legal users to access the content.
Physical access is the ability of a user or unauthorized user to physically
send or receive information with a communication system or device. By
restricting access to cable TV lines, the cable TV operator can prevent unauthorized access (of its signals). Some cable TV systems use locking RF connectors that required a special tool or key to allow the connecter to be
installed or removed. Because it is relatively easy to overcome the restrictions of physical access, cable TV system operators have been shifting to protecting content using electronic processes.
Service authorization is the process of validating a subscriber or device to
request or obtain services. Cable TV companies perform service authorization by installing conditional access systems. Conditional access systems
can use uniquely identifiable devices (sealed with serial numbers) and may
use smart cards to store and access secret codes that are used to validate the
identity (authentication) and control access (authorization) to services (content).
To protect premium (non-free) content from being viewed by unauthorized
people (unpaid), cable TV operators can modify (scramble) the video signals
of premium channels. Video signal scrambling changes the electrical signal
(often distortion of video, digital voice, or data) to prevent interpretation of
the signals by unauthorized users that are able to receive the signal.
Because the scrambling process is performed according to a known procedure or algorithm, the received signal can be descrambled to recover the
original digital stream through the use of a known code or filtering technique.
In 1971, the first system to use scrambling on a cable system was demonstrated [10]. The first scrambling suppressed the synchronization signal so