As the name implies, Conditional Access is access based upon certain conditions. The essential role of broadcasting is to convey information on a broad scale. Therefore, CA, which has the aim of restricting reception, is at odds with this role. However, the implementation of fee-based broadcasting made it necessary to prevent non-subscribers from receiving broadcasts and so scrambling of broadcast signals has become widespread. Although scrambling began with cable television (CATV), the advent of worldwide satellite-based broadcasts has fueled the development and implementation of large-scale CA systems. Under a CA System, only an authorized receiver can decrypt the broadcast content. CA offers service providers the ability to grant viewers access to specific programming and also facilitates parental control by restricting access to certain types of programming.

The Conditional Access technology generally covers the area of digital broadcasting like Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), and Digital Multimedia Broadcasting (DMB). It helps pay the media industry in ensuring the payment received from the viewer in return of programs viewed. There are two main concepts behind CA: Entitlement or Authorization & Scrambling or Encryption of the content.

Entitlement is a right or privilege whereas Scrambling is the process of transforming information using an algorithm to make it available for only those possessing the knowledge and means to decipher it. This special knowledge is usually referred to as the scrambling key. The result of the process is encrypted information. The keys which are used to de-scramble at the receiver end are never sent directly but are themselves encoded so that only an authorized (entitled) receiver gets the content. The scrambling key is dynamic and changes over a period of time. A CA system generally consists of two main subsystems:

• A scrambling subsystem that scrambles the signal to prevent non-subscribers from receiving it & descrambles the signal at the subscribers' receivers.
• An access control subsystem that processes access control messages to determine whether descrambling is to be performed.

Satellite broadcasting, in particular, features a very vast broadcast target, for which unauthorized reception can have far reaching effects. For this reason, highly secure systems are employed for both the scrambling subsystem and access control subsystem.

The general requirements that a scrambling systems must satisfy are:

• It must be difficult for a third party to perform unauthorized reception (security)
• Quality must not deteriorate on restoring the signal
• Efficient scrambling of all kinds of signals (as in multimedia broadcasts) must be possible
• Various business formats such as multi-channel services and billing schemes must be supported.

Working of a CA system

• The multiplexer collects the raw program data from their respective sources and entitlements (EMMs and ECMs) from SAS and sends them across to the scrambler.
• CWG generates a control word and sends it to the scrambler which then scrambles the program data using this CW and sends this scrambled data with ECM and EMMs to the modulator.
• Modulator modulates the data received and sends it to the transmitting antenna.
• At the receiving end, antenna receives the signals (scrambled program data) and sends it to the STB (which has a demodulator, de-scrambler, de-multiplexer and decoder in-built).
• Demodulator demodulates these signals and sends it to the de-scrambler which first separates the EMMs and ECMs from the scrambled data and then matches the ID (in EMMs) with the ID provided by the smart card. If this ID gets matched, it descrambles the data using the control word in ECMs.
• Descrambler then sends this data to de-Multiplexer which further transmits it to the decoder.
• The CA system should be so designed that the scrambling and descrambling processes maintain a high degree of fidelity with the original quality of the data that is being processed. These processes should get executed in a way that the time delay is minimal.

Benefits of using CAS. Consumers pay for only those channels which they want to view. Broadcasters get to know the exact number of subscribers with a Cable/DTH operator. The operators pay only a portion of subscription fee to the broadcasters for the absolute number of end users who opt for the channel, rather than all households having cable access. This system is also beneficial for the government as it generates revenues for them and the government in-turn facilitates the introduction and development of consumer friendly systems like pay per view.