Pertaining to the representation of data by means of digits, or discrete quantities such as numbers or signals that can be interpreted as numbers. By contrast, analog signals have meaning at all intermediate levels. In telecommunications, digital transmission systems make use of pulses or varying levels of electromagnetic energy, such as electricity, radio waves, or light. Digital communications originates in telegraphy, in which a mechanical key is used to close an electrical circuit for varying lengths of time to send a series of short pulses (dots) and long pulses (dashes) that, in specific combinations, represent specific characters or series of characters. Early mechanical computers used a similar concept for input and output. Contemporary computer systems communicate in binary mode through variations in electrical voltage. Digital signaling in a contemporary electrical transmission system involves a signal that varies in voltage to represent one of two discrete and well-defined states.Two of the simplest approaches are unipolar and bipolar signaling. Unipolar signaling makes use of a positive (+) voltage and a null, or zero (0), voltage. Bipolar signaling, makes use of a positive (+) or a negative (