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Frequency-shift keying (FSK) allows digital information to be transmitted by changes or shifts in the frequency of a carrier signal, most commonly an analog carrier sine wave. There are two binary states in a signal, zero (0) and one (1), each of which is represented by an analog wave form. This binary data is converted by a modem into an FSK signal, which can be transmitted via telephone lines, fiber optics or wireless media.
FSK is commonly used for caller ID and remote metering applications.
FSK is also known as frequency modulation (FM).
For example, a low-speed Hayes-compatible modem uses an unbit FM technique. When no digital information is transmitted, the frequency is 1,700 Hz. When a one is transmitted, the frequency shifts to 2,200 Hz. When a zero is transmitted, the frequency shifts to 1,200 Hz. The number of these frequency shifts per second is measured as the baud or modulation rate. Thus, a 2,400 baud modem can process zeros and ones from a computer at the rate of 2,400 bits per second using FSK. This is the simplest digital communication, where baud and bit rate are the same and measured in bits per second.
In more advanced modems and data transmission techniques, a symbol may have more than two states, not just zeros and ones. It may also represent more than one bit of information. However, a single bit always represents one of two states – either a zero (0) or a one (1). In this case, baud (or symbol rate expressed in symbols/second or pulses /second) and bit rate are different and must not be confused with one another.