MEASUREMENT DEVICE, TRANSMISSION DEVICE, AND NETWORK SYSTEM

A measurement device includes a memory, and circuitry coupled to the memory and configured to obtain first time stamp information transmitted from the first transmission device and added to a first frame in which an error has occurred in the transmission line, obtain second time stamp information transmitted from the second transmission device and added to a second frame in which an error has occurred in the transmission line, and specifies the error occurrence position in the transmission line on the basis of the first time stamp information, the second time stamp information, and a light speed, wherein the first time stamp information is added to the first frame by the first transmission device in time synchronization with the second transmission device, and the second time stamp information is added to the second frame by the second transmission device in time synchronization with the first transmission device.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2019-76332, filed on Apr. 12, 2019, the entire contents of which are incorporated herein by reference.

FIELD

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

BACKGROUND

According to the digital coherent transmission scheme, polarized light is modulated with data having been subject to multilevel modulation, and polarized beams of light having different directions of polarization are multiplexed and transmitted. A state of polarization (SOP) of the polarized light varies due to a phase cycle or slippage caused by a lightning strike on an optical fiber, vibration of the optical fiber, or the like. The variation of the SOP causes a bursty error of data signals. Such occurrence of an error becomes more remarkable as the degree of the multilevel modulation increases to increase the transmission capacity.

Therefore, it has been desirable to specify a fault position of an optical transmission line at which the error has occurred and to analyze the cause. With regard to specifying the fault position, there has been described a technique of specifying a fault point of a multi-branched optical line.

For example, Japanese Laid-open Patent Publication No. 2001-21445 and the like have been disclosed as a related art.

SUMMARY

According to an aspect of the embodiments, a measurement device includes a memory, and circuitry coupled to the memory and configured to obtain first time stamp information transmitted from the first transmission device and added to a first frame in which an error has occurred in the transmission line, obtain second time stamp information transmitted from the second transmission device and added to a second frame in which an error has occurred in the transmission line, and specifies the error occurrence position in the transmission line on the basis of the first time stamp information, the second time stamp information, and a light speed, wherein the first time stamp information is added to the first frame by the first transmission device in time synchronization with the second transmission device, and the second time stamp information is added to the second frame by the second transmission device in time synchronization with the first transmission device.

DESCRIPTION OF EMBODIMENTS

Since state variation of polarized light converges in a short period of time, it is difficult to specify a fault position from a pulse waveform incident on an optical transmission line as in the related art, and it takes a huge amount of time and effort to specify the fault position.

In view of the above, it is an object of the present embodiment to provide a measurement device, a transmission device, and a network system capable of easily measuring a fault position of an optical transmission line.

FIG. 1is a configuration diagram illustrating an exemplary network system. The network system includes a monitoring controller1, in-line amplifiers (ILAs)4aand4b, wavelength multiplexing devices6aand6b, and a time server7.

The wavelength multiplexing devices6aand6bare connected to each other via an optical transmission line9. The optical transmission line9is, for example, a multicore fiber, and includes two optical fibers9aand9b. One of the wavelength multiplexing devices transmits wavelength multiplex optical signals Sa to another wavelength multiplexing device6bvia the optical fiber9a, and the other wavelength multiplexing device6btransmits wavelength multiplex optical signals Sb to the wavelength multiplexing device6avia the optical fiber9b. Note that a distance L of the optical transmission line9between the wavelength multiplexing devices6aand6bis, for example, 800 (km).

With this arrangement, the wavelength multiplexing devices6aand6bexchange the wavelength multiplex optical signals Sa and Sb via the optical transmission line9. Here, the transmission speeds of the wavelength multiplex optical signals Sa and Sb are the same (e.g., 100 (Gbps) to 400 (Gbps)).

The wavelength multiplexing devices6aand6brespectively generate the wavelength multiplex optical signals Sa and Sb by multiplexing wavelengths of optical signals having wavelengths of λ1to λn. The wavelength multiplexing devices6aand6bIncludes a plurality of transponders (TPs)2aand2b, multiplexers (MUXs)3aand3b, and demultiplexers (DMUXs)4aand4b.

The transponders2aand2bare examples of the transmission device, which generate polarized multiplex optical signals having wavelengths of λ1to λn Including continuous frames, and output the signals to the multiplexers3aand3b. The multiplexers3aand3bare, for example, optical couplers, which multiplex the polarized multiplex optical signals having wavelengths of λ1to λn to generate the wavelength multiplex optical signals Sa and Sb, respectively.

The wavelength multiplex optical signals Sa and Sb are output to the optical transmission line9. The ILAs4aand4bare connected to the optical transmission line9. The ILAs4aand4bamplify the wavelength multiplex optical signals Sa and Sb.

The wavelength multiplex optical signals Sa and Sb are input from the optical transmission line9to the demultiplexers4band4aof the wavelength multiplexing devices6band6a, respectively. The demultiplexers4band4aare, for example, optical splitters, which demultiplex the wavelength multiplex optical signals Sa and Sb into each of polarized multiplex optical signals having wavelengths of λ1to λn in wavelength units. Each of the polarized multiplex optical signals having wavelengths of λ1to λn is received by the transponders2band2aas transmission destination.

The monitoring controller1and the time server7communicate with the wavelength multiplexing devices6aand6bvia a management network NW. The monitoring controller1is, for example, a network operation system (OpS), which monitors and controls the wavelength multiplexing devices6aand6b. The monitoring controller1is an example of the measurement device, which obtains, from each of the transponders2aand2b, transmission time of a frame in which an error has occurred (hereinafter referred to as “error frame”), and measures a fault position of the optical transmission line9from each transmission time.

The time server7distributes time information to the transponders2aand2busing, for example, a precision time protocol (PTP). As a result, the transponders2aand2bcan be in time synchronization with the time server7and add transmission time to the frame to be transmitted.

FIG. 2is a diagram illustrating exemplary operation of the transponders2aand2b. The transponder2asuccessively transmits a plurality of frames8ato the transponder2b, and the transponder2bsuccessively transmits a plurality of frames8bto the transponder2a. Note that the frames8aand8bare examples of a first frame and a second frame.

A reference sign Ga indicates a state in which the transponders2aand2bhave started to transmit frames. The transponders2aand2bare periodically subject to time synchronization with the time server7, for example, and add transmission time to each of the frames8aand8bto be transmitted. The transmission time is added to the overhead of each of the frames8aand8bas a time stamp “#1”, “#2”, or “#3”, for example. Note that a reference sign P indicates the intermediate position of the optical transmission line9, that is, for example, a position separated by a distance L/2 from each of the transponders2aand2b.

A reference sign Gb indicates a state in which an error has occurred in the frames8aand8bdue to a failure in the optical transmission line9. Since the transponders2aand2bare in time synchronization with the time server7, they simultaneously transmit the frames8aand8bto which the same time stamps are added, respectively. Accordingly, the frames8aand8bhaving the same time stamp (“#1005” in this example) usually pass through the intermediate position P of the optical transmission line9.

For example, it is assumed that a failure in the optical transmission line9has occurred at a position Q, which is a distance ΔL away from the intermediate position P on the side of the transponder2b, and an error has occurred in the frame8aof the time stamp “#802” and the frame8bof the time stamp “#1209” passing through the position Q. Here, examples of the failure in the optical transmission line9include a lightning strike on the optical transmission line9and vibration of the optical transmission line9, which indicate a failure that causes a bursty error in the frames8aand8bon the basis of polarization state variation of the polarized light of the optical signals.

A reference sign Gc indicates a state in which the transponders2aand2bhave received the error frames8aand8b. The transponder2breceives the frame8awith the time stamp “#802” transmitted from the other transponder2a, and detects an error. The transponder2btransmits the time stamp “#802” to the monitoring controller1as transmission time Ta of the frame8ain which the error has been detected.

The transponder2areceives the frame8bwith the time stamp “#1209” transmitted from the other transponder2b, and detects an error. The transponder2atransmits the time stamp “#1209” to the monitoring controller1as transmission time Tb of the frame8bin which the error has been detected.

The monitoring controller1measures, from the multiplied value of the difference between the transmission times Ta and Tb and a light speed Vc, the position Q (hereinafter referred to as “fault position Q”) of the optical transmission line9at which the error has occurred in the frames8aand8b.

The monitoring controller1calculates, using the formula (1) set out above, the distance ΔL between the fault position Q and the intermediate position P on the side of the transponder2b. The monitoring controller1calculates, using the formula (2) set out above, the distance Lq between the transponder2aand the fault position Q from the distance L and the distance ΔL of the optical transmission line9.

As described above, since the transponders2aand2bhave the same transmission speed and are in time synchronization with the time server7, the time stamp “# x” (x: transmission time) of the frames8aand8bpassing through the intermediate position P of the optical transmission line9is the same at all times. Accordingly, each of the value obtained by multiplying, by the light speed Vc, the difference (x−Ta) between the transmission time x and the transmission time Ta indicated by the time stamp added to the error frame8a, and the value obtained by multiplying, by the light speed Vc, the distance (Tb−x) between the transmission time x and the transmission time Tb indicated by the time stamp added to the error frame8bcorresponds to the distance ΔL.

Therefore, the value ((Tb−Ta)×Vc) obtained by multiplying, by the light speed Vc, the sum (Tb−Ta) of the difference (x−Ta) between the transmission time Ta and the transmission time x and the difference (Tb−x) between the transmission time Tb and the transmission time x is twice the distance ΔL. Accordingly, the distance Lq between the transponder2aand the fault position Q is calculated using the formula (1).

Furthermore, in a case where the distance ΔL between the fault position Q and the intermediate position P on the side of the transponder2ais calculated using the formula (3) set out above, the monitoring controller1can calculate the distance Lq between the other transponder2band the fault position Q using the formula (2) set out above.

Furthermore, while a constant value such as the light speed in a vacuum may be used as the light speed Vc, for example, it is possible to calculate, by using the propagation speed of light in the optical transmission line (e.g., 2.0×108(m/s)), the distance Lq of the fault position Q with higher accuracy while suppressing an error.

In the example ofFIG. 2, the transmission time Ta corresponds to the time stamp “#802” (802 (μs)), and the transmission time Tb corresponds to the time stamp “#1209” (1209 (μs)). Here, the distance L of the optical transmission line9is set to 800 (km), and the light speed Vc is set to 2.0×108(m/s).

At this time, the distance ΔL between the fault position Q and the intermediate position P is calculated to be 81.2 (km) (=(1208−802)×10×2.0×108/2) using the formula (1). Therefore, the distance Lq of the fault position Q from the transponder2ais calculated to be 440.6 (km) (=800/2+81.2) using the formula (2).

FIG. 3is a diagram illustrating a state of transmission of the frames8aand8bfor each time.FIG. 3illustrates the frames8aand8bat time 1007 (μs), 1008 (μs), 1009 (μs), 1015 (μs), 1016 (μs), and 1017 (μs) in the period from the time 1007 to 1017 (μs). Furthermore, a required time for the transponders2aand2bto transmit one frame8aand8bis assumed to be 1 (μs).

At the time 1007 (μs), an error occurs in the frame8awith the time stamp “#998” and the frame8bwith the time stamp “#1006” passing through the fault position Q. At the time 1008 (μs), the transponders2aand2breceive the frames8aand8bwith the time stamp “#999”, respectively.

At the time 1009 (μs), the transponders2aand2breceive the frames8aand8bwith the time stamp “#998”, respectively. At this time, the transponder2bdetects an error in the received frame8a, and discards the frame8a.

Accordingly, the transponder2bfails to detect the time stamp “#998” from the error frame8a. However, since the frame8ais not in burst transmission but is transmitted successively, the transponder2bcan identify the time stamp “#998” of the error frame8afrom the time stamp “#997” of the frame8areceived immediately before the error frame8a. The transponder2bnotifies the monitoring controller1of the time stamp “#998” as the transmission time Ta.

Thereafter, at the time 1015 (μs), the transponders2aand2breceive the frames8aand8bwith the time stamp “#1004”, respectively. Next, at the time 1016 (μs), the transponders2aand2breceive the frames8aand8bwith the time stamp “#1005”, respectively.

Next, at the time 1017 (μs), the transponders2aand2breceive the frames8aand8bwith the time stamp “#1006”, respectively. At this time, the transponder2adetects an error in the received frame8b, and discards the frame8b.

In a similar manner to the transponder2b, the transponder2acan identify the time stamp “#1006” of the frame8bfrom the time stamp “#1005” of the frame8breceived immediately before the error frame8b. The transponder2bnotifies the monitoring controller1of the time stamp “#1006” as the transmission time Tb.

In the present example, the transmission time Ta corresponds to the time stamp “#998” (998 (μs)), and the transmission time Tb corresponds to the time stamp “#1006” (1006 (μs)). Here, the distance L of the optical transmission line9is set to 2200 (km), and the light speed Vc is set to 2.0×108(m/s).

At this time, the distance ΔL between the fault position Q and the intermediate position P on the side of the transponder2bis calculated to be 800 (km) (=(1006−998)×106×2.0×108/2) using the formula (1). Therefore, the distance Lq of the fault position Q from the transponder2ais calculated to be 1900 km (=2200/2+800) using the formula (2).

Note that the distance ΔL between the fault position Q and the intermediate position P on the side of the transponder2ais calculated to be −800 (km) (=(998−1006)×106×2.0×106/2) using the formula (3) described above. Therefore, the distance Lq of the fault position Q from the transponder2bis calculated to be 300 (km) (=2200/2−800) using the formula (2).

In this manner, the monitoring controller1measures the fault position Q of the optical transmission line9at which an error has occurred in the frames8aand8bfrom the multiplied value of the light speed Vc and the transmission times Ta and Tb respectively obtained from the transponders2aand2b. Accordingly, the monitoring controller1can easily measure the fault position Q of the optical transmission line9without taking much time and effort.

The transponders2aand2bcalculate the distance Lq of the fault position Q in units of a distance corresponding to the required time for transmitting one frame8aand8bto add a time stamp to each of the frames8aand8b. In the present example, since the required time for transmitting one frame8aand8bis 1 (μs), the distance Lq of the fault position Q is calculated in units of 200 (m), which is a moving distance of 1 (μs) of the light speed Vc. Accordingly, accuracy in calculating the distance Lq of the fault position Q is dependent on the type and transmission speed of the frames8aand8b.

FIG. 4is a diagram illustrating an example of the format of the frames8aand8b. While examples of the frames8aand8binclude an optical transport unit (OTU) frame defined in Recommendation G. 709 of international Telecommunication Union Telecommunication Standardization Sector (ITU-T) in the present example, it is not limited thereto.

The frames8aand8binclude an overhead81and a payload80. The payload80contains data of client signals, such as Ethernet (registered trademark, the same applies hereinafter) signals, for example.

The overhead81includes a frame alignment signal overhead (FAS OH)82, an OTU OH83, an optical data unit-k overhead (ODUk OH)84, and an optical payload unit overhead (OPU OH)85. Note that the details of the overhead81are defined in ITU-T Recommendation G. 709.

The transponders2aand2badd the transmission time of the frames8aand8bto, for example, a reserve area830in the OTU OH83, reserve areas840and841in the ODUk OH84, or a reserve area850in the OPU OH85. Accordingly, the transponders2aand2bcan add the transmission time to the frames8aand8bwithout reducing the data band in the payload80. Note that the reserve areas830,840,841, and850are areas whose uses are not defined in ITU-T Recommendation G. 709.

Next, a configuration of the transponders2aand2bwill be described.

FIG. 5is a configuration diagram illustrating an example of the transponders2aand2b. The transponders2aand2binclude a central processing unit (CPU)20, a read only memory (ROM)21, a random access memory (RAM)22, a time stamp (TS) processing circuit23, a communication port24, and a PTP control chip25.

Furthermore, the transponders2aand2bfurther include a client interface (client IF)28, a framer chip26, an optical transmitter270, and an optical receiver271. Note that the TS processing circuit23, the PTP control chip25, the framer chip26, and the client IF28are circuits including hardware such as a field programmable gate array (FPGA) and an application-specific integrated circuit (ASIC). Note that the function of the TS processing circuit23may be implemented by software as a function of the CPU20.

The framer chip26includes a transmission frame processor260and a reception frame processor261, and is connected to the TS processing circuit23, the optical transmitter270, the optical receiver271, and the client IF28.

The client IF28receives client signals Ds from a client network and outputs the signals to the transmission frame processor260. The transmission frame processor260stores data of the client signals Ds in the payload80of the frames8aand8b, and generates the overhead81to add it to the payload80. The transmission frame processor260further stores the time stamp input from the TS processing circuit23in predetermined reserve areas830,840,841, and850in the overhead81. The transmission frame processor260outputs the frames8aand8bto the optical transmitter270.

The optical transmitter270transmits the frame8ato the other transponders2aand2bvia the optical transmission line9at a predetermined transmission speed in accordance with, for example, a digital coherent transmission scheme. The optical transmitter270includes a transmission light source such as a laser diode (LD), a polarization beam splitter, a polarization beam combiner, a modulator of multilevel modulation such as 16 quadrature amplitude modulation (QAM), an optical modulator, and the like. The optical transmitter270converts the frames8aand8binto polarized multiplex optical signals Fs and transmits the signals. The polarized multiplex optical signals Fs are input to the optical transmission line9from the multiplexers3aand3b. Note that the optical transmitter270is an example of a first transmission unit and a second transmission unit.

Furthermore, the optical receiver271receives the frame8aand8btransmitted from the other transponders2aand2bvia the optical transmission line9at a predetermined transmission speed in accordance with, for example, a digital coherent transmission scheme. The optical receiver271includes a local light source such as an LD, a polarization beam splitter, a polarization beam combiner, a demodulator of multilevel modulation such as 16QAM, a photodiode, and the like. The optical receiver271converts polarized multiplex optical signals Fr input from the demultiplexers4aand4binto the frames8aand8bof electric signals, and outputs the frames to the reception frame processor261. Note that the optical receiver271is an example of a first reception unit and a second reception unit.

The reception frame processor261converts the frames8aand8binto client signals Dr, and outputs the signals to the client IF28. The client IF28transmits the client signals Dr to the client network.

Furthermore, the reception frame processor261obtains a time stamp from the frames8aand8b, and outputs the time stamp to the TS processing circuit23. Here, the reception frame processor261detects an error in the frames8aand8b, and discards the error frames8aand8b. At this time, since the reception frame processor261fails to obtain the time stamp, it outputs detection signals indicating the detection of the error to the TS processing circuit23.

The TS processing circuit23includes a bus interface (bus IF)230, a time synchronization unit231, a time stamp adding unit232, and a time stamp detection unit233. The bus IF230relays communication among the time synchronization unit231, the time stamp detection unit233, and the CPU20via the bus29.

The time synchronization unit231periodically performs time synchronization processing with the time server7in accordance with a command from the CPU20, for example. At this time, the PTP control chip25communicates with the time server7via the communication port24on the basis of the PTP. The time synchronization unit231obtains highly accurate time from the time server7through communication of the PTP control chip25.

The time stamp adding unit232is an example of a first adding unit and a second adding unit, and adds transmission time to the frames8aand8b. The time stamp adding unit232receives, from the transmission frame processor260, notification indicating that the frames8aand8bto be transmitted are generated. The time stamp adding unit232obtains time from the time synchronization unit231in response to the notification of generation of the frames8aand8bto generate a time stamp. The time stamp adding unit232outputs the time stamp to the transmission frame processor260.

The transmission frame processor260inserts the time stamp into the overhead81of the frames8aand8b. Accordingly, the time at which the transmission frame processor260transmits the frames8aand8bis added to the frames8aand8b.

Furthermore, in a case where the reception frame processor261detects and discards normal frames8aand8b, the time stamp detection unit233obtains the time stamp of the frames8aand8bfrom the reception frame processor261. Furthermore, in a case where the reception frame processor261detects and discards error frames8aand8b, the time stamp detection unit233receives detection signals of the error of the frames8aand8bfrom the reception frame processor261.

The time stamp detection unit233specifies the transmission time of the error frames8aand8bin response to the reception of the detection signals, and outputs the transmission time to the CPU20via the bus.

FIG. 6is a diagram illustrating an exemplary method of specifying a transmission time. The time stamp detection unit233holds the time stamp obtained from the reception frame processor261, and specifies the transmission time on the basis of the time stamp of the frames8aand8bimmediately before the error frames8aand8b.

For example, in a case where an error is detected in the frame8awith the time stamp “#998”, the time stamp detection unit233specifies the transmission time “998” from the time stamp “#997” of the immediately preceding normal frame8a. The time stamp detection unit233outputs the specified transmission time to the CPU20.

Referring again toFIG. 5, the CPU20is connected to the ROM21, the RAM22, the TS processing circuit23, the communication port24, and the PTP control chip25via the bus29.

The ROM21stores a program for driving the CPU20. The RAM22functions as a working memory of the CPU20. The communication port24is, for example, a local area network (LAN) port, and relays communication among the CPU20, the monitoring controller1, and the time server7via the management network NW.

When reading the program from the ROM21, the CPU20forms a device controller200and a monitoring control interface (monitoring control IF)201as functions. The monitoring control IF201communicates with the monitoring controller1and the time server7via the communication port24.

The device controller200controls operation of the transponders2aand2b. The device controller200instructs, via the bus29, the time synchronization unit231to perform time synchronization.

Furthermore, the device controller200obtains the transmission time of the error frames8aand8bfrom the time stamp detection unit233via the bus29. The device controller200outputs the transmission time to the monitoring control IF201. The monitoring control IF201notifies the monitoring controller1of the transmission time. Note that the monitoring control IF201is an example of a notification unit.

Next, a configuration of the monitoring controller1will be described.

FIG. 7is a configuration diagram illustrating an example of the monitoring controller1. The monitoring controller1includes a CPU10, a ROM11, a RAM12, a communication port14, an input device15, and an output device16. The CPU10is connected to, via a bus19, the ROM11, the RAM12, the communication port14, the input device15, and the output device16in such a manner that signals can be input to and output from each other.

The ROM11stores a program for driving the CPU10. The RAM12functions as a working memory of the CPU10. The communication port14is, for example, a wireless local area network (LAN) card or a network interface card (NIC), which processes communication between the CPU10and the transponders2aand2b.

The input device15is a device for inputting information. Examples of the input device15include a keyboard, a mouse, and a touch panel. The input device15outputs the input information to the CPU10via the bus19.

The output device16is a device for outputting information. Examples of the output device16include a display and a touch panel. The output device16obtains information from the CPU10via the bus19, and outputs the information.

When reading the program from the ROM11, the CPU10forms a monitoring control unit100and a measurement unit101as functions. The monitoring control unit100communicates with the transponders2aand2bvia the communication port14to monitor and control the wavelength multiplexing devices6aand6b. The monitoring control unit100obtains the transmission times Tb and Ta of the frames8band8afrom the transponders2aand2b, respectively. Note that the monitoring control unit100is an example of an acquisition unit, and the transmission times Tb and Ta are examples of a first transmission time and a second transmission time.

Furthermore, the measurement unit101measures the fault position Q of the optical transmission line9at which the error of the frames8aand8bhas occurred from the multiplied value of the difference between the transmission times Tb and Ta and the light speed Vc. Note that a method of measurement is as described using the formulae (1) to (3) described above.

The measurement unit101outputs the fault position Q to the output device16in response to operational input from the input device15, for example. As a result, a user can know the fault position Q accordingly.

Next, a transmission process and a reception process of the frames8aand8bperformed by the transponders2aand2bwill be described.

FIG. 8is a flowchart illustrating an example of the transmission process of the frames8aand8b. The client IF28receives the client signals Ds from the client network (step St1) The client signals Ds are output to the transmission frame processor260.

Next, the transmission frame processor260generates the frames8aand8bcontaining data of the client signals Ds (step St2). At this time, the transmission frame processor260notifies the time stamp adding unit232of the generation of the frames8aand8b.

Next, the time stamp adding unit232obtains the time from the time synchronization unit231in response to the generation notification to generate a time stamp (step St3), and adds the time stamp to the frames8aand8b(step St4). The frames8aand8bto which the time stamp is added are output to the optical transmitter270. Next, the optical transmitter270transmits the frames8aand8b(step St5). In this manner, the transmission process of the frames8aand8bis executed.

FIG. 9is a flowchart illustrating an example of the reception process of the frames8aand8b. The optical receiver271receives the frames8aand8b(step St11). The frames8aand8bare converted into electric signals, and are output from the optical receiver271to the reception frame processor261.

The reception frame processor261performs error detection processing on the frames8and8b(step St12). A method for detecting an error is not limited, and may be a parity check, for example.

If no error is detected (No in step St12), the reception frame processor261obtains a time stamp from the overhead81of the frames8aand8b(step St3). The time stamp is output from the reception frame processor261to the time stamp detection unit233. Furthermore, the frames8aand8bare output to the client IF28.

Next, the client IF28generates the client signals Dr from the frames8aand8b(step St14), and transmits the signals to the client network (step St15).

Furthermore, if an error is detected (Yes in step St12), the reception frame processor261discards the error frames8aand8b(step St6). At this time, the reception frame processor261outputs error detection signals to the time stamp detection unit233.

Next, the time stamp detection unit233specifies the transmission time of the error frames8aand8bfrom the transmission time indicated by the time stamp of the immediately preceding frames8aand8b(step St17). The time stamp detection unit233outputs the transmission time to the CPU20.

The monitoring control IF201transmits the transmission time to the monitoring controller1via the communication port24(step St18). In this manner, the reception process of the frames8aand8bis executed.

Next, a process of measuring the fault position Q performed by the monitoring controller1will be described.

FIG. 10is a flowchart illustrating an example of the process of measuring the fault position Q. The monitoring control unit100obtains the transmission times Tb and Ta from the transponders2aand2b(step St21).

Next, the measurement unit101calculates the fault position Q using the formulae (1) to (3) described above (step St22). In this manner, the process of measuring the fault position Q is executed.

Other Examples

While the monitoring controller1measures the fault position Q in the example described above, one transponder2amay measure a fault position Q by obtaining transmission time Ta from another transponder2a. Since a monitoring controller1is unneeded in that case, a scale of a network system is reduced.

FIG. 11is a diagram illustrating operation of transponders2cand2daccording to another example. In a similar manner to transponders2aand2b, the transponders2cand2dadd time stamps of transmission times Ta and Tb to frames8aand8bto be transmitted, and transmit the frames to the other transponders2cand2dvia an optical transmission line9. Note that the transponders2cand2dare examples of a first transmission device and a second transmission device, respectively.

However, unlike the transponders2aand2b, the transponders2cand2ddo not notify the monitoring controller1of the transmission times Ta and Tb. One transponder2dadds, to a control frame8c, the transmission time Ta indicated by the time stamp added to the frame8areceived from the other transponder2c, and transmits the control frame8cto the transponder2cvia the optical transmission line9. The transponders2cand2dmeasure the fault position Q from the transmission times Ta and Tb. Hereinafter, a configuration of the transponders2cand2dwill be described.

FIG. 12is a configuration diagram illustrating a transponder2daccording to another example. InFIG. 12, the components same as those inFIG. 5are denoted by the same reference signs, and descriptions thereof will be omitted.

A CPU20forms a device controller200binstead of a device controller200. The device controller200bhas a function similar to that of the device controller200, and moreover, outputs transmission time Ta input from a time stamp detection unit233to a TS processing circuit23via a bus29.

The TS processing circuit23includes a bus IF230, a time synchronization unit231, a time stamp adding unit232, a time stamp detection unit233, and a control frame generation unit235. The control frame generation unit235receives the transmission time Ta from the device controller200bvia the bus IF230. The control frame generation unit235generates a control frame8cincluding the transmission time Ta.

The framer chip26includes a transmission frame processor260binstead of a transmission frame processor260. The transmission frame processor260bhas a function similar to that of the transmission frame processor260, and moreover, outputs the control frame8cto an optical transmitter270. The transmission frame processor260bnotifies the control frame generation unit235of the transmittable timing of the control frame8c, and the control frame generation unit235outputs the control frame8cto the transmission frame processor260bif there is the control frame8cto be transmitted.

In this manner, the device controller200bnotifies another transponder2cof the transmission time Ta added to a frame8ain which an error has occurred among the frames8areceived by the optical receiver271. Note that the device controller200bis an example of a time notification unit.

FIG. 13is a configuration diagram illustrating a transponder2caccording to another example. InFIG. 13, the components same as those inFIG. 5are denoted by the same reference signs, and descriptions thereof will be omitted.

A framer chip26includes a reception frame processor261ainstead of a reception frame processor261. The reception frame processor261ahas a function similar to that of the reception frame processor261, and moreover, outputs a control frame to a TS processing circuit23.

The TS processing circuit23includes a bus IF230, a time synchronization unit231, a time stamp adding unit232, a time stamp detection unit233, and a time acquisition unit236. The time acquisition unit236receives the control frame from the reception frame processor261a.

The time acquisition unit236obtains transmission time Ta added to the control frame. The time acquisition unit236outputs the transmission time Ta to a CPU20via the bus IF230and a bus29.

The CPU20forms a device controller200ainstead of a device controller200. The device controller200ahas a function similar to that of the device controller200, and moreover, measures a fault position Q from transmission times Ta and Tb.

The transmission time Ta of the error frame8ais input to the device controller200afrom the time acquisition unit236, and the transmission time Tb of the error frame8bis input from the time stamp detection unit233. The device controller200acalculates the fault position Q from the transmission times Ta and Tb using the formulae (1) to (3) described above. Note that the device controller200atransmits the fault position Q to a monitoring controller1via a communication port24, for example. As a result, a user can know the fault position Q accordingly.

In this manner, the time acquisition unit236obtains the transmission time Ta added to the frame8ain which the error has occurred among the frames8areceived by another transponder2d. The device controller200ameasures the fault position Q at which the error of the frames8aand8bhas occurred from the multiplied value of the difference between the transmission times Ta and Tb and a light speed Vc. Accordingly, the fault position Q can be easily measured in a similar manner to the example described above. Note that the device controller200ais an example of a position measurement unit.

Next, a reception process of the frame8aperformed by the transponder2dwill be described. Note that a transmission process of the frame8bperformed by the transponder2dis as illustrated inFIG. 8.

FIG. 14is a flowchart illustrating an example of a reception process of a frame8aperformed by a transponder2daccording to another example. InFIG. 14, the processes same as those inFIG. 9are denoted by the same reference signs, and descriptions thereof will be omitted.

After a time stamp detection unit233specifies transmission time Ta of an error frame8a(step St17), a control frame generation unit235generates a control frame8cincluding the transmission time Ta (step St30). The control frame8cis output from a transmission frame processor260bto an optical transmitter270. The optical transmitter270transmits the control frame8cto a transponder2c(step St31). In this manner, the transponder2dexecutes the reception process of the frame8a.

Next, a reception process of a frame8bperformed by the transponder2cwill be described. Note that a transmission process of the frame8aperformed by the transponder2cis as illustrated inFIG. 8.

FIG. 15is a flowchart illustrating an example of a reception process of a frame8bperformed by a transponder2caccording to another example. InFIG. 15, the processes same as those inFIG. 9are denoted by the same reference signs, and descriptions thereof will be omitted. Note that each processing of steps St40and St4to be described below may be executed before steps St16and St17.

After processing of step St17, an optical receiver271receives a control frame8cfrom another transponder2d(step St40). The control frame8cis input to a reception frame processor261a, and then input to a time acquisition unit236.

Next, the time acquisition unit236obtains transmission time Ta of an error frame8afrom the control frame8c(step St41). The transmission time Ta is output to a device controller200a.

Next, the device controller200acalculates a fault position Q from the transmission times Ta and Tb (step St42). In this manner, the transponder2cexecutes the reception process of the frame8b.

Note that, although an optical transmission line9includes two optical fibers9aand9bin each example described above, it may include only one optical fiber. In that case, bidirectional frames8aand8bare transmitted to a common optical fiber. Accordingly, in order to separate the frames8aand8b, a wavelength divisional multiplexing (WDM) coupler is provided between an optical transmitter270and an optical receiver271of each of transponders2ato2dand the optical fiber, for example. As a result, the transponders2ato2dcan transmit and receive the frames8aand8b.

The embodiment described above is a preferred example. However, the embodiment is not limited thereto, and a variety of modifications may be made without departing from the scope of the embodiment.