Central distribution point for a CATV system. Video
signals are received here from satellites and/or other sources,
converted to the appropriate channels
7. QAM: Quadrature Amplitude Modulation. A method of modulating digital
signals using both amplitude and phase coding. Used for downstream and
can be used for upstream.
8. QPSK: Quadrature Phase-Shift Keying. A method of modulating digital
signals using four-phase states to code two digital bits per phase
shift.
9. SID (Service ID): Used in the DOCSIS standard to define a particular
mapping between a cable modem (CM) and the CMTS. The SID is used for
the purpose of upstream bandwidth allocation and class-of-service
management.
10. Subscriber Unit (SU): An alternate term for cable modem.
11. Upstream: The data flowing from the CM to the CMTS.
12. Upstream frequency or Upstream bandwidth: The frequency used to
transmit data from the CM to the CMTS. Normally it is in the
5-42 MHz range for US systems and 5-65 MHz for European systems.
Conclusion
The aim of the paper is to present an overview of cable modem
technology which brings high-speed broadband networking to the home and
small business user through cable television lines. A Cable Modem is
a device that allows high-speed data access (such as to the Internet)
via a cable TV network
The available Cable Modems work with most of the
operating systems and hardware platforms, including Mac, UNIX, laptop
computers etc. Currently cable operators are in the process of
modifying the cable infrastructure by introducing fiber optic cables,
replacing the analog signals with digital transmissions, and replacing
the amplifiers so that the system becomes a two-way system
REFERENCES
1.ieee.org
2.ericsson.com
3.ciscon.com
4.wipro.com