Cable Modems-2002

8/7/2019 Cable Modems-2002

1/23

CEENET #8 8.2002 1

Internet Access via Cable TVInternet Access via Cable TV


2/23

CEENET #8 8.2002 2

Changes in the Cable NetworkChanges in the Cable Network

The cable network was designed to deliver TV

signals in one direction from the Head-End to the

subscribers homes

To provide TV services Cable Operators had to

recreate a portion of the over-the-air radio

frequency (RF) spectrum within a sealed coaxial

cable line Operators had to upgrade the cable network so that

signals could flow in both directions


3/23

CEENET #8 8.2002 3


Cable Operators assign a spectrum of signal

frequencies to the cable network

One spectrum is used for the signals that movefrom the Head-End towards the cable subscriber

Another spectrum of signal frequencies are used

for the signals that move from the cable subscriber

towards the Head-End


4/23

CEENET #8 8.2002 4


By replacing existing one way amplifiers with two

way amplifiers Cable Operators are able to

separate the upstream and downstream signals and

amplify each direction separately in the right

frequency range


5/23

CEENET #8 8.2002 5


A Traditional Cable network


6/23

CEENET #8 8.2002 6


A Modern Cable network


7/23

CEENET #8 8.2002 7

What is a Cable Modem?What is a Cable Modem?


8/23

CEENET #8 8.2002 8

How Fast is a Cable Modem?How Fast is a Cable Modem?

Cable modem speeds vary widely

Depends on the cable modem system

Cable network architecture

Traffic load.

In the downstream direction (from the network to

the computer), network speeds can be up to 27

Mbps BUT, this is an aggregate amount of bandwidth that is

shared by users.


9/23

CEENET #8 8.2002 9


Few computers will be capable of connecting at

such high speeds or have exclusive access to the

network

A more realistic number is 1 to 3 Mbps.

In the upstream direction (from computer to

network), speeds can be up to 10 Mbps.

However, most modem producers have selected a moreoptimum speed between 500 Kbps and 2.5 Mbps

AND, many cable operators limit the upstream

bandwidth to 128 or 384kbs


10/23

CEENET #8 8.2002 10


An asymmetric cable modem scheme is most

common. The downstream channel has a much

higher bandwidth allocation (faster data rate) than

the upstream,

primarily because Internet applications tend to be

asymmetric in nature.

Activities such as World Wide Web (http)navigating and newsgroups reading (nntp) send

much more data down to the computer than to the

network.


11/23

CEENET #8 8.2002 11


Mouse clicks (URL requests) and e-mail messages

are not bandwidth intensive in the upstream

direction.

Image files and streaming media (audio and video)

are very bandwidth intensive in the downstream

direction.


12/23

CEENET #8 8.2002 12

RealReal--world performanceworld performance

The theoretical performance of a Cable Modem is

based upon all other devices being able to work at

the same speed and performance as the Cable

Modem

However, in a similar way that the actual usable

bandwidth on a 10Mbps Ethernet connection

reduces to a 4Mbps, so too will the performance ofa Cable Modem connection be reduced


13/23

CEENET #8 8.2002 13


The Cable network itself will suffer the same

problems of Internet performance as any other

Internet Service Provider (ISP)

Although performance to services on the cable

network itself can be amazingly fast, access to 'the

outside world' will be slowed down by the

performance of other connections on the way.


14/23

CEENET #8 8.2002 14


As usage on your segment grows (as more

customers are added) the bandwidth must be

shared by more people

Adding more cable network segments is very expensive

for the cable operator

If you connect to a remote Internet site that itself

has a connection speed equivalent to a T1connection (1.5Mbps), then that is as fast as the

data can be served to you, no matter how fast your

receiving equipment is


15/23

CEENET #8 8.2002 15

Who Makes Cable Modems?Who Makes Cable Modems?

3Com, Cisco Systems, Com21, General

Instrument, Motorola, Nortel Networks,

Phasecom, Samsung, Terayon, Toshiba, Zenith

And many others


16/23

CEENET #8 8.2002 16

It MOdulates and DEModulates signals

Much more complicated than their telephone

counterparts Cable modems can be part modem, part tuner, part

encryption/decryption device, part bridge, part

router, part network interface card, part SNMP

agent, and part Ethernet hub

Cable Modem TechnologyCable Modem Technology


17/23

CEENET #8 8.2002 17

Typically, a cable modem sends and receives data

in two slightly different fashions

In the downstream direction

he digital data is modulated and then placed on a typical 6

MHz television channel, somewhere between 50 MHz and 750

MHz

64 QAM is the preferred downstream modulation technique,

offering up to 27 Mbps per 6 MHz channel

This signal can be placed in a 6 MHz channel adjacent to TV

signals on either side without disturbing the cable television

video signals.



18/23

CEENET #8 8.2002 18


The upstream channel is more tricky

Typically, in a two-way activated cable network, the upstream

(also known as the reverse path) is transmitted between 5 and

42 MHz

This tends to be a noisy environment, with RF interference andimpulse 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 direction, because

QPSK is more robust scheme than higher order modulation

techniques in a noisy environment

The drawback is that QPSK is "slower" than QAM.


19/23

CEENET #8 8.2002 19

Cable Modem ServicesCable Modem Services

The dominant service is high-speed Internet access

This enables the typical array of Internet services to be

delivered at speeds far faster than those offered by dial-

up telephone modems

Other services will include

access to streaming audio and video servers, local

content (community information and services)

access to CD-ROM servers

a wide variety of other service offerings. New service

ideas are being developed daily.


20/23

CEENET #8 8.2002 20

In North America, cable operators are packaging

high-speed data services much like they do basic

cable television service

Typically charging $40 - $60 per month for an

Internet service package

Includes software, unlimited Internet access,

specialized content and rental of a cable modem

Cost of Cable Modem ServiceCost of Cable Modem Service


21/23

CEENET #8 8.2002 21

Cost of Cable Modem ServiceCost of Cable Modem Service

At the low end of this pricing scale, a very robust

Internet service is available to consumers for

about the cost of a dial-up account with a local

Internet service provider and a second telephone

line

Even at $60 per month, cable is a far better value

than ISDN.


22/23

CEENET #8 8.2002 22

"Telco"Telco--Return"ModemsReturn"Modems

Not really a cable technology

Used more often with Direct Satellite video

systems Satellite down link is used for fast downstream

transmission

A telephone modem handles upstream

communication over the public telephone network.


23/23

CEENET #8 8.2002 23

Support for Multiple PCsSupport for Multiple PCs

A cable modem can provide Intenet access to

multiple PCs, if they are connected via a local area

network (LAN)

Cable modems typically have an Ethernet output,

so they can connect to the LAN with a standard

Ethernet hub or router

Each PC must have an assigned IP address The cable ISP usually sells at a premium of $5-$10 a

month per PC

NAT (Network Address Translation) can allow

multiple PCs to "hide" behind a single IP Address

Documents

Cable Modems-2002