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A carrier signal is a transmitted electromagnetic pulse or wave at a steady base frequency of alternation on which information can be imposed by increasing signal strength, varying the base frequency, varying the wave phase, or other means. This variation is called modulation. With the advent of laser transmission over optical fiber media, a carrier can also be a laser-generated light beam on which information is imposed.
Types of analog modulation of a carrier include amplitude modulation (AM), frequency modulation (FM), and phase modulation. Types of digital modulation include varieties of pulse code modulation (PCM), including pulse amplitude modulation (PAM), pulse duration modulation (PDM), and pulse position modulation (PPM).