This MIB module defines the managed objects and statistics objects for physical layer characteristics of optical line systems and optical transport devices. Performance monitoring (PM) parameters are used to gather, store, set thresholds for and report performance data for early detection of problems. Thresholds are used to set error levels for each PM parameter. During the accumulation cycle, if the current value of a performance monitoring parameter reaches or exceeds its corresponding threshold value, a threshold crossing alarm (TCA) is generated. The TCAs provide early detection of performance degradation. Glossary: OTS : Optical Transmission Section OTS-OCH : Optical Channel Information on OTS ATTN : Optical Attenuation GAIN : Optical Amplifier Gain TILT : Optical Amplifier TILT LBC : Laser Bias Current OSRI : Optical Safety Remote Interlock TX : Transmit RX : Receive VOA : Variable Optical Attenuator dB : Decibel dBm : Decibel-milliwatt mW : Milliwatt nm : Nanometer.