2. CATV
Cable TV
Originally community antenna TV
is a system for distributing television programs to subscribers
via radio frequency (RF) signals transmitted through coaxial
cables or digital light pulses through hybrid fibre coaxial (HFC)
networks.
To receive cable television at a given location, cable
distribution lines must be available on the local utility poles
Coaxial cable brings the signal to the customer's building
through a service drop, an overhead or underground cable.
The cable company's portion of the wiring usually ends at a
distribution box on the building exterior, and built-in cable
usually distributes the signal through the walls to jacks in
different rooms to which televisions are connected.
3. STANDARD CABLE TV SYSTEM
Head station distributes the
video programs over a coaxial
cable network .
Each program occupies a 6-
MHz channel .
The spectrum between 50
and 550 MHz
It can accommodate up to 80
channels.
Every subscriber receives the
same program.
4. LAYOUT
The network
connects the head stations of the service
providers to the user equipment.
it consists of optical fiber terminating at a
fiber node to which is attached a local
coaxial network that connects to 500
homes .
The fiber node
converts the down stream optical signal
originating at the head end into an
electrical signal
upstream electrical signal originating at
the users into an optical signal.
The Head stations
will access the video servers , web servers
and the internet via a backbone network.
Distributes the programs
Signals are bidirectional
down stream from head stations to users
up stream to head stations
Total Bandwidth is increased to 750 MHz
5. WIRELESS CABLE SYSTEM
An alternative distribution technology
in which all or a part of the
distribution system is wireless.
Wireless cable system (WLC)
subscribers directly access the signal
broad cast from the head end
stations.
Hybrid /fiber wireless system (HFWL)
a digital video signal is sent to the
curb over optical fiber and
the local co axial system is replaced
by a local wireless system.
The wireless portion of the network would
extend over short distances.
These systems may be less expensive
than cable when there is a high
geographical concentration of users.
6. CATV LAYERED NETWORK
We can decompose the
functions of catv network into
3 layers.
• Decomposition is different for
down stream and upstream.
• The proposed use of
frequency band is from 5 to
750 MHz
• Down stream and upstream
signal occupy different
frequency bands.
7. CONTINUE..............
Down stream
Physical Layer:
50 to 750 MHz
Conventional analog broadcasts that
can be received by existing television
sets occupy 6 MHz channels between
50 and 550 MHz (ie 38 Mbps over 6
MHz chnl)
The spectrum between 550 to 750 may
carry digital MPEG-2 programs , data
streams and down stream telephony
Using QAM-64 or 16-VSB modulators
each 6 MHz analog channel is
converted into a data link with a bit rate
of 27 to 38 Mbps
Such a link is used to carry 6 to 10
MPEG-2 programs at rates of 3.5 Mbps
or transport digital data to users
The Mpeg programs may be decoded
by set to boxes
Transmission of user data requires
cable modems
Network Layer:
_ The down stream signals are carried
by a circuit switched network.
8. CONTINUE..............
Up stream :
Physical Layer :
Upstream signal occupy 5 to 42 MHz
spectrum
This spectrum is usually divided in 2
MHz channels
Because the network has a tree and
branch structure , the transmission
path from users to the head end is
shared.
The effect is that the signal received
at the head end is the sum of the
user signals
Hence a Mac protocol is needed for
collision free access
Also the path can cause addition of
noises
The bits are modulated using QPSK
and forward error correction is used
The bit rate is about 3 Mbps for each
2 MHz channel.
LLC/MAC :
Uses a multiple access technique
(DOCSIS) for accessing data.
9. DATE OVER CABLE SERVICE INTERFACE
SPECIFICATIONS (DOCSIS)
Date Over Cable Service
Interface Specifications
An emerging standard
developed by MCNS
consortium (multimedia
Cable Network Systems )
The goal is to transparently
transmit IP traffic between the
user cable modem and the
cable modem termination
system (CMTS) at the head
end
The frames are divided into mini slots
When a user makes a request , the CMTS
may grant the user a certain number of
mini slots in the next frame .
The requests include modem id and
amount of bandwidth requested.
The grants are carried out in a down
stream frame
However more than one cable modem may
request at the same time resulting in
collision
The colliding modems learn abt this , bcoz
they do not receive any grants
They must back off for a random amount of
time before making another request.
The standard also specifies how user
ethernet packets or ATM cells are to be
framed .
What is DOCSIS ? Working
10. SERVICES OVER CATV
Also known as video dial tone
The subscribers can demand for
a video
Subscribers browse through a
large collection of video
programs and request a program
, using their set top boxes .
The head end transports the
requests (MPEG digital stream)
over an available channel
The set top box demodulates
and decompress the received bit
stream.
And generated the NTSC or
HDTV signal for display on the
TV set
Internet service providers
together with cable operators
offer , subscribers ,internet
access ,over a shared link (3
Mbps upstream and 38 Mbps
downsteram )
Typically 10 subscribers
share a resource at any time
.
Subscribers must purchase a
cable modem and usually
pay a monthly flat rate .
Video On Demand Internet Access
