A communication system with solidity against communication failures increased by redundantly connecting communication terminals using a plurality of lines has been devised. When a communication apparatus detects a communication failure in a line in use, such a communication system suspends the use of the line in which the failure occurs, establishes a link in another communication line, and resumes communication.
Japanese Patent Application Laid-open No. 2001-119345 discloses an optical communication system in which an OLT and a star coupler are connected by redundant two optical fibers (Patent Literature 1).
International Publication No. WO2008/126162 discloses a protection system that switches a line in use from a working optical fiber to a backup optical fiber when an OLT does not receive an upstream signal from ONUs.
The IEEE (The Institute of Electrical and Electronic Engineers) 802.3av standard specifies a communication protocol in which an optical line is used. In a PON system, because a plurality of ONUs communicate with an OLT in time-division multiple access using a common line, the ONUs need to accurately transmit data at allocated timing. Therefore, the OLT controls synchronization among the ONUs using a downstream control message. The OLT inserts a timestamp based on an own clock reference into the downstream control message and notifies the ONUs of reference time. The ONU extracts the timestamp from the received downstream control message and updates, as a PON (Passive Optical Network) counter value, a local timer serving as a timing reference for transmission and reception.
The OLT frequently transmits the downstream control message including the timestamp. Therefore, the ONU can check a difference between the timestamp included in a received signal and the own PON counter and always monitor out of synchronization and line abnormality. When the timestamp and the PON counter deviate by a degree equal to or larger than a threshold, the ONU has to detect a timestamp drift error, disconnect a logical link, and return to an initial state. In the ONU having returned to the initial state, a logical link is set anew by discovery processing by the OLT. The ONU can resume communication through necessary synchronization and resetting of control information.