Transmission system, transmission device, and transmission method

A transmission system includes: a first transmission device that receives a first signal from the work path; a second transmission device, coupled to a protection path in a redundant configuration with respect to the work path, that receives a second signal from the protection path; and a first communication device coupled to the first and second transmission devices, wherein the first communication device, when detecting switching information from the first transmission device, notifies the first transmission device of first switching notification information and notifies the second transmission device of second switching notification information, the first transmission device stops relaying the first signal to the first communication device in response to the first switching notification information from the first communication device, and the second transmission device starts to relay the second signal to the first communication device in response to the second switching notification information from the first communication device.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2016-112162, filed on Jun. 3, 2016, the entire contents of which are incorporated herein by reference.

FIELD

The embodiment discussed herein is related to a transmission system, a transmission device, and a transmission method.

BACKGROUND

The International Telecommunication Union Telecommunication Sector (ITU-T) and the Institute of Electrical and Electronic Engineers (IEEE) are promoting the standardization of optical transport network (OTN) technology. According to the OTN standards, different kinds of client signals are transmitted in a transparent manner in an optical network using wavelength division multiplexing technology.

Japanese Laid-open Patent Publication No. 2014-093558 discusses a related art.

SUMMARY

According to an aspect of the embodiments, a transmission system includes: a first transmission device, coupled to a work path, configured to receive a first signal from the work path; a second transmission device, coupled to a protection path in a redundant configuration with respect to the work path, configured to receive a second signal from the protection path; and a first communication device coupled to the first transmission device and second transmission device, wherein the first communication device, when detecting switching information form the first transmission device, notifies the first transmission device of first switching notification information and notifies the second transmission device of second switching notification information, the first transmission device stops relaying the first signal to the first communication device in response to the first switching notification information from the first communication device, and the second transmission device starts to relay the second signal to the first communication device in response to the second switching notification information from the first communication device.

DESCRIPTION OF EMBODIMENT

For example, an OTN transmission system includes a protection configuration in which paths are made redundant by using Y cables.FIG. 1illustrates an example of a transmission system. A transmission system100inFIG. 1includes an eleventh client (CL) device101A, an eleventh transmission device102A, a twelfth transmission device102B, a first controller103A, and an eleventh optical multiplexing and demultiplexing unit104A. The transmission system100further includes a twelfth CL device101B, a thirteenth transmission device102C, a fourteenth transmission device102D, a second controller103B, and a twelfth optical multiplexing and demultiplexing unit104B. The eleventh CL device101A is connected to the twelfth CL device101B by paths on an OTN105, for example, a work path105A and a protection path105B in the protection configuration.

The eleventh transmission device102A, the twelfth transmission device102B, and the first controller103A are installed in one network apparatus110A. The thirteenth transmission device102C, the fourteenth transmission device102D, and the second controller103B are also installed in one network apparatus110B.

The eleventh transmission device102A is connected to the thirteenth transmission device102C by the work path105A. The twelfth transmission device102B is connected to the fourteenth transmission device102D by the protection path105B. The eleventh optical multiplexing and demultiplexing unit104A connects the eleventh CL device101A to the eleventh transmission device102A and the twelfth transmission device102B with a Y cable. The eleventh optical multiplexing and demultiplexing unit104A splits and outputs an optical signal from the eleventh CL device101A to the eleventh transmission device102A and the twelfth transmission device102B. The eleventh optical multiplexing and demultiplexing unit104A outputs optical signals from the eleventh transmission device102A and the twelfth transmission device102B to the eleventh CL device101A.

The twelfth optical multiplexing and demultiplexing unit104B connects the twelfth CL device1018to the thirteenth transmission device102C and the fourteenth transmission device102D with a Y cable, and splits and outputs an optical signal from the twelfth CL device1018to the thirteenth transmission device102C and the fourteenth transmission device102D. The twelfth optical multiplexing and demultiplexing unit104B outputs optical signals from the thirteenth transmission device102C and the fourteenth transmission device102D to the twelfth CL device1018.

The eleventh optical multiplexing and demultiplexing unit104A splits and outputs an optical signal from the eleventh CL device101A to the eleventh transmission device102A and the twelfth transmission device102B. The eleventh transmission device102A outputs the optical signal from the eleventh optical multiplexing and demultiplexing unit104A to the thirteenth transmission device102C via the work path105A. The twelfth transmission device102B also outputs the optical signal from the eleventh optical multiplexing and demultiplexing unit104A to the fourteenth transmission device102D via the protection path105B.

In the thirteenth transmission device102C, while the work path105A is used, a laser for relay to the twelfth CL device1018is set in an ON state. Thus, the thirteenth transmission device102C outputs the optical signal received from the eleventh transmission device102A via the work path105A to the twelfth optical multiplexing and demultiplexing unit104B. In the fourteenth transmission device102D, while the protection path105B is on standby, a laser for relay to the twelfth CL device1018is set in an OFF state. Consequently, the fourteenth transmission device102D does not output the optical signal received from the twelfth transmission device102B via the protection path105B to the twelfth optical multiplexing and demultiplexing unit104B. The twelfth optical multiplexing and demultiplexing unit104B outputs the optical signal received through the thirteenth transmission device102C from the eleventh CL device101A to the twelfth CL device1018.

When a failure occurs in the work path105A, for example, when the thirteenth transmission device102C detects a path failure in the work path105A, the thirteenth transmission device102C notifies the second controller103B of the path failure. In response to detecting the path failure from the thirteenth transmission device102C, the second controller103B causes the thirteenth transmission device102C to turn off the laser for relay to the twelfth CL device1018. In response to detecting the path failure from the thirteenth transmission device102C, the second controller103B causes the fourteenth transmission device102D to turn on the laser for relay to the twelfth CL device1018. For example, when detecting a path failure from the thirteenth transmission device102C, the second controller103B switches a transmission path from the work path105A to the protection path105B. The fourteenth transmission device102D outputs the optical signal received from the twelfth transmission device102B via the protection path105B to the twelfth optical multiplexing and demultiplexing unit104B.

When the eleventh transmission device102A detects a path failure in the work path105A, the eleventh transmission device102A notifies the first controller103A of the path failure. In response to detecting the path failure from the eleventh transmission device102A, the first controller103A causes the eleventh transmission device102A to turn off a laser for relay to the eleventh CL device101A. In response to detecting the path failure from the eleventh transmission device102A, the first controller103A causes the twelfth transmission device1028to turn on a laser for relay to the eleventh CL device101A. For example, when detecting a path failure from the eleventh transmission device102A, the first controller103A switches the transmission path from the work path105A to the protection path105B. The twelfth transmission device102B outputs the optical signal received from the fourteenth transmission device102D via the protection path105B to the eleventh optical multiplexing and demultiplexing unit104A.

Consequently, the eleventh CL device101A switches the transmission path from the work path105A to the protection path105B. The twelfth CL device1016transmits an optical signal to the eleventh CL device101A via the twelfth optical multiplexing and demultiplexing unit104B, the fourteenth transmission device102D, the protection path105B, the twelfth transmission device102B, and the eleventh optical multiplexing and demultiplexing unit104A in that order.

In the transmission system100, for example, the eleventh transmission device102A, the twelfth transmission device102B, and the first controller103A are accommodated in the network apparatus110A. The first controller103A controls the eleventh transmission device102A and the twelfth transmission device102B in a centralized manner. Since the first controller103A controls the eleventh transmission device102A and the twelfth transmission device102B in the network apparatus110A, fast switching from the work path105A to the protection path105B is performed when a failure is detected in the work path105A.

For example, the transformation of networks into networks based on cloud computing has increased demand for cloud-based products. For example, network apparatuses may employ an open interface (software defined network: SDN) such that distributed network apparatuses are controlled in a centralized manner. For example, a small network apparatus that accommodates no controller103A (103B) for controlling multiple transmission devices in a centralized manner may become mainstream. In such a small network apparatus, fast switching from a work path to a protection path may fail to be performed within a specified time, because this network apparatus accommodates no controller for controlling transmission devices in a centralized manner, for example, no controller exchanging information with the transmission devices.

For example, a transmission system in which fast switching from a work path to a protection path is performed may be provided.

FIG. 2illustrates an exemplary transmission system according to an embodiment. A transmission system1inFIG. 2includes a first CL device2A, a first transmission device3A, a second transmission device3B, and a first optical multiplexing and demultiplexing unit4A. The transmission system1further includes a second CL device2B, a third transmission device3C, a fourth transmission device3D, and a second optical multiplexing and demultiplexing unit4B. The first CL device2A is connected to the second CL device2B by paths on an OTN5, for example, a work path5A and a protection path5B in a protection configuration. Each of the first transmission device3A and the second transmission device3B may be a small network apparatus that accommodates no controller for controlling the first transmission device3A and the second transmission device3B in a common manner. Each of the third transmission device3C and the fourth transmission device3D may be a small network apparatus that accommodates no controller for controlling the third transmission device3C and the fourth transmission device3D in a common manner.

The first transmission device3A is connected to the third transmission device3C by the OTN5. The second transmission device3B is connected to the fourth transmission device3D by the OTN5. A transmission path between the first transmission device3A and the third transmission device3C is set to the work path5A and a transmission path between the second transmission device3B and the fourth transmission device3D is set to the protection path5B.

The first optical multiplexing and demultiplexing unit4A connects the first CL device2A to the first transmission device3A and the second transmission device3B with a Y cable, and splits and outputs an optical signal from the first CL device2A to the first transmission device3A and the second transmission device3B. The first optical multiplexing and demultiplexing unit4A outputs an optical signal from the first transmission device3A or the second transmission device3B to the first CL device2A.

The second optical multiplexing and demultiplexing unit4B connects the second CL device2B to the third transmission device3C and the fourth transmission device3D with a Y cable, and splits and outputs an optical signal from the second CL device2B to the third transmission device3C and the fourth transmission device3D. The second optical multiplexing and demultiplexing unit4B outputs an optical signal from the third transmission device3C or the fourth transmission device3D to the second CL device2B.

The first CL device2A, the second CL device2B, the first transmission device3A, the second transmission device3B, the third transmission device3C, and the fourth transmission device3D may have substantially the same configuration or similar configurations.FIG. 3illustrates an exemplary first transmission device. For convenience of explanation, the first transmission device3A inFIG. 2is illustrated as a transmission device ofFIG. 3. Since the first CL device2A, the second CL device2B, the second transmission device3B, the third transmission device3C, and the fourth transmission device3D have substantially the same configuration as or a configuration similar to that of the first transmission device3A, the same components of these devices as those of the first transmission device3A may be designated by the same reference numerals and redundant description of the components and operations may be omitted.

The first transmission device3A ofFIG. 3includes an interface (IF)11and a CPU12. The IF11includes a first IF11A for connection to the work path5A on the OTN5and a second IF11B for connection to the first CL device2A. The first IF11A includes an optical communication unit21, an OTN framer22, a detection unit23, an information receiving unit24, an information transmitting unit25, and an IF communication unit26. The optical communication unit21may include a laser and a photodiode to transmit and receive optical signals to and from the work path5A. The optical communication unit21includes the photodiode for converting a received optical signal into an electrical signal. The OTN framer22executes OTN framing on the electrical signal. The OTN framer22decomposes the electrical signal into frames. The detection unit23detects alarm information from the electrical signal. The alarm information may be alarm information indicating, for example, occurrence of a path failure. The information receiving unit24extracts laser information from the electrical signal. The laser information may be notification information indicating operation states, or laser ON/OFF states for the work path5A and the protection path5B. The IF communication unit26serves as a communication unit that controls communication with the second IF11B.

The information transmitting unit25adds the laser information to the electrical signal from the IF communication unit26and transmits the resultant signal. The OTN framer22assembles the electrical signal into an OTN frame and outputs the OTN frame to the optical communication unit21. The optical communication unit21, which includes the laser for converting an OTN frame into an optical signal, converts the OTN frame into an optical signal and outputs the optical signal to the work path5A.

The second IF11B includes an optical communication unit21, an OTN framer22, a detection unit23, an information receiving unit24, an information transmitting unit25, and an IF communication unit26. The optical communication unit21may include a laser and a photodiode to transmit and receive optical signals to and from the first CL device2A. The first transmission device3A connects the first IF11A to the work path5A and connects the second IF11B to the first optical multiplexing and demultiplexing unit4A. When the first transmission device3A ofFIG. 3is set to the second transmission device3B, the first IF11A is connected to the protection path5B and the second IF11B is connected to the first optical multiplexing and demultiplexing unit4A. When the first transmission device3A ofFIG. 3is set to the third transmission device3C, the first IF11A is connected to the work path5A and the second IF11B is connected to the second optical multiplexing and demultiplexing unit4B. When the first transmission device3A ofFIG. 3is set to the fourth transmission device3D, the first IF11A is connected to the protection path5B and the second IF11B is connected to the second optical multiplexing and demultiplexing unit4B.

The CPU12controls the whole of the first transmission device3A. The CPU12includes a setting management unit31, a state management unit32, and a control unit33. The setting management unit31includes a setting table34. The setting table34is a table for managing which of the CL device (for example, the first CL device2A), the transmission device for the work path5A (for example, the first transmission device3A), and the transmission device for the protection path5B (for example, the second transmission device3B) is the device including this setting table34. The control unit33includes a state management table35. The control unit33controls the whole of the first transmission device3A. The state management table35is a table for managing laser information indicating ON/OFF states of lasers used to relay signals received from the work path5A and the protection path5B to the CL device (for example, the first CL device2A).FIG. 4illustrates an exemplary state management table. The state management table35ofFIG. 4manages ON/OFF information about work lasers and protection lasers. The work lasers in the optical communication units21are lasers used to relay a signal from the work path5A to the first CL device2A. The protection lasers in the optical communication units21are lasers used to relay a signal from the protection path5B to the first CL device2A.

The state management unit32manages a current operation state based on alarm information. The control unit33controls the work lasers or the protection lasers in the optical communication units21based on received laser information.

The control unit33of the first CL device2A includes a first control unit33A and a second control unit33B in addition to the state management table35. For example, when a path switching request (path failure) from the first transmission device3A is detected, the first control unit33A notifies the first transmission device3A of received laser information for controlling driving of the lasers in the optical communication units21in the first transmission device3A. Consequently, the control unit33in the first transmission device3A controls the driving of the lasers in the optical communication units21based on the received laser information. When a path switching request (path failure) from the second transmission device3B is detected, the second control unit33B notifies the second transmission device3B of the received laser information for controlling driving of the lasers in the optical communication units21in the second transmission device3B. Consequently, the control unit33in the second transmission device3B controls the driving of the lasers in the optical communication units21based on the received laser information.

The control unit33of the second CL device2B also includes the first control unit33A and the second control unit33B in addition to the state management table35. For example, when a path switching request (path failure) from the third transmission device3C is detected, the first control unit33A notifies the third transmission device3C of received laser information for controlling driving of the lasers in the optical communication units21of the third transmission device3C. Consequently, the control unit33in the third transmission device3C controls the driving of the lasers in the optical communication units21based on the received laser information. When a path switching request (path failure) from the fourth transmission device3D is detected, the second control unit33B notifies the fourth transmission device3D of the received laser information for controlling driving of the lasers in the optical communication units21of the fourth transmission device3D. Consequently, the control unit33in the fourth transmission device3D controls the driving of the lasers in the optical communication units21based on the received laser information.

FIG. 5illustrates an example of a format of an OTU frame. An OTU frame30includes an overhead (OH) area131, a payload area132, and a forward error correction (FEC) area133. The OH area131has a frame size of 16 bytes (Columns1to16)×4 rows. The payload area132has a frame size of 3803 bytes (Columns17to3824)×4 rows. The FEC area133has a frame size of 256 bytes (Columns3825to4080)×4 rows.

The OH area131includes a frame alignment OH located in Row1of Columns1to7, an OTU OH located in Row1of Columns8to14, an ODU OH located in Rows2to4of Columns1to14, and an optical channel payload unit (OPU) OH located in Rows1to4of Columns15and16. The OTU OH is an OTU OH area. The ODU OH is an ODU OH area. The OPU OH is an OPU OH area.

The frame alignment OH includes a frame alignment signal (FAS) and a multi-frame alignment signal (MFAS). The FAS is a frame synchronization signal. The MFAS is a multi-frame synchronization signal for detecting multi-frame synchronization. The OTU OH includes section monitoring (SM), general communication channel (GCC0), and reserved for future international standardization (RES). The SM is information indicating a monitoring state between OTU terminal points. The GCC0is information for supporting a communication channel between OTU terminal points.

The RES has a two-bit structure. In the two bits, received laser information, for example, the ON/OFF state of the lasers (W) used for the work path5A and the ON/OFF state of the lasers (P) used for the protection path5B are set.

The first transmission device3A permits the work path5A to be in use and sets the lasers used to relay a signal received from the work path5A to the first CL device2A to the ON state. The second transmission device3B permits the protection path5B to be on standby and sets the lasers used to relay a signal received from the protection path5B to the first CL device2A to the OFF state. The third transmission device3C permits the work path5A to be in use and sets the lasers used to relay a signal received from the work path5A to the second CL device2B to the ON state. The fourth transmission device3D permits the protection path5B to be on standby and sets the lasers used to relay a signal received from the protection path5B to the second CL device2B to the OFF state.

For example, in response to detecting a failure in the work path5A, the first transmission device3A sets the work path5A to be on standby and sets the lasers used to relay a signal received from the work path5A to the first CL device2A to the OFF state. For example, in response to detecting a failure in the work path5A, the second transmission device3B sets the protection path5B to be in use and sets the lasers used to relay a signal received from the protection path5B to the first CL device2A to the ON state. For example, in response to detecting a failure in the work path5A, the third transmission device3C sets the work path5A to be on standby and sets the lasers used to relay a signal received from the work path5A to the second CL device2B to the OFF state. For example, in response to detecting a failure in the work path5A, the fourth transmission device3D sets the protection path5B to be in use and sets the lasers used to relay a signal received from the protection path5B to the second CL device2B to the ON state.

FIG. 6illustrates an exemplary processing operation of a CPU in a transmission device involved in a path switching process. A transmission device10may be any of the first transmission device3A, the second transmission device3B, and the first CL device2A.

InFIG. 6, the setting management unit31in the CPU12in the transmission device10obtains setting information in the setting table34(operation S11) and determines based on the setting information whether the device is the first CL device2A (operation S12). If the device is the first CL device2A (Yes in operation S12), the control unit33in the CPU12determines whether there is received laser information (operation S13). The received laser information may be laser information received from another transmission device10, for example, the first transmission device3A or the second transmission device3B.

If there is no received laser information (No in operation S13), the control unit33determines that it is in an initial state and updates laser information in the state management table35(operation S14). The laser information indicates that the work lasers are ON and the protection lasers are OFF. After updating the laser information, the control unit33notifies the transmission device for the work path5A (the first transmission device3A) and the transmission device for the protection path5B (the second transmission device3B) of the updated laser information (operation S15). The process moves to operation S13to determine whether there is received laser information.

If there is received laser information (Yes in operation S13), the control unit33determines whether the received laser information indicates that the work lasers are OFF and the protection lasers are OFF (operation S16). If the received laser information indicates that the work lasers are OFF and the protection lasers are OFF (Yes in operation S16), the control unit33updates the laser information in the state management table35(operation S17). The process moves to operation S15, where the control unit33notifies the transmission device for the work path5A (for example, the first transmission device3A) and the transmission device for the protection path5B (for example, the second transmission device3B) of the updated laser information. If the laser information in the state management table35indicates that the work lasers are ON and the protection lasers are OFF before updating, the laser information in the state management table35is updated to indicate that the work lasers are OFF and the protection lasers are ON. If the laser information in the state management table35indicates that the work lasers are OFF and the protection lasers are ON before updating, the laser information in the state management table35is updated to indicate that the work lasers are ON and the protection lasers are OFF.

If the received laser information does not indicate that the work lasers are OFF and the protection lasers are OFF (No in operation S16), the control unit33determines the received laser information to indicate that the work lasers are ON and the protection lasers are OFF or that the work lasers are OFF and the protection lasers are ON (operation S18), and updates the laser information in the state management table35(operation S19). The process moves to operation S15, where the control unit33notifies the first transmission device3A and the second transmission device3B of the updated laser information. When the received laser information indicates that the work lasers are ON and the protection lasers are OFF, the updated laser information indicates that the work lasers are ON and the protection lasers are OFF. When the received laser information indicates that the work lasers are OFF and the protection lasers are ON, the updated laser information indicates that the work lasers are OFF and the protection lasers are ON.

If the device is not the first CL device2A (No in operation S12), the setting management unit31determines based on the setting information whether the device is the transmission device for the work path5A (for example, the first transmission device3A) (operation S20). If the setting management unit31determines that the device is the first transmission device3A (Yes in operation S20), the process proceeds to M1illustrated inFIG. 7. If the setting management unit31determines that the device is not the first transmission device3A (No in operation S20), the setting management unit31determines whether the device is the transmission device for the protection path5B (for example, the second transmission device3B) (operation S21).

If the setting management unit31determines that the device is not the second transmission device3B (No in operation S21), the process moves to operation S12, where the setting management unit31determines whether the device is the first CL device2A. If the setting management unit31determines that the device is the second transmission device3B (Yes in operation S21), the process proceeds to M2illustrated inFIG. 8.

In M1inFIG. 7, that is, if the device is the first transmission device3A (Yes in operation S20), the control unit33determines whether there is received laser information from the first CL device2A (operation S31). If there is no received laser information (No in operation S31), the state management unit32in the CPU12checks alarm information about the work path5A (operation S32), and determines based on the alarm information whether the work path5A connected to the device has a failure (operation S33).

If the work path5A has no failure (No in operation S33), the control unit33updates laser information in the state management table35(operation S34). The laser information indicates that the work lasers are ON and the protection lasers are OFF. After updating the laser information, the control unit33notifies the first CL device2A of the updated laser information (operation S35). The control unit33sets the lasers in the optical communication units21used to relay a signal received from the work path5A to the first CL device2A to the ON state (operation S36). The processing operation inFIG. 7terminates.

If the state management unit32determines based on the alarm information that the work path5A has a failure (Yes in operation S33), the control unit33updates the laser information in the state management table35(operation S37). The process moves to operation S35to notify the second CL device2B of the updated laser information. The updated laser information indicates that the work lasers are OFF and the protection lasers are OFF.

When determining that there is received laser information (Yes in operation S31), the control unit33determines whether the received laser information indicates that the work lasers are ON (operation S38). If the received laser information indicates that the work lasers are ON (Yes in operation S38), the process moves to operation S32, where the state management unit32checks alarm information about the work path5A.

If the received laser information does not indicate that the work lasers are ON (No in operation S38), the control unit33determines that the work lasers are OFF (operation S39). The control unit33sets the lasers in the optical communication units21used to relay a signal received from the work path5A to the first CL device2A to the OFF state (operation S40). The process returns to operation S31to determine whether there is received laser information.

In M2inFIG. 8, that is, if the setting management unit31determines that the device is the second transmission device3B (Yes in operation S21), whether there is received laser information from the first CL device2A is determined (operation S41). If there is no received laser information (No in operation S41), the control unit33sets the lasers in the optical communication units21used to relay a signal received from the protection path5B to the first CL device2A to the OFF state (operation S42). The process returns to operation S41, where the control unit33determines whether there is received laser information.

If there is received laser information (Yes in operation S41), the control unit33determines whether the received laser information indicates that the protection lasers are ON (operation S43). If the received laser information does not indicate that the protection lasers are ON (No in operation S43), the control unit33determines the received laser information to indicate that the protection lasers are OFF (operation S44). The process moves to operation S42, where the control unit33sets the lasers used to relay a signal received from the protection path5B to the first CL device2A to the OFF state.

If the received laser information indicates that the protection lasers are ON (Yes in operation S43), the state management unit32checks alarm information about the protection path5B (operation S45) and determines based on the alarm information whether the protection path5B has a failure (operation S46).

If the protection path5B has no failure (No in operation S46), the control unit33updates laser information in the state management table35(operation S47). The laser information is updated to indicate that the work lasers are OFF and the protection lasers are ON. After updating the laser information, the control unit33notifies the first CL device2A of the updated laser information (operation S48). The control unit33sets the lasers in the optical communication units21used to relay a signal received from the protection path5B to the first CL device2A to the ON state (operation S49). The process ofFIG. 8terminates.

If the state management unit32determines based on the alarm information that the protection path5B has a failure (Yes in operation S46), the control unit33updates the laser information in the state management table35(operation S50). The process moves to operation S48to notify the first CL device2A of the updated laser information. The updated laser information indicates that the work lasers are OFF and the protection lasers are OFF.

When received laser information indicates that the work lasers are OFF and the protection lasers are OFF, the first CL device2A executing the path switching process notifies the first transmission device3A and the second transmission device3B of laser information indicating that the work lasers are OFF and the protection lasers are ON. In response to receiving the laser information indicating that the work lasers are OFF from the first CL device2A, the first transmission device3A turns off the lasers in the optical communication units21used to relay a signal received from the work path5A to the first CL device2A. In response to receiving the laser information indicating that the protection lasers are ON from the first CL device2A, the second transmission device3B turns on the lasers in the optical communication units21used to relay a signal received from the protection path5B to the first CL device2A. Consequently, if a small network apparatus includes no controller, fast switching from the work path5A to the protection path5B may be performed within a specified time.

When received laser information indicates that the work lasers are OFF and the protection lasers are OFF, the first CL device2A notifies the first transmission device3A and the second transmission device3B of laser information indicating that the work lasers are OFF and the protection lasers are ON. In response to receiving the laser information indicating that the work lasers are OFF from the first CL device2A, the first transmission device3A turns off the lasers in the optical communication units21used to relay a signal received from the work path5A to the first CL device2A. In response to receiving the laser information indicating that the protection lasers are ON from the first CL device2A, the second transmission device3B turns on the lasers in the optical communication units21used to relay a signal received from the protection path5B to the first CL device2A. Consequently, if a small network apparatus includes no controller, for example, no controller exchanging information with the first transmission device3A and the second transmission device3B, fast switching from the work path5A to the protection path5B may be performed within a specified time.

When received laser information indicates that the work lasers are OFF and the protection lasers are OFF, the second CL device2B notifies the third transmission device3C and the fourth transmission device3D of laser information indicating that the work lasers are OFF and the protection lasers are ON. In response to receiving the laser information indicating that the work lasers are OFF from the second CL device2B, the third transmission device3C turns off the lasers in the optical communication units21used to relay a signal received from the work path5A to the first CL device2A. In response to receiving the laser information indicating that the protection lasers are ON from the second CL device2B, the fourth transmission device3D turns on the lasers in the optical communication units21used to relay a signal received from the protection path5B to the first CL device2A. Consequently, fast switching from the work path5A to the protection path5B may be performed within a specified time without any controller exchanging information with the third transmission device3C and the fourth transmission device3D.

In the above-described embodiment, when the work path5A has a failure, the first CL device2A provides received laser information indicating that the work lasers are OFF and the protection lasers are OFF to the first transmission device3A and the second transmission device3B. The timing of providing such information is not limited to an event in which the work path5A has a failure. The first CL device2A may provide such information to the first transmission device3A and the second transmission device3B to perform normal switching from the work path5A to the protection path5B.

In the path switching process depicted inFIGS. 6 to 8, the first CL device2A is set as a CL device, the first transmission device3A is set as a transmission device for the work path5A, and the second transmission device3B is set as a transmission device for the protection path5B. For example, the second CL device2B may be set as a CL device, the third transmission device3C may be set as a transmission device for the work path5A, and the fourth transmission device3D may be set as a transmission device for the protection path5B.

A removable module, such as a 10 gigabit small form factor pluggable (XFP) module, is compliant with the multiple source agreement (MSA) specifications. For example, it is assumed that an interface #1 and an interface #2 each include a removable module, such as an XFP module, and switching is performed when a failure is detected in the removable module in a work device. Fast switching within 50 ms may fail to be performed because it takes time to detect a failure. For example, it takes up to 200 ms to detect a failure in an XFP module according to the MSA specifications. In contrast, if a failure occurs in a removable module in the above-described first transmission device3A, fast switching within 50 ms may be performed.

If a failure occurs in a removable module, a signal transmitted from the removable module would become unstable. In the first CL device2A, since each information receiving unit24in the IF11does not receive laser information, the control unit33is notified of laser information indicating that the work lasers in the first transmission device3A are OFF and the protection lasers in the second transmission device3B are OFF. Since the laser information indicates that the work lasers are OFF and the protection lasers are OFF, the control unit33notifies the first transmission device3A and the second transmission device3B of laser information indicating the work lasers are OFF and the protection lasers are ON. The first transmission device3A turns off the work lasers based on the received laser information and the second transmission device3B turns on the protection lasers based on the received laser information, so that the transmission path is switched from the work path5A to the protection path5B. Consequently, Y-cable fast switching to the protection path may be performed within a specified time without any controller exchanging information with the first transmission device3A and the second transmission device3B.

Although received laser information is added to the RES in the OTU OH of the OTU frame, addition is not limited to this example. Received laser information may be added to RES in the ODU OH.FIG. 9illustrates an example of a format of an ODU frame. InFIG. 9, substantially the same elements as or elements similar to those in the format ofFIG. 5are designated by the same reference numerals. Redundant description of the elements and operations may be omitted.

Referring toFIG. 9, an OH area231includes a frame alignment OH located in Row1of Columns1to7, an OTU OH located in Row1of Columns8to14, an ODU OH located in Rows2to4of Columns1to14, and an OPU OH located in Rows1to4of Columns15and16. The OTU OH is an OTU OH area. The ODU OH is an ODU OH area. The OPU OH is an OPU OH area.

The ODU OH includes two RESs, tandem connection monitoring activation (TCMACT), TCM1to TCM6, fault type and fault location reporting channel (FTFL), path monitoring (PM), and experimental (EXP). The ODU OH includes GCC1and GCC2and automatic protection switching and protection communication control channel (APS/PCC). The TCMACT is information for identifying whether tandem connection monitoring is in an active state. The FTFL is information for providing notification of a failure type and a failure position. The PM is information for monitoring a path state. The EXP is information for identifying whether it is for experiment. The APS/PCC is an automatic protection switching and switching communication channel.

In RES#1 corresponding to two bits in a 16-bit configuration, laser information, for example, the ON/OFF state of the lasers used for the work path and the ON/OFF state of the lasers used for the protection path may be set. The laser information may be added to RES#2 instead of the RES#1. A field to which received laser information is added is not limited to the RES. Received laser information may be added to, for example, the GCC0, GCC1, and GCC2.

All or some of the illustrated units may be functionally or physically distributed or integrated in any unit depending on, for example, various loads or use conditions.

All or any one of various processing functions performed by the devices may be implemented on a CPU (or a microcomputer, such as a micro processing unit (MPU) or a micro controller unit (MCU)). All or any one of the various processing functions may be implemented on a program analyzed and executed by a CPU (or a microcomputer, such as an MPU or an MCU) or on wired-logic-based hardware.