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Figure 5.4
Convolutional coder block diagram.
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Modulation, Multiple Access, and Impairments
Figure 5.5
Improvement in BER performance versus code rate for coded binary data channels.
5.1.5
Scrambling and Encryption
While satellite communication offers many benefits from its wide-area coverage
capability, it brings with it a concern for the security of the underlying information.
Commercial users worry that their valuable data can be intercepted and used for
undeserved gains. Also, government agencies involved with public safety and other
security matters cannot allow senstitive information to fall into the wrong hands.
There is also the concern that someone might inject false data into an environment
and cause difficulties and even total failure of an important operation of some
type. For all those reasons, designers and operators of satellite networks must make
proper provision for security. As with any type of countermeasure, the steps taken
should be in proportion to the threat, to minimize the spent resources and to not
unnecessarily interfere with normal day-to-day use of the system. The following
sections provide some guidelines and technological options for addressing those
concerns. However, this subject is extremely complex and considers also that
security must deal with intentional misuse and intrusion, for which clever humans
are ultimately to blame. For those reasons, security technology must constantly be
reviewed to counter threats that can become more sophisticated as the opportunities
(and rewards) for abuse tend to increase. Another issue is that security technology
and encrypted information may be subject to import and/or export restrictions and
controls in particular countries.
5.1.5.1
Security Risks in Satellite Communication
If we were to take no security measures to protect our information, then the fact
that a satellite broadcasts information over a wide area allows unintended users
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Digital Baseband Signals and Hierarchies
163
with the proper receiving system to get any of our information. It is the same with
local radio broadcasting, but the satellite multiplies the opportunities. That, of
course, is an advantage in services designed to reach the largest possible audience.
However, there are a variety of services in which the network provider must control
access. The analog form of TV transmission is the most vulnerable because those
transmissions are easily monitored with inexpensive receiving equipment. Digital
transmission provides a certain degree of security because a somewhat higher
barrier is presented by the greater cost and complexity of the receiver. However,
the digital set-to-box is widely available and many services are not encrypted, or
free-to-air. We cover many of the commercial solutions offered on the market at
the time of this writing. This is not exhaustive and, as discussed below, the nature
of the threat escalates as our ability to counter it improves.
The following are representative of some reasons why network operators
require security:
•
Preventing piracy of the information content. This is the most fundamental
issue in broadcast application, leading first to the use of scrambling and
subsequently to encryption.
•
Controlling the customer base, also called conditional access. The network
operator has the power to turn individual service on and off either through
the network itself or through a separate ‘‘back channel.’’
•
Protecting intellectual property (IP) rights, derived from the fact that satellites
are used to distribute information such as TV, music and computer programs
that are the property of someone else. The rights of the IP holder must be
protected, for which conditional access and either scrambling or encryption
are appropriate.
•
Simplifying pay-per-use services to the end user. For example, PPV TV and
information file transfer are popular applications of satellites but demand
the type of control cited above.
•
Providing levels of service and privacy for sensitive information. This is not
a case of requiring payment but requiring that the information be blocked
for reasons of competitive sensitivity or even national security.
•
Theft of service and preventing disruption of the normal operation of the
network. This is a concern for all telecommunication services and will depend
on the nature of the satellite network.
5.1.5.2
Scrambling
For some commercial applications, enough of the security requirements given in
Section 5.1.5.1 can be provided by scrambling the information according to a
predefined pattern. Scrambling is achieved by altering the normal analog waveform
or bit pattern so that it cannot be used unless the receiver can restore the original
through foreknowledge of the scrambling pattern (Figure 5.6). Basically there are
two ways to do that. The cheapest is to simply add an interfering or jamming
signal into the baseband and then to filter it out at the receiving end. Alternatively,
the signal itself can be altered in time or frequency according to some pattern, such
that only authorized receivers can reverse the process to produce a proper signal.
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