Optical transmission system and transmission mode selecting method

A transceiver comprising:

• a transmission-mode-candidate transmitter configured to transmit to transmission apparatus to be communicated with transmission mode candidate information including transmission mode information which is a combination of a plurality of parameters related to transmission performance of the transceiver;
• a transmission-mode-candidate receiver configured to receive transmission mode candidate information including transmission mode information which is a combination of a plurality of parameters related to transmission performance of the transmission apparatus to be communicated with;
• a transmission-mode selector configured to select transmission mode information in descending order of priority out of transmission mode information common to the transmission mode candidate information of the transceiver and the transmission mode candidate information of the transmission apparatus to be communicated with;
• a signal transmitter configured to transmit, to the transmission apparatus to be communicated with, a signal modulated based on the selected transmission mode information;
• a signal receiver configured to receive signal light transmitted from the transmission apparatus to be communicated with and demodulate the received signal light based on the transmission mode information selected by the transmission-mode selector;
• a signal-quality detector configured to detect signal quality of the received signal light received by the signal receiver;
• and
• a signal-quality determiner configured to determine based on information indicating the signal quality detected by the signal-quality detector, whether the signal quality of the received signal light is permissible;
• wherein the when the signal-quality determiner determines that the signal quality of the signal is non-permissible, the transmission-mode selector is configured to select the transmission mode information having second highest priority.