In the modulation of a carrier, a change from one significant condition to another. Note 1: Examples of signal transitions are a change from one electrical current, voltage, or power level to another; a change from one optical power level to another; a phase shift; or a change from one frequency or wavelength to another. Note 2: Signal transitions are used to create signals that represent information, such as "0" and "1" or "mark" and "yyspace. " 2. For channel-associated signaling, a change in state of the A bit for 2-state signaling, A and/or B bit in 4-state signaling, or the A, B, C, and/or D bit for 16-state signaling.