You’ve doubtless heard about the dangers of WiFi, and the security risks resulting from wireless communications like Bluetooth over unencrypted channels. Though much of the threat in these technologies lies in the fact that information is being transmitted over them in a clear text form that’s easy for hackers to read, another aspect of the danger lies in the nature of the wireless transmission itself.
Conventional wireless communications usually take place at a constant frequency – and if a cyber-attacker determines what this frequency is, then the data streams are easy to intercept or disrupt. That’s where spread spectrum comes into the picture.
What Is Spread Spectrum?
Spread spectrum is a type of wireless communication in which there’s a deliberate variation made in the frequency of a transmitted signal. This is done over a relatively large proportion of the electromagnetic spectrum, and in commercial spread spectrum systems typically involves bandwidths ranging from ten to 100 times the frequency of the information that’s being sent. This ratio can rise to anywhere from 1,000 to 1 million times the information bandwidth, for military applications.
Spread spectrum (or SS, inappropriately enough) dates back to the Second World War, and was initially designed to provide counter-measures for communications, navigation beacons, and radar systems. SS signals occupy a much larger bandwidth than is actually needed by its transmitted data, and spread spectrum in essence trades a wider signal bandwidth in favor of a better signal to noise ratio.
Many commercial satellite systems have switched to spread spectrum in order to reduce their costs and increase their channel capacity.
Spread Spectrum Functions
To qualify as a spread spectrum signal, the signal bandwidth has to be much greater than the information bandwidth. In addition, some code or pattern has to be used to determine the transmission bandwidth from moment to moment. This pattern typically takes the form of a complex mathematical function or algorithm, designed specifically for a given transmission.
Spread spectrum functions are typically of a “frequency versus time” format, in which the transmitter begins sending information at a particular starting point then varies the frequency of its output at set times determined by the algorithm.
Frequency hopping is a digital strategy employed in optimizing spread spectrum transmissions. Under this scheme the transmitter frequency fluctuates, altering abruptly many times per second. Between these alterations or “hops” the transmitter frequency remains stable at a given level for a period of time known as the dwell time.
It’s the easiest method for modulating spread spectrum communications, as any radio having a digitally controlled frequency synthesizer can in theory be used as a frequency hopping unit. All that’s required is a pseudo noise (PN) code generator, which is used to select the transmission or reception frequencies.
Continuous Frequency Variation
While frequency hopping is generally a digital process, it’s possible to create the same effect by analog means in a process known as continuous frequency variation. This is due to the fact that a frequency hopped system can employ analog carrier modulation in a system designed using conventional narrow band radio technologies.
Direct sequence transmission gives the most robust and practical option for a digital spread spectrum. Information to be transmitted is digitally encoded using a locally generated pseudo-noise coding algorithm, which creates a local code that runs at a vastly higher frequency than that of the transmitted data. In addition, the composite pseudo-noise generated together with the data may be passed through a scrambler to randomize its output spectrum and make transmissions harder for outside parties to trace.
Interference And Interception
The true value of the spread spectrum method of data transmission lies in its resistance to tampering. With conventional wireless communications a constant-frequency signal may become subject to catastrophic interference, and be totally disrupted by another signal which is transmitted at or very near the same frequency. This may occur accidentally, or be perpetrated as a deliberate act of “signal jamming” such as the tactics used in wartime or by government and intelligence agencies.
Constant-frequency signals may also be readily intercepted by eavesdroppers with the right equipment, and are therefore unsuitable for transmissions of confidential or sensitive data.
With spread spectrum communications, so long as the spreading algorithm remains known only to those authorized to use it, the interception of a signal is a difficult task for any potential hacker. An eavesdropper would have to know or have equipment capable of determining the exact spread spectrum function being used, and its specific starting time, so as to create variations in the frequency of their receiver that exactly match those of the spread spectrum transmission.
The frequency hopping pattern or direct sequence spreading code effectively encrypts each transmitted signal. Further protection for data transmission may be assured by using cryptographic ciphers on the messages themselves.
Applications In Radio
Spread spectrum techniques may be employed to enhance the security of cognitive (or self-learning) radio system transmissions, where the dangers of signal jamming or eavesdropping may be reduced not only by frequency hopping and direct sequence deployments, but also the lesser known time-hopping and chirp spread spectrum techniques, or hybrid transmissions combining elements of two or more of these variants.
Internet and Network Communications
In addition to being hard to intercept or jam, spread spectrum transmissions are also difficult to spoof or exploit. So on a network (including corporate LANs and the internet), an attacker who doesn’t have authorization to access its resources has little chance of listening in and extracting valuable information, or injecting falsified information or data traffic. Spread spectrum messages may also be encrypted to any level necessary for maintaining confidentiality.
In the realm of cyber-security, it’s sometimes the case that what works as a defense or deterrent in preserving the integrity of computer networks may also be turned against them as a weapon. This is certainly true of sniffing.
Share this Post