micro controllers cable modems

8/8/2019 Micro Controllers Cable Modems

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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,


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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


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INTRODUCT

ION

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 thesefield 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.


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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


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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

sharedby 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


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(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.


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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 echocancellation 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.


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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 CableModem). 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 PCs. Macs 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


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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 thereturn 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-


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way set-top boxes) chips are also available as the

integration race drives more and more functions

into a single chip.

MACA 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 MACs 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 ASICs 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.


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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.www.ieee.org

2.www.ericsson.com

3.www.ciscon.com

4.www.wipro.com

micro controllers cable modems

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