2Amplitude-Shift Keying (Penetration Testing Wireless Systems using SDR)
When ASK modulation is used, the bits are designated by the amplitude of the signal. Figure clearly shows a plot of the signal being transmitted in carrier waves. A carrier wave is the amplitude of the carrier, and when there’s no wave, that’s the signal’s resting state. When the carrier line is high for a specific duration, which registers as a wave, that’s a binary 1. When the carrier line is at a resting state for a shorter duration, that’s a binary 0.
ASK modulation is also known as on-off keying (OOK), and it typically uses a start-and-stop bit. Start-and-stop bits are common ways to separate where a message starts and where it stops. Accounting for start-and-stop bits, Figure 12-1 could represent nine bits: 0-1-1-0-1-1-0-1-0. Frequency-Shift Keying Unlike ASK, FSK always has a carrier signal but that signal is instead measured by how quickly it changes—its frequency.
In FSK, a high-frequency signal is a 0, and a low-frequency signal is a 1. When the carrier waves are close, that’s a 1, and when they’re spaced farther apart, that’s a 0. The bits in Figure are probably 1-0-0-1-0-0-1-0-1.
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