JPH08186575A - No-hit switching system - Google Patents

Abstract

PURPOSE: To switch a transmission line or pass without hit when a system is switched from the one in current use to a spare system compulsorily for the transmission line or a VP connected by duplicating the transmission equipment of an ATM network. CONSTITUTION: The transmission equipment A on a transmission line upstream side, after inserting a monitoring cell into one of arbitrary VPs on the transmission line in advance, sends out by copying a signal on the transmission line. The transmission equipment B on a downstream side comprises a no-hit switching part 112 consisting of detecting parts 203, 204 which detect the monitoring cell running on the transmission lines of the system in current use and the spare system, a comparison part 207 which judges the delay relation of cell flows in both transmission lines by a detection result, buffer parts 205, 206 which equalize the transmission relay of both transmission lines based on the delay relation of the cell flow on the transmission line. and a selection part 208 which switches the transmission line when the transmission delay of both transmission lines are equalized, and executes the no-hit switching of the transmission line.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmission line or path for connecting two transmission devices in an ATM network in a duplicated manner.
The present invention relates to a hitless switching system for forcibly switching a transmission line or a path from an active system to a standby system without a service interruption for maintenance and operation.

[0002]

2. Description of the Related Art In an ATM network, information can be transferred in units of fixed length cells composed of a header section and an information field section, so that an arbitrary transmission rate can be achieved.
It is possible to handle audio / video / data media information with different transmission rates in the same network. In the ATM network, the transmission line or path between two transmission devices is 2
There has been conventionally studied a non-instantaneous switching method in which forced switching is performed without interrupting service to the user when switching from the active system to the standby system for redundancy and maintenance operation. The hitless switching can be realized by switching the transmission path or the path without cell loss and duplication.

[0003] As a conventional technique of a hitless switching method for a transmission line, there are "a hitless transmission path switching method for an ATM network" (Reference: Technical Report CS92-48) in Reference! And "ATM" in References.
A Study on Non-Instantaneous Duplex Switching Method in Transmission Equipment "
(Autumn Studies 1992 B-526).

In [Method 1] of the document !, a non-instantaneous-interruption switching method in which a SW cell that gives a switching timing is defined and switching is performed at the position of the SW cell is shown. In [Method 2],
A non-instantaneous switching system is shown in which SW cells are transmitted periodically, the active system and the standby system are synchronized each time a SW cell is received, and forced switching is performed by a switching control command. In [Method 3], a non-instantaneous switching method is shown in which the user cells of the active system and the standby system are bit-compared on the receiving side to maintain constant synchronization and forced switching is performed by a switching control command.

Further, "Document" discloses a phase-synchronous non-instantaneous interruption switching system based on phase synchronization of user cells.

[0006] As a prior art of the non-interruptive switching system for paths, there is a non-interruptive path switching method in ATM network (reference B-I Vol. J76-B-I No. 6) of reference #. . In [Method 1] of Reference #, the cell flow is branched and transmitted in parallel, bit comparison is performed using the delay difference absorption buffer provided on the receiving side, and the transmission delay difference between the standby systems is absorbed. The switching method is shown. In [Method 2], the current system,
There is shown a non-instantaneous interruption switching method in which the time alignment of the standby cell flow is performed by two switching delimiter cells sent simultaneously to the active system and the standby system. In [Method 3], immediately after the transmitting side has switched from the active system to the standby system, detection of the switching delimiter cell of the active path path causes the transmission delay difference absorption buffer provided in the standby system to start reading the effective cell. A method for switching between instantaneous interruptions is shown.

[0007]

However, in the conventional hitless switching system described above, the [system 1] of Reference!
In [Method 2] and [Method 2] and [Method 3] of Reference #, a switching cell such as a SW cell needs to be newly defined, and a switching cell generation unit and detection are performed due to the hardware configuration. There is a problem that the circuit scale becomes large because it is necessary to newly provide a section. Also, in [Method 3] of Reference !, [Method 1] of Reference "and [Method 1] of Reference #, since synchronization is established by bit comparison of cells using user cells, synchronization depends on the transmitted cell and synchronization is performed. There was a problem that the characteristics deteriorate.

[0008]

In order to solve the above problems, the present invention provides a dual transmission line for connecting transmission devices in an ATM network without interrupting communication between the transmission devices. In an uninterruptible switching system that forcibly switches from a redundant working transmission line to a backup transmission line, the signal of the working transmission line is reserved in the transmission device on the upstream side of the duplicated transmission line. A copying unit for copying to a system transmission line, and a means for inserting a monitoring cell on the upstream side of the copying unit for any one path on the working system and the standby system transmission line,
A detection unit that detects the monitoring cell flowing through the path on the both transmission lines in another transmission device on the downstream side of the transmission unit with respect to the copying unit, and a detection result of the monitoring cell by the detection unit. According to the comparison unit for determining the delay relationship between the signals flowing on the working and protection transmission lines, based on the delay relationship determined by the comparison unit, a buffer unit for equalizing the transmission delay in both transmission lines, A selection unit is provided for switching from the active transmission line to the standby transmission line when the transmission delays of both transmission lines are equalized by the buffer unit, and the transmission line is switched from the active system to the standby system without interruption. It is a hitless switching system.

Further, the system is a non-instantaneous-interruption switching system for switching the path from the active path to the backup path based on the above configuration without any instantaneous interruption.

[0010]

In the transmission device on the upstream side of the transmission line, a monitoring cell is inserted into any one of the active transmission line and the standby transmission line, and the signal of the active transmission line is copied to the standby transmission line. In another transmission device on the downstream side of the transmission path, the detection unit detects the monitoring cell flowing through the paths of both transmission paths, determines the delay relationship of the signals flowing through both transmission paths based on the detection result, and determines the determined delay relationship. Based on the above, a predetermined delay is inserted in the buffer unit to equalize the transmission delays of both transmission lines, and as a result, the transmission line is switched from the active system to the protection system to perform non-instantaneous switching of the transmission lines.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment, which is a non-instantaneous switching system for transmission lines, will be described.

FIG. 1 is a diagram showing a network configuration in the case of performing the non-interruptible transmission line switching of the first embodiment, and switching the duplexed transmission lines of the transmission device A100 and the transmission device B109. It is a thing. FIG. 2 is a configuration diagram illustrating the hitless switching unit 112 included in the transmission device B109. FIG. 1
FIG. 1 is a diagram including a configuration of a hitless transmission line switching of a conventional technique, but the present invention is particularly characterized by the configuration of the hitless switching unit 112.

The configuration of the transmission apparatus A100 shown in FIG. 1 will be described. The transmission line 101 for transmitting a cell to be input to the transmission device A100 is connected to the end of the transmission line in the transmission device A100 and an input interface unit 102 for performing cell synchronization. The input interface unit 102 is connected to the switch unit 103 that performs distribution to the output route of the transmission device A100. The switch unit 103 is connected to a copying unit 104 that copies cells for sending cell streams having the same contents to the active system and the standby system. The outputs of the active system and the standby system of the copying unit 104 are connected to the output interface unit 105 and the output interface unit 106, respectively, and further, the transmission line a10.
7, connected to the transmission line b108.

The transmission line a107 and the transmission line b108 are connected to the end of the transmission line in the transmission device B109, the input interface unit 110 and the input interface unit 111 for cell synchronization, and the instantaneous transmission for switching the transmission line without interruption. Disconnection switching unit 1
12 is connected.

The structure of the virtual path (hereinafter referred to as VP) will be described below. In the ATM network, a line from the ground to the ground is handled as a VP by using a virtual channel (hereinafter referred to as VC) as a unit, and in the transmission device, the route is distributed by the VP as a unit. When the VP is input to the transmission apparatus A100 on the transmission line 101, in the active system, the input interface unit 102 → the switch unit 103 → the copying unit 104 → the output interface unit 105 → the transmission line a107 → the input interface unit 110 → the no-interruption switching. Transmission device B along the route of section 112
It is input to the hitless switching unit 112 of 109.

On the other hand, in the standby system, the transmission device A10
Since the VP is copied in the copy unit 104 of 0, the copy unit 104 → the output interface unit 106 → the transmission line b10
8 → input interface unit 111 → non-interruption switching unit 112
Is input to the hitless switching unit 112 of the transmission device B109. In this way, the VP is a duplicated transmission line a1.
07 and the transmission path b 108 to provide a hitless switching unit 112.
Is input to. In this embodiment, the transmission line a107 is the active system and the transmission line b108 is the standby system.

Next, the configuration of the hitless switching unit 112 in the transmission device B109 will be described with reference to FIG. In the non-instantaneous interruption switching unit 112, the active input interface unit 11
The output signal 201 from 0 and the output signal 202 from the input interface unit 111 of the standby system are connected to the monitoring detection unit 203 and the monitoring detection unit 204, respectively. Each output signal from the monitoring detection units 203 and 204 is connected to the buffer unit 205 and the buffer unit 206. The output signals from the buffer units 205 and 206 are connected to the comparison unit 207 and also to the selection unit 208 that switches between the active system and the standby system. The output signal 209 of the selection unit 208 is output to the subsequent stage of the hitless switching unit 112. In addition, the non-instantaneous interruption switching unit 112 includes a control unit 11
0 is provided, and the monitoring detection unit 203, the monitoring detection unit 204, the buffer unit 205, and the buffer unit 206 are provided.
Also, various information for executing the hitless switching is set in the comparison unit 207, the state signals of the respective units are extracted, and the timing of the hitless switching is determined. As a result, the switching timing signal is selected by the selection unit 208. Send to.

A high throughput ATM network is realized by constructing the network based on the VP.
O is a maintenance operation function such as characteristic monitoring and failure notification for
AM (Operation, Administrati)
The effectiveness of deploying on and maintenance functions has been demonstrated.

The OAM cell that realizes this OAM function is
As described in ITU-T Recommendation I.610, there are an alarm cell of an alarm transfer function, a monitoring cell of a transmission quality monitoring function, and a loopback cell of a loopback function.

The present invention is premised on the use of a monitoring cell in order to carry out a hitless switching of a transmission line.

The operation of switching the transmission path without interruption will be described below using this monitoring cell.

In order to monitor VP bit errors, cell loss, and cell misdelivery in the monitoring target section of the VP which is the target of transmission quality monitoring, a monitoring cell is set every N VP cells on the upstream side of the monitoring section. Insert. In order to monitor the transmission quality, the monitoring cell has a sequence number for determining the parity of the transmitted user cell and the number of transmitted user cells, a sequence number added in the transmission order for the monitoring cell, and a payload error for detection. Has an error detection code in the payload. On the receiving side, the state of transmission quality is detected by comparing the information obtained from the user cell with the monitoring cell information.

There is a bit error on the transmission line, and the V
When a monitoring cell is generated on the upstream side of the P transmission quality monitoring target section, a bit error may occur and the monitoring cell may not correctly reach the receiving side. For this reason, transmission path switching may fail. In order to surely execute the transmission path switching, the error detection code of the monitoring cell itself is used to detect that the monitoring cell itself has no error.

Insertion of a monitoring cell is executed in the input interface section 102 of the transmission apparatus A100.
The insertion of the monitoring cell is not limited to the input interface unit 102 as long as it is upstream of the copying unit 104 in the transmission device A100 that is the branch point of the target VP.

Regarding the states of the active and standby transmission lines, the user cell flow on the transmission line a107 and the transmission line b108 is the transmission device A.
Since the cells are copied by the copying unit 104 of 100, the order of the cells in the user cell flow is determined by the transmission path a107 and the transmission path b10.
However, the positional relationship of empty cells is not always kept the same. Since the insertion of the monitoring cell of the VP that is monitoring the transmission quality is performed on the upstream side of the copying unit 104, the transmission path a1 including the monitoring cell is included.
In 07 and the transmission path b108, the cell order states are always kept the same.

Next, the operation of switching the transmission path without interruption will be described. The non-instantaneous-interruption switching unit 112 detects the monitoring cells respectively flowing in the VPs on the active and standby transmission paths. This detects and confirms the delay relationship between the working and protection transmission lines. Non-interruption switching unit 1 for the system with the smaller delay
A delay is inserted at 12 to match the timings of the active and standby transmission lines. As a result, the transmission line is switched. At this time, it is necessary that the cell transfer order of the monitoring cell and the user cell is the same in the transmission paths of the active system and the standby system, and the present invention has such a network configuration.

In the control unit 210, the transmission line 201,
The control signals 1101 and 1102 are used to instruct the monitoring cell detection units 203 and 204 to detect the monitoring cells of the active VP and the standby VP that flow on the 202. When detecting the VP monitoring cell, the monitoring cell detection units 203 and 204 notify the control unit 210 of the sequence number of the detected monitoring cell by the control signals 1101 and 1102. Further, the control unit 210 compares the reception timings of the monitoring cells of the same sequence number of the VP flowing in the active system and the standby system on the transmission lines 201 and 202 with the control signals 1101 and 1102. In this way, the delay relationship between the active system and the standby system is understood. At this time, in order to improve the reliability of the delay relation determination, it is possible to check the error of the monitoring cell itself by using the payload error detection code of the monitoring cell and improve the reliability.

When the delay relationship between the active system and the standby system is determined, the control unit 210 inserts a delay by the control signal 1103 or 1104 into the buffer unit 205 or the buffer unit 206 in the system where the cell flow arrives earlier. Instruct them to do so. At the same time, in order to confirm the coincidence of the cell flows of both systems, the control signal 1
The start of comparison is instructed by 105. In the comparison unit 207,
The cell flows of both systems are compared for identity, and when the cell flows match, the control signal 1105 is used to instruct the control unit 210 to match them. At the same time, the control unit 210 instructs the selection unit 208 to switch the system by the control signal 1106. As a result, the working transmission line is switched to the protection transmission line. In this way, when the cell flows of both systems coincide with each other, the system switching is executed by the selecting unit 208, so that the transmission paths are switched without interruption. When the cell flow comparison of both systems is performed, the switching can be surely executed by using the monitoring cell detected by the monitoring cell detection unit.

FIG. 5 shows an operation flow of the hitless switching. In the delay relationship between the working system and the protection system transmission line, the control unit 210 determines the delay relationship between the monitoring cells of the same sequence number flowing through the VPs of both systems (S301). After determining the delay relationship, the control unit 210 instructs the buffer unit 205 or the buffer unit 206 of the system in which the cell flow arrives early to insert a one-cell delay (S30).
2). The comparison unit 207 compares the cell flows of both systems for coincidence (S303). If the cell flows of both systems have a delay relationship, the process returns to S302, and when the delay relationships of both systems disappear and they match, the selection unit 208 switches from the active system to the standby system transmission path.

FIG. 6 shows the cell timing of the hitless switching unit. In this figure, the delay relationship between the active system and the standby system shows that the active system is advanced by two cells. (A) shows a state where the standby system is determined to have a two-cell delay in the delay measurement. (B) shows a state in which a delay corresponding to one cell is inserted. (C) shows a state in which a delay of one cell is further inserted into the active system and the cell flows of the active system and the standby system match. In (d), when the cell flows of both systems match, a matching display is made, a system switching instruction is given, and the system is switched in the selection unit 208. The state transition as described above is made,
Instantaneous switching of transmission lines is realized.

In the present embodiment, the method of delay insertion in the buffer and the comparison of cell flow coincidence in the comparison unit is such that, every time one cell is inserted in the system in which the cell flow arrives earlier, the delay of the monitoring cells in both systems is increased. If the monitoring cells with the same sequence number do not match when the relations are compared, 1 is added.
It shows a sequential insertion method in which a delay for a cell is repeatedly inserted to compare monitoring cells. Another method is to measure the difference between the delay amounts of both systems, and after the delay amount is confirmed, insert the delay cells in a batch into the buffer section of the cell flow system that arrives early, and It is also possible to apply the batch insertion method for matching the relationships.

As described above, the non-interruption switching section detects the monitoring cells respectively flowing in the transmission lines of the active system and the standby system, detects and confirms the delay relation between the active system and the standby system transmission line, and detects the one having the smaller delay. With respect to the system, a delay is inserted in the non-interruption switching unit, the timings of the active system and the standby system are matched, and the transmission path is switched, so that it is not necessary to define a new switching cell as a switching cell. It is possible to reduce the number of switching cell generation and detection circuit units. Further, since the user cell is not used for establishing the synchronization, the synchronization characteristic can be performed without depending on the transmission cell, and the synchronization can be maintained and the loss of synchronization can be surely detected. Further, on the downstream side of the hitless switching unit, by monitoring the transmission quality with a monitoring cell, whether the hitless switching is normally executed or the quality state of the transmission path after the switching is good, It is possible to confirm whether or not the transmission quality is deteriorated due to the path switching.

Next, as another embodiment, the non-instantaneous switching of the VP on the transmission path passing through different routes will be described.

FIG. 3 is a network configuration diagram in the case of performing the non-instantaneous VP switching, from the transmission device A100 to the transmission device C1.
A transmission path (a) 1 that reaches the transmission device B 109 via 13
Vp (a) 115 on 07 and V on transmission line (b) 108 reaching transmission device B 109 via transmission device D 114
The case where VP switching is performed in P (b) 116 is shown.

The VP input in the transmission device A100 is
It is connected to an input interface unit 117 which performs copying in order to send out cell streams having the same contents. Cell is transmission device A1
00 is input. The copied output is connected to the switch unit 103 that distributes the output to the output route of the transmission apparatus A100. The two copied outputs are input from the switch unit 103 to the output interface unit 105 and the output interface unit 106, respectively, and from there, the transmission line (a) 10
7, output to the transmission line (b) 108.

From the transmission device A100 to the transmission line (a) 10
7. The cell flow sent to the transmission line (b) 108 is input to the input interface unit 110 and the input interface unit 111 in the transmission device B109 via the transmission device C113 and the transmission device D114, respectively. Those outputs are connected to the hitless switching unit 112 via the switch unit 118.

Within the transmission device A100, the VP is used as a unit for distribution to the routes. The VP input to the transmission device A100 through the transmission line 101 is input interface unit 117 → switch unit 103 → output interface unit 105 → transmission line (a) 107 → transmission device C113 → transmission line (a) 107 → input interface. The signal is input to the non-instantaneous-interruption switching unit 112 of the transmission device B 109 through the route of the unit 110 → the switch unit 118 → the non-instantaneous-interruption switching unit 112. on the other hand,
The interface unit 117 of the transmission apparatus A100 copies the VP, and the copied VP is the input interface unit 117 → the switch unit 103 → the output interface unit 10.
6 → Transmission path (b) 108 → Transmission apparatus D114 → Transmission path (b) 108 → Input interface section 111 → Switch section 118 → Transmission apparatus B via the route of the non-instantaneous interruption switching section 112
It is input to the hitless switching unit 112 of 109. In this way, the VP is input to the hitless switching unit 112 via the duplicated route.

VP (a) 115 on the transmission line (a) 107
Is used as the active system and VP (b) 116 on the transmission line (b) 108 is used as the standby system, and VP is switched from the active system to the standby system.

The structure of the hitless switching unit 112 will be described with reference to FIG. The same parts as those in FIG. 2 are designated by the same reference numerals.

The non-instantaneous-interruption switching section 112 includes a switch section 118.
To the active system VP (a) 115 and the standby system VP (b) 116
Via the transmission line 211, the monitoring cell detection unit 20
3, 204 are connected. Further, it is connected to a buffer unit 205 and a buffer unit 206 that generate a delay. Further, it is connected to a comparison unit 207 for comparing cell flows, and the current system,
It is connected to the selection unit 208 that switches the backup system. The output of the selection unit 208 is output to the subsequent stage of the hitless switching unit 112. The non-instantaneous interruption switching unit 112 includes a monitoring cell detection unit 203, a monitoring cell detection unit 204, a buffer unit 205, a buffer unit 206, and a comparison unit 207.
The control unit 21 that sets various information for performing the hitless switching, determines the timing of the hitless switching by extracting the state of each unit, and instructs the selecting unit 208 about the switching timing.
Has 0.

Next, the operation of pathless instantaneous interruption switching will be described. It is defined in ITU-T Recommendation I.610 for each N VP cells on the upstream side of the monitoring target section in order to monitor the transmission quality of the target VP bit error, cell loss, and cell misdistribution. The prerequisite is that a monitoring cell is inserted.

Transmission line (a) 107 and transmission line (b) 108
The user cell flow is transmitted by the input interface unit 117 of the transmission apparatus A100 before the monitoring cell is inserted, and the monitoring cell is included in the transmission line (a) 107.
And the order state of the user cell flow flowing on the transmission path (b) 108 is the same.

The control unit 210 includes the monitoring detection unit 20.
3 and 204, V on the active transmission path (a) 107
VP (b) on P (a) 115 and transmission line (b) 108
Instruct to detect the monitoring cell flowing through 116. When detecting the designated monitoring cell, the monitoring cell detection units 203 and 204 notify the control unit 210 of the sequence number. The control unit 210 compares the reception timings of the monitoring cells having the same sequence number, which flow through the active system VP (a) 115 and the standby system VP (b) 116. As a result, the delay relationship between the active system VP and the standby system VP is grasped. When the delay relationship between the active system VP (a) 115 and the standby system VP (b) 116 is determined, the control unit 210 inserts a delay into the buffer unit 203 or the buffer unit 204 of the system in which the cell flow arrives earlier. Instruct. At the same time, a comparison unit 2 that compares the cell flows of both systems
The start instruction is given to 07. The comparing unit 207 displays a match on the control unit 210 when the cell flows of both systems match. The control unit 210 instructs the selection unit 208 to switch the system. VP
(A) 107 and VP (b) 116 Since the system switching is performed when the cell flows of both systems match, the VP non-instantaneous switching is performed. In addition, when performing cell flow comparison of both systems, switching can be reliably performed by using the monitoring cells detected by the monitoring cell detection unit.

The non-instantaneous switching unit detects a monitoring cell which flows through a designated VP on the active system and the standby system via different routes, thereby detecting and confirming the delay relationship between the active system and the standby system VP. For the smaller system, the non-instantaneous-interruption switching unit inserts a delay to match the timings of the active system and the standby system to perform VP switching, and thus it is not necessary to newly define a switching cell.

[0045]

As described in detail above, by using the monitoring cell as a switching cell for timing adjustment for switching the transmission line or the VP without interruption,
It is not necessary to newly define a switching cell, and it is not necessary to newly provide a switching cell generation / detection circuit. Therefore, the hardware configuration is correspondingly simplified. Further, since the user cell is not used for establishing the synchronization between the active system and the standby system, and the synchronization characteristic does not depend on the transmission cell, it is possible to reliably maintain the synchronization and detect the out-of-synchronization.

[Brief description of drawings]

[Figure 1] Network configuration for switching transmission paths without interruption

[Fig. 2] Configuration example of a hitless switching unit

[Fig. 3] Network configuration for non-instantaneous path switching

FIG. 4 is a configuration example of a path uninterruptible switching unit.

[Fig. 5] Operation flow for switching without interruption

FIG. 6 is a cell timing of a hitless switching unit.

[Explanation of symbols]

 102, 117 ... Input interface unit 104 ... Copying unit 112 ... Instantaneous interruption switching unit 203, 204 ... Monitoring cell detection unit 205, 206 ... Buffer unit 207 ... Comparison unit 208 ... Selection unit 210 ... Control unit

Claims (2)

[Claims] 1. A dual transmission line for connecting transmission devices in an ATM network is forced from a dual active transmission line to a standby transmission line without interrupting communication between the transmission devices. In a non-interruptionless switching system for dynamically switching, a copying unit for copying a signal of an active transmission line to a backup transmission line in a transmission device upstream of a duplicated transmission line, and an upstream of the copying unit. On the side, the working system, for any one path on the protection transmission path, a means for inserting a monitoring cell is provided,
A detection unit that detects the monitoring cell flowing through the path on both transmission paths in another transmission device that is downstream of both transmission paths with respect to the copying unit, and a detection result of the monitoring cell by the detection unit. According to the above, a comparison unit that determines a delay relationship between signals that flow on the working and protection transmission lines, and a buffer unit that equalizes transmission delays on both transmission lines based on the delay relationship determined by the comparison unit, A selection unit is provided for switching from the active transmission line to the standby transmission line when the transmission delays of both transmission lines are equalized by the buffer unit, and the transmission line is switched from the active system to the standby system without interruption. A non-interruption switching system characterized by that. 2. A duplicated path for connecting a transmission device in an ATM network is used without interruption of communication between the transmission devices.
In the non-interruptible switching system that forcibly switches from the redundant working path to the backup path, a copy unit that copies the working path signal to the backup path in the transmission device on the upstream side of the path. And a means for inserting a monitoring cell into the working path and the backup path on the upstream side of the copying section, and the working system in another transmission apparatus on the downstream side of both paths with respect to the copying section. A detection unit that detects the monitoring cell flowing through the path and the protection path; and a comparison unit that determines the delay relationship between the signals flowing through the working path and the protection path based on the detection result of the monitoring cell by the detection unit. And a buffer unit that equalizes transmission delays in both paths based on the delay relationship determined by the comparison unit, and when the transmission delays in both paths are equalized by the buffer unit. The switching from the working path to the backup path provided selector, hitless switching system and switches without interruption the path from the active system to the standby system.