Patent ID: 7852779

Claim:
A communication node that forms a communication system in which a plurality of communication nodes are connected to each other in a ring by Ethernet (registered trademark), the communication node comprising: a first port that includes an input unit that receives a frame and an output unit that outputs a frame; a second port that includes an input unit that receives a frame and an output unit that outputs a frame; and a communication processor that performs a process of receiving a frame input from the input unit of the first port and transmits the frame from the output unit of the second port when the first port and the second port are respectively in normal states, performs, when communication abnormality occurs between the communication node and a neighboring communication node, a loop-back process being a process of receiving a frame input from an input unit of a port on the other side of a port connected to the neighboring communication node and transmitting the frame from an output unit of the same port as the input unit to which the frame has been input, and when other communication node performs the loop-back process, transmits the frame from the output unit of the first port without performing a process of receiving a frame input from the input unit of the second port, wherein the communication processor includes a ring-establishment check-frame issuing unit that issues a ring-establishment check frame in which an issuance time is embedded when power is turned on or when an own communication node performs a loop-back process to configure a new ring, and, when receiving the ring-establishment check frame having circulated around the ring, stops issuing a subsequent ring-establishment check frame; a frame-circulation-time calculating unit that calculates a frame circulation time by subtracting the issuance time in the ring-establishment check frame from a time of reception of the ring-establishment check frame which has been issued by the ring-establishment check-frame issuing unit and has circulated around the ring; and a ring-establishment determining unit that determines whether any ring-establishment check frame passing around the ring is not detected for the frame circulation time or more after issuing of the ring-establishment check frame is stopped.