JPH0340985B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a monitoring system for an optical repeater transmission line configured by alternately cascading cables and regenerative repeaters. This method is especially suitable for high-speed digital relay transmission lines using fiber optic cables, and when the relay transmission line is not in operation, each regenerative repeater is individually selected and connected back by remote control, and its status is monitored. This is related to the method of doing so.

[Conventional technology]

Regarding this type of conventional monitoring system, the applicant for the present patent filed a patent application (Japanese Patent Application No. 56-152540, September 1983).
Filed on May 17th, hereinafter referred to as the "first application". ) was carried out.
The invention of this earlier application performs mark rate modulation (concentration modulation) on the main transmission signal of the relay transmission line using the monitoring frequency assigned to the repeater, detects the component of the monitoring frequency in each repeater, and outputs the detection output. At the detected repeater, the signal pulse train of the cable in one direction is looped back to the cable in the other direction, and at the terminal station, the above-mentioned monitoring frequency component is detected from this looped pulse train, and the number of phase inversions of the signal of that frequency component is counted. By doing so, the code error rate of the relay transmission path is monitored.

[Problem to be solved by the invention]

The method of this prior invention is an excellent method that allows the signal on the relay transmission line to be looped back by a repeater designated by remote control from the terminal station, but the inventor has proposed that this method is a 400 Mb/s digital optical transmission method. As a result of testing, we found the following: In other words, in this conventional method, a repeater is selected based on the monitoring frequency transmitted from the terminal station, but this monitoring frequency f _SV is calculated as f _SV = f ₀ /2m (where f ₀ is the clock frequency of the transmission line). (m is a natural number), the set frequency is limited, and the filter that extracts this monitoring frequency will be mounted on the repeater, so it must be constructed from small and highly reliable components. It was found that the applicable frequency band is limited, for example, if a monolithic crystal filter is used, the frequency band is limited to about 10 to 15 MHz.

Furthermore, with respect to the means for counting the number of phase inversions provided at the terminal station, there is, in principle, an upper limit to the count value depending on the monitoring frequency _fSV . In other words, according to the prior invention, the precondition is that the number of pulse marks (mark rate) always changes by a certain value or more every half cycle of the monitoring frequency _fSV .
As _fSV is increased, the occurrence of this mark rate becomes less periodic. Therefore, when this conventional method is applied to a long-distance multi-repeater transmission method, the monitoring frequency
It becomes impossible to obtain a large number of f _SVs , and there is a limit to the number of relay sections that can be monitored.

In addition, in the invention of the earlier application, information inside the repeater,
For example, it is not possible to remotely monitor analog information such as the bias voltage of an electro-optical converter and the operating voltage of an amplifier, and if this is to be monitored, it is necessary to provide a separate monitoring system for this purpose.

The present invention solves these problems and is a monitoring method that can be applied to long-distance multi-repeater transmission lines, which can increase the number of repeater sections that can be monitored, and which can be implemented in repeaters. The filter is small and highly reliable, and filters of the same standard can be used for each repeater, making it possible to remotely monitor the internal operating status of the repeater.This is a digital repeater monitoring method. The purpose is to provide

[Means to solve the problem]

In the present invention, the monitoring frequency f _SV is commonly used by a plurality of repeaters, and the mark rate (excluding 1/2) of the signal sent from the terminal station to the relay transmission path is set using the monitoring frequency f _SV.
At the same time, codes "1" and "0" are transmitted depending on the length of the signal mark rate fluctuation duration, and depending on the combination of the length and shortness of this mark rate modulation duration, repeater selection and internal information of the selected repeater are determined. It is characterized by controlling information selection.

That is, the first aspect of the present invention is that at least one of the pair of terminal stations is provided with means for dividing the main signal clock to generate a monitoring frequency, and a first constant mark rate. a constant mark rate pattern generating means for generating a pulse train of; a first constant mark rate pulse train generated by the mark rate pattern generating means; and a second pulse train of a second mark rate different from the first constant mark rate; A means for performing mark rate modulation of the main signal by alternately generating at the cycle of the monitoring frequency mentioned above, and a pulse train of two mark rates that is generated alternately at the cycle of the monitoring frequency generated by this means for performing mark rate modulation. means for superimposing and transmitting the binary digital code on the main signal as a combination of two types of mark rate modulation durations, long and short, based on the combination of the binary digital code corresponding to the repeater, The method includes an extraction means for extracting the monitoring frequency component from a received signal including the monitoring frequency spectrum by mark rate modulation, and a binary value by identifying the length of the mark rate modulation duration of the signal extracted by this extraction means. an identification means for decoding into a digital code and identifying from a combination of the decoded binary digital codes whether or not the repeater has been selected; and when the identification means identifies that the repeater has been selected; means for conducting a gate circuit that connects up and down signal paths; and means for generating a PN pattern signal in the one terminal station; The present invention is characterized by comprising means for sending a pattern signal to the loop transmission path, and means for measuring the code error rate of the loop transmission path based on the received PN pattern signal.

In addition to the configuration of the first invention, a second invention of the present invention provides that one terminal station receives a combination of binary digital codes corresponding to control information given to a selected repeater. The repeater is equipped with means for transmitting this binary digital code by superimposing it on the main signal as a combination of two types of mark rate modulation durations, long and short, and the repeater is equipped with a means for transmitting the binary digital code by superimposing it on the main signal as a combination of two types of mark rate modulation durations, long and short. Extracting means for extracting a component of the monitoring frequency; identifying the length of the mark rate modulation duration of the signal extracted by this extracting means, decoding it into a binary digital code, and relaying it by a combination of the decoded binary digital codes. identification means for identifying whether the repeater has been selected and the control information; and means for conducting a gate circuit connecting the up and down signal paths when the identification means identifies that the repeater has been selected. , means for selecting a monitored DC voltage in the repeater based on the control information identified by the identifying means; means for generating a signal with a frequency corresponding to the selected monitored DC voltage; After forming a return transmission line by connecting the transmission line, the main signal of the first constant mark rate (excluding the 1/2 mark rate) newly sent from the first terminal station is transferred to the monitored DC. means for modulating the mark rate at a frequency corresponding to the voltage, and further, the first terminal station receives a frequency spectrum from the received signal that has been modulated by the repeater and includes a frequency spectrum corresponding to the monitored DC voltage. It is characterized by comprising means for detecting.

The control signal is composed of a plurality of consecutive binary signals, and there are two types of duration: long and short. and means for controlling the repeater based on the code.

〔Example〕

FIG. 1 is a block diagram of a main part of a method according to an embodiment of the present invention. Among the devices classified by the dashed line, SV is a supervisory control device, and TR is a terminal relay device, which are located at the terminal station. RP is a repeater, and cable L is connected to both ends of the repeater. The cable L at one end is connected to the terminal relay device TR. Although only one relay section is shown in this figure, a large number of cables L and repeaters RP are alternately connected, and this relay transmission path is installed to another pair of terminal stations located far away.

In this embodiment system, the communication speed of the signal sent to the cable L is approximately 400 Mb/s.

The supervisory control device includes a frequency divider 2 that receives a clock signal from a transmission clock signal source 1 and generates a supervisory frequency _fSV , a pattern generator 3 that generates a mark rate modulation pattern, and a test signal PN pattern. A pattern generator 4 that generates a pattern is provided. The outputs of the frequency divider 2 and the pattern generator 3 are connected to the inputs of an exclusive OR circuit 6, and the output of the exclusive OR circuit 6 is connected to an AND circuit 7. The output of the pattern generator 3 is connected to an AND circuit 8, the output of the pattern generator 4 is connected to an AND circuit 9, and the AND circuits 7, 8, and 9 are configured to receive control signals from the control section 5. The outputs of the AND circuits 7, 8 and 9 are connected from the OR circuit 10 to the input of the transmitting circuit 11 of the terminal relay device. The output of the OR circuit 10 is the output of the AND circuit 7 when instructing the repeater to perform monitoring control, the output of the AND circuit 8 is used when monitoring the repeater monitoring information, and the output of the AND circuit 8 is used when measuring the bit error rate. The output of the AND circuit 9 is selected as necessary under the control of the control section 5. The transmission circuit 11 is a known optical signal transmission circuit that is supplied with a clock signal from the transmission clock signal source 1, converts the transmission signal into an optical signal, and transmits the optical signal to an optical fiber cable.

The output optical signal of this transmitting circuit 11 is sent out to an optical fiber cable 12, reaches a repeater, and is input to a receiving circuit 20. The receiving circuit 20 is a known receiving circuit of a regenerative repeater including a photoelectric converter, an equalization circuit, an identification circuit, a waveform shaping circuit, and the like. The output signal of this receiving circuit 20 is transmitted from the transmitting circuit 21 to the next section of cable. The transmitting circuit 21 includes an electro-optic converter.

The repeater is equipped with a similar circuit in the opposite downward direction, and the signal from the previous section is received and regenerated by the receiving circuit 22, and the signal is transmitted from the transmitting circuit 23 to the optical fiber cable 13.
Send to. The signal of this optical fiber cable 13 is input to the receiving circuit 14 of the terminal relay device. This receiving circuit 14 is a known regenerative receiving repeating circuit including a photoelectric converter, an equalization circuit, an identification circuit, a waveform shaping circuit, and the like.

The output signal of the receiving circuit 14 is connected to the inputs of AND circuits 15 and 16. A signal is connected to the AND circuits 15 and 16 from the control unit 5, the output of the AND circuit 15 is connected to the PN pattern receiving circuit 18, and the output signal of the AND circuit 16 is connected to the bandpass filter 17.
The clock is input to the frequency counter 19 via the PN pattern receiving circuit 18. The output (not shown) of the PN pattern receiving circuit 18 is displayed as a bit error rate. An output (not shown) of the frequency counter 19 is displayed on a monitoring monitor.

To explain the monitoring circuit provided in the repeater, the output signal of the receiving circuit 20 is branched and input to the bandpass filter 24. This output signal is the detection circuit 2
5, and its detection signal is connected to the input of the decoding circuit 26. The output of this decoding circuit 26 is input to an AND circuit 27, a conversion circuit 29, and an AND circuit 30. The conversion circuit 29 inputs analog signals such as bias voltages detected at various parts of the repeater, selects one of these analog signals by a switch circuit 41 and applies it to a voltage controlled oscillator 42, and outputs the oscillator output to an AND circuit 43. It is configured to be sent from AND circuit 27 and AND circuit 30
is a gate circuit for returning uplink signals to the downlink or returning downlink signals to the uplink, and the modulation circuit 28 is connected to the input of the AND circuit 27.
A modulation circuit 31 is provided at the input of the AND circuit 30. The output signals of the AND circuit 43 are connected to the modulation circuits 28 and 31, respectively.

The modulation circuit 28 branches the input signal and supplies it to the AND circuit 46 together with the output signal of the conversion circuit 29 via a delay circuit 45 which delays the input signal by a delay time τ (time equivalent to 1 bit of the main signal). 46 is combined with an input signal in an OR circuit 47 to produce an output signal. In the modulation circuit 28, a delay circuit 45 and an OR circuit 47 constitute a mark rate conversion circuit. For example, when the mark rate of an input signal is 1/4, the output is set to a mark rate of 1/2. When an AND circuit 46 is added to this circuit, the AND circuit 46 is turned on by the output of the voltage frequency conversion circuit 29 (for example, the frequency is _fV ).
It will be turned off. Therefore, when the AND circuit 46 is on, the modulation circuit outputs a mark rate of 1/2,
When the AND circuit 46 is off, the modulation circuit outputs a mark rate of 1/4. Therefore, the frequency f _V
Mark rate modulation is performed at , and as a result, the spectrum of frequency f _V is superimposed on the main signal component folded back in the downward direction. The same applies to the modulation circuit 31.

FIG. 2 is a block diagram of the decoding circuit 28. The input signal is passed through the waveform shaping circuit 51 to the register circuit 5.
Enter 3. A clock signal input to this register circuit 53 is given from a monostable multivibrator 52 driven by the output signal of the waveform shaping circuit 51. This register circuit 53 has 12 registers in this example.
It consists of bits, the most significant bit and the least significant bit are guard bits, and of the 10 bits in between, the upper 5 bits are for identifying the repeater (address),
The lower 5 bits are for identifying information selection (command)
It is. The upper five bits are connected to the input of the AND circuit 54 in a bit combination unique to that repeater. The lower 5 bits are each combined according to information,
Connected to the inputs of AND circuits 55 and 56. AND circuit 5 is connected to the input of AND circuits 55 and 56.
Connect the output of 4. Latch circuits 57, 58 and 59 are connected to the outputs of AND circuit 54, AND circuits 55 and 56, respectively.

The operation of the device configured in this way will be explained.
In this system, when monitoring is performed, the transmission of normal information signals on the relay transmission path is stopped and only the monitoring signal is transmitted. In other words, signal transmission to the downstream transmission path of the opposite end station is stopped, and does not affect the monitoring operation.

The pattern generator 3 generates a pulse train signal with a mark rate m ₁ (m≠1/2). Since the frequency divider 2 generates the monitoring frequency _fSV , the mark rates are m ₁ and m ₂ (m ₂ = 1 - m ₁ ) at the output of the exclusive OR circuit 6.
A signal is obtained that alternately changes at the monitoring frequency _fSV . That is, the main signal is subjected to mark rate modulation using the monitoring frequency _fSV , and a main signal including the monitoring frequency spectrum can be generated.

The manner in which this mark rate modulation is performed will be explained with reference to FIGS. 4A and 4B.

The exclusive OR circuit 6 receives a monitoring signal g with a monitoring frequency _fSV from the frequency divider 2, and a pulse train signal h with a mark rate _m1 from the pattern generator 3. Since these two signals g and h are exclusive ORed, pulse train signals with a mark rate m ₁ and a mark rate m ₂ are output alternately every half cycle of the monitoring signal, as shown in Figure 4B. .

The control section 5 first sends control signals to the AND circuits 7 and 8, opens and closes the AND circuit 7 using long and short signals, and determines τ ₁ and τ ₂ shown in a of FIG. 3. By opening and closing the AND circuit 8 with this reverse phase signal, m ₁ is sent during the period neither τ ₁ nor τ ₂ in FIG. 3a, thereby preventing the transmission signal from being disconnected. That is, when AND circuit 7 is conductive, AND circuit 8 is closed, and when AND circuit 7 is closed, AND circuit 8 is conductive. This situation is shown in Figure 3 Sc. The mark rate modulation duration of the long signal is τ ₁ , and the mark rate modulation duration of the short signal is τ ₂ . The transmission repetition period of this control signal is τ, and the code of a long signal is "1" and the code of a short signal is "0". In the example shown in FIG. 3, "10101..." is transmitted.

At the output of the transmitting circuit 11, the signal structure is as shown in FIG. 3a. That is, when the AND circuit 7 is conductive, the mark rate changes alternately to m ₁ and m ₂ at the monitoring frequency _fSV , and while the AND circuit 7 is closed, a signal with a constant mark rate m ₁ is transmitted from the AND circuit 8. be done. In this embodiment method, the speed of the transmission signal is approximately
400 Mb/s, and the monitoring frequency f _SV is approximately 10 MHz.

During mark rate modulation (times τ ₁ and τ ₂ in Figure 3)
Only, since the f _SV spectrum is included in the main signal,
This monitoring frequency _fSV appears at the output of the bandpass filter 24 of the repeater, and its signal waveform is as shown in FIG. 3b. This signal is detected by the detection circuit 25, and its output signal is as shown in FIG. 3c. This signal is sequentially input to the decoding circuit 26.

In the decoding circuit 26, the input signal is shaped by the waveform shaping circuit 51 shown in FIG. 2 as shown in FIG. 3d, and together with the reproduced clock signal shown in FIG. 3e,
It is input to the register circuit 53. Register circuit 53
The shift operation is as shown in Fig. 3 f, and the continuous
The 12 bits are sequentially stored in the register circuit 53.

In this way, the execution sequence of supervisory control is to specify the repeater (upper 5 bits of the address signal), conduct the up and down signals of the designated repeater, and specify command information (lower 5 bits of the command signal) by transmitting a 12-bit code. is executed, followed by execution of the specified command.

First, an example of specifying a repeater address will be explained.

If the upper 5 bits of the signal input to the AND circuit 54 match the combination set for this repeater, this means that this repeater has been selected, and an output signal is sent to the AND circuit 54 to lock the latch. Circuit 57 is set. This latch circuit 5
7 conducts the AND circuit 27 of the repeater, and the upstream signal passes through the OR circuit 47 and the AND circuit 27, is connected to the downstream transmission path, and is transmitted via this repeater. , a loop transmission path from one terminal station is constructed. When the supervisory control is executed, the transmission of the signals flowing through the downlink transmission path has already been stopped at the opposite end station, and no signal mixing occurs.

The case where the bit error rate is measured in this state will be explained.

The control unit 5 of the terminal station monitoring and control device is an AND circuit 7
and 8 are both closed, AND circuit 9 is made conductive, and a PN pattern signal is transmitted from PN pattern generator 4 in order to measure the bit error rate. At this time, the control unit 5 makes the AND circuit 15 conductive, and a predetermined PN pattern receiving circuit 18 receives the PN pattern signal returned by the repeater. By measuring the error rate of the received signal with this PN pattern receiving circuit 18, it is possible to know the bit error rate of the signal transmission path going back and forth from the terminal station to the repeater.

Next, the case of collecting information inside the repeater will be explained. Here, we will describe the state in which a loop is configured with a repeater, starting with the specification of command information.

When a predetermined sign is given to the contents of the lower 5 bits of the register circuit 53, the AND circuit 55 or 56 becomes conductive depending on the sign, and the latch circuit 58 or 59 is set.
The switch circuit 41 of the repeater is caused by the latch circuit.
Select one of the analog input signals. As a result, a signal with a frequency f _V corresponding to the selected analog signal is sent out from the voltage controlled oscillator 42,
The AND circuit 46 is opened and closed at this frequency. At this time, the control unit 5 of the terminal station makes the AND circuit 8 conductive and sends a signal with a constant mark rate (except for the mark rate 1/2).
Send. A constant mark rate signal (for example, 1/4) transmitted from the terminal station is modulated by the modulation circuit 28 of the repeater at the frequency (period) of the voltage controlled oscillator 42, so that, for example, the mark rate 1/4 and 1/2 are periodically changed. It is then sent back to the terminal station via the downlink transmission path.

This operation will be explained using FIGS. 5A and 5B. A main signal with a mark rate of 1/4 is input to the modulation circuit 28. The signal j is given a delay of τ (corresponding to 1 bit of j) by the delay circuit 45 and becomes a signal k. Since a signal with a period f _V inputted from the conversion circuit 29 is given, the AND circuit 46 switches the delayed signal k at the frequency f _V and outputs the signal o. In the OR circuit, the main signal j and signal o are combined, so a signal with a mark rate of 1/2 and a signal with a mark rate of 1/4
The signals from the voltage controlled oscillator 42 are modulated and a signal that appears alternately every half period of the signal from the voltage controlled oscillator 42 is output.

At this time, the control unit 5 conducts the AND circuit 16 and guides the received signal including the frequency spectrum modulated by the mark rate by the repeater to the frequency counter 19, and measures the frequency to determine the frequency of the voltage controlled oscillator 42. In other words, the voltage of the analog input can be known.

In this way, it is possible to turn back the relay transmission path at the selected repeater, measure the code error rate of the turn-back loop, and monitor information inside the repeater at the turn-back point at the terminal station.

Although the above example uses an optical repeater system, the present invention can also be implemented using a digital repeater using electrical signals.

In the above example, one type of monitoring frequency f _SV is used, but
By making this into a small number of plurality, repeaters to be selected can be grouped according to the monitoring frequency _fSV .

〔Effect of the invention〕

As explained above, according to the present invention, the repeater to be selected is determined by the sign of the mark rate modulation signal duration time, so there is no limit in principle to the number of repeaters that can be selected, and the number of repeaters that can be selected is not limited in principle. It can be applied to a large number of long-distance, multi-reply transmission lines. There is no need to change the frequency of the filter mounted on the repeater for each repeater, and only one type or a small number of types are required, so the optimal frequency can be selected, so use one that is small and highly reliable. be able to. Further, according to the present invention, there is an excellent effect that not only the bit error rate of the loop-back connection section but also the operating status of various circuits inside the repeater at the loop-back point can be monitored from a distance.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention. FIG. 2 is a block diagram of the decoding circuit. FIG. 3 is a signal waveform diagram of each part for explaining the operation. Figure 4 A and B are exclusive OR circuits 6
FIG. 3 is a diagram illustrating mark rate modulation in . 5A and 5B are diagrams illustrating mark rate modulation in the modulation circuit 28. FIG. 1... Clock signal source, 2... Frequency divider, 3...
Mark rate m ₁ pattern generator, 4...PN signal pattern generator, 5...control unit, 6...exclusive OR circuit, 11...transmission circuit, 14...reception circuit, 17...bandwidth Filter, 18...PN pattern receiving circuit, 19...Frequency counter, 20, 22
...Reception circuit, 21, 23...Transmission circuit, 24...
... bandpass filter, 25 ... detection circuit, 26 ... decoding circuit, 28, 31 ... modulation circuit, 41 ... switch circuit, 42 ... voltage controlled oscillator, 45 ... delay circuit, 53 ... register circuit, 57, 58, 59
...Latch circuit.

Claims (1)

[Claims] 1. Includes a pair of terminal stations, a cable provided between the terminal stations, and a digital regenerative repeater inserted into the cable and relaying bidirectional signals transmitted to the cable. , In a method for monitoring a repeater of a relay transmission path, the method includes means for modulating and transmitting a main signal with a monitoring signal to at least one of the pair of terminal stations, wherein the at least one terminal station has: Divide the main signal clock to obtain the monitoring frequency ( _fSV )
means (2) for generating a signal of a first constant mark rate (m ₁ but not 1/2);
a constant mark rate pattern generating means (3) for generating a pattern signal of; this first constant mark rate and a second constant mark rate other than this constant mark rate (m ₂ but not 1/2); means (6) for generating a mark rate modulated signal that appears alternately every half cycle of the signal at the monitoring _frequency ; The control means includes switching means (7, 8) for selecting one of the switching means, and a control means (5) for controlling the switching means based on a combination of binary digital codes determined by the selected repeater. , two types of mark rate modulation durations, long and short, are set to select the mark rate modulated signal corresponding to the binary digital code, and a binary digital signal is created by a combination of the two types of mark rate modulation durations, long and short. The repeater includes means for controlling to generate a combination of codes, and the repeater includes an extraction means for extracting a component of the monitoring frequency from a received signal including a monitoring frequency spectrum, and a mark rate of the signal extracted by the extraction means. identification means for identifying the length of modulation duration and decoding it into a binary digital code, and identifying whether or not the repeater has been selected from a combination of the decoded binary digital codes; means for conducting a gate circuit connecting the up and down signal paths when it is determined that the signal path has been selected; and means for generating a PN pattern signal in the one terminal station; The method further comprises means for transmitting the PN pattern signal to the loop transmission path after forming a loop transmission path for uplink and downlink signals, and means for measuring the bit error rate of the loop transmission path based on the received PN pattern signal. Features a digital repeater monitoring method. 2 A pair of terminal stations, including a cable provided between the terminal stations, and a digital regenerative repeater inserted into the cable and relaying bidirectional signals transmitted to the cable, respectively; In a method for monitoring a repeater of a relay transmission line, in which at least one of the terminal stations is equipped with a means for modulating and transmitting the main signal with a monitoring signal, the at least one terminal station has a clock for the main signal. Frequency monitoring (f _SV )
means (2) for generating a signal of a first constant mark rate (m ₁ but not 1/2);
a constant mark rate pattern generating means (3) for generating a pattern signal of; this first constant mark rate and a second constant mark rate other than this constant mark rate (m ₂ but not 1/2); means (6) for generating a mark rate modulated signal that appears alternately every half cycle of the signal at the monitoring _frequency ; switching means (7, 8) for selecting one of the switching means; and control means (5) for controlling the switching means based on a combination of binary digital codes determined by the selected repeater and the applied control signal. The control means sets two types of mark rate modulation durations, long and short, during which the mark rate modulated signal passes in correspondence with the binary digital code, and controls the duration of the two types of mark rate modulation durations, long and short. The repeater includes an extraction means for extracting a component of the monitoring frequency from a received signal including the monitoring frequency spectrum; Identifying the length of the mark rate modulation duration of the extracted signal and decoding it into a binary digital code, and identifying whether the repeater has been selected or not and the above control information based on the combination of the decoded binary digital codes. means for conducting a gate circuit connecting up and down signal paths when the identifying means identifies that the repeater has been selected; After forming a return transmission line by connecting the up and down transmission lines, a means for selecting a DC voltage to be monitored from among the DC voltages to be monitored, a means for generating a signal with a frequency corresponding to the selected DC voltage to be monitored, and the above-mentioned up and down transmission lines. means for modulating the mark rate of the main signal of the first constant mark rate (excluding the 1/2 mark rate) newly transmitted from the terminal station at a frequency corresponding to the monitored DC voltage, Further, the first terminal station is equipped with a means for detecting a frequency from a received signal containing a frequency spectrum corresponding to the monitored DC voltage that has been subjected to mark rate modulation in the repeater. monitoring method.