RX LOS (Receiver Loss of Signal) indicates the module's receiver (RX) is not detecting sufficient optical power to establish a valid link. One of the most common reasons for LOS alarms. This design note outlines the characteristics of the MAX3991 LOS detector, and describes how to set the optical assert power in a 10Gbps receiver for a specified BER. To maintain stability, most SFP, SFP+, SFP28, and QSFP modules provide two key diagnostic indicators: TX Fault and RX LOS. This article explains what they mean, how they work. optical communication technologythe optical module is a tool to realize the mutual conversion of photoelectric signals, and is one of the key components in optical communication equipment.