Patent ID: 8737833

Claim:
A communication line switching method for an optical communication system in which a station-side optical line terminal apparatus (OLT) and user-side optical line terminal apparatuses (ONUs) are connected via a plurality of redundant physical lines, the communication line switching method comprising: a discovery step of the OLT registering the ONU; a monitoring step of the registered ONU monitoring a time stamp drift error that is generated when a difference between a time stamp included in a received signal and a local time measured by the own apparatus is larger than a value set in advance and, when the time stamp drift error occurs, shifting to a deregistered state and waiting for registration by the discovery step; a switching step of the OLT switching a physical line from a working physical line to a backup physical line of the physical lines; and a holdover step of the ONU in the registered state shifting to an holdover state for suppressing a shift to the deregistered state due to occurrence of the time stamp drift error by the monitoring step in a holdover period set in advance even if line abnormality is detected.