LOS

Loss of signal, or LOS, is the condition where a communications channel, optical fiber, or wireless link ceases to carry detectable data. In electrical engineering, LOS is not a failure mode but a state: the equipment detects the absence of an expected signal and responds according to its programming. The signal itself may be an RF carrier, a modulated baseband waveform, or in fiber-optic systems, a continuous light stream. When LOS occurs, the receiving equipment cannot extract usable information and typically enters a fault state within milliseconds.

The practical meaning of LOS depends entirely on context. In a fiber-optic transport system running at 10 Gbps, LOS means the photodiode has stopped receiving light above a threshold, usually around 20 to 30 percent of nominal power. In cellular base stations, LOS refers to loss of synchronization with the reference clock or backhaul signal. In analog RF systems, LOS describes the absence of a carrier frequency within the receiver's bandwidth. Each system defines its own detection threshold and alarm timescale.

Detection and response

Modern equipment detects LOS automatically. Fiber-optic receivers incorporate optical power monitors that trip an alarm when light intensity drops below a configurable threshold, typically within one to ten milliseconds. In digital systems, a LOS detector observes the incoming bit stream and declares loss of signal when it fails to lock onto the expected pattern or clock edge for a specified duration, often 32 to 256 bit periods. Equipment often responds to LOS by generating a local fault signal, halting transmission downstream, or switching to a backup channel.

The real cost of LOS is not the condition itself but the cascade it triggers. A single fiber cut or transceiver failure generates LOS at the receiver end. If that receiver is part of a chain carrying clock reference signals to multiple transceivers, those slaves may themselves generate secondary LOS alarms. In protected systems, LOS triggers automatic switchover to redundant paths. In unprotected systems, LOS means service interruption. The severity depends on how deeply the failed signal is embedded in the network hierarchy.

LOS is distinguished from other fault states by its simplicity: it means the signal is gone, not degraded, not corrupted, not out of specification. A signal with high bit error rate, high jitter, or low power but still present is not LOS. This distinction matters because different remedies apply. A power-depleted signal may respond to optical amplification; a LOS condition requires finding and restoring the signal path itself, whether that means reseating a connector, replacing a failed laser, or rerouting traffic around a severed cable.