Communication apparatus, communication system, and communication control method

A communication system comprising:

• a pair of communication apparatuses coupled to a ring line, one of the pair of communication apparatuses includes
• a first port that is coupled to an other one of the pair of communication apparatuses through the ring line,
• a second port that is coupled to an external communication apparatus through a second access line, the external communication apparatus being disposed external to the ring line, the second access line being configured redundantly with a first access line between the external communication apparatus and the other one of the pair of communication apparatuses,
• a third port that is coupled to the other one of the pair of communication apparatuses through a control line for transmitting and receiving a control signal concerning the first access line, and
• first circuitry that detects a fault in the control line, detects a fault in the ring line, detects a fault in the second access line, switches the second access line from an active line to a standby line in accordance with the detection of the fault in the second access line, and while the fault in the control line is detected, shuts down the first port in accordance with the detection of the fault in the second access line, and
• the other one of the pair of communication apparatuses includes
• an access port that is coupled to the external communication apparatus through the first access line, and
• second circuitry that detects a fault in the first access line, detects the shutdown of the first port, and while a fault in the control line is detected and no fault in the first access line is detected, switches the first access line from a standby line to an active line in accordance with the detection of the shutdown of the first port,
• the first circuitry causes the third port to stop the transmission and reception of the control signal in accordance with the detection of the fault in the ring line, and shuts down the third port in accordance with the detection of a fault in the second access line while the fault in the ring line is detected.

A communication control method that uses a communication apparatus including a first port, a second port, and a third port, the first port being coupled to a first communication apparatus through a ring line, the second port being coupled to a second communication apparatus through a second access line, the second communication apparatus being disposed external to the ring line, the second access line being configured redundantly with a first access line between the first communication apparatus and the second communication apparatus, the third port being coupled to the first communication apparatus through a control line for transmitting and receiving a control signal concerning the first access line, the communication control method comprising:

• detecting a fault in the control line;
• detecting a fault in the ring line;
• detecting a fault in the second access line;
• switching the second access line from an active line to a standby line in accordance with the detection of the fault in the second access line;
• and
• while the fault in the control line is detected, shutting down the first port in accordance with the detection of the fault in the second access line;
• and
• causing the third port to stop the transmission and reception of the control signal in accordance with the detection of the fault in the ring line, and shutting down the third port in accordance with the detection of a fault in the second access line while the fault in the ring line is detected.