JP3087147B2 - Synchronous maintenance control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a synchronization maintaining control system which synchronizes a cell of a duplex system so that the cell can be switched without an instantaneous interruption and maintains the synchronization state. In various communications, the transmission path and transmission device have a duplex configuration,
The aim is to improve the reliability of the communication network. ATM (Async
In the hronous transfer mode) method, digital audio signals, digital image signals, various data, and the like are formed into fixed-length packets and transmitted as cells. When a duplex configuration is used for the active system and the standby system, When performing system switching, there is a demand for enabling instantaneous interruption-free duplex switching in which switching is performed so as not to cause loss or overlap of valid cells.

[0002]

2. Description of the Related Art In a duplex system, for example, as shown in FIG. 13, a transmitting side 31 distributes transmission data to transmission sections 34 and 35 by a distribution section 33, and the transmission sections 34 and 3
5 to the transmission lines 38 and 39, respectively.
On the receiving side 32, the data is received by the receiving units 36 and 37, respectively, and either one of the received data is selected and output by the selector 41. That is, as indicated by the two-dot chain line, the transmission unit 34, the transmission path 38, and the reception unit 36 constitute a zero-system transmission system, and the transmission unit 35, the transmission path 39, and the reception unit 37 transmit one-system transmission. Make up the system.

In the above-described duplex system, cells in the ATM system are transmitted from the transmission side 31 to the reception side 32, and are switched to the standby system by the selector 41 when a failure occurs in the active system. In this case, as a method of switching without any instantaneous interruption, a switching trigger cell is transmitted from the transmission side 31, and when the switching trigger cell is simultaneously read from buffers (not shown) of the receiving units 36 and 37 of the reception side 32, 0 is output. A method of controlling the selector 41 by judging that the cells of the system and the system 1 are synchronized can be considered. That is, when the selector 41 is controlled to switch between the 0-system and the 1-system, a cell synchronization is established between the 0-system and the 1-system.

The cells of the 0-system and the 1-system are received by the receiving units 36 and 3
7, a buffer (not shown) absorbs the transmission delay difference between the transmission lines 38, 39, etc., and is always in a synchronized state.
When switching to the standby system due to the occurrence of a failure in the active system or the like, a method is also conceivable in which the selector 41 is immediately controlled to perform instantaneous interruption.

[0005]

The above-mentioned system using the switching trigger cell identifies the switching trigger cell,
At the same time, it is not possible to control the selector 41 to perform the instantaneous interruption switching until the data is read out from the buffer. Therefore, for example, there is a disadvantage that the time required for switching from the active system to the standby system becomes longer.

The system in which the buffers of the receiving units 36 and 37 (although not shown) are always in a synchronized state and the selector 41 performs the instantaneous interruption switching has a disadvantage that the synchronization is lost due to the discard of the cell after the establishment of the synchronization. Further, when transmitting a special cell for checking the normality or the like of the transmission system of the working system, there is a disadvantage that an extra cell is inserted and the synchronization is lost. When such out-of-synchronization occurs, when switching from the active system to the standby system and immediately controlling the selector 41, there is a disadvantage that cells are lost or duplicated. An object of the present invention is to perform control so that the synchronization can be maintained after the synchronization is established.

[0007]

A synchronization maintaining control system according to the present invention will be described with reference to FIG. 1. A first-in first-out memory for inputting valid cells of one and the other of a duplex configuration. (FIFO) buffers 1 and 2 and a synchronization control unit 3 for controlling these buffers 1 and 2 to read valid cells synchronously from the buffers 1 and 2 and switch the cells by the selector 4 without interruption. The synchronization control unit 3 synchronizes between one system (0 system) and the other system (1 system), and then reads a valid cell from a buffer of one or the other system. When there is no valid cell in the buffer of the other or one system when the state is possible, the reading of the buffer of the one or the other system is stopped, and an empty cell is sent to the one and the other system, The other or one When the active cell is input to the buffer, in synchronism from the system buffer between one and the other reads the valid cell, it is to maintain synchronization.

At the time of discarding an effective cell of one or the other system (system 0 or 1), a dummy cell is inserted at the position of the discarded effective cell, and this dummy cell is input to the buffer as an effective cell.

When a special cell is inserted into one or the other system (system 0 or system 1), a dummy cell is inserted into the other or one system at a position corresponding to the insertion position of the special cell,
The dummy cells are input to the buffers 1 and 2 as valid cells together with the special cells.

A special cell insertion cell is transmitted to one or the other system (system 0 and system 1), and a special cell is inserted into the special cell insertion cell in one or the other system.

A special cell detector for detecting a special cell inserted in one or the other system (system 0 or system 1) is provided. When the special cell detected from the buffers 1 and 2 is detected by the special cell detector, The synchronization control unit 3 stops reading the buffer of the system in which the special cell is not inserted, and transmits an empty cell in synchronization with the special cell.

When the number of valid cells stored in the buffers 1 and 2 is detected, and when the number of valid cells exceeds a preset value, the synchronization control unit 3 determines that the buffers 1 and 2 are in the reading stopped state. In this case, the reading stop is released and the effective cell reading is controlled.

[0013]

According to the first aspect of the present invention, the buffer (FIF
O) 1 and 2 input only valid cells, and the synchronization control unit 3
The reading of valid cells from the buffers 1 and 2 is controlled by a read control signal. In a synchronous state where the same valid cells can be read simultaneously, the data is valid for the buffer 2 due to, for example, transmission delay of the first system. If you run out of cells,
When a valid cell is read from the 0-system buffer 1 and then a valid cell is input to the 1-system buffer 2 and read in synchronization with the valid cell of the buffer 1, the valid cells of the 0-system and 1-system are read. Misalignment occurs. Therefore, when the buffer 2 becomes empty, the reading of the buffer 1 is stopped. When a valid cell is input to the buffer 2, the valid cell in the buffer 1 and the valid cell in the buffer 2 are read out in synchronization. Therefore, once the synchronization is established, even if there is a variation such as a transmission delay, the synchronization state is maintained, and the selector 4 is controlled by the system selection control signal to switch between the 0 system and the 1 system. In addition, instantaneous interruption switching can be performed.

According to the second aspect of the present invention, the cell may be discarded due to a bit error in the header of the cell or the like. Since the discarded cell is an empty cell, it is not stored in the buffer. . Therefore, when the cell is discarded, even if the effective cells are read out from the buffers 1 and 2 of the system 0 and the system 1 synchronously, only cells having different contents are read out at the same time, and the actual synchronization state is lost. Are in different states. Therefore, a dummy cell is inserted instead of the discarded cell, and this dummy cell is input to the buffer as a valid cell. As a result, the contents of the valid cells after the dummy cell become the same, and the actual synchronized state can be maintained.

In the third aspect of the present invention, the normality of the working system may be confirmed by inserting a special cell into the working system.
In this case, no special cell is inserted into the standby system, so that one more effective cell is input to the buffer of the system into which the special cell is inserted than the buffer of the other system. , 2 and read out synchronously,
The state is different from the actual synchronization state. Therefore, for example, when a special cell is inserted into the 0 system, a dummy cell is inserted into the 1 system. Therefore, the special cell is input to the buffer 1 and the dummy cell is inserted into the buffer 2, and the special cell and the dummy cell are read out in synchronization with each other, so that the actual synchronization state can be maintained.

In the fourth aspect of the present invention, the timing for inserting a special cell is generally predetermined, and the special cell insertion cell is transmitted to the 0-system and the 1-system. For example, when the system 0 is a working system, a special cell insertion cell is detected in the system 0 and a special cell is inserted. Therefore, when the valid cell into which the special cell is inserted is input to the buffer 1 of the system 0, the cell for inserting a special cell is input to the buffer 2 of the system 1 as a valid cell. Since the special cell insertion cell into which the special cell is inserted and the special cell insertion cell into which the special cell is not inserted from the buffer 2 of the first system are read out synchronously,
Synchronous state can be maintained.

According to a fifth aspect of the present invention, in a system in which a cell for inserting a special cell or a dummy cell is not inserted, when a special cell of one system is detected by a special cell detecting section, the other system is inserted. Stop reading the buffer and send an empty cell instead. For example, when a special cell of the system 0 is read from the buffer 1, the reading of the buffer 2 of the system 1 is stopped, and an empty cell is transmitted in synchronization with the special cell of the system 0. Therefore, even if a special cell is inserted into the system 0, the synchronization state thereafter can be maintained.

According to a sixth aspect of the present invention, in the buffer 1,
2 has a finite storage capacity, for example,
When the arrival of valid cells input to the buffer 1 of the system 1 is delayed, the reading of the buffer 1 of the system 0 is stopped, the buffer 1 overflows, and the valid cells are discarded. Therefore, based on the capacity information of the buffers 1 and 2, the synchronization control unit 3 determines whether or not the number of stored effective cells exceeds a predetermined number. Is canceled, and the cell is temporarily out of synchronization to avoid discarding the valid cell.

[0019]

FIG. 2 is a flow chart of one embodiment of the present invention. In the principle explanatory diagram of FIG. 1, the buffers 1 and 2 are constituted by first-in first-out memories, The unit 3 controls reading of valid cells from the buffers 1 and 2 in synchronization with a read control signal,
The selector 4 switches between system 0 and system 1 by a system selection control signal. In FIG. 2, the 0 series and 1
One of the systems is indicated as an X system and the other is indicated as a Y system. The synchronization control unit 3 determines whether or not the buffers (FIFOs) of the X system and the Y system are free from the capacity information (FIFO) from the buffer (FIFO). (A) and (b).
If the X-system and Y-system buffers (FIFOs) are not empty, that is, if valid cells are stored, the valid cells of both systems are synchronously read (c). If the X-system buffer is empty, the reading of the Y-system buffer (FIFO) is stopped (d), and an empty cell is sent to the Y-system (e).
If the Y-system buffer is empty, the X-system buffer (F
The reading of the IFO is stopped (f), and an empty cell is sent to the X system (g). The synchronization control unit 3 has a control function of a microprocessor or the like, and performs read control of the buffers 1 and 2 and transmission control of the above-mentioned empty cell.

FIGS. 3, 4 and 5 are diagrams for explaining the operation of the embodiment of the present invention. Reference numerals 1 and 2 denote buffers (FIFO) of 0 system and 1 system, Y, Z and A shown in FIG. , B, C,... Indicate valid cells, and the dotted line indicates an empty cell, and shows a case where synchronization is established using the valid cell B. (A1) of FIG.
This shows a state in which the valid cells Y are input to the buffer 1 while the buffers 1 and 2 with the system are empty, and the valid cells Y are input to the buffer 2 with a delay of three cells. (A2) is 0
A valid cell Y is read from the buffer 1 of the system, a vacant cell is sent to the system 1 for the valid cell Y, and a valid cell Z is accumulated in the buffer 1 to wait for the next reading.

FIG. 3 (A3) shows a state in which valid cells B for establishing synchronization are stored in the buffer 1 and valid cells Y are stored in the buffer 2; When input to the buffer 1, a synchronization control unit (not shown)
Detects the valid cell B for establishing synchronization, stops reading the buffer 1 of system 0 until the valid cell B for establishing synchronization is accumulated in the buffer 2 of system 1, and sends an empty cell to system 0. I do. In this case, since buffer 1 of system 1 is empty, 1
An empty cell will be sent to the system. Accordingly, the state shifts from the state (A4) to the state (A5) in FIG.
As shown in the figure, a valid cell B for establishing synchronization
When the data is accumulated in the buffer 2, the suspension of reading from the buffer 1 of the system 0 is released, and the valid cells B are read out from the buffers 1 and 2 in synchronization. That is, the state shown in (A6) of FIG.
Synchronization is established.

After the synchronization is established, as shown in (A7) of FIG. 4, when the valid cells D, E, F, and G of the system 1 are delayed from the system 0, when the buffer 2 of the system 1 becomes empty, The reading of the buffer 1 of the system is stopped, and an empty cell is sent to both systems. Therefore, the valid cells D, E,
F is stored. Next, as shown in (A8), when the valid cells D of the first system are accumulated in the buffer 2, the read stop is released, and the valid cells D are synchronously read as shown in (A9). . Therefore, synchronization is maintained even in the state (A10).

As described above, (A11) in FIG.
In (A14), since the buffer 1 of the system 1 becomes empty, the reading of the buffer 1 of the system 0 is stopped, and when a valid cell is input to the buffer 2 of the system 1, the buffers 1
2 can be read and synchronization can be maintained.

FIG. 6 is an explanatory diagram of the problem of cell discarding in one embodiment of the present invention, and shows a case where the effective cell G of the first system is discarded as shown in (B1). Therefore, (B
As shown in 2), valid cells F are accumulated at the head of buffers 1 and 2 of system 0 and system 1, and the valid cells F are stored in (B3).
After being read out synchronously as shown in FIG.
, The valid cells G are accumulated at the beginning of the buffer. However, since the valid cells G are not input to the 1-system buffer 2, the reading of the 0-system buffer 1 is stopped when the buffer 2 becomes empty. , (B4), an empty cell is transmitted to both systems, and then, as shown in (B5), 1
When the effective cells H are accumulated in the system buffer 2, (B6)
As shown in (1), the valid cell G from the 0-system buffer 1 is read out and sent out simultaneously from the 1-system buffer 2, and the state is out of phase synchronization. This state continues after (B7). Will do. That is, when no cell is discarded, the synchronization can be maintained by the above-described embodiment, but when the cell is discarded, the phase is out of phase.

Therefore, in another embodiment of the present invention, as shown in FIG. 7, a discarded cell X is used as a dummy cell and this is input to a buffer as a valid cell. That is,
As shown in (B1) and (B2) of FIG. 6, when the valid cell G is discarded, in this embodiment, (C1) of FIG.
As shown in the figure, the discarded valid cell G is assumed to be a dummy cell X. For example, if a cell header is discarded due to a bit error, the header is modified so that it can be handled as a valid cell. The dummy cell X is input to the buffer 1 of the first system after the valid cell F as shown in (C2), and as shown in (C3), the valid cell G is at the head of the buffer 1 and the dummy cell X is at the head of the buffer 2. Is stored, and as shown in (C4), the valid cell G and the dummy cell X are read simultaneously, and then, as shown in (C5), the valid cell H is stored at the head of the buffers 1 and 2. Thereafter, as shown in (C6) and (C7), the phase synchronization is maintained.

FIG. 8 is an explanatory diagram in the case of using the special cell of the embodiment of the present invention. Reference numerals 1 and 2 denote buffers, 3 denotes a synchronization control unit, 4 denotes a selector, and 5 and 6 denote special cell detection units. . A special cell is inserted into the transmission path of the working cell, the special cell is extracted at a check point, and the normality of the working system is confirmed by a bit error rate or the like. Special cell detectors 5 and 6 for detecting the special cell are provided, and a detection signal is applied to the synchronization controller 3. The special cell detectors 5 and 6 show the configuration of the order in which special cells are detected from valid cells read from the buffers 1 and 2, and are stored in the valid cells input to the buffers 1 and 2 or the buffers 1 and 2. The configuration may be such that a special cell is detected from the valid cells.

FIG. 9 is an explanatory diagram of a problem when a special cell is inserted. As shown in (D1), when a special cell (special) is inserted between the effective cells G and H of the first system, that is, , 0 system becomes the standby system, and 1 system is the working system. When a special cell (special) for confirming the normality of the transmission path is inserted, up to (D2), (D3) and (D4) Are maintained in phase synchronization. In this (D4), the valid cell H at the head of the buffer 1 and the special cell (special) at the head of the buffer 2
Is stored.

In the next state (D5), a valid cell H is read out to the system 0 and a special cell (special) is read out to the system 1 at the same time.
6) In the state, since the valid cell I is simultaneously read in the 0 system and the valid cell H in the 1 system, (D5), (D6), and (D7)
Thereafter, the phase is out of phase and the synchronization cannot be maintained.

Therefore, as shown in FIG. 8, special cell detectors 5 and 6 are provided, and the synchronization controller 3 controls the insertion of dummy cells or the stop of reading of the buffer. FIG. 7 is an explanatory diagram of the operation of the embodiment when the operation is performed. In the embodiment of this operation explanatory diagram, the special cell detectors 5 and 6 are configured to detect a special cell before being input to the buffers 1 and 2, and (E1) in FIG.
This shows a state in which a special cell (special) is inserted between the valid cells G and H of the first system. When this special cell (special) is detected by the special cell detector 6, a dummy cell is set between the valid cells G and H of the zero system. Insert (da). This insertion is performed on the buffer 1.

Therefore, as shown in (E2), (E3), and (E4), the dummy cell (da) is accumulated in the buffer 1 and the special cell (*) is accumulated in the buffer 2, and (E5)
As shown in the figure, since the dummy cell (da) and the special cell (*) are simultaneously read, (E6) and (E6) after (E5) are read out.
As shown in 7), phase synchronization is maintained.

Instead of inserting a dummy cell (da) corresponding to a place where a special cell (special) is inserted, the embodiment shown in FIG. The special cell detectors 5 and 6 in this case are configured to detect the special cells stored in the buffers 1 and 2. Therefore, as shown in (F1), the special cell (special) is set between the effective cells G and H of the first system.
Is inserted, the phase synchronization is maintained as shown in (F2) and (F3), and as shown in (F4), the special cell
Is accumulated at the head of the buffer 2, the special cell detection unit 6
The synchronization control unit 3 stops reading of the 0-system buffer 1 by adding the detection signal to the synchronization control unit 3. Therefore, the valid cells H are stored at the head of the buffer 1.

Next, as shown in (F5), a special cell (special) is read from the buffer 2 of the system 1 and an empty cell is transmitted to the system 0. Thereby, as shown in (F6) and (F7), the same valid cell is read from buffers 1 and 2, and
Phase synchronization will be maintained.

The special cells are inserted at predetermined intervals,
Since it is common to regularly monitor the normality of the working system, special cell insertion cells are sent to the system 0 and system 1 in advance, and special cells are inserted into the special cell insertion cells. This special cell can be extracted at the health monitoring point.
In this case, the cells for inserting a special cell into which no special cell is inserted are also input to the buffers 1 and 2 as valid cells, thereby applying the embodiment of the present invention described with reference to FIGS. , The synchronization after the establishment of the synchronization can be maintained.

FIG. 12 is an explanatory view of still another embodiment of the present invention. The same reference numerals as those in FIGS. 1 and 8 denote the same parts, and 7 denotes a cell coincidence detecting unit. The cell coincidence detection unit 7 detects whether or not the valid cell B for establishing synchronization shown in FIG. 3 is simultaneously input to the selector 4 for the 0-system and the 1-system, for example.
The cell contents are compared, and if they match, it is determined to be in a synchronized state. In this case, it is also possible to determine that synchronization is established when a predetermined number of cells match continuously, and to determine that synchronization is lost when a predetermined number of non-matching cells continue. When the special cells are detected by the special cell detectors 5 and 6, the special cell and the dummy cell, the empty cell or the cell for inserting the special cell are input to the cell coincidence detector 7. Thereby, the cell match detection unit 7 can prevent the cell contents from being compared.

The special cell detectors 5 and 6 are provided with special cells input to the buffers (FIFO) 1 and 2 or special cells or buffers 1 and 2 stored in the buffers 1 and 2 as in the above-described embodiment. It can be configured to detect a special cell in any state of the special cells read from the device.

The synchronization control unit 3 receives the capacity information indicating the number of effective cells stored in the buffers 1 and 2 and determines whether or not the number exceeds a predetermined number. Since the possibility of overflow has increased, the suspension of reading is canceled. For example, in a state where valid cells are not input to the buffer 2 due to a failure in the transmission system 1, the buffer 2 becomes empty, and the reading of the buffer 1 in the 0 system is stopped. If the number of valid cells accumulated in the buffer 1 exceeds a predetermined number and the state is continued, the buffer 1 overflows and the valid cells are discarded. Therefore,
As described above, when the number of effective cells stored based on the capacity information exceeds a predetermined number, the stop of reading is canceled, and discarding of effective cells due to overflow of the buffer 1 is avoided. In this case, the state is out of synchronization.

The present invention is not limited to the above-described embodiments but can be variously added and changed. The present invention is applied to a duplex system in which data is packetized and transmitted. Can be possible.

[0038]

As described above, according to the present invention, the buffers 1 and 2 of one system and the other system absorb the phase difference between the cells of one system and the other system, and have the same contents. Simultaneously read valid cells to establish synchronization, and in that state,
When the buffer of one system becomes empty, the reading of the buffer of the other system is stopped. Until a valid cell arrives at one system, valid cells that have arrived at the buffer in the other system first. , The synchronization can be maintained. Therefore, when the selector 4 is controlled by the system selection control signal to perform switching between one system and the other system, instantaneous interruption switching can be performed.

In some cases, a cell may be discarded due to a header error or the like. In this case, a dummy cell is inserted in place of the discarded cell. This dummy cell is input to the buffer as a valid cell, so that synchronization is maintained. be able to.

When a special cell is inserted into one of the systems,
Maintaining synchronization by inserting a dummy cell into the other system and controlling the synchronization control unit 3 to simultaneously read a dummy cell from the buffer of the other system when reading a special cell from the buffer of the other system. Can be. Further, by transmitting the special cell insertion cell to one system and the other system, it is possible to maintain synchronization when a special cell is inserted without performing operations such as insertion of a dummy cell.
When the special cell of one system is read from the buffer, the reading of the buffer of the other system is stopped, and an empty cell is sent to the system. Synchronization can be maintained by the same control as the control for stopping the reading of the buffer.

When the number of valid cells stored in the buffers 1 and 2 exceeds the set value, the suspension of reading is released, thereby avoiding the overflow of the buffer caused when a failure occurs in one of the systems. Control can be performed so that no disposal occurs.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a flowchart of one embodiment of the present invention.

FIG. 3 is an operation explanatory diagram of one embodiment of the present invention.

FIG. 4 is an operation explanatory diagram of one embodiment of the present invention.

FIG. 5 is an operation explanatory diagram of one embodiment of the present invention.

FIG. 6 is an explanatory diagram of a problem of cell discarding in one embodiment of the present invention.

FIG. 7 is an operation explanatory diagram of another embodiment of the present invention.

FIG. 8 is an explanatory diagram when a special cell according to an embodiment of the present invention is used.

FIG. 9 is an explanatory diagram of a problem of a special cell in the embodiment of the present invention.

FIG. 10 is an operation explanatory diagram when a special cell according to an embodiment of the present invention is used.

FIG. 11 is an operation explanatory diagram when a special cell according to an embodiment of the present invention is used.

FIG. 12 is an explanatory view of still another embodiment of the present invention.

FIG. 13 is an explanatory diagram of a duplex system.

[Explanation of symbols]

 1, 2 buffer (FIFO) 3 synchronization control unit 4 selector

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Haruo Yamashita 1015 Uedanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fujitsu Limited (72) Inventor Hiroshi Ota 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation In-company (72) Inventor Hiromi Ueda 1-6-1 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation (56) References JP-A-5-336157 (JP, A) JP-A-5-227191 (JP) JP-A-4-222138 (JP, A) JP-A-4-346538 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04L 12/28 H04L 12/56 H04L 1/22

Claims (6)

(57) [Claims] 1. Buffers (1) and (2) each composed of a first-in first-out memory for inputting valid cells of one and the other of a duplex configuration, and said buffers (1) and (2) And a synchronous control unit (3) for controlling the synchronous control unit. The synchronous control unit (3) reads out the valid cells from the buffers (1) and (2) in synchronization with each other and switches between the one system and the other system without instantaneous interruption. In the synchronization maintaining control method, the synchronization control section (3) can read a valid cell from the buffer of the one or the other system after synchronizing between the one system and the other system. In the state of the above, if there is no valid cell in the buffer of the other or one system, the reading of the buffer of the one or the other system is stopped, and an empty cell is sent to the one and the other system, A valid buffer is stored in the buffer of the other or one system. There when inputted in synchronization from the system buffer between the one and the other reads the valid cells, maintaining synchronization control method characterized by maintaining synchronization. 2. At the time of discarding an effective cell of the one or other system, inserting a dummy cell at a position of the discarded effective cell,
2. The synchronization maintaining control method according to claim 1, wherein the dummy cell is input to the buffer as a valid cell. 3. When a special cell is inserted into the one or the other system, a dummy cell is inserted into the other or one system at a position corresponding to the insertion position of the special cell, and the dummy cell is inserted together with the special cell. 2. The synchronization maintaining control method according to claim 1, wherein the buffer is input to the buffer as a valid cell. 4. A special cell insertion cell is transmitted to the one and the other systems, and a special cell is inserted into the special cell insertion cell in the one or the other system. The synchronization maintaining control method according to claim 1. 5. A special cell detecting unit for detecting a special cell inserted into the one or the other system, wherein when the special cell detected from the buffer is detected by the special cell detecting unit, the synchronization control unit ( 3. The synchronization maintaining control method according to claim 1, wherein in 3), reading of a buffer in a system in which the special cell is not inserted is stopped, and an empty cell is transmitted in synchronization with the special cell. 6. The synchronization control unit (3) detects the number of valid cells stored in the buffers (1) and (2), and when the number of valid cells exceeds a preset value, the synchronization control unit (3) 2. The synchronization maintaining control method according to claim 1, wherein, even when the buffers (1) and (2) are controlled to be in a read stopped state, the read stop is canceled to control the effective cell reading.