Delay adjustment device and delay adjustment method

Abstract

A delay adjustment device according to one embodiment adjusts a delay for a communication system comprising an active system transmission device that transmits a signal in an active system line connecting a transmission device and a reception device, and a redundant system transmission device that transmits a signal in a redundant system line connecting the transmission device and the reception device. The delay adjustment device comprises: a measurement unit that measures an active system delay caused by the active system transmission device without instantaneous interruption of the active system line; an adjustment unit that adjusts a delay in the redundant system line so as to eliminate a difference between the active system delay measured by the measurement unit and a redundant system delay caused by the redundant system transmission device; and an adjustment amount control unit that, when the active system line has been switched to the redundant system line, performs control so as to gradually decrease an adjustment amount in the delay adjusted by the adjustment unit while making a change amount of the delay per unit time less than a predetermined value.

Description

Delay adjustment device and delay adjustment method

[0001] The present invention relates to a delay adjustment device and a delay adjustment method used when switching from a working line to a redundant line.

[0002] When replacing transmission equipment used in a redundant line with new equipment, it is necessary to prevent momentary line interruptions based on factors such as the buffer length of the line aggregation device that aggregates the working and redundant lines and the delay tolerance between the lines.

[0003] For example, each line is replaced with a device having the same performance as the conventional transmission device so that the delay difference between the working line and the redundant line is less than the delay tolerance between the lines.

[0004] Furthermore, when an optical fiber is used for a line, a technique is known in which leakage light is observed to grasp the usage status of the optical fiber in order to prevent the optical fiber from being accidentally cut (see, for example, Non-Patent Document 1).

[0005] Hiroyuki Iida, Takui Uematsu, Kazutaka Noto, Hidenobu Hirota, and Kenji Inoue, "Local Light Injection and Detection Technology for Optical-fiber Identification", [online], NTT Technical Review, Vol. 19, No. 8, pp. 46-51, Aug. 2021, [Retrieved July 5, 2020], Internet <URL: https: / / doi.org / 10.53829 / ntr202108ra2>

[0006] However, in the past, even if the new equipment was faster and had lower latency than the transmission equipment being replaced, in order to prevent the delay time difference between the lines from widening, it was necessary to deliberately set the performance of the new equipment to a lower level so that the delay difference between the working and redundant lines was less than the delay tolerance between the lines. In this case, even if all lines could be replaced, the high-performance new equipment would continue to be used with a lower-performance setting.

[0007] Furthermore, even if the performance of the transmission equipment to be replaced is the same as that of the new replacement equipment, if the true delay time during current communication is unknown, the delay difference between the systems may become larger than the allowable range during replacement, and there is a risk of line disconnection occurring when switching from the current line to the redundant line.

[0008] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a delay adjustment device and a delay adjustment method that make it possible to switch from a working line to a redundant line and to renew transmission equipment within the line without causing an instantaneous line interruption.

[0009] A delay adjustment device according to one embodiment of the present invention adjusts delay for a communication system including a working transmission device that transmits a signal within a working line connecting a transmitting device and a receiving device, and a redundant transmission device that transmits a signal within a redundant line connecting the transmitting device and the receiving device, and is characterized by comprising: a measurement unit that measures a working delay due to the working transmission device without causing an instantaneous interruption of the working line; an adjustment unit that adjusts the delay in the redundant line so as to eliminate a difference between the working delay measured by the measurement unit and the redundant delay due to the redundant transmission device; and an adjustment amount control unit that, when the working line is switched to the redundant line, controls the adjustment amount in the delay adjusted by the adjustment unit to gradually decrease while keeping the amount of change in delay per unit time below a predetermined value.

[0010] Furthermore, a delay adjustment method according to one embodiment of the present invention is a delay adjustment method for adjusting delay in a communication system including a working transmission device that transmits a signal in a working line connecting a transmitting device and a receiving device, and a redundant transmission device that transmits a signal in a redundant line connecting the transmitting device and the receiving device, the delay adjustment method comprising: a measurement step of measuring a working delay by the working transmission device without causing an instantaneous interruption of the working line; an adjustment step of adjusting the delay in the redundant line so as to eliminate a difference between the working delay measured in the measurement step and the redundant line delay by the redundant transmission device; and an adjustment amount control step of controlling, when the working line is switched to the redundant line, to gradually reduce the adjustment amount in the delay adjusted in the adjustment step while keeping the amount of change in delay per unit time below a predetermined value.

[0011] According to the present invention, it is possible to switch from a working line to a redundant line without causing a momentary line interruption, and to renew transmission equipment within the line.

[0012] 1A is a diagram illustrating an example of the configuration of a communication system before updating a transmission device that transmits a signal in a redundant line. FIG. 1B is a diagram illustrating an example of the configuration of a communication system after updating a transmission device that transmits a signal in a redundant line. FIG. 1C is a diagram illustrating an example of the configuration when a delay adjustment device according to an embodiment is used in a communication system. FIG. 1D is a diagram illustrating an example of the configuration of a measurement unit. FIG. 1E is a diagram illustrating an example of the configuration of an analysis delay unit. FIG. 1F is a diagram illustrating an example of the configuration when a delay adjustment device is used when updating a new redundant transmission device to a new transmission device. FIG. 1G is a flowchart illustrating a first procedure for updating a transmission device in a line of a communication system using a delay adjustment device according to an embodiment. FIG. 1H is a flowchart illustrating a second procedure for updating a transmission device in a line of a communication system using a delay adjustment device according to an embodiment. FIG. 1I is a diagram illustrating an example of the hardware configuration of a delay adjustment device according to an embodiment.

[0013] A communication system in which a delay adjustment device according to an embodiment is used to switch a working line to a redundant line and to renew transmission equipment within the line will be described below with reference to the drawings.

[0014] 1A and 1B are diagrams illustrating an example of the configuration of a communication system 1 in which transmission equipment within a line is to be replaced. Fig. 1A is a diagram illustrating an example of the configuration of the communication system 1 before replacing transmission equipment that transmits signals within a redundant line. Fig. 1B is a diagram illustrating an example of the configuration of the communication system 1 after replacing transmission equipment that transmits signals within a redundant line. The communication system 1 transmits, for example, optical signals.

[0015] As shown in Figure 1(a), in a communication system 1 before the transmission equipment is replaced, an aggregation device 2 serving as a transmitting device and an aggregation device 3 serving as a receiving device are connected by a working line 4 and a redundant (standby) line 5 constructed using, for example, optical fiber.

[0016] In addition, in the communication system 1 before the transmission equipment is replaced, for example, two transmission equipment 10 are provided on the working line 4, and for example, two transmission equipment 10 are also provided on the redundant line 5. The transmission equipment 10 transmits signals with a predetermined delay. Note that any number of transmission equipment 10 may be provided on each of the working line 4 and the redundant line 5.

[0017] The delay difference between the redundant line 5 and the working line 4 is absorbed by the function (e.g., buffer length) of the aggregation device 3, making it possible to switch from the working line 4. However, each of the transmission devices 10 included in the redundant line 5 is replaced with a new transmission device in the event of a device failure or as a preventive replacement.

[0018] In the case of equipment failure or preventive replacement, conventionally, the transmission device 10 provided in the redundant line 5 has been updated so that the performance is the same. However, since the delay difference between the working line 4 and the redundant line 5 after updating is not necessarily the same as the delay difference before updating, conventionally, there was a possibility that a line disconnection would occur when switching between the working line 4 and the redundant line 5.

[0019] 1B, in the communication system 1, for example, it is assumed that two transmission devices 10 in the redundant line 5 are each replaced with a new transmission device 20. The new transmission device 20 transmits signals at a higher speed and with less delay than the transmission device 10.

[0020] In such a case, a delay adjustment device 6 according to an embodiment is used. Fig. 2 is a diagram showing a configuration example in which the delay adjustment device 6 according to an embodiment is used in a communication system 1. In this example, two transmission devices 10 provided in a redundant line 5 are replaced with two new transmission devices 20.

[0021] The delay adjustment device 6 includes, for example, a measurement unit 60 and an analysis delay unit 62, and adjusts the line delay.

[0022] The measurement unit 60 measures the working system delay caused by the transmission device 10 provided on the working system line 4 without momentarily interrupting the working system line 4. The analysis delay unit 62 has a function of adjusting the delay for the redundant system line 5 so as to eliminate the difference between the working system delay measured by the measurement unit 60 and the redundant system delay on the redundant system line 5.

[0023] Fig. 3 is a diagram showing an example of the configuration of the measurement unit 60. As shown in Fig. 3, the measurement unit 60 clamps, for example, an optical fiber A used in the working line 4 and measures the optical signal passing through the optical fiber A, so that the working line 4 is not disconnected.

[0024] Specifically, the measuring unit 60 passes an optical fiber A between the convex block 600 and the concave block 602, detects leakage light from the optical fiber A using a probe 604, and outputs the detected light to the signal analyzing unit 70 described later.

[0025] Fig. 4 is a diagram showing an example of the configuration of the analysis delay unit 62. As shown in Fig. 4, the analysis delay unit 62 includes, for example, a signal analysis unit 70, a comparison unit 71, an adjustment control unit 72, a storage unit 73, a delay unit (adjustment unit) 74, and a transmission control unit 75.

[0026] The signal analysis unit 70 has, for example, two light receiving elements 700 and a processing unit 702. Each light receiving element 700 receives the leaked light detected by the probe 604, performs photoelectric conversion, and outputs the signal to the processing unit 702. The processing unit 702 analyzes the signal (frame) detected by the probe 604 and photoelectrically converted by the light receiving element 700, and performs processing to measure the delay of the signal transmitted through the working line 4.

[0027] The comparator 71 compares the redundant delay in the redundant line 5 with the working delay in the working line 4 analyzed by the signal analyzer 70 , and outputs the comparison result (delay difference) to the adjustment controller 72 .

[0028] The adjustment amount control unit 72 uses data stored in the storage unit 73 and the like to control the amount of delay (adjustment amount) of delay unit (adjustment unit) 74 that adjusts the delay for the redundant line 5 .

[0029] The storage unit 73 stores the amount of delay (adjustment amount) controlled by the adjustment amount control unit 72, etc.

[0030] The delay unit 74 adjusts the delay in the redundant line 5 in accordance with the control of the adjustment amount control unit 72 so as to eliminate the difference between the current line delay measured by the measurement unit 60 and the redundant line delay due to the new transmission device 20 or the like (to make the delay difference zero). For example, when the new transmission device 20 transmits signals at a higher speed and with a lower delay than the transmission device 10, the delay unit 74 is a delay buffer circuit or the like that adjusts the delay in the redundant line 5 to be larger (adds a delay amount).

[0031] The transmission control unit 75 performs line output with a delay caused by the delay unit 74 .

[0032] In this way, the delay adjustment device 6 adjusts the delay in the redundant line 5 so as to eliminate the difference between the current system delay and the redundant system delay (to make the delay difference zero), so it is possible to switch the current system line 4 to the redundant line 5 and renew the transmission equipment within the line without causing a momentary interruption to the line in the communication system 1.

[0033] In addition, after the current line 4 is switched to the redundant line 5 in the communication system 1, the adjustment control unit 72 controls the delay unit 74 to gradually reduce the adjustment amount in the delay adjusted while keeping the amount of change in delay per unit time below a predetermined value (e.g., 1 μsec / day).

[0034] For example, when the delay unit 74 adjusts (adds a delay amount) to increase the delay in the redundant line 5, the adjustment control unit 72 controls so that the delay unit 74 gradually decreases (decreases the delay amount) the increased delay.

[0035] The amount of delay reduction per unit time is set to be smaller than the allowable delay drift amount in the communication system 1 .

[0036] In other words, the adjustment control unit 72 has two operations and functions: a delay tracking mode that controls the delay amount in the redundant line 5 to follow the delay amount in the working line 4, and a mode that reduces the delay adjustment amount for the redundant line 5 (delay amount reduction mode if a delay amount is added).

[0037] Therefore, after the transmission device 10 in the redundant line 5 is replaced with a new high-speed, low-latency transmission device 20, the communication system 1 can be used without degrading the performance of the new transmission device 20.

[0038] 5 is a diagram showing an example of a configuration in which the redundant line 5 in which the transmission device 10 has been replaced with the new transmission device 20 is regarded as a new active system, and the active line 4 is regarded as a new redundant system (standby system), and a delay adjustment device 6 is used when replacing the new redundant system transmission device 10 with the new transmission device 20. Note that the communication system 1 may be provided with a plurality of redundant systems, a plurality of analysis delay units 62, or a plurality of delay adjustment devices 6, and may be configured such that the measurement unit 60 measures the delay of the active system and the analysis delay unit 62 adjusts the delay of the redundant system.

[0039] In the configuration example shown in Figure 5, as in the case explained using Figure 2, in the delay adjustment device 6, the measurement unit 60 measures the delay of the new active system, and the analysis delay unit 62 adjusts the delay of the new redundant system so as to eliminate the delay difference between the new active system and the new redundant system.

[0040] Then, after the new active system is switched to the new redundant system, the adjustment amount control unit 72 performs control so as to gradually reduce the adjustment amount in the delay adjusted by the delay unit 74 while keeping the amount of change in delay per unit time below a predetermined value.

[0041] When the delay amount adjusted by the delay unit 74 becomes 0, the analysis delay unit 62 (or the delay adjustment device 6) can be removed while switching between the active system and the redundant system. At this time, the communication system 1 can be used without degrading the performance of the new transmission device 20.

[0042] Next, a procedure for switching the working line to the redundant line and updating the transmission equipment within the line using the delay adjustment device 6 without causing a momentary interruption of the line of the communication system 1 will be described.

[0043] 6 is a flowchart showing a first procedure for updating a transmission device in a line of the communication system 1 using the delay adjustment device 6. Here, as shown in FIG. 2, a measurement unit 60 is used for the current line of the communication system 1, and normal operation of the current line is confirmed. Also, for the redundant line, while the transmission device 10 is updated to the new transmission device 20 and is stopped, an analysis delay unit 62 is inserted and operates in a delay tracking mode.

[0044] In step 100 (S100), the transmission control unit 75 (see FIG. 4) included in the analysis delay unit 62 (see FIG. 2) stops the transmission of the signal.

[0045] In step 102 (S102), the adjustment amount control unit 72 initializes the delay amount stored in the storage unit 73 to zero.

[0046] In step 104 (S104), the new transmission device 20 in the redundant line 5 starts transmitting a signal. At this time, the transmission control unit 75 has stopped signal transmission, so no signal is transmitted to the aggregation device 3.

[0047] In step 106 (S106), the comparator 71 receives and compares the data of the active system and the redundant system (the active system delay and the redundant system delay). For example, it is assumed that the data of the active system and the redundant system are as follows:

[0048] Active system: 1234A1234B1234C12 Redundant system: A1234B1234C1234D1

[0049] In other words, by analyzing and comparing the frames of the working system and the redundant system, it is possible to measure the delay difference. In this case, the redundant system data is transmitted four orders of magnitude earlier than the working system data.

[0050] In step 108 (S108), the comparator 71 calculates the delay amount (delay difference) between the active system delay and the redundant system delay. In the above example of the active system and redundant system data, the redundant system data is four orders of magnitude faster than the active system data, so the comparator 71 calculates the delay time based on the transmission speed as follows:

[0051] In other words, the difference in delay time between the active system and the redundant system here corresponds to 4 / (number of characters transmitted per second).

[0052] In step 110 (S110), the adjustment amount control unit 72 writes the delay amount (delay difference) into the storage unit 73.

[0053] In step 112 (S112), the adjustment amount control unit 72 sets the delay amount (delay difference) in the delay unit 74 based on the delay amount (delay difference) stored in the storage unit 73.

[0054] In step 114 (S114), the adjustment control unit 72 determines whether the delay amount of the active system relative to the redundant system is 0 or not, and if the delay amount is not 0 (S114: No), returns to processing of S106, and if the delay amount is 0 (S114: Yes), proceeds to processing of S116.

[0055] In step 116 (S116), the adjustment control unit 72 determines whether the delay amount of the current system relative to the redundant system is 0 and stable, and if it is not stable (S116: No), returns to processing of S106, and if it is stable (S116: Yes), proceeds to processing of S118.

[0056] In step 118 (S118), the adjustment amount control unit 72 prohibits the writing of the delay amount (delay difference) to the storage unit 73.

[0057] In step 120 (S120), the adjustment amount control section 72 controls the line delay so that the amount of delay by the delay section 74 is fixed to the current amount of delay.

[0058] At step 122 (S122), the transmission control unit 75 starts transmitting a signal to the aggregation device 3 for the first time at this point. At this time, the amount of delay in the redundant system for the aggregation device 3 is the same as the amount of delay in the working system.

[0059] Furthermore, after switching from the current system to the redundant system, the redundant line 5 can be regarded as the new current system and the current line 4 as the new redundant system, and delay adjustment can be performed using the same procedure, thereby updating all transmission devices 10 on the current line 4 and the redundant line 5 to new transmission devices 20.

[0060] At this time, the analysis delay unit 62 adjusts the amount of delay, so that it is possible to update all transmission devices 10 on the current system line 4 and redundant system line 5 to the new transmission device 20 while maintaining the current system delay of the current system line 4 equipped with the transmission device 10 before the update.

[0061] 7 is a flowchart showing a second procedure for updating the transmission equipment in the line of the communication system 1 using the delay adjustment device 6. Here, as shown in FIG. 5, it is assumed that an analysis delay unit 62 is inserted into both the active system and the redundant system, and a measurement unit 60 is provided in the new active system. Furthermore, the lines of the active system and the redundant system maintain the delay amount that they had when they were equipped with the transmission equipment 10 before updating.

[0062] In step 200 (S200), the adjustment amount control unit 72 in the redundant system releases the prohibition of writing the delay amount to the storage unit 73.

[0063] In step 202 (S202), the adjustment amount control unit 72 in the active system changes operation from the delay follow-up mode to the delay amount reduction mode. For example, the adjustment amount control unit 72 in the active system reduces the delay adjustment amount by, for example, 1 μsec / day.

[0064] In step 204 (S204), the comparator 71 in the redundant system receives and compares the data of the active system and the redundant system (the active system delay and the redundant system delay). For example, the data of the active system and the redundant system are as follows:

[0065] Active system: 1234A1234B1234C12 Redundant system: A1234B1234C1234D1

[0066] In other words, the redundant data here is transmitted four orders of magnitude earlier than the active data.

[0067] In step 206 (S206), the comparator 71 in the redundant system calculates the delay amount (delay difference) between the active system delay and the redundant system delay. In the above example of data between the active system and the redundant system, the data in the redundant system is four orders of magnitude faster than the data in the active system, so the comparator 71 in the redundant system calculates the delay time based on the transmission speed as follows:

[0068] In other words, the difference in delay time between the active system and the redundant system here corresponds to 4 / (number of characters transmitted per second).

[0069] In step 208 (S208), the adjustment amount control unit 72 in the redundant system writes the delay amount (delay difference) into the storage unit 73.

[0070] In step 210 (S210), the adjustment amount control section 72 in the redundant system sets the delay amount (delay difference) in the delay section 74 based on the delay amount (delay difference) stored in the storage section 73.

[0071] In step 212 (S212), the adjustment control unit 72 in the redundant system determines whether the delay amount of the redundant system is 0 or not, and if the delay amount is not 0 (S212: No), returns to processing of S204, and if the delay amount is 0 (S212: Yes), proceeds to processing of S214.

[0072] In step 214 (S214), the adjustment control unit 72 in the redundant system determines whether the delay amount of the redundant system is stable at 0, and if it is not stable (S214: No), returns to processing of S204, and if it is stable (S214: Yes), proceeds to processing of S216.

[0073] In step 216 (S216), the adjustment control unit 72 in the current system determines whether the delay amount of the current system is 0 or not, and if the delay amount is not 0 (S216: No), returns to processing of S204, and if the delay amount is 0 (S216: Yes), proceeds to processing of S218.

[0074] In step 218 (S218), the adjustment amount control unit 72 in the redundant system prohibits writing of the delay amount (delay difference) to the storage unit 73.

[0075] Then, the analysis delay unit 62 and the like are removed from the redundant system, the new transmission device 20 in the redundant system is directly connected to the aggregation device 3, the redundant system and the working system are switched, and similarly the analysis delay unit 62 and the like are removed from the new redundant system (the working system before switching), and the new transmission device 20 in the new redundant system (the working system before switching) is directly connected to the aggregation device 3. In this way, it is possible to remove the delay adjustment device 6 without affecting the communication system 1.

[0076] In the communication system 1 after the transmission equipment has been replaced, the delay adjustment device 6 is no longer necessary and may cause an increase in the failure rate of the system. Therefore, the delay adjustment device 6 is removed after the transmission equipment has been replaced.

[0077] In this way, by using the delay adjustment device 6 for the communication system 1, the delay amount of the transmission device to be replaced and the delay amount of the replaced transmission device can be made the same before switching, and the current line can be switched to the redundant line without causing a line interruption, thereby replacing the transmission devices in the line. Furthermore, by gradually reducing the delay amount added after replacement of all transmission devices in all systems by an amount of change less than a predetermined value, it is possible to use the new transmission device 20 without degrading its performance after replacement of the transmission device 10 with the high-speed, low-delay new transmission device 20.

[0078] In addition, each function of the delay adjustment device 6 may be configured in part or in whole by hardware such as a PLD (Programmable Logic Device) or an FPGA (Field Programmable Gate Array), or may be configured as a program executed by a processor such as a CPU.

[0079] For example, the delay adjustment device 6 can be realized using a computer and a program, and the program can be recorded on a storage medium or provided via a network.

[0080] 8 is a diagram illustrating an example of a hardware configuration of a delay adjustment device 6 according to an embodiment. As illustrated in FIG. 8, the delay adjustment device 6 has, for example, an input unit 80, an output unit 81, a communication unit 82, a CPU 83, a memory 84, and an HDD 85 connected via a bus 86, and functions as a computer. The delay adjustment device 6 is also configured to be able to input and output data to and from a computer-readable storage medium 87.

[0081] The input unit 80 is, for example, a keyboard and a mouse. The output unit 81 is, for example, a display device such as a display that outputs images. The communication unit 82 is, for example, a wired or wireless network interface, and may have a function as an output unit that outputs data to the outside.

[0082] As described above, the CPU 83 controls each component of the delay adjustment device 6 and performs predetermined processing, etc. The memory 84 and the HDD 85 are storage units that store data, etc.

[0083] The storage medium 87 is capable of storing programs and the like that cause the delay adjustment device 6 to execute the functions of the delay adjustment device 6. The architecture that configures the delay adjustment device 6 is not limited to the example shown in FIG.

[0084] The functions performed by the components described herein may be implemented in circuitry or processing circuitry, including general purpose processors, application specific processors, integrated circuits, ASICs (Application Specific Integrated Circuits), a CPU (a Central Processing Unit), conventional circuits, and / or combinations thereof, programmed to perform the described functions.

[0085] A processor includes transistors and other circuits and is considered to be circuitry or processing circuitry. A processor may be a programmed processor that executes programs stored in memory.

[0086] In this specification, a circuitry, unit, or means is hardware that is programmed to realize or performs the described functions, which may be any hardware disclosed herein or any hardware known to be programmed to realize or perform the described functions.

[0087] If the hardware is a processor considered to be a type of circuitry, the circuitry, means, or unit is a combination of the hardware and software used to configure the hardware and / or processor.

[0088] 1...Communication system, 2...Aggregation device (transmission device), 3...Aggregation device (reception device), 4...Working line, 5...Redundant line, 6...Delay adjustment device, 10...Transmission device, 20...New transmission device, 60...Measuring unit, 62...Analysis and delay unit, 70...Signal analysis unit, 71...Comparator, 72...Adjustment control unit, 73...Memory unit, 74...Delay unit, 75...Transmission control unit, 80...Input unit, 81...Output unit, 82...Communication unit, 83...CPU, 84...Memory, 85...HDD, 86...Bus, 87...Storage medium, 600...Convex block, 602...Concave block, 604...Probe, 700...Photosensitive element, 702...Processing unit

Claims

1. A delay adjustment device that adjusts delay for a communication system that includes a working transmission device that transmits signals within a working line connecting a transmitting device and a receiving device, and a redundant transmission device that transmits signals within a redundant line connecting the transmitting device and the receiving device, comprising: a measurement unit that measures the working delay caused by the working transmission device without causing an instantaneous interruption to the working line; an adjustment unit that adjusts the delay in the redundant line so as to eliminate the difference between the working delay measured by the measurement unit and the redundant delay caused by the redundant transmission device; and an adjustment amount control unit that, when the working line is switched to the redundant line, controls the amount of delay change per unit time to be less than a predetermined value, while gradually reducing the adjustment amount in the delay adjusted by the adjustment unit.

2. The delay adjustment device according to claim 1, wherein the adjustment unit adjusts the delay in the redundant line to increase it, and the adjustment amount control unit controls the delay increased by the adjustment unit to gradually decrease it.

3. A delay adjustment method for adjusting delay in a communication system comprising a working transmission device that transmits signals in a working line connecting a transmitting device and a receiving device, and a redundant transmission device that transmits signals in a redundant line connecting the transmitting device and the receiving device, comprising: a measurement step of measuring the working line delay caused by the working transmission device without causing an instantaneous interruption to the working line; an adjustment step of adjusting the delay in the redundant line so as to eliminate the difference between the working line delay measured in the measurement step and the redundant line delay caused by the redundant transmission device; and an adjustment amount control step of controlling, when the working line is switched to the redundant line, to gradually reduce the adjustment amount in the delay adjusted in the adjustment step while keeping the amount of change in delay per unit time below a predetermined value.

4. A delay adjustment method according to claim 3, characterized in that in the adjustment step, the delay in the redundant line is adjusted to be larger, and in the adjustment amount control step, the delay increased by the adjustment step is controlled to be gradually reduced.

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