JP2806766B2 - Cell input / output line accommodating apparatus and system switching 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 cell input / output line accommodating device for data communication in a digital electronic switching system, and more particularly to a method of exchanging existing various communication information into cells of an asynchronous transfer mode system, and using this cell as a unit. The present invention relates to a cell input / output line accommodating device having a redundant configuration for performing system switching between a main operating system and a standby system, and a system switching control method thereof.

[0002]

2. Description of the Related Art A conventional input / output line accommodating device has a capacity of 150M.
In general, the communication system handles communication information at a lower speed than a communication system using cells of an asynchronous transfer mode system (hereinafter, referred to as an ATM cell) which handles high-speed communication information such as bps or more.

In particular, an external input / output line which does not have a redundant configuration is accommodated, and a main operating system (hereinafter referred to as A
CT system) and standby system (hereinafter referred to as SBY system)
When the communication information transfer method is a synchronous method, the communication information is input from an external input line having no redundant structure in accordance with a synchronization signal for synchronizing a frame for transferring the communication information. The communication information is switched from the ACT system to the SBY system.

[0004] When the communication information transfer method is an asynchronous method such as a packet, AC transfer is performed during a series of system switching operations.
A communication information reception additional signal for temporarily suppressing the communication information input to the T system is sent to the connected external device, and the communication information already retained and held in the ACT system is transferred to the SBY system. After that, the output destination of the communication information is switched in the input line accommodating unit, and a communication information receivable signal is output to the external connection device to cancel the input suppression of the communication information, thereby terminating the system switching operation. Was.

[0005]

Therefore, when processing the high-speed communication information in such a conventional input / output line accommodating apparatus, if the communication information transfer method accommodates a synchronous line as in the former case. In this case, since the line accommodating unit does not have a redundant configuration, it is difficult to synchronize the output switching operation of the communication information in the line accommodating unit with the system switching operation of the system function unit having the redundant configuration. there were.

Therefore, there is a problem that frame loss occurs at the time of system switching operation and communication information is likely to be lost. Further, as communication information becomes faster, a function and switching function of the line accommodating unit becomes higher and higher. There is a drawback that the communication quality is greatly affected by the performance of the components that make up the communication.

When the communication information transfer method accommodates an asynchronous line as in the latter case, it takes time to transfer the communication information already held and held in the ACT system to the SBY system, so that the switching is performed. Since a large delay occurs as an operation and a control procedure is complicated, there is a problem that it is not suitable for performing real-time communication. Further, since a communication information reception disable signal is output to the external connection device to suppress the output of the communication information, the external connection device is overloaded with traffic.

The present invention has been made to solve such a problem, and has no loss of communication information at the time of system switching operation, reduces transfer delay and suppression of transmission of communication information, and provides a cell suitable for real-time communication. An object of the present invention is to provide an input / output line accommodating device and a system switching control method thereof.

[0009]

In order to achieve the above object, a cell input / output line accommodating apparatus according to the present invention has a system function unit, in the case of an ACT system, which performs communication from an external input line at the time of communication. If additional information such as a synchronization establishment flag pattern attached to the information is specified to be deleted by an instruction / setting before the start of communication, this is deleted, and the remaining communication information can be retained for several tens of cells. After temporarily holding the communication information in the buffer,
Assigned sequentially to the subscriber communication information storage area in the M cell A
Output as a TM cell, stop inputting communication information from the external input line unit to the buffer in response to the system switching preparation instruction, thereby discarding subsequent communication information,
All the communication information remaining in the buffer is output as ATM cells to make the buffer empty, a buffer empty state notification for permitting test cell insertion is made, and subsequent communication information is discarded and buffered even after an instruction to restart the system processing. The vacant state is continued, and in the case of the SBY system, the discard of the communication information after the additional information is discarded at the time of communication is stopped according to the system switching preparation instruction, the input to the buffer is resumed, and the communication information is held. On the other hand, there is provided a line input cell processing means for converting the communication information held therein and the subsequent communication information into ATM cells according to the system processing restart instruction and sequentially outputting the ATM cells.

Further, as a system function unit, a test cell for system switching confirmation is inserted in response to a test cell insertion instruction from the control unit and provided to the control unit in response to the completion of the insertion. Test cell insertion means for notifying this, and various cells output from the line input cell processing means in response to a cell holding instruction from the control means are held, and held in response to a cell output instruction from the control means. A buffer means for sequentially outputting a cell and a subsequent cell, and a test cell among the cells output from the buffer means is detected and notified to the control means, and detected in response to a cell output instruction from the control means. Test cell detecting means for discarding the test cell.

Further, in addition to the system function unit, at the time of system switching, a system switching preparation instruction is output to the line input cell processing means of both systems, and then a test cell insertion instruction is output to the test cell inserting means of both systems. In response to the notification of the completion of the test cell insertion from both systems, a system processing restart instruction is output to the line input cell processing means of both systems, and either of the first test cell detection from the test cell detection means of both systems is output. In response to the notification, a cell holding instruction is output to the buffer means of the corresponding system, and in response to the subsequent notification of test cell detection from the other system, the test cell detection means of both systems and the SBY system buffer means are output. Control means for outputting a cell output instruction.

Further, the system switching control method in the cell input / output line accommodating apparatus according to the present invention provides an ACT for converting communication information from an external input line into an ATM cell during communication and outputting the ATM cell.
The communication information of the system is discarded, the ATM cell processing is stopped after the conversion of the residual communication information into the ATM cell, and the discard processing of the SBY system for discarding the communication information at the time of communication is stopped and the communication information is held. A test cell for system switching confirmation is inserted into both systems, and in response to the completion of the insertion, the ACT system continues the above-mentioned state and converts the communication information held in the SBY system and the subsequent communication information into ATM cells. Is started and A and subsequent test cells output by the ATM cell processing are detected until both test cells inserted into both systems are detected.
The TM cell is held again, and the output of the ATM cell held again in the SBY system and the output of the subsequent ATM cell are restarted in response to the detection of the test cell in both systems, thereby switching the data communication processing operation of both systems during communication. Is what you do.

Therefore, the line input cell processing means sets the AT
The communication information of the ACT system which has been converted into M cells and output to the cell switching device via the buffer means is discarded in response to the system switching preparation instruction from the control means, and the output of the remaining communication information into the ATM cell is completed. The ATM cell output is later stopped, and the SBY communication information discarded by the line input cell processing means is discarded and retained.

In response to a test cell insertion instruction from the subsequent control means, test cells for system switching confirmation are inserted from the test cell insertion means of both systems, and a system processing restart instruction from the control means in response to the notification of completion of the insertion. As a result, the communication information discarding process is continued by the ACT line input cell processing means, and the output of the communication information held in the SBY line input cell processing means to an ATM cell is started.

As a result, in the ACT system, a test cell is inserted at the end of a valid cell before system switching, and in the SBY system, a test cell is inserted at the head of a valid cell after system switching. It is detected by the test cell detection means of both systems and notified to the control means respectively.

Here, in response to the cell holding instruction from the control means in response to the test cell detection notification of either of the two systems, the cell of the system in which the test cell has been detected first is held by the input buffer means.

Then, in response to a cell output instruction from the control means in response to the test cell detection notification from the other system,
The test cells inserted into both systems are discarded by the test cell detecting means of both systems, and the ATM cells held in the input buffer means of the SBY system are sequentially output to the cell switching device.

[0018]

Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram of a cell input / output line accommodating apparatus according to an embodiment of the present invention. FIG. 2 is a detailed block diagram of the line input cell processing unit 3A (3B) of FIG.

In FIG. 1, reference numeral 20 denotes a cell input / output line accommodating device, which converts input communication information into ATM cells and converts the communication information into ATM cells through a predetermined system internal line 80.
, A system function unit 22 having the same configuration as the system function unit 21, and an input for accommodating the external input line 60 and distributing the input communication information to the system function units 21 and 22. A line accommodating unit 1 is configured by an output line accommodating unit 2 accommodating the external output line 70 and outputting communication information from the system function units 21 and 22 to the external output line 70.

Reference numeral 30 denotes an exchange system control device for controlling the system switching operation between the system function units 21 and 22, and reference numeral 40 denotes various types of ATM cell exchange processing which are connected to the cell input / output line accommodating device 20 and the system internal line 80. It is a cell switching device.

Reference numeral 50 denotes a cell input / output line accommodating device 2.
0, switching system control device 30, and cell switching device 4
0 shows a cell switching system consisting of zeros.

The system function units 21 and 22 have the same configuration contents. As an input system, the additional information processing unit 3A1 (3B1) for deleting additional information included in the communication information from the input line accommodating unit 1. ), A buffer unit 3A2 (3B2) and a buffer unit 3A2 (3B) that temporarily hold the communication information after deleting the additional information to convert the communication information into ATM cells.
A test for inserting a test cell in accordance with an instruction from the cell-forming unit 3A3 (3B3) for converting the communication information held in 2) into an ATM cell, a line input cell processing unit 3A (3B) including them, and the switching system control device 30 Cell insertion part 5A (5
B), an input buffer unit 6A (6B) that temporarily holds input cells, and detects and discards test cells in order to adjust the timing of cell output in accordance with the system switching process and various line switching processes at the subsequent stage. The test cell detector 8A (8
B) and a cell output switching section 9A (9B) for selecting a system internal line 80 and outputting a cell in accordance with an instruction from the switching system control device 30.

Further, as an output system, a cell input switching unit 10A (10) for selecting a system internal line 80 and inputting a cell according to an instruction from the switching system controller 30.
B) In order to adjust the timing of cell output, the output buffer unit 7A (7) temporarily holds the input cells.
B), and a line output cell processing unit 4A (4B) that converts ATM cells into communication information based on a predetermined transfer method and outputs the communication information to the line accommodating unit 2.

As described above, a plurality of system function units 21 and 22 having equivalent functions are provided, and
System, that is, a main operation system function unit that actually operates, and the other is an SBY system, that is, a standby system function unit that does not actually operate, and the system operation state of these system function units 21 and 22 can be switched as needed. I / O line accommodating device 20
Is realized.

Therefore, the communication information input from the external input line 60 is distributed and output at the same timing to the line input cell processing units 3A and 3B of the system function units 21 and 22 in the input line accommodation unit 1.

Here, as the current operation status, the system function unit 2
1 is an ACT (main operation) system and the system function unit 22 is an SBY
In the case of the (standby) system, the distributed and output communication information is added to the line input cell processing unit 3A of the system function unit 21 by additional information such as a synchronization establishment flag pattern accompanying the communication information. Additional information processing unit 3 inside 3A
A1 (FIG. 2) deletes and temporarily stores the communication information from which the additional information has been deleted in the buffer unit 3A2, which can hold an information amount of about several tens of cells, into ATM cells. further,
The communication unit 3A3 sequentially assigns communication information to the subscriber communication information storage area in the ATM cell and converts the communication information into an ATM cell.
The data is temporarily stored in the input buffer unit 6A via the test cell insertion unit 5A. Thereafter, the signal is input to the cell output switching unit 9A via the test cell detecting unit 8A, and from there, the system internal line 8 selected by the instruction of the switching system control unit 30.
0 to the cell switching device 40.

The communication information distributed and input by the input line accommodating unit 1 to the line input cell processing unit 3B of the system function unit 22 whose current operation status is the standby system is discarded inside the line input cell processing unit 3B. Discarded and no further processing takes place.

Next, the system switching operation according to the present invention will be described with reference to FIGS. In addition, as the state before the start of switching, the system function unit 21 is set to the ACT (main operation) system or the system function unit 2.
2 is an SBY (spare) system. Description of the switching operation on the output line side, which is not particularly related to the present invention, as the system switching operation is omitted.

First, FIG. 3 shows a normal communication state. The communication information input from the external input line 60 is transmitted to the input line accommodating unit 1 by the current ACT system function unit 21 and the current SB.
Each line input cell processing unit 3A, 3 in the Y system function unit 22
B are output at the same timing. This communication information is converted into ATM cells by the line input cell processing unit 3A of the system function unit 21 and output to the cell switching device 40 through each subsequent processing unit. The processing section 3B internally discards it and does not output it to the subsequent processing section.

In the figure, "communication information 1" indicates communication information distributed and output to the system function units 21 and 22 in the input line accommodation unit 1, and "communication information 2" indicates communication information on the external input line 60. The “communication information 1” is input before the “communication information 2”. Also, "cell 1"
~ "Cell 4" is input before "communication information 1" and is converted into an ATM cell by the line input cell processing unit 3A of the system function unit 21, and indicates that the ATM cell is sequentially converted from "Cell 1". Is shown.

At this time, the system function unit 21
Since the T system and the system function unit 22 are of the SBY system, “system 1” to “cell 4” exist in the system function unit 21 in each processing after the line input cell processing unit 3A. In the functional unit 22, since the communication information is discarded by the line input cell processing unit 3B, no cell exists in the subsequent processing.

The "communication information" input via the external input line 60 according to the present invention generally indicates an existing data communication system including an ATM and a general transfer format.

Accordingly, the number of "cells" output by the ATM cell processing changes according to the adopted communication system, that is, the amount of information contained in one "communication information". The “cell” does not always correspond to 1: 1.

Here, when the switching system control device 30 outputs a system switching preparation instruction, the cell input / output line accommodating device 20 enters a state of system switching preparation shown in FIG. In response to the system switching preparation instruction, first, the line input cell processing unit 3A of the system function unit 21 starts the operation of discarding the newly input communication information, and performs the ATM cell conversion processing after the conversion of the residual communication information into the ATM cell. Cancel, and the subsequent communication information holding instruction is ignored in this system. However, it is assumed that there is no residual communication information in the case of FIG.

The line input cell processing unit 3B of the system function unit 22 stops the communication information discarding operation in response to the system switching preparation instruction, and transmits the communication information sequentially input in response to the subsequent communication information holding instruction. Hold.

Subsequently, in response to a test cell insertion instruction from the switching system controller 30, each test cell insertion unit 5
A and 5B insert a test cell for system switching confirmation. As a result, the test cells are simultaneously output from the system function units 21 and 22, and the test cell insertion units 5A and 5B output a test cell insertion completion notification to the switching system control device 30 in response to the end of the test cell insertion. .

The switching system control unit 30 outputs a system processing restart instruction in response to the reception of the test cell insertion completion notification from both systems, and communicates with the line input cell processing unit 3A of the current ACT function unit 21. Instructs the continuation of the information discarding operation, and at the same time, the line input cell processing unit 3B
To the held communication information and input line accommodation unit 1
The subsequent communication information input from is designated as an ATM cell output.

In FIG. 4, in response to a test cell insertion instruction after "cell 4" is output from line input cell processing section 3A, test cell insertion sections 5A and 5B transmit "test" which is a test cell for system switching confirmation. Is inserted. In the current ACT function unit 21, "test" is inserted after "cell 4".

Further, in response to the test cell insertion completion notification indicating the end of the test cell insertion, the communication information held by the current SBY system line input cell processing unit 3B in response to the system processing restart instruction from the switching system controller 30. And subsequent communication information “cell 5” input from the input line accommodation unit 1
"Cell 6" is output.

Therefore, “cell 4” in the current ACT system is the last cell storing valid communication information before system switching, and cells after “test” in the current SBY system store communication information after system switching. It becomes a cell.

That is, the "test", which is a test cell for system switching confirmation, indicates an end point of effective communication information in each system at the time of system switching processing.

Next, as shown in FIG. 5, the inserted test cells are input to input buffer units 6A and 6B and detected by test cell detectors 8A and 8B, respectively. Outputs a test cell detection notification to switching system controller 30 in response to this.

The switching system control unit 30 is connected to the input buffer unit 6 of the system corresponding to the test cell detection notification received earlier.
Instruct A or 6B to hold cells after the test cell.

In FIG. 5, the current SBY system has the current AC
The "test" is detected before the T system, and the current S
The cells after “test” are held in the BY input buffer unit 6B.

In the current ACT system, each cell ("cell 4") is output from the input buffer 6A until "test" is detected by the test cell detector 8A.

Subsequently, the switching system controller 30 outputs a cell output instruction in response to receiving the test cell detection notification from the other system, and as shown in FIG. 8B is instructed to discard the test cell, and the input buffer unit 6B is instructed to release the cell holding and to output the cell.

As a result, the system function unit 21 that was the current ACT system before the switching becomes the new SBY system, and the system function unit 22 that was the current SBY system becomes the new ACT system, completing a series of system switching operations. I do.

In FIG. 6, the switching system controller 30
The "test" of both systems is discarded in response to the cell output instruction from, and the cell and subsequent cells held up to then in the system function unit 22 (new ACT system) are transferred from the input buffer unit 6B to the "cell 5". ”At the beginning and output to the cell switching device 40 via the predetermined system internal line 80 by the cell output switching unit 9B.

In the system function unit 21 (new SBY system), the communication information input to the line input cell processing unit 3A is discarded without being converted into an ATM cell, that is, in a spare state. It is not output to subsequent processing units.

The communication information holding instruction output to each system from the switching system controller 30 as the system switching preparation described above may be executed as a series of processing operations by the system switching preparation instruction output immediately before. .

Further, the system switching preparation instruction and the subsequent communication information holding instruction output from the switching system controller 30 to both systems are output to both systems in response to the reception of the test cell insertion completion notification from both systems. As a method of outputting these system processing instructions to the line input cell processing units 3A and 3B of both the current ACT system and the current SBY system, the individual instructions are output by control signals at the same timing. System switching in a higher-speed communication system is enabled.

Further, the control signals are made the same, and the circuit input cell processing units 3A and 3B which receive the control signals receive
By discriminating the instruction indicated by the control signal according to the current operation state of each system, the effect is further increased.

[0053]

As described above, according to the present invention, as the system switching operation, both the ACT system and the SBY system output of the ATM cells are stopped, the test cell is inserted while holding the communication information, and the test cell insertion is completed. After that, the output of the SBY-based ATM cell is restarted, and it is determined that the output of the ATM cell before switching in the ACT system has been completed according to the test cell detection in the output stages of both systems, and the inserted test cell is discarded. S
The ATM cells after switching are sequentially output from the BY system.

Therefore, the system switching operation can be performed smoothly even during communication without causing a loss of communication information, and the external connection device caused by the communication information transfer delay and the transmission suppression control during the system switching operation. , And has a special effect that high-speed real-time communication can be realized.

[Brief description of the drawings]

FIG. 1 is a block diagram of a cell input / output line accommodating apparatus according to an embodiment of the present invention.

FIG. 2 is a detailed block diagram of a line input cell processing unit 3A (3B) of FIG. 1;

FIG. 3 is an operation explanatory diagram showing a normal operation before system switching.

FIG. 4 is an operation explanatory diagram showing a state during preparation for system switching (test cell insertion).

FIG. 5 is an operation explanatory diagram showing a state during preparation for system switching (test cell detection).

FIG. 6 is an operation explanatory diagram showing completion of system switching.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Input line accommodation part 3A, 3B Line input cell processing part 3A1, 3B1 Additional information processing part 3A2, 3B2 Buffer part 3A3, 3B3 Celling part 5A, 5B Test cell insertion part 6A, 6B Input buffer part 8A, 8B Test cell detection Unit 9A, 9B Cell output switching unit 20 Cell input / output line accommodating device 21, 22 System function unit 30 Switching system control device 40 Cell switching device 50 Cell switching system 60 External input line 70 External output line 80 System internal line

Continuation of the front page (56) References JP-A-4-346538 (JP, A) JP-A-3-266547 (JP, A) JP-A-5-227196 (JP, A) Technical report of IEICE, SS E91-149 (1992-1-30); 73-78 Proceedings of IEICE Spring National Convention, B-722 (1991-3-15), P3-274 Proceedings of IEICE Autumn National Convention, B-487 (1991-8-15), P3-206 (58) Field surveyed (Int. Cl. ⁶ , DB name) H04L 12/28 H04L 12/56

Claims (3)

(57) [Claims] 1. A system function unit which is connected to a cell switching device which performs data exchange processing by an ATM cell, exchanges communication information given from an external input line for data communication into an ATM cell, and supplies the ATM cell to the cell switching device. A redundant configuration is provided in which a plurality of units having the same configuration are provided as an operation system and a standby system, communication information from an external input line is distributed to respective system functional units at the same timing, and both systems can be switched and controlled during communication. In the input / output line accommodating apparatus provided, the system function unit operating as the main operation system converts the remaining communication information into ATM cells at the time of switching control, and thereafter converts incoming communication information into ATM cells. A cell input cell processing unit for stopping the operation, a test cell insertion unit for outputting a test cell after transmitting the ATM cell output last by the line input cell processing unit, Buffer means for holding a test cell, a test cell detection unit for detecting the test cell from an output from the buffer means, and an output means for outputting an output from the test cell detection unit until detection of the test cell, On the other hand, the system function unit that operates as the standby system includes a line input cell processing unit that sequentially stores communication information arriving after the switching control operation instruction and converts the communication information into an ATM cell during the switching control. A from the processing unit
Prior to the output of the TM cell, test cell insertion means for outputting a test cell, buffer means for sequentially holding the test cell and the ATM cell, and a test cell detection unit for detecting the test cell from the output from the buffer means Output means for outputting an ATM cell after the detection of the test cell. 2. Communication information transmitted and received via an external input line and an external output line for data communication, and an AT.
A plurality of system functional units having the same configuration are provided as a main operating system and a standby system for mutually exchanging M cells and realizing data communication between each external line and the cell switching device, and transmitting communication information from an external input line. In the input / output line accommodating device with a redundant configuration that distributes to the respective system function units at the same timing and enables switching control of both systems during communication, in the case of the main operation system, from the external input line during communication ATM that can delete additional information such as a synchronization establishment flag pattern attached to the communication information specified by the instruction / setting before the start of communication and delete the remaining communication information in an amount of about several tens of cells. After being temporarily stored in a small cell buffer, the communication information stored in the buffer is sequentially allocated to a subscriber communication information storage area in the ATM cell and output as an ATM cell. The input of communication information from the external input line unit to the buffer is stopped according to the instruction, and the subsequent communication information is discarded, and all the communication information remaining in the buffer is output as ATM cells. The buffer is emptied, a buffer vacancy notification for test cell insertion permission is performed, the subsequent communication information is discarded even after the system processing restart instruction, and the buffer vacancy is continued. The communication information after the additional information is discarded during the communication is discarded in response to the system switching preparation instruction, the input of the buffer is stopped, the input to the buffer is resumed, and the communication information is held. Line input cell processing means for converting the communication information and subsequent communication information into ATM cells and sequentially outputting the cells, and a test cell insertion finger provided at the subsequent stage of the line input cell processing means. Test cell insertion means for inserting a test cell for system switching confirmation according to the above and outputting an insertion completion notification in accordance with the completion of insertion, and various types of the cells output from the line input cell processing means in response to a cell holding instruction. Buffer means for sequentially outputting the held cells and subsequent cells in response to the cell output instruction, and detects the test cell among the cells output from the buffer means and outputs a test cell detection notification A system function unit comprising test cell detecting means for discarding the test cell detected in response to the cell output instruction, and outputting the system switching preparation instruction to the two line input cell processing means at the time of system switching. The test cell insertion instruction is output to both test cell insertion means, and the both line input cells are output in response to the test cell insertion completion notification from both systems. The system processing restart instruction is output to the processing means, and the cell holding instruction is output to the buffer means of the corresponding system in response to one of the first test cell detection notifications from the two test cell detection means. And a control unit that outputs the cell output instruction to both of the test cell detection means and the buffer means of the standby system in response to the test cell detection notification from the other system thereafter. Cell input / output line accommodation device. 3. The communication information transmitted / received via an external input line and an external output line for data communication and an ATM cell for data exchange processing in a cell exchange device are mutually converted to convert between each external line and the cell exchange device. A plurality of processing systems for realizing data communication having the same configuration as a main operation system and a standby system are provided, and communication information from an external input line is distributed to the respective processing systems at the same timing. A system switching control method for switching control of the data communication processing operation of the first embodiment, wherein communication information from the external input line during communication is transmitted to an AT.
Discarding the communication information of the main operation system which is converted into an M cell and outputting the data, stopping the ATM cell processing after the conversion of the residual communication information into the ATM cell, and discarding the communication information at the time of communication. And hold the communication information, insert a test cell for system switching confirmation in both systems, in response to the completion of the insertion, the main operating system, while continuing the state, in the standby system The ATM of the held communication information and the subsequent communication information
The cell processing is started, the ATM cells subsequent to the test cells output by the ATM cell processing are held again until both of the test cells inserted into the two systems are detected, and the test in the two systems is performed. The two test cells are discarded in response to the cell detection, and the ATM cells held again in the standby system and the subsequent ATM cells are discarded.
A system switching control method for switching between data communication processing operations of the two systems during communication by restarting cell output.