Conditional Access System


Television in India

Television industry is launched in early 80s in India. At that time there was only one national channel Doordarshan, which was government owned. Doordarshan was broadcasted terrestrially. In the early 90’s, Indian government allowed private and foreign broadcasters to engage in limited operations. At the start of private channels, cable television is started in India. Cable television does a good business in 90s.

In the year 2003, Indian government launched conditional access system. It first launched in metro cities. CAS or conditional access system is a digital mode of transmitting TV channels through a set-top box (STB). The transmission signals are encrypted and viewers need to buy a set-top box to receive and decrypt the signal. It starts a new era of CA system and satellite TV in India.

Conditional Access System

Conditional Access means inserting some condition criteria in broadcasted digital multimedia data so that only authorized user can view that program. It is a technology used to control access to digital television (DTV) services to authorized users by encrypting the transmitted programming. Under the Conditional Access System (CAS), user needs a set-top box (STB) to receive services (pay channels).

Components of CA System

A CA system consists of several basic components:

Subscriber Management System (SMS): The SMS is a subsystem of the CA system that manages the subscriber’s information and requests entitlement management messages (EMM) from the Subscriber Authorization System (SAS). An EMM provides general information about the subscriber and the status of the subscription. The EMM is sent with the entitlement control message (ECM). The ECM is a data unit that contains the key for decrypting the transmitted programs.

Subscriber Authorization System (SAS): The SAS is a subsystem of the CA system that translates the information about the subscriber into an EMM at the request of the SMS. The SAS also ensures that the subscriber’s security module receives the authorization needed to view the programs, and the SAS acts as a backup system in case of failure.

Security module: The security module, usually in the form of a smart card, extracts the EMM and ECM necessary for decrypting the transmitted programs. The security module is either embedded within the set-top box or in a PC Card that plugs into the set-top box.

Set-top box: The set-top box houses the security module that gives authorization for decrypting the transmitted programs. The set-top box also converts the digital signal to an analogue signal so an older television can display the programs.

Working of CA System:

Under the DVBconditional access system standards are defined in the specification documents for DVB-CA (Conditional Access), DVB-CSA (the Common Scrambling Algorithm) and DVB-CI (the Common Interface). These standards define a method by which a digital television stream can be obfuscated, with access provided only to those with valid decryption smart cards. This is achieved by a combination of scrambling and encryption.

Working at broadcast center

SMS manages subscription information of users. It takes input from billing and customer service about the service subscribed by user and stores it in its database. It provides subscription information to SAS whenever it requests.

SAS get subscription information from SMS, translate it in EMM format, finally encrypt it and send to multiplexer.

SAS also provides control information to generate ECM packets. ECM packets contain private Conditional Access information, which carries the control word in a secure manner.ECM packet send to the multiplexer after encryption.

Multiplexer get the audio, video and other data of a service with ECM and EMM packet, multiplexed it in single stream and send it to scrambler.

DVB scrambler scrambled the output stream of multiplexer using control word and sends it to modulator, which further broadcasted it after modulation.

Working at Receiving End:

At the receiving end, the set-top box descrambles the program streams and decodes the MPEG-2 data.

The tuner portion of the set-top box receives the incoming signal, demodulates it and sends the resulting data to the transport stream generator. This reconstitutes the transport stream, which is passed to the MPEG-2 decoder.

DVB descrambling system sends CA specific data (ECM and EMM) to the demultiplexer which separates it in ECM and EMM packets and sends them to smart card.

Smart card contains algorithm for EMM and ECM decryption.

EMM contains the service key which decrypts by the EMM decryption algorithm using master/smart key.

ECM decryption algorithm decrypts ECM and extracts the control word using the service key. The encrypted Control Word, carried by the ECM, is related to particular program material. This key allows the transport stream to be descrambled so that the viewer can see a particular program or view the program material for a particular session.

The contents of ECMs and EMMs are not standardized and as such they depend on the conditional access system being used.

Process of scrambling and descrambling can be summarized by following diagram:

Benefits of CAS

Consumers: Consumers get the option to choose the channels they want to pay for and view, rather than receiving the whole set of channels that the Cable Operator makes available to them.

Cable Operators: Cable operators get the opportunity to pay a part of the subscription fees to the broadcasters only for the actual number of end users who opt for the channel, rather than all households having cable access. This allows operators to price their channel offerings according to their popularity.

Broadcasters: Broadcasters have a long-standing grouse that the Cable Operators under-declare the actual number of subscribers, and hence pass on only a fraction of the paid subscriptions. With a system like this in place, it is possible to address the exact number of subscribers with a cable operator.

Advertisers: It gives a far more accurate indicator of programme popularity with only the actual subscribers of each channel being accounted for.

Government: Since the issue of addressability ensures a fair degree of transparency in accounting across the entire value chain, it minimizes the loss of revenues to the government through mis-reporting or non-disclosure of actual revenue figures. The government also facilitates the introduction and development of consumer friendly systems like pay per view, interactive programming, etc.

Definitions

Control Word (CW): data object used for scrambling.

Encryption: The method of processing the continually changing electronic keys needed to descramble the broadcast signals, so that they can be securely conveyed to the authorized users, either over-the-air or on smart cards.

Entitlement Control Message (ECM): A cryptogram of the control word and the access conditions. An ECM is a specific component of the electronic key signal and over-the-air addressing information. The ECMs are used to control the descrambler and are transmitted over-air in encrypted form.

Entitlement Management Message (EMM): A message authorizing a viewer to descramble a service. An EMM is a specific component of the electronic key signal and over-the-air addressing information. The EMMs are used to switch individual decoders, or groups of decoders, on or off and are transmitted over-air in encrypted form.

Scrambling: The method of continually changing the form of the broadcast signal so that, without a suitable decoder and electronic key, the signal is unintelligible.

Service: A sequence of events, programmes or data, based on a schedule, assembled by a service provider to be delivered to the viewer.

Smart Card: A device that looks rather like a credit card; it is used as a token of entitlement to descramble broadcast signals. Most of the major European access control systems use smart cards. Other systems that bury the same functionality inside the decoder do not usually allow the system to be changed to combat piracy or to add new services. Smart cards can be issued by the Subscriber Management System which can validate them by pre-programming them with keys to authorize access to certain tiers of programmes and/or data services. As part of the same issuing and validation process, the card may be personalized to make each one valid for one particular decoder only.

Subscriber Authorization System (SAS): The centre responsible for organizing, sequencing and delivering EMM and ECM data streams under direction from the Subscriber Management System.

Subscriber Management System (SMS): The business centre which issues the smart cards sends out bills and receives payments from subscribers. An important resource of the Subscriber Management System is a database of information about the subscribers, the serial numbers of the decoders and information about the services to which they have subscribed. In commercial terms, this information is highly sensitive.

Satellite television: It is television delivered by the means of communications satellite and received by a satellite dish and set-top box. In many areas of the world it provides a wide range of channels and services, often to areas that are not serviced by terrestrial or cable providers.

References:

http://en.wikipedia.org/wiki/Conditional_access_system

http://www.ebu.ch/en/technical/trev/trev_266-ca.pdf

http://www.nhk.or.jp/strl/publica/bt/en/le0012.pdf

http://www.atsc.org/standards/a_70a_with_amend_1.pdf

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