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CEENET #8 8.2002 1
Internet Access via Cable TVInternet Access via Cable TV
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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
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CEENET #8 8.2002 3
Changes in the Cable NetworkChanges in the Cable Network
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
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CEENET #8 8.2002 4
Changes in the Cable NetworkChanges in the Cable Network
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
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CEENET #8 8.2002 5
Changes in the Cable NetworkChanges in the Cable Network
A Traditional Cable network
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CEENET #8 8.2002 6
Changes in the Cable NetworkChanges in the Cable Network
A Modern Cable network
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CEENET #8 8.2002 7
What is a Cable Modem?What is a Cable Modem?
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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.
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How Fast is a Cable Modem?How Fast is a Cable Modem?
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
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How Fast is a Cable Modem?How Fast is a Cable Modem?
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.
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CEENET #8 8.2002 11
How Fast is a Cable Modem?How Fast is a Cable Modem?
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.
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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
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RealReal--world performanceworld performance
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.
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RealReal--world performanceworld performance
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
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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
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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
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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.
Cable Modem TechnologyCable Modem Technology
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CEENET #8 8.2002 18
Cable Modem TechnologyCable Modem Technology
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.
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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.
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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
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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.
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"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.
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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
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