The number of bits per second (bps) transmitted. Bit rate and baud rate are often used interchangeably, and incorrectly so. Baud rate refers to the number of signal events, signal changes, or signal transitions occurring per second over an analog circuit, such as changes from positive voltage to zero voltage, from zero voltage to negative voltage, or from positive voltage to negative voltage. The relationship between baud rate and bit rate depends on the sophistication of the modulation scheme used to manipulate the carrier.The bit rate and baud rate can be the same, if each bit is represented by a signal transition in a unibit modulation scheme. The bit rate can be higher than the baud rate, as a single signal transition can, and often does, represent multiple bits. Further, multiple bits can be transmitted before a signal transition occurs. A modulation scheme that impresses multiple bits on a baud makes more effective use of analog bandwidth, which is always in limited supply. A purely digital transmission system uses an entirely different approach. Rather than varying the signal state of an analog carrier, a purely digital system is a two-state system that involves simply turning a signal on and off. In an electrically based telegraph system, for example, a dot ( . ) is a short electrical pulse transmitted by holding a telegraph key down for a short time, thereby closing an electrical contact, and a dash ( __ ) is a longer pulse. Fiber optic transmission systems (FOTS) can achieve bit rates of many Gbps through diode laser infrared light sources that pulse on and off billions of times per second. See also analog, baud rate, bit, carrier, digital, FOTS, and modulation.