CAS Demystified 

According to the latest notification from the Ministry of Broadcasting, CAS (Conditional Access System) is to be implemented soon, in the four Indian metros of Delhi, Mumbai, Kolkata and Chennai. This has raised apprehension among different interest groups including the satellite channels, the cable TV service providers and consumers.

Case for CAS



Why do we need CAS? We need it because with more and more channels being aired, programming has become less unique and sometimes even irrelevant, to the audience. Hence, the now familiar argument, "why should we pay for programs that we do not watch?" For example, many cable TV operators in India provide German, French, Mandarin and Korean channels as part of their offerings. These channels are often viewed only by a miniscule proportion of the total audience.

At the cablewallah's end, CAS can be used to lock out program-pirates from illegally plugging into cable TV channels or programs. CAS is thus, an attempt to address these issues and bring about transparency in the satellite TV market.

CAS zones



The CAS system can be broadly classified into six parts or 'zones'.

Zone 1: The Content origin path, where the original programming comes from, usually the network run by the satellite channel.



Zone 2: The Interactive software multiplex, where the interactive content is generated.


Zone 3: The accounting backbone, where the tracking and billing services are implemented


Zone 4: The return-path network or the path used by the feedback from the consumer's set-top box.


Zone 5: The content delivery route, which is a combination of multiplexers and deplexers that mix and separate the signals and are responsible for the transmission and feedback of the cable programs along the whole route.


Zone 6: The consumer set or the combination of the set-top box and the television together with relevant access-cards and mechanisms.

CAS Processes

At the control room

Here the content from the satellite channel is downloaded by the systems in Zone 2. Information about the type of channel, content and some accounting information like whether it is a paid channel or a

pay-perview program and if particular sets of subscribers are authorized to download and view the program are generated on a separate set of

systems in Zone 2. Inputs from Zone 1 and 2 are injected into the encoder/multiplexer complex in

Zone 5.

Another set of systems in Zone 3 generates program schedules and performs various accounting operations. This information is also fed into the encoder/multiplexer that combines this as a separate stream in the dispatch signal.

Finally, the encoder/multiplexer combines all the inputs fed in and forms the digitally scrambled "dispatch signal", beamed over its cabling systems and into the consumer's set-top box.

The scrambling is carried out in different ways, the main one using PGP encryption. The PGP-encrypting private key used is the encoder's own private key.

At the subscribers' home



When the signal arrives at the set-top box, it is in a scrambled form. Any hacker/pirate attempting to gain access to the signal will receive the scrambled signal and cannot use it. The set-top box contains a card reader into which a special access card is plugged in. This access card contains the subscriber's own private key. The box uses this private key and the public key embedded in the transmitted signal to decode the channels and sends it as pure audio-video stream to the television set.

Now comes the tricky part. One of the uses of CAS is to tell the cable-guy what channels the user is viewing and let the accounting system do the billing accordingly. When the user views a particular channel or program, depending on the embedded information in the stream it has received, the set-top box decides whether to inform the accounting servers or not. For example, channels in the US regularly beam 'pay per view' programming, where you pay extra for particular content (like a Formula 1 race or an India-Pakistan cricket match).

Given this information, the box encodes the "this channel/program is being watched" information along with the user's private PGP key and transmits it over the feedback channel.

This is different from the incoming network. The return signal can be sent over a cellular GSM frequency or a regular PSTN line as per the setup of your provider. The CAS server decodes this signal with its private key and the embedded public key.

Satellite-channel angle



The information in the accounting servers can be used as statistics by the Satellite channels to better or re-configure their programming.

It can also be used to decide how much of what type of advertising they need to generate more revenues, which can then be used to re-price the bouquets offered to the subscribers.

Sujay Sarma