This counter measures, in seconds, how long the optical channel signal experienced loss of signal during the interval. Because loss of signal at the OCh layer means the wavelength itself dropped out, this is one of the clearest physical-layer fault indicators an operator has for that channel. A technician seeing nonzero loss-of-signal seconds on one wavelength but not its neighbors on the same fiber would suspect a problem specific to that channel's transponder rather than the shared fiber plant.