JPL's Wireless Communication Reference Website

Chapter: Network Concepts and Standards 
Section: Broadcast Systems, Digital Video Broadcasting (DVB)

Conditional Access to Television Service

Since more and more TV-services are offered to the consumer by an increasing number of service providers, the revenues from the TV licence fee and from publicities do not ensure anymore the financial equilibrium of the production and the distribution of the services. Pay TV is a solution for this problem. A popular definition of Pay TV broadcasting is that it is a form of television broadcasting where the service provider is in direct control of the customer's access to the service. The advantages for the provider of having direct control are twofold. In the Figure below a basic setup of a Pay TV system is represented:
               Basic configuration of a Pay TV system
The working of a Pay TV system is now briefly explained and at the same time, some often used terminology is introduced.

It is easiest to understand the system from the receiver side, where the system usually consists of a set-top box (also referred to as the decoder box), some detachable module such as a smart card, and the television set itself. The system will only work if the card is plugged into the box. The system works roughly as follows.

Before transmission, the normal, or clear signal is scrambled. This means that it is distorted in such a manner that normal viewing of the service becomes impossible. At the receiver side, de-scrambling is carried out to convert the scrambled signal back to the clear signal. The clear signal is then fed into the television set. The de-scrambler itself is a piece of hardware located inside the set-top box, but its control is carried out by the conditional access system, located on the smart card. The conditional access system only allows the de-scrambler to operate if the customer is authorized to watch the service.

Authorization is done by sending a safely encrypted authorization message (Entitlement Management Message, EMM) to the customer which, after reception and decryption, is stored on the card. The authorization messages are usually sent well in advance of the service that they authorize. A second type of (encrypted) messages describes each service, and these messages (Entitlement Control Messages, ECM's) are sent together with the actual service data. Upon receiving an ECM, the smart card checks if the customer is authorized to receive the service, and only if this is the case, the de-scrambler is turned on by issuing of the appropriate key. These keys are often conveyed in encrypted form in the ECM's themselves. For a more detailed description of the control messages click here.
Photo: Smart cards for the heart of the security of many Conditional Access TV systems. This one can support access to up to four different broadcasters. Courtesy: Philips Digital Video Systems (DVS). 

On the transmission side, the system usually comprises several large subsystems that may be physically located in different buildings. A typical setup consists of the following equipment. First there is the equipment for playback and scheduling, usually located in the studio. There is also a computer based Subscriber Management System, (SMS), needed to do all the administration and billing. The SMS includes an order entry system that passes the orders on to the Subscriber Authorization System, (SAS). The SAS on its turn converts the orders into EMM's, which are then injected into the service. Other equipment necessary include a generator and an injector for the ECM's, and several control systems. All in all, the equipment on the transmission side is quite extensive and it is usually highly tuned to a specific conditional access system.

Over the past decade, several conditional access systems have been introduced. The following are only a few examples from Europe:

All these systems make use of various addressing schemes and allow different forms of authorization. The companies that have pioneered these systems have spent huge amounts of time and effort on the research, development and manufacturing of the boxes, Subscriber Management Systems, Subscriber Authorization Systems, message injectors and not in the least in the setting up of the subscriber base itself. Their investments should not be underestimated. Again and again improvement of the boxes and smartcards is required to discourage hackers from reverse engineering the key management system in the smartcards and selling their own pirate cards.

For a more complete overview of video broadcasting systems that are currently available on the market and hacks on them click here. For a short overview of the hardware in the boxes that is subject to the hackers interest click here.

Service providers are reluctant to accept storage devices. A subscriber to a pay-TV channel may copy a pay service on a VHS tape or other media and pass it on to its neighbour. To avoid that this neighbour can use the pay service without paying the service provider copy(right) protection methods are build in the de-scrambler boxes. For more information on copy(right) protection methods click here.

The incompatibility of the systems however poses a problem to the customer; if he wishes to receive another service, he has to rent or buy an additional decoder box and smart card. Especially if the other service is new on the market, it is really not likely that the customer is willing to do that. A solution to the problem could be to let this new service be accessed through the already installed boxes, which means that the two service providers would have to come to some mutual agreement. Recent examples of this can be found in the UK where BSkyB has reached such agreements with TV Asia, UK Gold and many more.

Further reading topics on Pay TV:


JPL's Wireless Communication Reference Website Daan van Schooneveld, Gerhard C. Langelaar and Jean-Paul M.G. Linnartz, 1993, 1999.