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Baud (Bd) is a data transmission unit that shows how many signaling elements or symbol changes (electronic state change) are sent per second in a line code or a digitally modulated signal. It is not the measure of data transfer speed, but the measure of modulation. This should not be confused with the actual gross data transfer rate, which is expressed in bits per second. Although the two are related, they are not equal.
Baud rate is the measure of how much the electronic state changes per second. Each state change involves more than a single bit of data, so it cannot be equal to bits per second. Baud is represented as an SI unit, so the first letter is written in uppercase (Bd). The unit is named after Emile Baudot, the inventor of the Baudot code used in telegraphy.
Baud has been misunderstood as being equal to bit rate because these two rates are the same in older modems and simple communication links that use only one bit per symbol. In this case, each state change is only represented as a one or a zero, making Baud rate and bit rate equal. Modern electronic transmission techniques have more than two states and may represent more than one bit.