CATVPresented by

Hrudya

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.

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.

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

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.

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.

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.

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.

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

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