- Conditional access system or CAS is
a system that has been developed as a means for providing the protection to
content by setting certain criteria that should be met before the access to it
is granted.
- The concept of conditional access is however related closely to the
digital television systems such as the satellite television.
- The standards for
the conditional access system have been defined under the following
specification documents of the digital video broadcasting standard (DVB):
- DVB – CA
(conditional access)
- DVB – CSA (common
scrambling algorithm)
- DVB – CI (common
interface)
- All of these standards together
define a method for obfuscating a digital television stream, thus providing
access to only those possess the ‘valid description smart cards’.
- All of these
specifications are available on the standards page of the DVB web site.
- Conditional
access is the result of combination of encryption and scrambling technologies.
- Firstly, the data stream is subjected to scrambling with a secret key of 48 –
bits usually called the control word.
- The control word is generated in such a pattern in
which the successive word cannot be predicted.
- It is recommended by the DVB
that a physical process should be used for that.
- For a person to unscramble the
data stream it is required that he/ she should know about the current control
word.
- In order to provide the protection to control word during the course of
transmission, encryption methodology is used.
- The control word will be
de-crypted by the conditional access system only when there is authorization.
- This
authorization is given in the form of EMM or entitlement management message.
- They
are specific for each subscriber.
- Different ECMs can also transmit the control
word at one allowing the usage of many conditional access systems at once and
simultaneously.
- The simulscrypt of the DVB allows all the multiplex operators
to cooperate while at the same time saves bandwidth.
- Channels such as the hot
bird satellites and the CNN international make use of several CASs in parallel.
- The data from the decryption is read from the cards and updated if required
through either of the following:
- CAM or conditional
access module
- PC card – format
card reader
- Built – in ISO/ IEC
7816 card reader
- Conditional access systems include
two major types namely analog systems and the digital systems both categorized
at shown below:
- Analog systems:
Ø
Nagravision
Ø
Videocrypt
Ø
Videocipher
Ø
Eurocrypt
- Digital systems:
Ø
Abel
Quintic
Ø
ABV
(alliance broadcast vision)
Ø
Accessgate
- These systems are the key component
for all the digital TV operations.
- Their purpose is to secure the investments
of the operators through the encryption of the signals.
- The payment from the
customers in order to watch TV is also ensured by the conditional access
systems.
- But the TV operators usually have limited knowledge about the
conditional access systems.
- Encryption is involved with the whole conditional
access system.
- Operators have the analyzers for finding the cause of the common
signal problems.
- But they do lack tools that can debug the CASs.
- Most of the
conditional access system monitoring tools follow the ETSI TR 101 290 standard
(a DVB promoted standard) even though the CAS is not fully covered by this
specification.
- Analyzing tools as such let the operators to know about the problems
of conditional access system before the customers notice it.
- But practically
also, it is not possible to take care of all the problems that may affect the
subscriber.
